EG4 Charger Question

I have two 206amp/hr SOK batteries in parallel that are at 100% at 14.4v. I just tried charging them with my 12V 15A EG4 charger but it would only charge them up to 13.31v. So I checked the output voltage during charging and it was only 13.39v at the terminals and 13.48v on the clamps. Does this mean that the charger is malfunctioning?

Disconnect the battery from the charger terminals. With the charger energized, what voltage do you measure?

Checking the voltage on the charger clamps I get 14.58 volts, so I guess the charger is OK. And now I've got an email from the dealer where I bought the batteries and she says that since I've got two 206amp/hr batteries in parallel then my 12v 15A EG4 will take over 27 hours to charge my two batteries from empty to full.
This is awful news because it means that if I am going to be independent of grid power in my motorhome for a few months, I am going to need a couple of pick-up truck loads of cans of gasoline to recharge the batteries with my little 4cycle 1600w Hyundai generator. There is no where near enough sunshine during the winter for my solar cells to pull this off.

Ah. I didn't pick up on the fact that the batteries were still charging. I thought it had stopped.

Anyway, the vast majority of charging of LFP will be in the 13.2-13.8V range, so what you describe is normal. The charger and the battery must be at the same voltage, so when you connect the charger to the lower voltage battery, they meet one another. The battery voltage rises in accordance with the amount of current flowing into the battery and its state of charge.

206Ah * 2 = 412Ah

412Ah /15A = 27.5 hours.

The issue here is that you have purchased a very small charger for a large battery. Good news is that 412Ah is pretty respectable, and the two batteries should be able to handle much higher current than 15A.

Does your motorhome already have an AC to DC converter for 12V power? Most do. If so, what is it?

You might consider a PM4 converter from Powermaxx. I have the 12V/100A version, and it's been serving me well for over 5 years. The 75A version for $189 would probably be good since your charge limit is 100A. It would get your batteries charged in about 5-6 hours.

Hi Super Eggo;
Thanks for the fantastic information. You are really helping me to win the battle for independence. 412Ah/15A = 27.5hrs. I didn't know the formula.
Actually, I removed the original AC to DC converter that was in my motorhome because it was taking up so much room in the battery box and I didn't think that I had any use for it because it was only running the lights and the fridge. I never need to use the fridge because fridges are only for people who need grocery stores and I have replaced the original lights with full spectrum 4 foot fluorescent tubes.
And I couldn't imagine charging up my new LFP batteries with an ancient converter from lead-acid 1977, that would be extremely disrespectful to them.
Hey yeah, the 12v/100A PowerMaxx sounds great because I've really got to keep the charging time down to as little as possible in order make the generator fuel last as long possible.
And I want my wonderful batteries to last as long as possible so I will take Will Prowse's advice and do this by cycling them around 50% as much as possible. Like maybe 85% max charge and 30% min charge.
And you are telling me that the absolute maximum charging current that they can handle can handle is 100amps? And then you suggest that I use a 75amp charger. Is a 100amp charge going to be hard on them? And if so, what can I do to make it a bit easier? Would it be easier on them if I added another battery, like a 100amp one? No, I can't do that because then it would take 7 hours longer to do a zero to max charge.
How many amp hrs are you regenerating with your 100A PowerMaxx?

You can't have it all. You either choose max life or fast charging. It's pretty much impossible to charge and discharge to specific levels without a high quality battery monitor that accurately calculates the state of charge.

Generator fuel is both a function of run time and power generated. A generator running under light load for 4 hours will use less fuel than a generator running under max power for 4 hours. Generators tend to be most efficient at 70-80% power. You need to get away from thinking purely about run time and think about $ per Ah produced.

Nothing wrong with using an old converter if it produces the correct voltages.

I didn't recommend the 100A for a reason. The maximum charging current of each battery is 50A. 2X 50A =100A; however, it's pretty much impossible to make both batteries perfectly accept 50% of the current. The 75A should be able to safely be split by both batteries and stay under the 50A limit for each battery.

Amp hours are simply calculated by Amps * time, i.e., 50A for 2 hours is 50A * 2h = 100Ah.

I should be able to charge and discharge to approximate levels by just charging for a limited period, like one hour and then turning off the charger and checking the battery voltage so that I'm not charging them up to 100% every time in order to increase their lifespan.
And I could get the fastest possible charging time by buying a 50 amp charger and just charging one battery. And then using that battery down to about 30% and then using the 50amp charger to charge the other battery. And this would also result in longer life for both batteries.

I see now that Powermaxx does not make a 50amp charger, just a 55amp one. So I would need to either charge them one at a time with the Powermaxx 45amp charger or charge them both with the Powermaxx 80amp charger, what do you think?

Thos said:

I should be able to charge and discharge to approximate levels by just charging for a limited period, like one hour and then turning off the charger and checking the battery voltage so that I'm not charging them up to 100% every time in order to increase their lifespan.

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That sounds better in theory than it is in practice. Are you really going to charge them up and then completely disconnect them for about 30 minutes with no charging or discharging to get an accurate read on the resting voltage?

Thos said:

And I could get the fastest possible charging time by buying a 50 amp charger and just charging one battery. And then using that battery down to about 30% and then using the 50amp charger to charge the other battery. And this would also result in longer life for both batteries.

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Right... so now you're going to run the generator 2X as long charging each battery separately?

Thos said:

I see now that Powermaxx does not make a 50amp charger, just a 55amp one. So I would need to either charge them one at a time with the Powermaxx 45amp charger or charge them both with the Powermaxx 80amp charger, what do you think?

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The next step below 100A was the 75A PM4, so that's the one I recommended.

I didn't know that I have to wait 30 minutes to get an accurate reading on the resting voltage. Although I have done extensive voltage level tests that I have kept a record of and it really bothers me that the two batteries apparently drop from 90% to about 75% if they are resting for 2 or 3 hours. I wonder how long that goes on for.
Yeah, I guess that's a dumb idea to charge the batteries separately because then I will be running the generator for twice as long.
I am seeing that the idea here is to charge the two batteries up as quickly as possible to minimize fuel consumption but without using a 100amp charger because you said that; "it's pretty much impossible to make both batteries perfectly accept 50% of the current"
OK, so I want to get as close as possible to a 100amp charge but it must be less than a 100amp charge. And Powermaxx does not make a 90amp charger so what I could do then is buy two 45amp Powermaxx chargers and plug them both into my 1600w gas generator and then run them together into the two batteries so that I would be giving them a 90amp charge.

Thos said:

I didn't know that I have to wait 30 minutes to get an accurate reading on the resting voltage. Although I have done extensive voltage level tests that I have kept a record of and it really bothers me that the two batteries apparently drop from 90% to about 75% if they are resting for 2 or 3 hours. I wonder how long that goes on for.

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They don't. You've mistakenly assumed voltage is a reliable means of determining SoC, and/or your testing didn't include the effects of charge and/or discharge current. The moment you input current, the voltage increases giving you an unreliable SoC correlation. The moment you stop charging, the voltage drops - again giving you an unreliable SoC correlation.

I can guarantee you that if you truly had no load on those batteries over the 2-3 hour period, they did not lose any charge at all. The most accurate description is that after the 2-3 hours of rest, if the resting voltage suggested 75% SoC, then that was the actual SoC it was at when you thought it was 90%.

Thos said:

Yeah, I guess that's a dumb idea to charge the batteries separately because then I will be running the generator for twice as long.
I am seeing that the idea here is to charge the two batteries up as quickly as possible to minimize fuel consumption but without using a 100amp charger because you said that; "it's pretty much impossible to make both batteries perfectly accept 50% of the current"

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You got it.

Thos said:

OK, so I want to get as close as possible to a 100amp charge but it must be less than a 100amp charge. And Powermaxx does not make a 90amp charger so what I could do then is buy two 45amp Powermaxx chargers and plug them both into my 1600w gas generator and then run them together into the two batteries so that I would be giving them a 90amp charge.

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You can do what you want, but you're picking fly turds out of pepper at this point. Again, fuel consumption isn't only about run time. You will burn less fuel at a lower power setting, so the fuel savings between 75A and 90A is going to be a very small number. Rather than two units consider there are other options: Progressive Dynamics, Iota, etc.

OK, right, I understand that fuel consumption is not only about run time. So when you say that I will burn less fuel at a lower power setting I guess you mean that I will burn less fuel when I am using a battery charger that is putting out a lower amperage.
Yeah, if there's any way I can do it without using two units that would be great. I checked out Powermaxx and they list an 80amp but when I click on "choose an option" to pick a size there is no 80amp option available. Then I looked at Progressive Dynamics and their 80amp unit has a "special 20amp outlet" so I would need an adapter.
So thanks a million for mentioning Iota, because they actually have a 90amp charger.
However, my little 1600w generator does not have a 30amp power socket. It's just got two 15amp ones. So I need to make an adapter by running two standard 3-prong 120v plugs into the back of a 30amp socket to plug the charger into. Unless the Iota charger uses a regular 3-prong plug and not a commercial 30amp plug. I need to phone them and find out.
Do you think that kind of adapter will work OK ? Because if it doesn't then I have wasted $700 on a battery charging generator that I now have no use for.

Make sure the Iota has a desirable charge profile. I'm a big fan of the Powermax because they can operate as constant voltage power supplies or 3 phase chargers with adjustable bulk/boost charge voltage.

Generator currents are listed in AC amps @ 120VAC, not charging amps.

15A exceeds your generator's 1600W rating.

15A * 120V = 1800W

1800W/12V = 150A

Read the generator manual and confirm that you can pull at least:

90A * 14.4V = 1296 * 1.15 (inefficiency) = 1490W

1490W/120VAC = 12.4A from the 15A outlet.