Determining Batteries State of Charge

gregbyst · November 2020

Hello,

I finally wired my whole set up together! Big thanks to those who helped me out on here. It is working well so far, but I am struggling to determine an accurate state of charge.

I called Renogy to ask what the figures are and they gave me some numbers. Then I called again to double check and they gave me RADICALLY different answers. Alarming since following the second figures would mean discharging my AGM's bowl 50% according to the second figures.

Since then I learned that if the batteries are under load the state of charge is going to reflect differently than if they are not under load. I also understand I can charge them and then wait awhile for the phantom voltage to diminish.

In my case, I am constantly using the batteries. How do I determine an accurate state of charge to program my battery monitor with (and then hopefully never worry again)?

So far I have just been using 26 volts (I have a 24 volt battery bank) as the full charge but I think they can actually hold voltage above that. Which actually brings up another question... If my batteries can go above the rated "full charge" figures from Renogy (say 26.5 volts) should I set that as the "full" and 26 volts would actually be like 90% (just for example; not accurate)?

Thanks!

BB. · November 2020

A couple of suggestions...

First, figure out what your average load is during the day... Say it is 15 amps @ 24 volts. Then, if you are aiming for 10% rate of charge, you want the solar charger output current to be (at least) 15 amps load + 10% * AH rating of battery bank... Made up example:

(0.10 rate of charge * 600 AH battery bank + 15 amps load) = 75 Amps from charge controller (minimum)
75 Amps * 29 volts charging * 1/0.77 panel+charge controller derating = 2,825 Watt array (minimum for load+charging)

And then there is monitoring your battery bank voltage... You are correct, the estimated state of charge is affected by charging/discharging current, state of charge, temperature, etc. Here is an article/chart that shows the basics for a Flooded Cell Lead Acid battery bank:

http://www.scubaengineer.com/documents/lead_acid_battery_charging_graphs.pdf

For your battery bank... It is not charging unless you measure (at 77F/25C) around 28.0 to 28.8 volts on the battery bus. Ideally, you want around 10% (minimum) rate of charge (if the bank is discharged to (roughly) less than 80% SoC) and you see >~14.0 volts on the bus. As the bank recharges in "bulk mode" (bulk is the maximum current from charge controller--current limited charging), the battery bus will eventually reach the charging voltage setpoint (around 28.8 VDC is typical for AGM). At that point, you want to hold 28.8 volts for 2-6 hour or so (the deeper the discharge, the longer the Absorb Cycle time). Once the voltage has been held for 2-6 hours (estimated)--Then the battery is "full".

You can also monitor the battery charging current (use a DC Current Clamp type DMM) and when the charging current falls below ~1% (or even down to 0.1%) of AH rating as charging current--The battery is "full" (monitoring the "tail" of the charging current is an ideal method to terminate charging--or drop down to float voltage at ~27.2 volts for float) (as always, check your battery specifications for charging current/voltages/etc.).

Once the battery bank is charged, and the sun has set... You will see the battery bus voltage fall. Nominally, a fully charged AGM is around 26.4 volts to 25.6 volts at full charge and after a few hours of "resting"...

As the bank starts to discharge--You will see the voltage fall.... As a guess, you probably do not want to see less than ~23.0 volts on the battery bus under "moderate" loads--That is (very roughly) 50% +/- state of charge...

You cannot (easily) measure the specific gravity of a sealed/AGM battery... So you are left with estimating SoC using a voltmeter (and looking at current). And/or getting a battery monitor or similar.

There are several classes of Battery Monitors. A simple one just monitors the battery bank voltage and estimates state of charge (links are for your research--I do not have experience with any of this equipment):

https://www.solar-electric.com/midnite-solar-mnbcms.html
http://smartgauge.co.uk/products.html

And there are those that use a current shunt (precision resistor) in the battery leg to measure current flow in/out of the battery bank (as well as voltage):

https://www.solar-electric.com/bogart-engineering-tm-2030-a-battery-monitor.html
https://www.solar-electric.com/victron-energy-bmv-712-smart-battery-monitor.html

And there are a few charge controllers with integrated battery monitors--The Midnite MPPT family have that capability.

Lastly, besides using an AC/DC Current Clamp DMM--There are a lot of low cost battery/DC monitoring systems out there--Not sure if I would trust a $2,000 battery bank to them, but for some experimenting/understanding, they are hard to ignore:

https://www.amazon.com/s?k=dc+battery+monitor+meter&crid=O9E1AR7RN4R1&sprefix=dc+battery+monitor,aps,407&ref=nb_sb_ss_ts-a-p_2_18

The big issue is that most battery banks are killed by undercharging / over discharging. AGMs... You do want to monitor them to make sure they are not getting too hot during charging, and that they are not bubbling/hissing (over gassing, venting).

As always, details matter with solar power systems.

-Bill

Photowhit · November 2020

gregbyst said:

In my case, I am constantly using the batteries. How do I determine an accurate state of charge to program my battery monitor with (and then hopefully never worry again)?

So without knowledge of your system we can only give generalities.

You appear to understand 'generalities' pretty well. As Bill said there are 2 basic types of batteries monitors, one that is basically voltage based and usually right like a broken clock, twice a day. And one that is shunt based, measuring current going into and out of your battery bank.

Please give a description of your system, size of array, charge controller, battery bank, battery monitor, even inverter and, if you like, and loads...

It is very frustrating to hunt down battery charging recommendations, only to find you aren't talking about a Renogy battery, but a Renogy charge controller!

You can add this as a signature like mine below which is 3-4 years out of date below...
▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼

New_Mexico_Will · November 2020

This is especially true of Renogy batteries, because they make more than one type of AGM. They make traditional AGMs, as well as their hybrid battery, which is a gel AGM, and those batteries have different characteristics. This may also explain why you got different answers from different people.

Link to hybrid specs: https://www.renogy.com/content/RBT200GEL12-G1/GEL200-Datasheet.pdf

Link to traditional AGM specs: https://www.renogy.com/content/RNG-BATT-AGM12-200/AGM200-Datasheet.pdf

Marc Kurth · November 2020

Take a look at their published Depth-of-Discharge vs Cycle Life curves. They show that with an average daily discharge of 30%, their batteries will be down to about 80% of their original capacity in 3.5 years. Also at 30% DOD, in 4.5 years your battery bank will have around 60% of its design capacity.

It would appear that these are designed for more of a backup type duty than daily cycling. Based on their curves, I would stay in a 15% to 20% average daily depth-of-discharge range in order to get a reasonable service life. (They do not publish those numbers - I am estimating)

littleharbor2 · November 2020

Marc Kurth said:

Take a look at their published Depth-of-Discharge vs Cycle Life curves. They show that with an average daily discharge of 30%, their batteries will be down to about 80% of their original capacity in 3.5 years. Also at 30% DOD, in 4.5 years your battery bank will have around 60% of its design capacity.
It would appear that these are designed for more of a backup type duty than daily cycling. Based on their curves, I would stay in a 15% to 20% average daily depth-of-discharge range in order to get a reasonable service life. (They do not publish those numbers - I am estimating)

Marc, Isn't 80% of original capacity the standard for useful life of a battery bank? Seems I heard this before.

gregbyst · November 2020

BB. said:

A couple of suggestions...

First, figure out what your average load is during the day... Say it is 15 amps @ 24 volts. Then, if you are aiming for 10% rate of charge, you want the solar charger output current to be (at least) 15 amps load + 10% * AH rating of battery bank... Made up example:
(0.10 rate of charge * 600 AH battery bank + 15 amps load) = 75 Amps from charge controller (minimum)
75 Amps * 29 volts charging * 1/0.77 panel+charge controller derating = 2,825 Watt array (minimum for load+charging)
And then there is monitoring your battery bank voltage... You are correct, the estimated state of charge is affected by charging/discharging current, state of charge, temperature, etc. Here is an article/chart that shows the basics for a Flooded Cell Lead Acid battery bank:

http://www.scubaengineer.com/documents/lead_acid_battery_charging_graphs.pdf

For your battery bank... It is not charging unless you measure (at 77F/25C) around 28.0 to 28.8 volts on the battery bus. Ideally, you want around 10% (minimum) rate of charge (if the bank is discharged to (roughly) less than 80% SoC) and you see >~14.0 volts on the bus. As the bank recharges in "bulk mode" (bulk is the maximum current from charge controller--current limited charging), the battery bus will eventually reach the charging voltage setpoint (around 28.8 VDC is typical for AGM). At that point, you want to hold 28.8 volts for 2-6 hour or so (the deeper the discharge, the longer the Absorb Cycle time). Once the voltage has been held for 2-6 hours (estimated)--Then the battery is "full".

You can also monitor the battery charging current (use a DC Current Clamp type DMM) and when the charging current falls below ~1% (or even down to 0.1%) of AH rating as charging current--The battery is "full" (monitoring the "tail" of the charging current is an ideal method to terminate charging--or drop down to float voltage at ~27.2 volts for float) (as always, check your battery specifications for charging current/voltages/etc.).

Once the battery bank is charged, and the sun has set... You will see the battery bus voltage fall. Nominally, a fully charged AGM is around 26.4 volts to 25.6 volts at full charge and after a few hours of "resting"...

As the bank starts to discharge--You will see the voltage fall.... As a guess, you probably do not want to see less than ~23.0 volts on the battery bus under "moderate" loads--That is (very roughly) 50% +/- state of charge...

You cannot (easily) measure the specific gravity of a sealed/AGM battery... So you are left with estimating SoC using a voltmeter (and looking at current). And/or getting a battery monitor or similar.

There are several classes of Battery Monitors. A simple one just monitors the battery bank voltage and estimates state of charge (links are for your research--I do not have experience with any of this equipment):

https://www.solar-electric.com/midnite-solar-mnbcms.html
http://smartgauge.co.uk/products.html

And there are those that use a current shunt (precision resistor) in the battery leg to measure current flow in/out of the battery bank (as well as voltage):

https://www.solar-electric.com/bogart-engineering-tm-2030-a-battery-monitor.html
https://www.solar-electric.com/victron-energy-bmv-712-smart-battery-monitor.html

And there are a few charge controllers with integrated battery monitors--The Midnite MPPT family have that capability.

Lastly, besides using an AC/DC Current Clamp DMM--There are a lot of low cost battery/DC monitoring systems out there--Not sure if I would trust a $2,000 battery bank to them, but for some experimenting/understanding, they are hard to ignore:

https://www.amazon.com/s?k=dc+battery+monitor+meter&crid=O9E1AR7RN4R1&sprefix=dc+battery+monitor,aps,407&ref=nb_sb_ss_ts-a-p_2_18

The big issue is that most battery banks are killed by undercharging / over discharging. AGMs... You do want to monitor them to make sure they are not getting too hot during charging, and that they are not bubbling/hissing (over gassing, venting).

As always, details matter with solar power systems.

-Bill

Thanks Bill,

My battery bank is 400ah at 24 volts. They are AGM Renogy Batteries. Here are the specs:

Image: https://us.v-cdn.net/6024911/uploads/editor/ud/m2fir3oreqkq.jpg

As you can see, the rate of charge is only 60 amps. So, with a 400ah bank a 10 percent rate of charge plus my average daily load (5 amp load except for occasionally running the microwave or coffee maker) is 45 amps. The highest I have seen from my solar array is a 20 amp charge. Is this bad?

I also have a generator and my inverter doubles a charger. It is capable of charging at 120 amps and I have it set to about halfway (60 amps)... is this bad?

I have a coulemeter (not sure if I spelled it right) and my MPPT Epever charge controller also tells me the battery voltage (as well as the amps it's being charged at (from the solar anyways)).

My battery monitor tells me the load in amps, the voltage, and the amp hours still left in the bank. I just don't know if I can trust the information. How does it know that the amp hours are all the way charged? Even my charge controller will tell me that I am not fully charge, but I will have 26 volts under load (surely that's fully charged?).

I do currently see my bank get to around 29.2 volts for the bulk charge and then drop down to 27. (something) for the float charge. So it does seem to be working correctly that way.

ALSO:

Today I figured out something wrong with my battery monitor situation. My amp hours never rise on my battery monitor when charging from the solar because the charge controller is connected DIRECTLY to the batteries. I was informed by someone that this was the correct thing to do. Should in fact be attaching the negative and positive of the charge controller to the bus bars (that way the shunt would know when the solar is charging the batteries)?

gregbyst · November 2020

So, the batteries are full once the bulk phase of the charge cycle has completed? I thought they were almost full and then the float cycle tops them off from there...? BB. said:

For your battery bank... It is not charging unless you measure (at 77F/25C) around 28.0 to 28.8 volts on the battery bus. Ideally, you want around 10% (minimum) rate of charge (if the bank is discharged to (roughly) less than 80% SoC) and you see >~14.0 volts on the bus. As the bank recharges in "bulk mode" (bulk is the maximum current from charge controller--current limited charging), the battery bus will eventually reach the charging voltage setpoint (around 28.8 VDC is typical for AGM). At that point, you want to hold 28.8 volts for 2-6 hour or so (the deeper the discharge, the longer the Absorb Cycle time). Once the voltage has been held for 2-6 hours (estimated)--Then the battery is "full".

gregbyst · November 2020

New_Mexico_Will said:

This is especially true of Renogy batteries, because they make more than one type of AGM. They make traditional AGMs, as well as their hybrid battery, which is a gel AGM, and those batteries have different characteristics. This may also explain why you got different answers from different people.

Link to hybrid specs: https://www.renogy.com/content/RBT200GEL12-G1/GEL200-Datasheet.pdf

Link to traditional AGM specs: https://www.renogy.com/content/RNG-BATT-AGM12-200/AGM200-Datasheet.pdf

Mine is the second one! And maybe! They don't tell me the state of charge figures on those data sheets tho... and now Renogy decided not to answer the phone anymore.

gregbyst · November 2020

Photowhit said:

gregbyst said:

In my case, I am constantly using the batteries. How do I determine an accurate state of charge to program my battery monitor with (and then hopefully never worry again)?

So without knowledge of your system we can only give generalities.

You appear to understand 'generalities' pretty well. As Bill said there are 2 basic types of batteries monitors, one that is basically voltage based and usually right like a broken clock, twice a day. And one that is shunt based, measuring current going into and out of your battery bank.

Please give a description of your system, size of array, charge controller, battery bank, battery monitor, even inverter and, if you like, and loads...

It is very frustrating to hunt down battery charging recommendations, only to find you aren't talking about a Renogy battery, but a Renogy charge controller!

You can add this as a signature like mine below which is 3-4 years out of date below...
▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼

Thanks! I will update my user tag thing.

I have:

- 4 of the 12 volt, 200ah AGM renogy batteries (I sent a screenshot of the specs above).
- 150/60 MPPT Epever Charge Controller
- 3000 watt (9000 peak) inverter/charger/auto-transfer switch
- PV array of 6 260 watt, 30.7 Vmpp, 8.5 Impp.

Also, I live in a fifth wheel!

gregbyst · November 2020

Here's the battery shunt: https://www.amazon.com/gp/product/B07FGFFHC6/ref=ppx_yo_dt_b_asin_title_o07_s00?ie=UTF8&psc=1

Photowhit · November 2020

gregbyst said:

Here's the battery shunt: https://www.amazon.com/gp/product/B07FGFFHC6/ref=ppx_yo_dt_b_asin_title_o07_s00?ie=UTF8&psc=1

That does look to be a shunt based battery meter. I don't know if it allows for losses based on battery type or resets after reaching fully charged, but it should be better than a voltage meter.

The reason I suggest a voltage based monitor is right twice a day is that all it 'sees' is the 'system voltage', both charging and loads effect the 'system voltage'

Here's a bit about how charging works for lead acid batteries, AGM batteries can usually take a bit higher charge rates closer to 85-90% of capacity. I suggest using Renogy's charging profiles;

Charging cycles;

The voltage you are seeing is the system voltage and not the battery voltage. If you are connected to charging or a load it will effect the system voltage.

During charging, there are basically 3 stages of charging, Bulk, Absorb, and Float.

BULK;

First thing when charging starts you will be in bulk, the voltage rises from what ever the system voltage was to a set point, around 14.5 volts. At that point the Charge controller stops the voltage from rising. Higher voltage can damage sealed batteries.

ABSORB;

Once the battery hits the preset point the charge controller keeps it at that point. Your batteries are roughly 80% full. Flooded batteries will start accepting less current at 80-85% full AGM/Sealed may go a little longer before accepting less current.

On many controllers you can set this point, Some will have different presets for Flooded, and sealed batteries, or flooded, AGM, and sealed batteries.

The charge controller has a couple ways to know when to switch to float, Most inexpensive Charge controller are just timed for 1.5-2 hours. Some will also see less current flowing through the charge controller and shut it down when minimal current is flowing through the controller. On more expensive charge controller. You can set battery capacity to give the Controller a better idea of when to stop. you can also set a longer Absorb time. Or set 'end amps' a amount of amps flowing through the charge controller to stop Absorb and switch to the final stage.

FLOAT;

Once the Controller has determined the battery is fully charged it reduces the voltage to a point where very little current is flowing to the battery. This will prevent the battery from over charging and heating up.

While in 'Float' the charge controller watch for voltage drop, which would indicate a load. If the voltage begins to drop the charge controller will allow as much current to flow from the panels/array to compensate and maintain the voltage. If the voltage can be maintained, the load will in essence be running directly off the array/solar. If the voltage drops below the preset float voltage, the controller may start a whole new cycle if it stays there for a period of time.

The system voltage drop you see at night when the sun goes down is the charge controller moving into a resting mode with no energy to contribute to the system.

gregbyst · November 2020

Thank you. I had read about those but you gave me some information I didn't know.

My battery monitor requires that I "set it". I can set it to 100%. To do this, should I wait until my batteries are charged, drop down to float voltage, shut off the charge source, and wait a while for the voltage to drop to the resting state before setting it to 100%?

Also, is the "cycle-use" voltage the max voltage the batteries can handle in bulk phase? (referencing the above specs for my batteries)

gregbyst · November 2020

Also, in terms of the wiring set up, should be charge controller and inverter both be connected to the same busbar ( not directly connected to battery)?

And should they both pass through the shunt so that I can see what the charging amps are, as well as so the monitor can tell that it was recharged?

BB. · November 2020

I think different mfg. have different End of Life specs... The 80% of capacity (20% loss of capacity), was, as I recall, the Concorde battery company's EOL spec. (at least for warranty purposes).

The realitly is EOL is when the battery bank no longer functions for your needs... If you use 25% per day for your needs, and rarely go down to 50%, then you can lose almost 50% of battery capacity before they fail to meet your needs... Of course, you may not want that to happen on a December-January dark and stormy night--So, you probably want to change out the bank before they fail for you needs.

The 5%/10%/13%+ rate of charge is one mark in the sand... It usually keeps a property designed battery bank "happy"... 10%+ is really needed for daily operation. 5% only for a small system/emergency backup/weekend sunny weather system...

The other is having enough solar panel to replace the battery energy used overnight (and during bad weather). That is based on your location/hours of sun per day/and your other needs (more panels, less genset runtime).

For people in very sunny locations (southwestern US), 10% can be more than enough for a typical system (25% discharge per day, for 2 days, to 50% maximum planned discharge). In places with less sun and/or occupied in Winter--then the second calculation based on loads and hours of sun per day (and allowed genset runtime)--May be a larger array than a 10%-13% array calculation "requires".

Usually, really like to figure out the loads first, then design the battery bank... Then size the solar array for the bank and daily loads (for location and by season).

Wiring up a parallel battery bank--There is the Smartgauge website so the parallel strings properly share charging/discharging current:

http://www.smartgauge.co.uk/batt_con.html

Measuring your daily loads (AH or WH) is a good first estimate of how deep you are cycling the battery bank. 25% discharge suggested for optimum full time off grid usage... 50% daily discharge can work for a weekend system:

400 AH * 0.25 discharge = 100 AH per day discharge to 75% state of charge
100 AH * 24 volts * 0.85 inverter eff = 2,040 Watt*Hour daily load

For you solar array, a 6 x 260 Watt array should produce (in bulk charge, cool/clear day, near noon, a few times a year):

6 panels * 260 Watts * 0.77 panel+controller derating * 28.8 volts (or less) charging = 41.7 Amps typical best case (more or less)

This does depend on your controller's charge settings, what mode it is in (bulk, absorb, float), and what (if any) loads you have on the battery bank... If you are getting less than 1/2 that--Then I would wonder if everything is OK or not (check voltage/current at the Vpanel input for the controller, check Vbatt for controller and Vbatt-bus voltage for excessive voltage drop, etc.). If you have a DC current clamp meter, you can measure the current from each solar panel string or if you have a combiner box+breakers--cycle each string off/on. Ideally, each string should be carrying its share of current.

Generally, with a battery current monitor--You want the battery bank connected in series/parallel and the positive lead connected to Vbatt-bus...

For the precision shunt (resistor), you want one end connected to the Battery negative bus, and the other end to be the "true" battery Vbatt-negative-bus. This is where all the negative load and charge controller connections are made. You want the shunt to "see" all battery current. If you wired your charge controller to the battery directly (not through the shunt)--The charger will be "invisible" to the battery current monitor.

Of course, it is the two heavy cables that connect to the large terminals on the resistor and to the battery bank/negative system bus. The two small leads just measure the voltage drop and give the 'current flow' information to the battery monitor.

The Coulumb meter is named after Charles-Augustin de Coulomb--An early researcher into electricity.

https://en.wikipedia.org/wiki/Coulomb

As Photowhit says... Float does not charge the battery--It just keeps up with "self discharge" of the battery (0.1% or less for AGM), and also supplies current to your daytime loads.

It sounds like your charge controller is reaching ~29.2 volts--The specifications for your battery (two in series). And, you want the controller to hold that voltage for something like 2-6 hours (deeper cycling, longer time).

If your controller is in bulk (current limited by sun and solar panels) and less than 29.2 volts--That is when you will measure maximum current flow charging. Once your battery bank reaches 29.2 volts (absorb charge mode), the battery bank starts to limit its input charging current (normal operation).

You can always try and put a 1,500 Watt heater on your AC inverter output around solar noon, and pull a lot of current (1,500 Watts / 24 volts = 62.5 amps)--Then you will see what your solar array + charge controller can truly output.

Note that with MPPT controllers and AC inverters--We are dealing with "constant power" devices... Power=Voltage*Current... So, the exact battery bus voltage will affect the current supplied/drawn... I.e.:

1,500 Watts * 1/0.85 AC inverter eff = 1,765 Watts from battery bus
1,765 Watts / 29.2 volts (battery charging) = 60.4 amps
1,765 Watts / 23.0 volts (battery battery somewhere 50% discharge under load) = 73.6 amps

And for the MPPT controller, similar behaviours:

6 * 260 Watt array * 0.77 panel+controller deratings = 1,201 Watt typical "best case" harvest
1,201 Watts array / 29.2 volts charging (absorb setpoint) = 41.1 Amps best case
1,201 Watt array / 25.2 volts (charging discharged/heavy loads) = 47.6 Amp best case at lower battery bus votages

Note that AGM batteries are nearly 100% efficient for Amps*Hours (Coulumb) measurements... You take out 10 AH for loads, then you restore 10 AH during charging. Note that Watt*Hour are not that efficient. 10a*29.2v= 292WH charging. And 10a*25.2v= 250WH discharging.

I have not read the meter specs... But some will reset back to 100% full when 29.2 volts is held for ~1 hour (or whatever settings you use)... This is to help prevent "drift" in the meter readings.

From the Amazon link, there is this setup procedure (did not find a manual with a quick look)

Question:
How do i set this to 170 amp hours? i purchased the one that goes to 350 ah. i have 2 85 amp hour batteries. the monitor only goes to 100 amp hrs.

Answer:

Model "350A" means it can handle current up to 350 amps (NOT Capacity), 170 amp hours is your battery capacity, this item can measure capacity up to 999 amp hours.
At first use, the percentage and capacity displayed is not the actual correct value
How to initialize capacity (Ah) and percent (%) reading?
First. the default capacity reading is 100 amp hours. Press Key A/Ah to capacity reading interface (x.xxx Ah), and hold the key A/AH for 3 seconds (long press), default reading "100.0 Ah" blinks, press key "%" or key "A" to change the value (long press key to change the value faster), you can add value up to 999 Ah in this way.
Second, the percentage displayed is not the actual correct value, charge the battery fully, press key % to percentage reading interface, and hold the key % for 3 seconds to set the capacity full (100%).
By Battery Monitor on August 21, 2019

-Bill

gregbyst · November 2020

Wow, thanks for the info! I'm going to have to read that a few times. Will check back tomorrow. Thanks again

gregbyst · December 2020

Sorry for the delay in reading that. Life has been busy.

So, If I am understanding correctly, then I can set my battery monitor to full when I see the voltage switch from ABSORB to FLOAT? And I am assuming that I should unplug the loads AND the charge source (solar or generator) to wait for the phantom voltage to drop? How long should I wait?

Also, I am getting nowhere near a 10% rate of charge. I will sometimes get 20 amps, but my load is usually 5 amps and the 10% rate of charge for my 24 volt bank at 400ah is 40 amps. Is this harmful for my batteries?

BB. · December 2020

AGM batteries can survive on a lower rate of charge (as I understand)--However, you still want to make sure they do get "fully charged" (>~90% SoC) at least once or several times per week (assuming you are cycling daily). And AGM batteries tend towards 90-98% efficiency.

When we look at a system for "balanced" design... The loads define the battery bank (i.e,. 25% discharge per day, for 2 days, to 50% maximum planned discharge--Gives longer life, and a 10%+ rate of charge gets the bank back fully charged in a day or so after some bad weather).

And the amount of sun (hours per day) vs loads, and that 5/10/13% rate of charge work out well together.

Off grid systems generally don't get a chance to rest for 3 hours for their battery banks so that you can estimate your state of charge... So, instead, measuring you loads and having sufficient charging current (and hours of sun per day) is what we really aim for...

If you want a system that you don't have to babysit--You really need to know (for example) what your loads are... For example 5 amps (at 12 volts) is just a piece of the puzzle. Is that 5 amps for 5 hours per day (5a*5h=25ah per day) or is that 24 hours per day (5a*24h=120ah per day)...

if you get 20 amps charging for 5 hours per day (summer usage), that is (in this example: 20a*5h of sun = ) 100 AH per day usage.

So if you use 5a*5h per day (25 AH of loads) will easily be filled by an array with 20a*5h of sun per day (100 AH charging) in general (obviously bad weather/genset/genset for winter still requires some thought/planning here).

So--Cannot really tell you what is your system's performance (regarding keeping the batteries "happy") without more information...

Daily loads in AH/WH per day (at xx volts)
Base loads (such as propane fridge, LED lighting) vs optional loads (laptop/networking/camera/video/etc... that you can skip during bad weather, etc.).
AC inverter? Any tools/high surge current loads?
Where (hours of sun per day)
When (seasonal--3 season use, summer weekend use, full time usage)?
Backup genset, using vehicle alternator
Solar array--How big of array can you fit/wish to pay for?
Solar array--Mounted flat to roof, tilting (better for winter/farther north trips)?

If you do not know your loads yet (have not measured them, new to RV life, etc.)... I can estimate how much power you can harvest from your system (based on location and panel angles, panel wattage, battery bank capacity)...

The 3,000 Watt AC inverter is (typically) way overkill for an RV on 12 volts. For a 12 volt battery bank, generally a 1,200 to 1,800 Watt array is about maximum (you are talking about ~200 Amps @ 12 volts for a 2,000 Watt load--Very heavy cables). An 800 AH battery bank @ 12 volts is good for a max inverter of ~2,000 Watts max anyway.

For a typical RV--Closer to a 300-600 Watt inverter is better for normal loads... Although, a 1,200+ Watt AC inverter could be used for a microwave.

Some folks are using Induction cooktops and Microwaves and even washer/drier units... But that is a lot to expect from a 12 volt system. (and many will use LiFePO4 batteries for better performance/lighter weight).

Yes--You could set to 100% on your battery monitor when the battery transition to float (you don't have to catch the transition--Just "set/reset" the monitor while the battery bank is at "float voltage").

-Bill

gregbyst · December 2020

Thanks Bill,

I will be creating a SOC chart for my batteries soon with figures from it being both under a load and not.

The way I designed my system was probably not the best, but I was in a hurry and confused. I used these principles:

1. Want to run the Air Conditioner and microwave without the generator.
2. Create the largest solar array possible for my trailer.
3. Get the biggest battery bank I can afford.

So basically, I got as many batteries as I could and the inverter that would allow me to run the AC and microwave. My solar array is currently only 6 panels, but I will expand it with my ground array consisting of another 3. That will allow me to boost my power from the array considerably and I can bring out the ground array when necessary, but leave it away when not.

We are living in the RV full time and the typical constant amperage taken (at 24 volts) is about 5-7 amps during the day and maybe 3 during the night unless we turn the inverter off (then it is about 1 amp).

We will be traveling full-time across the U.S within a few months hopefully so the weather will be changing constantly for us. My solar is mounted flat on the roof unfortunately, but I think I'll make that up with the ground array and additional 3 panels I plan on putting on the roof.

Do you know why my inverter shuts off when running the Air conditioner for more than 10 minutes btw?

Maybe I should make that question into a new thread...

BB. · December 2020

This is your system discussion... The history and details help, so I would just keep posting here.

Do you have a voltmeter (or use the Battery Monitor)? Most 12 volt AC inverters shutdown at 10.5 volts... And you probably don't want to run the bank much below ~11.5 volts under moderate loads (i.e., 5 minutes under 11.5 volts--Probably getting towards 50% (really rough estimate).

Measuring the voltage on the input to the AC inverter should tell you if the battery bank + wiring is capable or not (note: suggest a maximum of 0.5 volts from battery bus to AC inverter input under "full load").

Pulling much more than 1,800 Watts from a 12 volt battery bank is tough... AGM batteries can usually supply much higher discharge current than FLA batteries... But you still need to look at the wiring/voltage drop too.

-Bill

Photowhit · December 2020

gregbyst said:
1. Want to run the Air Conditioner and microwave without the generator..

I'm sure you understand this will be very difficult to do! I think you should first get a handle on how large a load the air conditioner is. I think minimum wattage for a standard roof top air conditioner on an RV is 900 watts for a 1200btu, here's some sort of range based on btu ratings;

Image: https://us.v-cdn.net/6024911/uploads/editor/3s/spt7lx5xhkho.jpg

https://www.ramsond.com/wattage-chart/

gregbyst said:
We are living in the RV full time and the typical constant amperage taken (at 24 volts) is about 5-7 amps during the day and maybe 3 during the night unless we turn the inverter off (then it is about 1 amp).

So your current loads are 6 ampshours at daytime system voltage of around 29 volts or 6x29=174watts x 12= 2088 watt hours + night time loads 2ahx24v=48 watts x 12 hours =600 watt hours. You aren't doing much more than meeting your needs now! or roughly 2700 watthours per day (without further system losses) or 2700x30=81,000 watt hours a month.

So you aren't getting much more than the average 3 hours of daylight this time of year the flat panels will provide.

I've run an air conditioner on a minimal system, It was a fixed system and I built the cabin in the shade with 6" thick walls. During the summer running a small room air conditioner (5200btu 440 watts running) I just might have been able to run it 24/7 with an appropriate battery bank. I had just a 24 volt 210 ah system, about 1/2 of what you have. With a 1600 watt array. Unfortunately you will have to park in the sun to get full exposure to your array, so you will have a much higher heat gain. You are running a larger less efficient air conditioner, You have less insulation and even adding 800 more watts of array I think it's going to be very difficult to meet your needs.

Lots to be taken into account, The duty cycle of the air conditioner will be very high because you will need to park in the sun, I ran nearly 80% duty cycle for the first hour cooling down the room and 50% or so the next hour down to 30% or so as the cabin cooled off so I could sleep. I typically ran the air conditioner about 5 hours at night When I started with a smaller array, The batteries were just getting topped off and I did not run the air conditioner during the day. When I increased the array to 1600 watts I was able to run the air conditioner for a few hours in the afternoon as the batteries reached absorb...

Image: https://us.v-cdn.net/6024911/uploads/editor/c4/v8o6yx5a9r4u.jpg

You might consider retrofitting a minisplit air conditioner, I've read a couple of accounts of tiny homes getting close with them. They will be much more efficient than your roof top air conditioner. I'm NOT someone to suggest looking around at youtube videos, but you might look to see what you can find on running a air conditioner off solar in a RV.

gregbyst · December 2020

BB. said:

This is your system discussion... The history and details help, so I would just keep posting here.

Do you have a voltmeter (or use the Battery Monitor)? Most 12 volt AC inverters shutdown at 10.5 volts... And you probably don't want to run the bank much below ~11.5 volts under moderate loads (i.e., 5 minutes under 11.5 volts--Probably getting towards 50% (really rough estimate).

Measuring the voltage on the input to the AC inverter should tell you if the battery bank + wiring is capable or not (note: suggest a maximum of 0.5 volts from battery bus to AC inverter input under "full load").

Pulling much more than 1,800 Watts from a 12 volt battery bank is tough... AGM batteries can usually supply much higher discharge current than FLA batteries... But you still need to look at the wiring/voltage drop too.

-Bill

You're talking about measuring to see if I am getting a voltage drop from the batteries to the inverter correct?

gregbyst · December 2020

Photowhit said:

gregbyst said:
1. Want to run the Air Conditioner and microwave without the generator..

I'm sure you understand this will be very difficult to do! I think you should first get a handle on how large a load the air conditioner is. I think minimum wattage for a standard roof top air conditioner on an RV is 900 watts for a 1200btu, here's some sort of range based on btu ratings;

https://www.ramsond.com/wattage-chart/

gregbyst said:
We are living in the RV full time and the typical constant amperage taken (at 24 volts) is about 5-7 amps during the day and maybe 3 during the night unless we turn the inverter off (then it is about 1 amp).

So your current loads are 6 ampshours at daytime system voltage of around 29 volts or 6x29=174watts x 12= 2088 watt hours + night time loads 2ahx24v=48 watts x 12 hours =600 watt hours. You aren't doing much more than meeting your needs now! or roughly 2700 watthours per day (without further system losses) or 2700x30=81,000 watt hours a month.

So you aren't getting much more than the average 3 hours of daylight this time of year the flat panels will provide.

I've run an air conditioner on a minimal system, It was a fixed system and I built the cabin in the shade with 6" thick walls. During the summer running a small room air conditioner (5200btu 440 watts running) I just might have been able to run it 24/7 with an appropriate battery bank. I had just a 24 volt 210 ah system, about 1/2 of what you have. With a 1600 watt array. Unfortunately you will have to park in the sun to get full exposure to your array, so you will have a much higher heat gain. You are running a larger less efficient air conditioner, You have less insulation and even adding 800 more watts of array I think it's going to be very difficult to meet your needs.

Lots to be taken into account, The duty cycle of the air conditioner will be very high because you will need to park in the sun, I ran nearly 80% duty cycle for the first hour cooling down the room and 50% or so the next hour down to 30% or so as the cabin cooled off so I could sleep. I typically ran the air conditioner about 5 hours at night When I started with a smaller array, The batteries were just getting topped off and I did not run the air conditioner during the day. When I increased the array to 1600 watts I was able to run the air conditioner for a few hours in the afternoon as the batteries reached absorb...

You might consider retrofitting a minisplit air conditioner, I've read a couple of accounts of tiny homes getting close with them. They will be much more efficient than your roof top air conditioner. I'm NOT someone to suggest looking around at youtube videos, but you might look to see what you can find on running a air conditioner off solar in a RV.

That's really cool!

It is true that I am already barely making my power consumption needs right now...

But I have three additional panels to add as a ground array AND three more to get on the roof once I buy some frames for them. With this boost, I should have a lot more wiggle room for the A/C. Also, it's not even necessary to run it right now, but I would like to be able to run it for a little while without using the generator once the weather is warmer(and save some gas/money).

gregbyst · December 2020

I should be able to be running my air conditioner now for at least a couple hours until 50%, but I get 10 minutes or so lol

Photowhit · December 2020

gregbyst said:

I should be able to be running my air conditioner now for at least a couple hours until 50%, but I get 10 minutes or so lol

Could be many things, I meant to address that, usually the fans come on long before the compressor kicks in, are you sure the compressor is already kicked in? My window unit takes at least 5 minutes before kicking in the compressor. It's out in the heat and it wants t measure the temperature of the inside air. That might just be on ECO mode.

The next bit will be the wiring, I think you have adiquat heavy gauge wiring, but can't find it in all the replies. A 3000 watt 24 volt inverter, Should have pretty stout cabling from your battery bank, also sometimes the inverter can be a good distance from the battery bank in an RV. So what we need to look at involves voltage drop, If you have 1/0 wire and you are 10 feet away from the battery bank, Again we need to know the load, but for a 1200 watt load, run during the evening (no solar input) on a full battery. It would look something like this;

1200 watt load AC + 150 watts for the inverter inefficiency total load 1350watts. 1350/24.7= 54.6 amps

So it would look like this;

Image: https://us.v-cdn.net/6024911/uploads/editor/zm/3zzhnxe1m91j.jpg

voltage drop calculator here;
https://www.calculator.net/voltage-drop-calculator.html

Not too bad, The draw is roughly 1/8th of your battery bank per hour, batteries are calculated at a draw of 1/20th the the effective size of your battery bank will be less than 400 amps (rated at c/20)

The spike starting up will be much higher, but need to get some real numbers to work with.

Determining Batteries State of Charge

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