6-6volt battery bank series parallel voltage

[Silver_Surfer]

I have a battery bank that had a low voltage battery 4volts, in 1 of 6 in series parallel. I understand that it is best to replace all batteries in a bank at same time which I’m going to do. Boat was in Mexico and just wanted to get back in Bay Area and replace them then. Well I’m back and had to remove a victron charger inverter that got wet during the trip and used the mastervolt chargemaster that was isolated to charge the single 12v starter battery. Been 2 weeks on shorepower at home marina and I check my volt meter and shows no capacity for house bank?
When I check bank I find the low volt battery swollen and another battery just parallel to it also swollen but putting out 9volts?
Checked remaining batteries at around 6-7volts
I guess I’m wondering what would cause a battery to have a high volt reading? The chargemaster can do 3 banks and I have number one position at house bank and number 2 for starter which is a single 12v battery
I selected dip switches for agm as all the batteries are agm, but wondering if the charging two sets of voltage banks is an issue?
Going to check dip settings to make sure but before I buy 6 batteries just want to see if there is anything I’m missing.
I plan to go back to individual chargers, victron multiplus 2000 charger inverter for the house bank and chargemaster for the starter battery
Thanks in advance

[StuM]

I'm guessing that the swollen 9V one is the one in series with the 4 Volt one?
You should have removed the bad battery and the one it is in series with before charging

[ItDepends]

The battery that was at 4 volts started the problem. One of the three cells inside it developed an internal short, and was essentially removed. Since each cell generates about 2 volts, your 6 volt battery goes to a four volt battery.

BUT... that 4 volt battery is in series with a 6 volt battery, and the charger is driving the two of them up to 12 volts. The other battery in the pair is being grossly overcharged, that's what has driven its voltage through the roof.

Both of these batteries are at serious risk of thermal runaway and exploding. If they are still connected to the charger, RUN directly to your boat, do not walk, do not pass GO, and disconnect them from the charger and the rest of the batteries in the bank!!!

[Silver_Surfer]

Yes when I took the reading and saw the swelling I disconnected the mastervolt charger. I just wanted to see if maybe there was a something I didn’t recognize before replacing with new batteries. This would be a good time to ask recommendations, I’m looking at Trojan 6volt T-105 AGM’s or maybe the mastervolt 6volt AGM’s. The ones installed are Westmarine 6v AGM. Size of batteries is a concern as the existing box for it doesn’t allow a lot of expansion. the two batteries on left are the bad ones

[wingssail]

Originally Posted by Silver_Surfer

Yes when I took the reading and saw the swelling I disconnected the mastervolt charger. I just wanted to see if maybe there was a something I didn’t recognize before replacing with new batteries. This would be a good time to ask recommendations, I’m looking at Trojan 6volt T-105 AGM’s or maybe the mastervolt 6volt AGM’s. The ones installed are Westmarine 6v AGM. Size of batteries is a concern as the existing box for it doesn’t allow a lot of expansion.Attachment 263174 the two batteries on left are the bad ones

It's a bit hard to read that photo with all the extra wires running around but it looks like you have three sets of two six volt batteries in parallel giving a 12 volt bank of, probably, I dunno, summit over 650 amps.

Whatever.

To fit that space I'd look at T105's or some other generic 6 volt batteries.

Oh, I see you have an LTH starting battery, I bought LTH Golf Cart batteries. I honestly think for the money you go cheap. AGM, LifePo etc. Bigger investment and can't see that its worth it.

[Silver_Surfer]

yes the LTH was purchased in ensenada as the starter battery went bad, and that was the only choice for AGM down there. i'm not going lithium so maybe your idea is correct. will check into it, thanks for the reply.

[Searles]

That battery box is far too tight for all those batteries,no room to cool ,heat build up is deadly to all types ,good ventilation is essential for safety and battery life.⛵️⚓️

[Silver_Surfer]

thanks, but the clear plastic lid over it is gapped by about 3/4" around the circumference edge of box. the area is 35" x 26.5" square, with a gap all the way around so they're should be enough air flow. thanks again for the reply.