# How to get the most amp hour from your battery bank

Registered Users Posts: 8 ✭✭
edited October 2020 #1
I’m currently installing 8 x 6v 400ah batteries for a 24v system. 24v 2000w inverter.
Can someone show me a wiring diagram to get the most out of my system.
Also the correct wire size in mm2 would be great.
«1

What brand/model of Charge controller and AC inverter are you using?

Do you have a copy of the installation manual?

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 8 ✭✭
Plasmatronic PL 80 controller.  Redarc inverter. The wire size from the inverter to the batteries is 2 (AWG) 34mm2
do I still use that wire size for the  battery links is what I’m looking for.
The PL 80 is rated for 80 amps maximum... You can use this NEC chart for cable sizing (NEC is relatively conservative):

https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm (conservative)

The AC inverter--Need to know the Watt and voltage rating.

Say you have an 1,800 Watt @ 12 VDC inverter (about the largest I would suggest for a 12 volt battery bank):
• 1,800 Watt * 1/0.85 ac inverter eff * 1/10.5 battery cutoff voltage = 202 Amps
And you also need to check your wire AWG, length, and current for Voltage drop. For 202 amps @ 5 feet @ 12 volt and a maximum suggested 0.5 volt drop (for 12 volt wiring):
https://www.calculator.net/voltage-drop-calculator.html

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=0.6465&voltage=12&phase=dc&noofconductor=1&distance=5&distanceunit=feet&amperes=202&x=0&y=0

## Result

Voltage drop: 0.40
Voltage drop percentage: 3.32%
Voltage at the end: 11.6

So about 5 feet (one way run for this calculator) would be the maximum suggested for an 1,800 Watt @ 12 VDC inverter on 2 AWG cable.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 8 ✭✭
edited October 2020 #5
This is great info, thanks.
The system is 24v, hence a Redarc pure sign 24v 2000w inverter. And the length of the cable is about 3foot. So I hope not as much voltage drop. And I presume the same cable (34mm2) is the size to connect all the batteries together?
These are the solar panels I’ve bought secondhand.
I was told by the solar installer to join them all pos to pos, neg to neg to keep the voltage @ 36v.
Also the company that sold the PL80 and the solar guy said that it will be ok, for a 24v system.

Cheers. John.
Hi John,

The basic products look fine (24 volt battery bus, 36.8 Vmp panels in parallel, 2000 Watt 24 VDC input AC inverter). As always, make sure you have the manual for your products (here is the PL controller, I think):

http://www.plasmatronics.com.au/common/tech1.html#PL
https://www.redarc.com.au/24v-inverter-2000w
https://www.redarc.com.au/Content/Images/uploaded/RS Series Pure Sine Inverters Instruction Manual.pdf

Yes, 2 AWG looks fine for connecting batteries together and running to the AC inverter's DC input.

Note for wiring the PL 80 controller, you can use smaller cable. However, for a 24 volt battey bus, the charge controller wiring drop shoudl be around 0.1 to 0.2 volts maximum. And you would need a smaller fuse to protect the wiring to charge controller (if 80 amps, a 100 Amp fuse/breaker would be good). Make sure that you always connect the battery bank first, then the solar array (and disconnect the array first, then the battery bank). If you have the solar array connected without the battery bank, it can confuse the charge controller, and in some cases even damage it.

Connect the DC power to the AC inverter directly to the battery bank, do not use the output power connections from the PL 80--The inverter draws way too much current and will damage the PL 80.

And the DC drop for the inverter wiring can be upwards of 1.0 volt max drop on a 24 volt battery bank.

Fuses are fine--But good quality DC breakers (rated for 24 VDC minimum), are nice too... They can be use for on/off switches, and large Amp rated fuses can be expensive (and you need a few on hand for spares so you don't ever get left in the dark).

How many solar panels do you have?

A few rule of thumb design suggestions. For solar power, generally suggest you charge at 5-10-13%+ rate of charge to the battery bank. 5% can work for weekend systems during sunny weather. 10%+ suggested for full time off grid usage.

For your 24 Volt @ 800 AH battery bank (4x 6 volt batteries for 24 volts times 2x parallel strings of 400 AH batteries for 800 AH bank).
• 800 AH * 29.0 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 1,506 Watt array minimum
• 800 AH * 29.0 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 3,013 Watt array nominal
• 800 AH * 29.0 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 3,917 Watt array typical "cost effective" maximum
Note that you have an 80 Amp PWM charge controller, The data sheet does not say what the maximum supported array wattage is, so the math for a typical controller would suggest:
• 80 Amps * 36.8 volts Vmp * 0.80 solar safety derating = 2,355 Watt max suggested array size
Maximum suggested solar panels for single PL 80 controller:
• 2,355 Watt maximum array * 1/190 Watt panels = 12.4 = 12 panels in parallel maximum
Because you have 3 or more parallel panels, you need a "combiner box" that has a Series Fuse (or breaker) of 10 amps to protect against short circuits in your array (12 panels = 12 fuses/breaker total).

And as you can see for your 800 AH battery bank, if you wanted >2,355 Watt array, you would need to parallel 2 (or more) of your PL 80 charged controllers together to your main battery bus. Paralleling solar charge controllers to the same battery bus is just fine. Large battery banks need lots of solar power for proper charging. Many/most flooded cell lead acid battery mfg. require 10% minimum rate of charge for best life.

And to complete the math... Assuming you live in/around Melbourne Australia, with a fixed array facing north:

### MelbourneAverage Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 52° angle:
(For best year-round performance)

 Jan Feb Mar Apr May Jun 5.75 5.74 5.04 4.22 3.33 2.87 Jul Aug Sep Oct Nov Dec 3.00 3.48 4.10 4.80 5.30 5.53
Lots of sunny weather except for May through July. Say your average is 4.0 hours of sun per day (and cut back on power usage during poor sun and/or use a backup genset when needed) using a 2,355 Watt array (max suggested for the PL 80):
• 2,355 * 0.52 off grid solar AC system end to end efficiency * 4.0 hours of sun per day = 4,898 Watt*Hours per day average harvest
For base loads (i.e., refrigerator, LED Lighting, Cell Phone charging), you should not plan on using more than 50-65% of your predicted power (save clothes washing, vacuuming, water pumping for sunny days):
• 4,898 Watt*Hours per day * 0.50 fudge factor = 2,449 WH per day base loads
• 4,898 Watt*Hours per day * 0.65 fudge factor = 3,184 WH per day base loads
And based on your battery bank--Suggest 2 days of storage and 50% maximum "planned" discharge (never go below 20% or you may permanently damage the bank):
• 24 volts * 800 AH * 0.85 AC inverter eff * 1/2 days storage * 0.50 max discharge (for longer battery life) = 4,080 WH per (cloudy day or overnight) battery usage
Your system is fairly large--And you might think about either getting a MPPT type charge controller (put 2 or 3 panels in series x X parallel strings) and/or going to a 48 volt battery bank. Both of these can make for easier wiring jobs (smaller wire, longer distance from array to battery shed, cut combiner box to 6 or even 3 fuses/breaker for a 12 panel array).

Anyway, probably enough for this post. Questions?

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 8 ✭✭
edited October 2020 #7
Yes, lots of questions! But they need answers..
I do have the manuals, but they’re 4hrs travel away on our property.
14 solar panels. Which will be mounted @ approximately 25deg. 6 facing easterly, 8 westerly. The property is west of Sydney.
I’ll be using 8 AWG from the panels to the PL80, approximately 10-12m.
I was told by the solar installer that I got panels from that a10amp, one for pos, one for neg. I’ve bought one used for AC that you connect the active and neutral to that’ll trip if there’s are short or fault.

So 800ah is the best I’ll get from the battery bank? I’ve put temporary connections on, but I’ll put the correct size on before startup. Is this correct?

I have petrol generators 2k & 7k if needed. And the property is a holiday house for the few years.

Cheers. John.

You have some sort of sealed batteries... Make sure your charging voltage setpoint is correct... AGM is typical around 14.4/28.8 and GEL is around 14.2/28.4 volts

The optimum way to connect the battery bank--You want to balance the parallel wire runs for the batteries so that they share charging/discharging current equally:

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

The 8 AWG wire run for 12 meters @ 5.18 Amps @ 36.8 volts will have a drop of:
https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=2.061&voltage=36.8&phase=ac&noofconductor=1&distance=12&distanceunit=meters&amperes=5.18&x=57&y=38

# Voltage Drop Calculator

## Result

Voltage drop: 0.26
Voltage drop percentage: 0.70%
Voltage at the end: 36.54

Which is fine (too much drop, and you reduce the charging voltage available at the battery bank terminals).

I cannot figure out your wiring for the battery bank from your picture... (not at all sure I understand the green wire connection). Please double check before connecting. I have no idea--But I fear that your present wiring is not correct)

Just make sure you have 4x 6 volt batteries in series (+ to - for each series battery connection) and then connect the two series string in parallel (+ to + and - to - for each string) for your 24 volt output.

Take the Left (negative bus) and connect with the second string Negative. And on the right, connect the positive output to the second string positive output. Notice that one connection is at the lower left, and the other is at the upper right--This is to ensure that both 24 volt strings of batteries have the same overall wiring path resistance (balance current flow between parallel strings).

14 solar panels in parallel may be OK for your controller... 14*5.18a=72,52 amps nominal under full sun. Not a lot of margin for unusual conditions (clouds can sometimes focus more light on an array for a few minutes, reflections from sand/snow can do similar).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 8 ✭✭
I got the battery battery wiring configuration from a googled pic, ( someone’s home setup)
so 800ah is the best achieve?
edited November 2020 #10
I am not quite sure what you are asking... My suggestion is that a nice maximum battery bank AH capacity is 800 AH... If you charge at 10% rate of charge, that is 80 Amps... Which is about the larger typical "home solar" charger you can get.

And when you talk about AC inverters, an 800 AH @ 24 volt battery bank would support upwards of 4,000 Watt AC inverter (suggest around 2,400 to 3,600 Watts "comfortable maximum"). Which is a lot of current (and heavy wiring):
• 4,000 Watts * 1/0.85 AC inverter eff * 1/42 volts battery cutoff = 196 Amps max continuous
And ~2x max surge or almost 400 Amps current for a few seconds (starting motor/well pump/etc.).

So--If you want a larger system, going with the next higher voltage (48 volts) would be the next step (typically solar chargers work on 12/24/48 volt banks, but AC inverters and AC Chargers--You typically need new 48 VDC rated hardware).

The other thing is to look at the battery options... You can get larger battery AH capacities from 200 to >1,200 AH per battery. It is usually better to keep to 1/2/3 parallel strings and use larger AH batteries (there are large 2 volt cells out there) to keep the number of cells and connections down--less maintenance, fewer things to go wrong).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 9,362 ✭✭✭✭✭
I question that one heavy cable connecting the + to - in the dead center of the bank.  It looks ODD.
ODD is usually wrong.  electrical wiring, has a nice symmetry and it becomes very easy to spot something wrong
Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,

• Registered Users Posts: 107 ✭✭
I don't understand how that green wire in the middle isn't shorting the bank?
• Solar Expert Posts: 52 ✭✭
Found this on the Full River site. Maybe trying to match this?

4-Canadian Solar CS330 in series/TraceSW4024/Midnite Classic 250 with Whiz Bang jr/8 L16370ah 4S2P/ Propane Honda EU2000/Propane Champion3800/Propane refrig/Wood heat/Propane tankless water heater.
Off grid in Upper peninsula Michigan
I suggest avoiding cross tying batteries together... First, if the batteries are working correctly, there will be no current in the cross connects.

Second, it is easy to take a quick voltmeter check across each battery... When you cross connect, then instead of two separate battery voltages, you have one combined voltage--So you cannot check each battery (other than a specific gravity check for flooded cell batteries).

Lastly, if you do have problems, then you still have a weak battery mixed in with the good. And that leaves the "good battery" carrying all the charging/discharging current--And more or less, just like disconnecting the weak battery anyway.

If all is working OK, cross connecting the batteries won't hurt anything (or make anything better either)...

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 8 ✭✭
Hey BB.
What I mean the most I can get, it, the best way to connect the batteries to to achieve the highest ah.
The green wire is a bridge. This configuration is a trial. I’ll be using what you suggested.
• Registered Users Posts: 107 ✭✭
The capacity of the battery is what it is.  The only gains to be had by different wiring schemes is that of efficiency, which you are maximizing at 48 volts (nominal),
edited October 2020 #17
About the only other thing you can do to affect a battery bank (besides keeping them properly charged), is to keep them at a reasonable temperature.

Batteries are spec'ed at 75F/25C... Keeping them cool will extend their life (for every 18F/10C increase in temperature, their life is cut by 1/2). In very hot areas, keeping batteries in a cellar (still need to be well ventilated) can help keep them cool.

In the future, if your batteries are in a very hot climate, LiFePO4 chemistry batteries can do well (conversly, in very cold climates, plain old lead acid batteries are hard to beat).

Otherwise, under charging/over discharging a battery bank is usually the cause of most early life failures of battery banks.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 6,032 ✭✭✭✭✭
edited October 2020 #18
Bat cables look too small and this thread sounds like a fire waiting to happen. Did not read it all as I have my own fire problems!

Full River AGM's, especially at 24v need 4/0 connections or current limiting properly sized!

There are plenty of 100 amp charge controllers from Schneider, Outback, Midnite, & Magnum BTW
"we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
http://members.sti.net/offgridsolar/
E-mail [email protected]

Dave, the Travelersrest said that was just a temporary connection until the correct battery cables could be obtained.

And, by the way, what do you mean by current limiting? Are you talking about fuses and circuit breakers, or something else?

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 6,032 ✭✭✭✭✭
Yep the usual protection for a battery system.   Wiring is not a form of current limiting if you care about safety
"we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
http://members.sti.net/offgridsolar/
E-mail [email protected]

• Registered Users Posts: 8 ✭✭
Bat cables look too small and this thread sounds like a fire waiting to happen. Did not read it all as I have my own fire problems!

Full River AGM's, especially at 24v need 4/0 connections or current limiting properly sized!

There are plenty of 100 amp charge controllers from Schneider, Outback, Midnite, & Magnum BTW
What size is 4/0.
I understand mm2 rather than AWG
• Registered Users Posts: 107 ✭✭
120 mm2
• Solar Expert Posts: 3,854 ✭✭✭✭✭✭
To gain the highest Ah number with 8 400Ah 6V batteries would be to wire them all in parrallel for 3200Ah at 6V, impractical of course but the point is, the stored energy is what it is. No matter what the nominal voltage, the result is the same 19 200Wh, the crux of the matter is which is the best configuration and the answer to that is to use the 8 batteries in series for 48V nominal.

The rational behind this is that it is far easier to maintain ballance as there is a single path for current to flow during charging and dischargeing. Using higher capacity 2v cells is the best way to increase capacity as it reduces the number of cells, monoblock batteries have a tendency to have a single cell fail which can go unnoticed until it's to late to recover it as it's virtually impossible to target the individual cell without damaging the remaining cells doing so, subjecting them to long periods of overcharging. With individual cells it's possible to focus on the individual offender or to simply replace it.

Personally I've developed a disdain for lead acid batteries, primarily for their high temperature intolerance, this  coupled with the inability to build parrallel first then series to achieve the desired capacity then series to provide nominal voltage , not to mention the ability to protect the bank at a cellular level, as is possible with LFP. Having said that  LA do have their place in extreme cold but outside that they have little to offer in terms of performance, including value versus life expectancy.

1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS
Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.
5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
• Registered Users Posts: 8 ✭✭
edited November 2020 #24
The batteries are in!  Didn’t get a photo to show!
The system is working fine. Upgraded the wire size to the same required for the inverter.
The initial startup demanded a lot of amps, to fill the battery bank I presume!
Thanks BB for your much needed input. Cheers.
• Solar Expert Posts: 5,657 ✭✭✭✭✭
mcgivor said:
Personally I've developed a disdain for lead acid batteries, primarily for their high temperature intolerance, this  coupled with the inability to build parrallel first then series to achieve the desired capacity then series to provide nominal voltage , not to mention the ability to protect the bank at a cellular level, as is possible with LFP. Having said that  LA do have their place in extreme cold but outside that they have little to offer in terms of performance, including value versus life expectancy.
Amazing that I have the exact feeling for Lithium chemistry! I've spent nothing on my system since setting it up in 2012. A lithium battery bank would require adding an additional heating source for my home or battery bank as well as find a place for the system inside my home. I don't need one with lead acid. (I have bought a used Magnum4000 to make my existing 1800 watt inverter redundant about a year ago) Can't make numbers work for new lithium batteries, without doing a smaller battery bank and a generator. No way to make it as cheap as lead acid...yet. I'm absolutely positive the time will come, but of course it makes no sense to spend money before I need to. I suspect my forklift battery will survive another 2-3 years, It's in it's 10th year now (it came from my cabin system, NO, I did NOT enjoy moving a 1000lb+ battery)
Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
- Assorted other systems, pieces and to many panels in the closet to not do more projects.
• Solar Expert Posts: 5,657 ✭✭✭✭✭
"... 800ah is ..."
I don't think anyone approached you about the sizing of the  array to the battery bank.

I have no problem using a PWM charge controller for charging your battery bank. The battery bank is somewhat over sized for your array.

When looking at the voltage drop, I'd use the Normal Operating Cell Temperature values for the panels;
PDF here;
https://static.trinasolar.com/sites/default/files/Comax_DC01A_Datasheet_Feb13_EN.pdf

So each panel will provide about 4 amps and you have 14 of them so roughly 56 amps max into a 800 amp battery bank or about a 7% charge rate, In general for full time use we would suggest 10-13%.

Sounds like 'holiday use' would be weekend and once in a while week long use. Should be okay and you have generators to back up so sounds good, just so you are aware. I hope the arrays are somewhat north east and north west, near Melbourne and angled near your latitude (37 degrees), this virtual tracking will help with daily loads.

Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
- Assorted other systems, pieces and to many panels in the closet to not do more projects.
• Registered Users Posts: 1
Newbie here and dang, Ya'll are some smart people!  I will have Solar questions in the near future, but right now I want to read a lot of threads so my questions do not become redundant.  Finally found a place where people KNOW!!
• Registered Users Posts: 2
Thank you for some very enlightening discussion. I am learning but dunno what to think with several configurations found on the web. Not sure if this is right... I am connecting 8 6v (225Ah) to product 12v (900Ah) to supply my RV. This will be supplemented by solar charging but will primarily use shoreline power or generator for charging. My question: Will this wiring configuration produce 12v?

Thanks so much for your help!
KC

• Solar Expert Posts: 5,657 ✭✭✭✭✭
KC_Seven said:
My question: Will this wiring configuration produce 12v?
It will, but it's very hard for 4 strings of batteries to evenly share the load and charging.
If yu must have that many strings, try reading and understanding the info on how to maintain even resistance through each string found here;

http://www.smartgauge.co.uk/batt_con.html
Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
- Assorted other systems, pieces and to many panels in the closet to not do more projects.
• Solar Expert Posts: 9,362 ✭✭✭✭✭
Yes, but look at the smartguage page for method 4, which is "more equal" than your method.
With AGM, because of their low internal resistance, the wiring becomes the dominant factor for equal current sharing, otherwise your bank fails prematurely.   You need to get yourself a clamp-on DC amp meter to be able to measure and verify the 4 pairs are sharing equally.
Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,

• Registered Users Posts: 2
Thank you “Whit” & “Mike”! The author in this link takes the time to give examples in his explanation to instill a better understanding. Good stuff! That said, considering that my crude drawing was intended to depict connection scheme and NOT to indicate cable length, I this scheme shows a good “balance” between the batteries. I intend to cut, crimp assemble my cables using the best cable and connectors to achieve the least amount of resistance.

But with exception of the of my project being series-parallel, 6v to 12v, isn’t the scheme I posted above IDENTICAL to “smartgage Method 4” example? What am I missing here? Since the new batteries have not arrived yet I am unable to connect them to test & measure. Here’s the Method 4 image...

Thanks Gents!