Battery boiling - hot to touch

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Well-known member
Oct 15, 2006
I recently bought a 1999 Jayco Designer 5th wheel.  It has 2 batteries wired together with a fuse iin the storage tray. 

While checking the trailer out, battery A (manf. Jan06) was fine on fluid level, battery B (manf Mar05) was low on almost all cells.  I topped it off with distilled water.

Two days later I checked the fluids again and battery A is still fine, Battery B was low again.  Added about 3/4 gallon distilled water to the cells.  Went to check after a couple of hours, and can hear the water boiling inside.

Battery A is slightly warm to the touch.  Battery B is much warmer, almost hot.

Any ideas what is going on?  A neighbor has a 2000 Jayco and his battery is not even warm to the touch.

My Power source is a 55 amp from Todd Engineering sales.  It has a hi/lo switch and its currently set to the LOW setting.  It also had 2 leds, and the green one is currently lit, which says all is fine.

Should I disconnect battery B from the system and run on one for now?  I'm hooked to shore power so don't need the battery right now. 

Also, if I pull the fuse at the battery, will that stop the batteries from being charged?

You have a bad battery ... the plates have warped and there is a short.  The water is boiling out ... be careful it is venting hydrogen gas which VERY flammable.  Turn off the charger, let the battery cool and buy new batteries.  Most of the time when one goes bad the other is damaged as well. 
Regarding battery A/B.  Are those 12 volt batteries (Six filler caps each) or SIX volt batteries (3 filler caps each)

If they are 12's disconnect the hot-boiling battery

If they are sixes, then they are wired in series to make a 12 volt battery  Two sixes in series are the same as one BIG 12 volt

Treat them as such (Replace both, at the same time) since in this case one battery will have been damaged by the other's failure
I agree you have a bad one. I'd charge them overnight then either take the trailer or batteries to a good battery store and have them load tested. Also lead acid batteries (caps on them) can be tested with a hydrometer to find a weak or bad cell. AGM (absorbed glass mat) batteries won't have caps as they have the acid suspended in a fiberglass mat so it won't leak (maintaince free).
If you have a DC voltmeter you can do some of the testing yourself. A fully charged 12 volt battery will show about 12.6-12.7 volts. A fully discharged one will still show 11.9-12 volts with no load. With your charger running it will show about 13.7 to 14.5 volts depending on the charger. A good charger will drop back to about 13.1 volts when the battery gets a full charge. If it stays up high it's not a good charger for extended charging and will damage the battery about like you describe. A fully charged battery will hold up to about 11 volts or more with a load tester. A weak or bad battery will usually drop below 10 or more under a load test. You can also unhook them when fully charged and wait overnight and check them with your voltmeter. They should still show 12.6-12.7. Some will show 12.8-12.9 shortly after charging and will slowly drop. In your trailer there are some things that draw a bit of power with everything shut off. Also a stored battery will loose about 1% of it's charge a month.
Storage is hard on them (sulfating). A bit of a charge every couple weeks will agitate the acid and slow down the sulfating. A hot battery is dangerous so take caution. Also the heat pretty much tells you that theres a short, but a bad charger that charges much over 14.5 volts will boil them also. Thats not very common though.
Get rid of the hot battery - it is a goner and perhaps even dangerous. And it is sucking the life out of the other battery as well.  The only real question is whether the other "good" battery is worth saving. A load test will determine that. However, it is generally advisable to replae both batteries of a pair at the same time, since newer ones charge & discharge at a different rate than an older one. Unless the other battery is less than about 18 months old, I would replace it too.
Thanks for the info.  I pulled the bad battery last night and the remaining battery seems to be doing fine.  It is taking a charge, not hot (or even warm). 

Since this one seems to be okay for now, I will just run with 1 battery.  It is a 12 volt. 

Since I'm in a park I don't see much of a need for 2.  1 Battery should handle any intermittent power loss. 

The other bad battery was still warm to the touch in the morning, about 9 hours after pulling it out!!
Re "have it load tested"
My experience to load testers is that it is just a step in the sales process for a new battery. If you ask the battery salesman about a problem he will suggest you have the battery load tested, and the load test will either suggest that the battery SHOULD be replaced, or that it PROBABLY is faulty. Either way, it's a new battery. OK, call me cynical.
A load tester is a valid way to check a battery.  I carry one in the coach and have used it to check not only my batteries but others as well.  It's a quick way to eliminate the battery as a problem or see that it's bad.
Hi Rankjo  I use a load tester all the time on snowmobiles & PWC's. Since they mostly have AGM (Absorbed Glass Mat) batteries now you can't use a hydrometer like a lead acid battery. It's a voltmeter that puts a predetermined load when you push the button. A good fully charged battery needs to hold up to a certain voltage level for some period of time. Those specs are shown in service manuals for the various products. We even test new ones after they are charged for delivery. Every now and then I find a bad new one. The warranty people don't argue if I use the right tools and chargers. It also saves people agrevation and unneeded service problems. The biggest issue with using one is you have to have the battery fully charged to 12.7 or 12.8 volts before they tell you the truth.
Re my post above.
Don't get me wrong, I'm not against load testers. It's just that when you take a battery in to have it tested it always always fails, or is at least doubtful.
Maybe I should get my own tester, sounds like a good idea from what you say.
I like to have new batteries in top condition, and work to keep them that way with distilled water, and solar top-up chargers, trickle chargers, and so forth. I am also quick to replace a battery that uses water, and if one battery goes, I usually replace both.
Generally, I get good service out of my batteries and I have learned from experience that dicking around with a doubtful battery is an aggravation and no good in the end.

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