Sulphur smell from battery box

Your batteries are going bad

 

Alternator could be over charging.
Cables shorting causing batteries to overheat.
Or, What opus said.

 

How are you testing the batteries? If they just came out of the truck after running a while or off the charger you've got to pull the surface charge off first before testing. 100 amp load for a few seconds, let them recover a few minutes then load them up to half the CCA rating for 15 seconds. If they recover to 14 volts a minute or 2 after the load test they're definitely shot. Any smoking cells during the test they're shot.

Batteries generally do not boil over until about 15.5 volts IIRC. If they're boiling at normal charging voltage (13.5 - 14.5 volts) chances are the batteries are on the way out.

 

If you're smelling sulfur, you should be able to find the bad battery easily. Look for a swollen battery case, or feel for a hot battery after the trucks been running a while.

@AModelCat posted the proper full test procedure, but if one's already bad, it doesn't take that much to find it. Even one of those handheld load testers will let you find batteries getting towards the end of life, the voltage drop under load is much greater than a good battery.

 

Don't forget your safety glasses working with batteries.

 

Absolutely. I've had several blow up during load testing.

Got nothing else that@AModelCat didn't mention.

 

Once they start off gassing they’re toast. And voltage test isn’t the way to check, you need to load test them.

 

I'd even add in a faceshield, maybe have a garden hose handy if one blows its top. You'd want that acid off your skin as soon as possible. That gas is flammable so you need to wait until they're done gassing before removing any cables. One spark and its Hindenburg 2.0 right in your face.

 

Shoot all batteries at the base with temp gun. The sulfated one will be warm or even hot to the touch compared to the others.