Marco Meile avatar image

5% mid-point voltage deviation in final 10% of charge


I'am using two serial 12V 122Ah AGM batteries to get 24V. I'am measuring the Mid-Point Voltage via a SmartShunt and a Multimeter.

Between the Batteries there is a thirdparty balancer which measures the voltage of each battery and charges the one with less voltage with up to 2A to equalize.

Also the Battery are AGM of the type "China" :)

+ ---[BAT1]---[BAT2]---- -

I'am charging the batteries with a victron IUoU Charger which delivers maximum 15A

Absorption Voltage is set to 29,4V, compensated by 60mV/degrees measured at the surface of BAT1

so far so easy.

Usually Absorbtion starts at a tail current of around 10A and from around 6-8A tail current (approx. 95% SOC) the Mid-Point deviation reaches 4% or more.

Today: at 94%, 4A Tail Current BAT2 got 15,3V out of the 29,6V feed. For testing reason, the balancer has been removed.

The Age of the batteries is 1,5 Years, i never discharge it more than 70%SOC.

What could be the issue here, higher inner resistance of one Battery? is there a way to mitigate that?

battery chargingsmartshuntAGM Battery
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2 Answers
Alexandra avatar image
Alexandra answered ·
@Marco Meile

Most likely the difference in resistance in the batteries.

Try swapping their positions. Sometimes taking them out of the bank and charging each one separately with a good 12v charger helps. We do this as maintenance on built banks.

And the battery balancer is there for this very reason. It is a common problem in banks built up with 12v thats why the product exists.

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Ok, thank you so much!

I have one more question regarding the balancer:

in my setup, i feed the charge to the batteries directly while having a balancer in place which doesn not solve the problem with the too high voltage for one of the batteries.

would a "real" balancer get the charge from the charger and feed the batteries? because i don't see a way to prevent of overcharging except of lowering the absorption voltage to below 29V but then i'am at risk the other battery is not fully charged. propably the better solution when charge the undercharged battery every other week... what a crap! :)

So, i now set the Absorption voltage to 29V so the problematic battery will not exeed the 15V+compensation voltage. The missing charge at the other Battery "should" be feeded by balancer. right?

A balancer should either actively move the higher charge from one battery to another or bleed down (this is more common as it is simpler) the voltage of the higher battery.

I dont know what your one does.

Here is a demo of what the victron one does. It is triggered in a certain voltage range on the bank so it is not active all the time, just near the top in absorption phase.

hmm, i think my balancer is just sh*t

In both diagrams, the midpoint is measured on the exact midpoint of two batteries. how important is this? the connection between the two batteries is maybe 30cm. do you think the unequal resistance is a problem? i attached the midpoint wire directly to one of the batteries.

@Marco Meile

Yes, unequal resistance is the issue. Ohms law.

Both in the battery internally and in the wiring, possibly.

I think the ideal connection is midpoint in the connection between the batteries, but for all practical purposes you end up connecting to one batteryor the other. In theory if you have a big link between then batteries then you should be fine.

Is your shunt measuring the midpoint at the same points the balancer is connected to?

Marco Meile avatar image
Marco Meile answered ·

I have now swapped both batteries and on SOC 100 i've put the less charged battery on a 12v charger, mostly foating around for a few hours.

after installing everything again the less charged battery became higher charged and the the midpoint deviation swapped, so the previously less charged battery had a higher voltage. of 0.4%

the 0.4% keeped consistend after one night and 10% DOD which keeps me believing the balancer is not balancing at all.

but now while charging up to 98% again, the unequal charging swapped again. So, now we have the same behavior as before, same battery gets higher voltage, even in his new position.

So it seems to be the inner resistance of a battery but not because of sulfate. Maybe because of agein. maybe it gets less in the future by his new position.

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