Duncan Hall avatar image
Duncan Hall asked

Overcharging when Multiplus is on shore power from Solar

I have a domestic battery bank of 3 Trojan 31 XHS batteries purchased February 2016. They are charged by a Victron multiplus at the recommended charging voltages for Trojan batteries with Trojans recommended temperature compensation. Fortuitously I monitor the battery state remotely using a raspberry PI running Victorns Venus software which connects to a Victron BMV 700, the solar charge controller (MPPT 75/15). Last week I noted the current rising and the battery temperature increasing. I went to the boat and found that one of the batteries had failed. Once disconnected from the charger it read under 11V. Fortunately the other pair seem OK reading about 12.8V fully charged with no load after a days rest.

In the remaining batteries the plates are clearly distorted

The boat is not in daily used and has suffered very few deep discharges. It is typically used about 8 weeks per year so the cause of failure is not deep cycling the batteries. The battery bank is used to drive the inverter in the multiplus. When it does this the current is typically up to 100A. Could this be the cause of failure or is something else in my use causing a problem.

The multiplus on on and it follows Trojans recommended prescription for i.e.
Ansorbtion 14.82V
Float 13.5
Max charge current 17A per battery
Temp comp -30mV/deg C
Repeated Absorption time 1hr
Repeat Absorption Interval 7 days

Storage mode set which gives 13.2V after 1 day at float.

However the solar panels are not so constrained so that it goes on bulk mode each day.

There is only about 200W solar on deck in partial shade which in Sept for example raised voltage to about 14.5V so did not get to absorption on the solar controller. The net current was typically 3A compared to the storage current of 0.2 A for the bank.

I suspect that in this regime I have overcharged the battery.

When in the Marina, I leave the mains on since it is used to run the dehumidifier. I wonder whether the best course of action is to turn the bulk voltage down to 13.2 on the solar system when connected to the grid.

MPPT ControllersMultiPlus Quattro Inverter ChargerVenus OS
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1 Answer
JohnC avatar image
JohnC answered ·

Hi Duncan.

Yes, I too think that for batts effectively in storage, you've been pushing them too hard. *Especially* given you have them in parallel, where imbalance can develop and that aggressive 14.82V isn't really welcome.

Everyone has their own opinion on how to treat pb's, so accept mine in that spirit.

I'd set the Multi to 13.2V with all the settings, Abs, Float, and turn Equalize off.

If you wanted to use the solar, set all slightly higher, say 13.4V, so the Multi charger will back off when the sun is up. You could leave Equalize on there, and even if it can't manage it totally, will attempt a bit of a stir-up periodically.

Of course once away from the marina, and daily cycling the batts, you may need to revert the settings, but I'd resist going above 14.4V Abs anyway (+/- temp comp, which happens by itself).

I wouldn't get too excited about 'distorted' plates if they still work. If you look closely you may also notice bulges in the side of the casings.. carry on.

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