Is a voltage drop to 12.0V to be expected with a 21.3A load with a fully charged ~216Ah battery bank? I've never loaded the batteries that much before, so really have no idea what to expect.It's a pair of 3 year old GCs, ~216Ah new, maintained by solar: daily charging up to 14.8V then 13.1V float. Haven't checked their sp.gr. recently but it was ok 6 months ago.When resting at 13.1 float, I turned on the newly installed inverter and then the frig (256W according to the inverter, so 21.3A at 12V), and the voltage at the batteries dropped to 12.0V.Turning off the frig, the battery voltage went quickly back up to 12.5V or so, and the solar controller was then in mppt.

You need an ammeter to follow the action. The voltage drop you are seeing is a little higher than it "should be" but you also have the solar on which confuses things. The amps in from the solar would make the amps draw from the fridge net less from the battery and hold up the voltage a bit higher, so perhaps your actual situation is worse (but it is not a calamity, just more than it should be)You can help by improving the inverter-battery path (fatter wires and tighter connections) but there might also be less capacity in the batteries than you think, since you have not been checking on them all that much and they might need at least a good equalization session to bring their "at full" SG back up someI get about 0.8 drop with a 70 amp draw on four 6s but that is just my set-up. Others would get more or less than that with theirs. with two batteries instead of four you get more drop. With AGMs instead of Wets you get less drop.Some inverters are better than others too for amps draw. My older 2000w Vector (now moved to the truck camper from the 5er) pulls 90 amps to run the kettle but my newer Can Tire Xantrex 2000w using the same wires and batteries pulls 80 amps to run the same kettle.

I would make the battery DC VOLTAGE measurement across the terminal posts first to be sure you are not having any voltage drop due to too small of battery cables...I use 4AWG cables with properly crimped ring terminals for everything I hook up to my battery setup... My DC VOLTMETER measurements is done at my main BANK 1 and 2 BATTERY SWITCH which is about two feet away from the batteries. I think I usually see about a .1VDC drop when my three 12VDC batteries are running my 600WATT PSW Inverter running at half power which is around 22 AMPS DC current.I was originally going to use 2AWG ANCOR MARINE GRADE CABLE between all of the battery terminals and the three four position BLUE SEA switches I have setup to control four 12VDC batteries in Parallel. Ordered the 2AWG cable and ring terminals but never installed them.I would RULE OUT your cabling first before pointing to the low performance of the battery setup. I would also check the terminal connections at the batteries as well. They may not be tight and may need cleaned up some...0.5VDC voltage drop seems excessive to me for a 21 DC AMPs load measured at the battery terminal posts...Roy Ken

The drop is normal but with a 10% load on a new battery-bank that size you have 5 hours run time on the refrigerator (not counting any other loads.I would suspect your inverter will sound "low battery" warning in about that time frame due to total demands on the batteries.You did not state how large your solar panels were, but you would never make it through the night with that small a battery-bank.

Thanks all.It was the voltage at the batteries that dropped to 12.0V. (The inverter reported 11.9V. 0.1V is about what I expect for cable V drop with that load)And yes the solar was active at the time but overcast. And I was just using the frig load as a test. No plans to run it routinely.If the sun comes out, I'll equalize first, then try again with the solar turned off.

Battery Voltage drop under load | Good Sam Community

26 Replies

brulaz
Explorer
Jul 01, 2015
Well, the weight issue could be solved if I bought a heavier duty truck.
Then I could load up the trailer's tongue with a lot more lead. That's definitely not a cheap option though (the new truck, that is).

You say AGM is *less* sensitive to partial recharge than FLAs? I thought AGM wasn't recommended for pure solar systems because they were *more* sensitive? Or is the problem that you just can't equalize AGMs to fix them after a week or so of partial recharge? All you can do with AGMs is "conditioning" (which is a milder form of "equalization" as I understand it). And "conditioning" is more difficult and not as effective?

My MPPT controller is programmable. The constant voltage Absorb stage can be terminated when the current drops to a programmable amount or after a programmable time, whichever comes first. And I can manually "Equalize" at a constant, programmable V up to 16V. But that's about it. Not sure if that's good enough for AGM "conditioning".

Guess I'll take another look at that LifeLine manual ...
MEXICOWANDERER
Explorer
Jul 01, 2015
The Freas is not unbreakable but the column is made from Pyrex. I detailed a way to construct a PVC tube to protect the unit while in storage.

A quality AGM battery may be a significant step-up if performance is what you seek. Their penalty as compared to lithium is weight. Lifeline says their product will endure 500 full discharges to 0% SOC. But under load their voltage remains very high. But a strong solar panel would help in the partial recharge quandary. AGM is less sensitive to partial recharge than are flooded batteries. But a person needs good instrumentation to condition them.

I get weak-kneed when I price lithium.
brulaz
Explorer
Jul 01, 2015
MrWizard wrote:
brulaz wrote:
Wiz- My question was about the Battery V drop. Not the source of the current draw. Thought the info was sufficient.
Mex- Think the bubbles were uniform, but will redo this several times and see what happens.
Vulcan- LiFePO4 advantages to me are:
Extreme light weight and configurability
Can be left at 10-90% SOC indefinitely
Relatively constant voltage

AGMs are much heavier and apparently do not last long when only partially charged for weeks. IOW a pure solar system without a generator is not good forAGMs.

But there disadvantages with LiFePO4 as well

Well simply staying the wattage draw.."I have a 256w load" my village drops appears to be too much... it's simple and straight forward
But saying " I'm running my fridge on inverter" with no distinction
Makes me and possibly others think residential 120v fridge...
There is lots of talk about that topic and inverters and batteries
Very little about running an RV fridge on inverter
And the avg conversion to residential does not use that much power
Sorry for getting confused about what you were doing

Understood. Frankly, I can't imagine trying to keep track of so many conversations. But this site has been incredibly useful to me and I appreciate it.
MrWizard
Moderator
Jun 30, 2015
brulaz wrote:
Wiz- My question was about the Battery V drop. Not the source of the current draw. Thought the info was sufficient.
Mex- Think the bubbles were uniform, but will redo this several times and see what happens.
Vulcan- LiFePO4 advantages to me are:
Extreme light weight and configurability
Can be left at 10-90% SOC indefinitely
Relatively constant voltage

AGMs are much heavier and apparently do not last long when only partially charged for weeks. IOW a pure solar system without a generator is not good forAGMs.

But there disadvantages with LiFePO4 as well

Well simply staying the wattage draw.."I have a 256w load" my voltage drop appears to be too much... is simple and straight forward
But saying " I'm running my fridge on inverter" with no distinction
Makes me and possibly others think residential 120v fridge...
There is lots of talk about that topic and inverters and batteries
Very little about running an RV fridge on inverter
And the avg conversion to residential does not use that much power
Sorry for getting confused about what you were doing
MrWizard
Moderator
Jun 30, 2015
brulaz wrote:
Wiz- My question was about the Battery V drop. Not the source of the current draw. Thought the info was sufficient.
Mex- Think the bubbles were uniform, but will redo this several times and see what happens.
Vulcan- LiFePO4 advantages to me are:
Extreme light weight and configurability
Can be left at 10-90% SOC indefinitely
Relatively constant voltage

AGMs are much heavier and apparently do not last long when only partially charged for weeks. IOW a pure solar system without a generator is not good forAGMs.

But there disadvantages with LiFePO4 as well

Well simply staying the wattage draw.."I have a 256w load" my village drops appears to be too much... it's simple and straight forward
But saying " I'm running my fridge on inverter" with no distinction
Makes me and possibly others think residential 120v fridge...
There is lots of talk about that topic and inverters and batteries
Very little about running an RV fridge on inverter
And the avg conversion to residential does not use that much power
Sorry for getting confused about what you were doing
brulaz
Explorer
Jun 30, 2015
MEXICOWANDERER wrote:
Disadvantages?

Yeah!

The big disadavantage is having to buy a big wheelbarrow

To tote the bale of banknotes when you go to purchase lithium

I'm still nervous about the overheating issue with lithium...

Heh. Yeah their list of disadvantages is long.

One of the reasons most people recommend a Battery Management System for them is the over-charging/over-heating issue. As over-charging permanently screws the battery. And charging at temperatures below 0C also permanently screws the battery. And letting them drop to zero SOC also permanently screws the battery, so a BMS is useful there too.

But then the BMS adds another level of complexity with relays and circuit boards that can go wrong. And like you say, the batteries are not cheap.

Definitely a challenge! :)

So for now I'd just like to keep these GCs running longer. But need to get a new Sp. Gr. bobber first (woudn't need that with the LiFePO4s ;))
MEXICOWANDERER
Explorer
Jun 30, 2015
Disadvantages?

Yeah!

The big disadavantage is having to buy a big wheelbarrow

To tote the bale of banknotes when you go to purchase lithium

I'm still nervous about the overheating issue with lithium...
brulaz
Explorer
Jun 30, 2015
Wiz- My question was about the Battery V drop. Not the source of the current draw. Thought the info was sufficient.
Mex- Think the bubbles were uniform, but will redo this several times and see what happens.
Vulcan- LiFePO4 advantages to me are:
Extreme light weight and configurability
Can be left at 10-90% SOC indefinitely
Relatively constant voltage

AGMs are much heavier and apparently do not last long when only partially charged for weeks. IOW a pure solar system without a generator is not good forAGMs.

But there disadvantages with LiFePO4 as well
MEXICOWANDERER
Explorer
Jun 30, 2015
Look inside. Are all cells bubbling equally when you are equalizing? If you have a "smarter than me" automatic equalization - do it three or four times then try your volt drop test again. You may have sulfated batteries and they may recover nicely. You've nothing to lose at this point...
Vulcan_Rider
Explorer
Jun 30, 2015
brulaz wrote:
All I need is a couple of grand for LiFePO4. The boss will love that.

Identifying the weak 6V and giving it (only) some special attention might buy you considerable time on the whole string.....might.

Then is your choice of new battery type just a case of "gotta have the latest and greatest" or do you perceive a significant practical advantage that justifies the additional cost ?

I think AGM might be a better choice.

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Battery Voltage drop under load

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