I upgraded my boat batteries and charging system 6 months ago.
6x 220AH GEL diagonal wired in series parallel with carefully identical cable lengths.
Charged with a Multiplus programmed for Victron GEL batteries and set to 70A max charge rate. Monitored with a Smart shunt. 4x95mm cables to the Multi.
System worked great, they have been cycled down to 85-90% about 4 times no problems, then this week, lost shore power and had a voltage alarm at 92%, 22V.
After a full charge to float, I consumed 30A out of them for approx 1.5hr, watching the voltage steadily drop from 26-24.5V as seems typical, then suddenly they 'collapsed' to 22V.
I disconnected them all, charged with a 12V compact multi I have and painstakingly performed steady discharge tests on each single battery, they all perform as expected until about 90%, then the voltage 'collapses'.
I have not seen this behaviour before, any experts out know what might be the cause?
Thanks in advance.