Coach battery boils when charged

[mck67]

Hi folks. I have an unusual problem, at least in my experience.
We came back from to our Vegas at an RV park, after spending a few hours at a beach. Smelling a strong sulfate smell, I checked the coach batteries (we have two) and found one of them was extremely hot and boiling off fluid. The other was fine. We were connected to shore power and had been for a couple of days. We have about 3500 miles on our 2015 Vegas and this problem seems to have cropped up suddenly.
I took the battery out of the circuit and ran with one coach battery to get home.
Any ideas as to what the heck happened?
I'll contact Harris and see what they have to say.

[FW28z]

From a generic point of view, boiling usually comes from applying too high of a charge to a fully charged battery (i.e. overcharge condition).

The only way I could fathom this occurring is if the two batteries are so mismatched that the charger thinks it needs to apply a high charge while one battery is fully charged and begins to boil while the other battery still needs a charge.

That means the boiling battery might actually be the good battery. Of course, it could have sustained damage due to boiling.

The condition you experienced could occur if the boiling battery became 100% charged long before the other battery did, which could occur in a severe mis-match of batteries.

It is pretty complicated when this happens, as the fully charged battery will be attempting to discharge into the partially discharged battery, and the charger will be trying to charge both batteries.

The problem is, with the batteries paralleled, the charger cannot sense what one battery's charge needs are vs. the other. So who knows what charge rate the charger is applying in this situation?

This is one of the disadvantages of paralleling batteries without any kind of isolation between them - if one goes bad, it can fail the other.

[JamieGeek]

If you are putting two 12V batteries in for the coach it may be a good idea to use a marine selector switch:
http://www.amazon.com/Perko-8501DP-M.../dp/B00144B6AE

This would make it easy to isolate one or the other battery (and thus in your current situation all you'd have to do is switch off the boiling battery).

[mck67]

Thanks, good information.
The batteries are identical make and model and came with the RV when I bought it in April. I'm now just using the one battery that didn't boil. It charges just fine without the suspect battery in the circuit. I'm going to take it in under warranty and see what happens. This may end up in the Harris vs Thor debate mentioned in another thread.

[blw2]

I could be wrong, but I think that can be a symptom of a bad converter.
I'd suggest doing some research and troubleshooting to make sure the converter is operating properly..... maybe getting stuck in bulk charge mode

Another thought for brainstorming... is on the track of a bad battery.
The old batteries in my golf cart would boil. Turns out I had several bad cells scattered through all the batteries, that were keeping the bank from charging fully so the charger just kept on doing what it does....
Do you have a hydrometer to check the batteries?

[FW28z]

Originally Posted by blw2

I could be wrong, but I think that can be a symptom of a bad converter.

I agree with that as well.

I have pretty much convinced myself that a boiling battery is the result of too much charging current in overcharge.

This then necessarily means the charger is outputting too much voltage for that battery.

Whether it is the result of a bad converter, a bad battery, or a combination of the two is the thing to determine.

For example, a shorted cell could cause the remaining cells of the battery to go into overcharge at normal voltages.

Lots of possibilities.

[mck67]

I would expect the convertor, except it boils when it's being charged by the vehicle alternator (i.e. driving down the road). I was thinking bad cell(s) also.

[FW28z]

So if it boils with the alternator, it's a good bet it is a bad battery.

While I had pretty good luck with the two Harris batteries in my coach (they lasted 5 seasons), I think I'd just replace them with better batteries (Interstate, etc), and be done with it as it seems many have had bad experiences with Harris.

You don't want to be messing with bad batteries each time you want to use the RV.

I just replaced my batteries last week. I did a lot of looking around for batteries, and ended up buying two Interstate batteries. I have had good luck with Interstate deep cycle (had them on my last two boats).