Can a car battery overheat temporarily?

I drive an old VW bus, and I’ve been having a problem with starting, where it seems like the battery doesn’t have enough juice to crank the engine, but it only happens intermittently. The only pattern to it that I can discern is that it does it after I’ve been driving for a while, and then I stop and shut the engine down for a short period, and then try and restart. If the engine is cold, it starts right up, and if I let it sit for an hour or so after driving, it also starts right up. This makes me think that maybe the battery is getting too hot and not working properly, and then once it cools off some it’s okay again.

Is it possible for a car battery to have an overheating problem like this?

Sounds like something is getting hot, though I’m not convinced it’s the battery. Could the starter motor be getting hot?

I don’t know. Try doing this:

When the engine is cold, measure the battery voltage (with a digital meter) before starting the bus. Then have someone measure the battery voltage while you’re cranking the engine. And then measure the battery voltage after the engine has started.

Do the same for when the engine is hot. Then get back with us.

Agreed, though I’m unsure why you specifically called for a digital meter. It’s not as though the impedance of the car’s electrical system is very high compared to to an analog voltmeter. True, the precision of a digital meter is greater, but the accuracy won’t suffer much because of meter loading, and a difference of a few hundred millivolts isn’t going to tell the tale here, in any case.

It’s not unusual for an aging battery to have trouble cranking a hot engine. When the engine’s hot, tolerances are closer; things are “tighter”.

Consider replacing the battery. If possible, you might consider one with a higher capacity.

I was pretty much planning to replace the battery, I just wanted to make sure I wasn’t doing it unecessarily. I just checked the voltage on the battery (hasn’t been run in a couple of hours), and it checked out right at 12, if I’m reading the voltmeter correctly (which I’m not sure I am, I’m not very experienced with those things). I’ll check it again when I get home from work, at which point everything should be nice and hot. If it shows it strong at that point, then it must be something else, right?

It’s important to also measure the battery voltage while the starter is cranking. Open-circuit voltage is not a good indicator of battery condition, per se, since even a weak battery can also measure the full 12 V OC. It’s what it does under full load that’s important, in this case. If your battery voltage drops more than one or two volts from the OC reading while cranking, then you’ve got a reason to suspect the battery.

The only reason I’m recommending a digital meter is for better resolution (typically 0.1 V for an el-cheapo handheld DVM). It’s kind of hard to get that resolution with an analog meter (especially the small, cheap ones), and he might need the resolution when making a comparison between:

open-circuit cold engine vs. open-circuit hot engine
cranking cold engine vs. cranking hot engine
running cold engine vs. running hot engine

Well if it was at 12.0 V then the battery is in desperate need of a charge. A fully charged car battery has a V[sub]OC[/sub] of 12.6 V. (Q.E.D.: yet another reason to use a digital voltmeter!) When the car is running the battery voltage should be between 13.2 V and 15 V; the actual value depends on battery charge, temperature, phase of the moon, etc.

And Q.E.D. is correct… it would be nice to know what the battery voltage is when the engine is cranking, both when the engine is cold and hot.

Come to think of it, if you just wanted to check to see if the battery/wiring is getting too hot, you could perform a simpler procedure:

When the engine is cold (and off), measure the battery voltage. Then turn the headlights on and measure the battery voltage. Use a digital voltmeter! When the engine is hot (and off), again measure the battery voltage. Then turn the headlights on and (again) measure the battery voltage. These four data points should tell us if the battery is the cause.

If you were driving a Honda i’d blame the main relay. Classic symptoms.

Anyway, aren’t batteries less efficient when hot? I’m picturing the chemistry in my head and I can’t seem to see any reason why they would be, but yet I still seem to recall that efficiency goes down with heat.

Is that so? I’ve always heared that cold cranking is harder, because the oil has higher viscocity at lower temperatures.

It is usually (always?) the case that the rates of chemical reactions decrease with decreasing temperature. Batteries are no exception. It’s not really a question of efficiency so much as one of capacity.

It sounds like a situation where the starter is causing the problem to me. I’ve had cars where the starter would get hot and the starter wouldn’t be able to turn over the engine. I would take a wrench and hit the starter, and the car would start right up(This worked for a while anyway). There’s just not enough info to say for sure.

I’m certainly not an expert on batteries, but I believe you’re correct, i.e. the self discharge rate tends to increase with decreasing temperature, and the available energy tends to increase with increasing temperature.

sorry, I worded my post really badly.

What i meant to say is that according to the picture in my head batteries should work better when they are hot due to increased kinetic energy. However, I recall always being told that batteries do not work as well when hot. Is this true? or do they work better when hot?

I was thinking of something else when I replied. In general, batteries do havea slightly higher output when warmer, however, too much heat can begin to cause other things to start happening iside the battery. Bad things. Some chemistries, like alkaline, NiCd and NiMH can suffer mightily from overheating, since the heat drives out water, reducing capaciity significantly over the long term. This is one reason rapid chargers are a Bad Thing™ for these sorts of batteries. AFAIK, lead-acid batteries are pretty immune to such effects, however, unless the heat levels are VERY high, i.e. nearly to the point of boiling the electrolyte.

Dog
It’s similar at both ends of the temperature range. When very cold the oil viscosity is high. When very hot, rings and bearings are expanded. Both result in hard cranking.

Automotive batteries are not known for having problems when they’re hot. As suggested by others, the most likely cause of the symptom described in the OP is a worn starter. A worn engine is also a possibility, but much less common.

Honda main relay failures do not affect cranking.