Thought folks might be interested in the following from the NordHavndreamers forum:
Re: Heat - Batteries - Explosion - Sunk
Posted by: "marc.siedenburg" email@example.com
Fri Nov 18, 2011 6:03 am (PST)
I have had a experience like this, but we were in time to stop a potential disaster just in time. I am happy to share this information. On my Stentor Yacht I have 6x 200Ah Mastervolt Gel service batteries outside of the engine room, charged by two Victron Phoenix inverter/chargers (4-step charging). During delivery of the boat the installation was checked by Victron. A very crucial part of the installation is the thermo compensation, and there was one sensor mounted to the first battery.
Three years later, after a long trip and on a hot summer's day, we moored in our Marina and attached the shore power. After dinner my wife noticed that the floor around the battery compartment was warm. I opened the hatches to the batteries to find out that two of the six batteries were "cooking" and seriously venting gases and moisture (sulphuric smell) trough the safety-vents. The batteries were to hot to touch by hand. I opened all doors and to vent the area, shut down the main switch (which could have lead to an explosion!, but I had shut down the inverter/chargers and all onboard 24Volt systems, so there was no load) and disconnected the two batteries from the other four. Those two batteries have been hot for an entire week! The cases were deformed from the heat, but did not open up in any way. I now think we were just in time to prevent a disaster.
Later inspection of the system revealed that the thermo sensor was not connected to the Phoenix chargers (this is a requirement) but to the battery monitor only...
And still then one sensor would not be enough. In a series-parallel connected grid, there are always small differences between the batteries and in my opinion all batteries should have a thermo-sensor connected to the charger(s) for thermo compensation. However; most chargers don't have this capability.
I spoke to Victron about the two hot batteries. They explained that during heat-buildup, two lead-plates in the batteries can "warp" and make a internal short circuit, thus progressing to even more heat. They called it a thermal runaway.
I concluded from this that the thermal compensation for charging is very, very important. Not only the chargers should have this compensation (reducing the absorption voltage when heat is measured) but also on the DC generators of the main engine.
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