Adding a backup generator/ ac charger to off grid system

kansas

I have a new, small off-grid set up. My charge controller (have a separate post on this) isn't working and my batteries have a lower SOC than I would like. I already have a 3000 watt Honda generator. My thought is to buy a three stage AC battery charger, wire it to the battery bank, plug it into the generator and top off my batteries whenever needed. It would be a very useful backup, make for happier batteries, and give my system more autonomy. The "battery bank" is two 6 volt, 242 Ah US batteries wired it series.

Question: If I hardwire the charger leads to the batteries, I assume I will need to open the PV charging circuit before I run the gas generator. Correct?

Any other circuits I need to open? Controller to battery? Battery to inverter?

Will the AC charger effect the Trimetric meter I will be installing in a few days?

Do I need a three stage (bulk, absorption and float) charger? Seems to me that would be the safest way to go and prevent any unintended overcharge.

Anything else to consider?

Thanks - Bill

Chris

Re: Adding a backup generator/ ac charger to off grid system

You shouldn't have to disconnect any of those devices wile charging.

When you install the shunt for the Trimetric all of your negative need to pass threw it (ei.....nothing other then the battery terminal connected to one end of it) So you will need some form of bussbar or breaker panel to divide up all you wiring.
The Trimetric will read the current going both ways no matter if it is coming from the charge controller or the AC charger.

BB.

Re: Adding a backup generator/ ac charger to off grid system

If you do not have grid power, only backup genset... You really do not need a three stage charger. A "simple" Iota charger should be fine (you may have to adjust the output voltage up or down for your specific battery bank needs).

You are going to shut down the genset once the batteries are charged. If you will have grid power on occasion, you can get the plug-in IQ4 module for 3 state charging.

-Bill

kansas

Re: Adding a backup generator/ ac charger to off grid system

Bill -

I looked at the Iota chargers. They all seem to be multi-stage and pretty pricey. I was thinking more something from the auto parts store since the cabin is pretty low usage and would probably need generator back up the next few winter months while my daughter is there writing the great American novel. Thoughts?

Bill

kansas

Re: Adding a backup generator/ ac charger to off grid system

I should add that I plan to wheel the generator out of the shed and plug it in to the AC charger only occasionally, or leave it nearby if I need it more than occasionally. It won't be an automatic connect or need to sense a low battery. I'll just monitor the SOC and shut the gen down when SOC is 100% or so.

BB.

Re: Adding a backup generator/ ac charger to off grid system

"Multi-Stage" charger is always a difficult term to separate from the marketing usage.

The Iota (without IQ4) is a fixed output voltage charger with current limited output. In marketing terms, it is a two stage charger (bulk:limited output current, absorb:limited output voltage). There is a switch/connector on the side where you can have "float" or "charge" voltage settings (switch in, "high voltage"; switch out, "float voltage").

The IQ4 automates that Charging/Floating setup.

The nice thing about the Iota chargers is that they are known for outputting rated output for as long as needed... Many off the shelf chargers are not near capable of outputting high current into a relatively large storage bank of batteries (your bank can easily take 20-40+ amps during bulk charging).

A typical "car charger" capable of 20-40 amps @ 12 volts is not cheap either at something like $100:

http://www.amazon.com/Schumacher-SE-1555A-Automatic-Battery-Charger/dp/B006PAGJXQ/ref=sr_1_8?s=automotive&ie=UTF8&qid=1411319197&sr=1-8&keywords=battery+charger

A 10 amp charger is something like $50:

http://www.amazon.com/Schumacher-SE-1052-Manual-Starter-Charger/dp/B000AM8BF4/ref=sr_1_9?s=automotive&ie=UTF8&qid=1411319589&sr=1-9&keywords=battery+charger

In the end, a "balanced" design is my goal. Battery bank => charger size (10%-20% of bank AH capacity is typical range) => generator size...

Ideally, you want to run the generator somewhere around 50-80% of its rated capacity most of the time... Over 80%, it can overheat "consumer grade" gensets. And much below 50% of rated capacity, they suck a lot of fuel (fuel usage is not much lower as AC loads are reduced, for gasoline gensets, between 50% and 0% of rated load--I.e., fuel usage at 50% of load and 0% of load is almost the same--For fixed RPM type gensets).

The inverter-generators from Honda (like eu2000i) or Yamaha, tend to be quieter and more fuel efficient than fixed RPM gensets below 50% of rated load.

• 20 amps * 14.5 volts DC * 1/0.80 charger eff = 362.5 Watts average (estimated) load for 20 Amp "car charger"

I.e., you don't want to run a very nice 7kWatt Champion genset on a 20 amp 12 vdc charger (50% of 7kW = 3.5 kW output):

http://www.championpowerequipment.com/generators/41532/

• 5.9 gallons of fuel / 8 hours (at 50% or less load) = 0.74 Gallons per hour
• A Honda eu2000i will run 400 Watts for ~9+ hours on ~1.1 gallons of gasoline.

If you want to recharge your 242 AH battery bank at 20 amps from 50% to 80% state of charge:

• 242 AH * 0.3 discharge capacity * 1/20 amps = 3.63 hours of run time

-Bill

kansas

Re: Adding a backup generator/ ac charger to off grid system

Thanks for your patience, Bill. After looking at the prices of a "auto parts store" charger I see that the difference in price with the IOTA is not that much. The quality more than justifies the difference. My Honda 3000 watt generator has a maximum amperage of 25, is rated at 21.7A and has a commercial rated OHV engine. From the above I conclude that the Iota DLS-15 amp charger is the best choice. I'm buying a lot more stuff on your recommendation. Do you work for NAWS? Just kidding. Thanks, Bill.

vtmaps

Re: Adding a backup generator/ ac charger to off grid system

BB. wrote: »

(fuel usage is not much, for gasoline gensets, between 50% and 0% of rated load).

huh?

BB. wrote: »

Ideally, you want to run the generator somewhere around 50-80% of its rated capacity most of the time...

And that's a bit tricky with an Iota charger. Those chargers have huge inrush currents and low power factor... you need a generator with a rating much higher than the Iota's power consumption.

--vtMaps

BB.

Re: Adding a backup generator/ ac charger to off grid system

Hmm... Wonder if I had a stroke and nobody told me.

Ideally, you want to run the generator somewhere around 50-80% of its rated capacity most of the time... Over 80%, it can overheat "consumer grade" gensets. And much below 50% of rated capacity, they suck a lot of fuel (fuel usage is not much lower as AC loads are reduced, for gasoline gensets, between 50% and 0% of rated load--I.e., fuel usage at 50% of load and 0% of load is almost the same--For fixed RPM type gensets).

-Bill

BB.

Re: Adding a backup generator/ ac charger to off grid system

kansas wrote: »

Thanks for your patience, Bill. After looking at the prices of a "auto parts store" charger I see that the difference in price with the IOTA is not that much. The quality more than justifies the difference. My Honda 3000 watt generator has a maximum amperage of 25, is rated at 21.7A and has a commercial rated OHV engine. From the above I conclude that the Iota DLS-15 amp charger is the best choice. I'm buying a lot more stuff on your recommendation. Do you work for NAWS? Just kidding. Thanks, Bill.

Now the confusion... We really need to work as Watts to keep things straight:

• 21.7 Amps * 117 VAC = 2,540 Watts @ 117 VAC (or there about)

The battery bank is charging at ~12 volts:

• 12 volts * 21.7 amps = 260 Watts

So--With the voltage of an AC generator at 120 VAC, if we want the same power at 12 VDC (ignoring a bunch of other stuff), we would need 10x the current:

• 40 amps * 12 VDC = 480 Watts
• 4 amps * 120 VAC = 480 Watts

So, if you run your Honda 3000 -- Assuming that 2,400 Watts is its rated continuous output and you want to run it at 50% of rated output:

• 2,400 Watts * 0.50 = 1,200 Watts
• 1,200 Watts * 1/14.5 volts charging * 0.80 charger eff = 66 Amps @ 12 volt charge controller

So--Very roughly, you could put upwards of a 66 amp @ 12 volt battery charger on your 3,000 Watt genset without problems.

http://www.solar-electric.com/lib/wind-sun/DLS-55-specs.pdf

You could run upwards of a 45 Amp 12 volt Iota to run on a eu2000i genset (the 55 amp version should technically run on the eu2000i, but probably is too big for that genset).

-Bill

kansas

Re: Adding a backup generator/ ac charger to off grid system

Thanks again. Could you - and I promise not to ask anything else - explain your math to get the approximate run time to charge from 50% to 80% with a 55 amp. charger. I can duplicate your answer of 3.63 hrs. when charging at 20 amps, but can't come up with the equation that would result in a run time of approximately half of the 3.63 hours? Bill

BB.

Re: Adding a backup generator/ ac charger to off grid system

Something like:

• 242 AH * 0.3 discharge capacity * 1/55 amps = 1.32 hours of run time

If you wanted to only charge for 1/2 of 3.63 hours:

• 3.63 hours * 1/2 = 1.815 hours
• 242 AH * 0.3 capacity = 72.6 Amp*Hours to recharge
• 72.6 AH / 1.815 Hours = 40 amp charging rate

Is one of those two the question you wanted answered?

Note--Lots of approximations going on here... Very roughly, a deep cycle battery will take any reasonable charging current up to ~80-90% of capacity. That is "bulk charging" phase--When the charge controller is at maximum output current.

When the battery becomes ~80-90% state of charge, the battery voltage hits the "absorb" set point--And the charge controller will hold the 14.6 volt (or so) charging voltage and the charging current will slowly drop to ~1-2% of the battery bank's AH capacity (2%*242AH=4.8 amps "finishing current"). At that point, the charger would turn off or fall back to float voltage.

New poster "leaf" has a really nice set of charts that compare battery voltage against different rates of discharging and charging (as well as resting voltage readings).

leaf wrote: »

Am trying to upload the charts I am using...

Attachment not found.Attachment not found.

[note charts are from: I believe those charts are from Home Power #36, August- September 1993. Lead-Acid Battery State of Charge vs. Voltage ©1993 Richard Perez.
Here is a link: http://www.scubaengineer.com/documen...ing_graphs.pdf

vtMaps]

I don't quite a agree with the resting voltage line (I think the voltage is a bit low)--But it shows how to estimate a battery's state of charge while operating.

Note, where the charts "flatten out"--the room for error estimating state of charge is pretty high.

-Bill

You can see a "knee" in the second "charging current" chart when the voltage rises very quickly... And the higher the charging current, the more to the left (and higher) the knee becomes.

Below 80% SOC (roughly), a lead acid battery is much more efficient at charging (little energy turned into heat and Hydrogen/Oxygen gasses). So, if you are trying to charge with a genset--Charging quickly up to 80% SOC in the early morning, and letting the solar panels take over for the rest of the day (or recharging again the following morning) will be more fuel efficient than trying to recharge to ~100% every day with the genset (during stormy weather, etc.).

And this is where LiFePO4 batteries are great--They are virtually "flat voltage line" from 20% to 80% state of charge--And almost no heat generated--And very efficient. Many of the rules of thumbs we use around the limitations of lead acid deep cycle batteries. Other chemistries (and even types of Lead Acid, such as AGM, GEL, etc.) have other requirements/limitations/advantages.

Does this help?

-Bill

kansas

Re: Adding a backup generator/ ac charger to off grid system

Super. Answered my question and gives me lots of interesting reading. Thanks all - Bill in Lawrence, Kansas