boiling battery - sign of a bad battery, or bad alternator?

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dlb
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boiling battery - sign of a bad battery, or bad alternator?

Post by dlb »

Fyi, this is on my corolla but I figure the discussion will be pertinent to t4's as well.

I always thought that a battery boiling over was the sign of a dying battery, but I just looked it up because the battery in my corolla has started boiling over a bit and from what I've read it seems like boiling is more a sign of overcharging, which then kills the battery.

So I measured the voltage when the engine is running at 2000 rpm and it's almost 14.5 V. I read 14 V is ideal so is 14.5 V too high? And if so, do I really need to replace the alternator over this or is it more of a voltage regulator issue? Replacing an alternator because it's putting out slightly too much voltage seems silly to me.
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Re: boiling battery - sign of a bad battery, or bad alternator?

Post by NWMO »

Hey David,

Most modern cars (as I'm sure you are aware) have the voltage regulator built into the alternator. I've never changed the VR on one of the integral units, but it may be possible. Definitely overcharging, swap out the VR if you can, if not, you're stuck swapping out the alternator.

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Re: boiling battery - sign of a bad battery, or bad alternator?

Post by Petros »

usually this means the voltage regulator is bad. if you put a volt meter on the running car you will get 17-18 volts. it will ruin the battery fast. likely now you need both a new regulator and a new battery.

Replace the voltage regulator. If it is built into the alternator you can usually just buy the internal parts and dismantle the alternator and replace just the regulator. or you can just replace the whole alternator (usually faster to get). You might replace the whole alternator assembly to get the car back on the road, and than take apart the bad one and take your time rebuilding it yourself (fairly fast and easy job usually, kits are available on the internet), and than keep the old one as a spare.
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dlb
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Re: boiling battery - sign of a bad battery, or bad alternator?

Post by dlb »

Like I said, I measured the voltage and it's charging at just under 14.5 V. Not crazy high, in fact I thought that was fine until I looked online. I guess my concern at this point is that I replace the alternator or VR only to find it still puts over 14 V.
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Re: boiling battery - sign of a bad battery, or bad alternator?

Post by Mark »

If it was me I wouldn't worry about 14.5V. *Disclaimer*...that opinion isn't based on any kind of science. 14.5v is just (barely) within my comfort level for alternator output. I've had 16v before, which is obviously too high. How do you know the battery is boiling over? I've had 2 batteries so far where the post seals failed and the sloshing-around electrolyte leaked out and crystallized around the posts or pooled on top of the battery.
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Re: boiling battery - sign of a bad battery, or bad alternator?

Post by dlb »

Mark wrote: Thu Sep 17, 2020 9:45 pm How do you know the battery is boiling over? I've had 2 batteries so far where the post seals failed and the sloshing-around electrolyte leaked out and crystallized around the posts or pooled on top of the battery.
It leaves wet marks running from the filler caps to the edge of the battery. The fluid level wasn't very low though so I think it's just barely boiling over...more like simmering maybe.
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Re: boiling battery - sign of a bad battery, or bad alternator?

Post by Petros »

when it is over charging you can watch it boiling out of each vent. regularly changing gives off hydrogen gas (flammable BTW), which is normal, and why each cell has a vent.

a fully charged battery should show 13.2 volts (2.2 volts per cell x 6 = 13.2) and normal charging would be about 14 volts. but that should drop off to a small level when it is charged, that is when the voltage regulator cuts it off. if it does not it will continue to charge a fully charged battery.
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Re: boiling battery - sign of a bad battery, or bad alternator?

Post by Mark »

dlb wrote: Thu Sep 17, 2020 9:57 pm
Mark wrote: Thu Sep 17, 2020 9:45 pm How do you know the battery is boiling over? I've had 2 batteries so far where the post seals failed and the sloshing-around electrolyte leaked out and crystallized around the posts or pooled on top of the battery.
It leaves wet marks running from the filler caps to the edge of the battery. The fluid level wasn't very low though so I think it's just barely boiling over...more like simmering maybe.
My last battery had electrolyte leaking out of the caps too. Even when I removed it from the car and was carrying it around, the sloshing was causing the acid to leak out pretty bad. I tried to prolong it by dumping baking soda everywhere around the battery, but eventually I came to my senses and replaced it since spilt battery acid and steel/paint don't mix well. Maybe remove the battery and gently tilt/shake it a bit and see if liquid comes out. I assume if the battery was being overcharged and boiling over/offgassing it would be noticeable if you listened around the battery with the hood up.
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Re: boiling battery - sign of a bad battery, or bad alternator?

Post by ALiveSR5 »

Here's an unfounded thought. If one or more of the cells is going bad, would that cause the charging system to be over active?

Do you have a hydrometer to test the electrolyte specific gravity?

Have you disconnected one of the battery terminals then check the battery voltage after it sits for 24 hours?

Have you tried using an amp meter to see if there is an excessive draw, with all lights and accessories turned off?
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ALiveSR5
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Re: boiling battery - sign of a bad battery, or bad alternator?

Post by ALiveSR5 »

dlb wrote: Thu Sep 17, 2020 6:41 pm Like I said, I measured the voltage and it's charging at just under 14.5 V. Not crazy high, in fact I thought that was fine until I looked online. I guess my concern at this point is that I replace the alternator or VR only to find it still puts over 14 V.
Maybe these troubleshooting pages from the official Toyota 1985 Tercel Service Manual will help:
TercelChargingSystemCheckP1.pdf
TercelChargingSystemCheckP2.pdf
TercelChargingSystemCheckP3.pdf
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No other vehicle that I have ever owned had a heart and soul like my 1985 Tercel SR5 4WD Wagon. :D
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