question

Eric M. avatar image
Eric M. asked

Temperature compensation causing overcharging?

Trying to understand some curious charging effects on my 12v system. I have a 600Ah AGM batteries bank (in use 3 years), just upgraded components to Victron: SmartShunt with temperature sensor. Charging from a SmartSolar and MultiPlus with DVCC (shared everything) setup on a Cerbo (using SmartShunt temp), and an Orion Smart just doing its thing. All are set to recommended charge profile and temp compensation (-13mV/*F) from the battery manufacturer.


The issue: When I connect MultiPlus to shore power (15a), it works great, gets my batteries up to float and then storage, things are fine. But if I leave it plugged in on storage or float for a few hours, the batteries heat up bigtime. In float/storage they're taking appropriate minimal charge of ~5a, but still eventually heat up to ~110*F in a ~40*F cabinet.

Due to temp compensation, when engaged the Orion/Solar think it needs to bulk charge the full batteries because the MP is only charging at 12.7v or so. This keeps the batteries hot and the voltage low, so the Orion/Solar will try to bulk the full batteries all day. Turning off all charging overnight and letting the batteries cool down allows all devices to charge properly the following days, but any time my MP charges for a while this happens...

Additionally, the 2 times I've tried to equalize with the MP, the batteries go well over 110*F, fill the van with hydrogen off-gas, and set off my CO detectors.


I know these high temps are bad for the batteries, and I'm worried what they'll do in ~80*F ambient summer temps, if they're overheating in the dead of winter in Colorado...

Does anyone have any idea on a fix for this?

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JohnC avatar image
JohnC answered ·

Hi @Eric M.

You don't actually say much about Voltages, so difficult to comment on Temp Comp. And it's unusual for batts to overheat this much, even when overcharged to a minor degree.

I'm going to take a stab and guess/suggest that your 600 Ah @ 12V is made up of multiple paralleled batts. And a cell in one has failed/shorted. So it only has 5 cells left and those are now being overcharged by 20+%. That'll cause heat and extreme gassing, and even when not under charge, it will continue with the power coming from the remaining good batts.

Wild guess maybe, but the symptoms suggest this as the most likely scenario. 3 years of service from batts set up like this is typical.

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Eric M. avatar image Eric M. commented ·

John, thank you! Yes, I have 3x 200Ah Renogy AGM batts in parallel. Here are the voltages in my Solar config:

1647357071973.png

Your suggestion was my suspicion, but it has only ever happened in long float/storage on the MP - after several non-charge hours they cool down and function normally on just Orion/Solar for months until I plug into shore power again.. And everything I read says AGM cells don't ever really short like other LA batts.

The reason I suspect temp compensation is because the MP floats the hot batteries at 12.7v, and when Solar kicks on, it stays in bulk around 12.7v, forcing up to 40a into the hot "full" batteries, not realizing they are full due to the low temp comp'ed voltage...


I guess step 1, I'll separate the batteries under 0 load and check them for lower nominal voltages.

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1647357071973.png (192.6 KiB)
JohnC avatar image JohnC ♦ Eric M. commented ·
@Eric M.

At 12.7V the batts shouldn't be able to accept 40A. Unless they're faulty (one or more).

Your solar config may be what the batt maker sez, but I'd probably keep them down to (say) 14.4V Abs and 13.6V Float (+-TempComp). Or even less. They'll still charge at lower V's, just take a little longer.

Parallel batts can suffer imbalance as they age, and pushing them hard will highlight this. Unfortunately it can bring on early failure. Heat from within is the first symptom, and I often recommend a 'touch test' to see if you can detect a different T between them. The writing is on the wall if you can detect this, but a less aggressive charge profile may well extend their life significantly.

That said, I think you may have at least one for the recycler.

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