Battery balancing with 2 x 6V AGM batteries - your next failure mode waiting to happen?

rh5555

We had one of our AGM batteries fail on our last trip.  They're both gone now, and replaced with lithium, but a little postmortem analysis points to a failure mode that I was completely unaware of, and that others might find interesting.

I was browsing through this excellent document from Victron when I stumbled upon this:

3.4. Lead-acid battery bank balancing
When creating a lead-acid battery bank with a higher voltage, like 24 or 48V you will need to connect multiple 12V batteries
in series. But there is one problem with connecting batteries in series, and this is that batteries are not electrically identical.
They have slight differences in internal resistance. So, when a series string of batteries is charged, this difference in
resistance will cause a variance in terminal voltages on each battery. Their voltages become “unbalanced”. This “unbalance”
will increase over time and will lead to one of the batteries being constantly overcharged while the other battery is constantly
undercharged. This will result in a premature failure of one of the batteries in the series string.

Maybe the two 6V AGM cells in my TAB400 got out of balance and that led to the premature failure of one of them.

Had I realized that this could have been an issue, I would have used the feature of my Victron BMV-712 which allows for monitoring of the mid-point voltage to see if the batteries were getting out of balance.  I could then have used a 6V battery charger on the weak battery to bring it back to full charge, and hopefully in balance.

I recommend that any TAB owner with the 2 x 6V AGM battery configuration and who has a BMV-712 (or a Victron SmartShunt) to wire it up to measure the mid-point voltage and check on the balance of their setup.  Maybe they can avoid the problem we had.

Interestingly there are automatic battery balancers out there, but not, it seems, for 6V batteries.

YanniLazarus

Well, thanks for a timely seasonal present- something else to obsess over! You made me curious enough to do some web surfing research, and it does seem as though this imbalance can occur and can be a problem.  Hopefully  not too common with matched new batteries. My front tub stacked 6V AGMs are pretty inaccessible so no likelyhood I'll be able to check the individual batteries... so here's hoping they're fine!

I wonder how often this has been a T@B 6V AGM pairing problem... and it was correctly diagnosed. 

rh5555

It seems that I am one of many who are switching to Lithium because of premature failure of the factory AGMs, so this may be a more common problem than one would have thought.  I am hoping that others will chip in if they have the Victron battery monitor so that the community can see this if this is indeed a developing problem.

ChrisFix

@rh5555
Count me in as one of those with a 2020 T@B 400 with dead 6V Harris batteries, currently switching to Lithium...

1DRR406

This should help with extending life of the AGM battery bank. Using the app allows you to control this equalization cycle. Memory serves me right, initial set up was not enabled or frequent enough, can not remember which. Minimum should be at least once a month.

Victron MPPT solar charger manual

3.9.3. Equalization charging
Some lead-acid battery types need a periodic equalization charge. During equalization the charge voltage will be increased above
the regular charge voltages to achieve cell balancing.
If an equalization charge is required it can be enabled using the VictronConnect App.

YanniLazarus

I have 2 stacked 6v agm's in my 2020 320S front tub - for around 2 years now... a pain to access for anything. It seems like equalizing is important- and news to me!  I have the old- school WFCO, so have regularly used a NOCO genius 10 to charge my batteries. The NOCO will charge 6v agms only when wired in series to produce 12v... (it only has 6v non agm charging capacity) 

I am having trouble finding a 6v agm charger with sufficient capacity, and would prefer to just disconnect my batteries, check their voltage, then put a load on the higher voltage battery to bring it down to match the lower voltage one- then re connect - changing the order of the batteries.  Too simple?  Advice? Easiest way to put a load on a 6v battery? 

rh5555

The non-AGM 6V setting of your NOCO will do a fine job of recharging the weaker cell.  An AGM setting would just have done it faster.  If you don't already have one, now might be a great time to install a Victron SmartShunt with the extra sense wire attached to your battery mid-point.  This will keep track of of when (if) you need to re-balance.

YanniLazarus

Update: my 3 year old 6v agm batteries show no voltage imbalance despite only ever being charged in series by a NOCO genius 10.  I'm considering this a Christmas present. I am now separately charging each battery to full, then will recheck individual voltage and reassemble. Edited to add no balance issues between my 2 6v agm batteries recharged regularly with a NOCO genius 10.

FreewheelinFranklin

Wow, I just learned that I wired my two 12v AGMs in parallel incorrectly. Guess I have a new busbar installation project for my battery bank this winter!