Help!         My Batteries are getting hot!     What should I do?

Hot batteries usually means battery failure.  A common mechanism for failure is that one cell in one battery shorts out.  In wet cell batteries as the plates charge and discharge it is normal for small parts of the plate to become loose and drop to the bottom.  A space is left at the bottom of the battery to collect the droppings.  However when enough droppings accumulate they can short out to the adjacent plate.  When that happens your 12 Volt battery with one shorted cell becomes a 10 Volt battery.  Your charging system tried to work harder than ever to charge it up, and in the process all the other cells in the battery and in the bank become overcharged until they fail too.  Along the way they can get very hot as all that energy being pumped in to the battery has to go somewhere and if it cant go as charge it just shows up as heat. 

As the batteries become hotter the electrolyte boils off producing an unpleasant 'Rotten Eggs" odor and an excess of sulfuric acid gas may discolor plastics in the battery area.  Eventually as all the electrolyte is lost the batteries will stop functioning at all but the danger is that before this happens a runaway situation may occur.  There is a very interesting article on Thermal Runaway at this LINK  including some gruesome photos, but in summary, heat accelerates the chemical reaction and thus the batteries get hotter and hotter until something stops them.

The mechanism of failure in AGM and GEL batteries is slightly different but the principle is the same.

So to get back to the original question:  What should you do if your batteries get hot?  Turn off the charger.  Ventilate the battery compartment. Then test them for a failed battery, replace as necessary, test your charging system and if possible fit temperature sensors on your chargers to prevent this happening again.  Note that unless you spotted this failure very early it is likely that all the batteries have been damaged by overcharging and so I would recommend that the entire bank be replaced.

As this is an interactive blog feel free to add your questions or comments below.


Date 7/26/2016

IF a 12v battery is measured to be 13.3v but it can't start engine which is designed to do so and was working before, what will be the reason?

Peter Kennedy

Date 7/27/2016

There could be many reasons but one example would be if there were almost no electrolyte in the battery. If only the very bottom of the plates had electrolyte the battery might still show 13.2 volts but be unable to do anything because most of the plates were not in contact with electrolyte. So you can see how the same concept could apply if the plates were sulphated, only the part of the plates in direct contact with the electrolyte would be doing anything.

Paul T

Date 6/10/2018

My question is simple but the answer is so hard to find, even after reading many articles. What do you do if one of your batteries goes into "thermal runaway"? This happened to me recently when equalizing AGM batteries. On of the batteries was hissing gas through the cell caps and half-hour after disconnecting the charger, it continued to "boil" and the sides were very very hot to the touch. So what is the best course of action to take in this situation to possibly prevent battery damage, but more importantly to avoid damage to your boat or to persons aboard?

Peter Kennedy

Date 6/12/2018

Equalizing batteries is no fun. You are doing a deliberate overcharge so it is quite likely that they will get warm, fizz, spit, and smell bad. I think you have to sick around to supervise so that this doesnt happen to excess. AGM batteries would be less tolerant of this process than wet cell batteries. Be easy on them because electrolyte lost cant be replaced.

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