Battery on a salt water boat (i.e. not mine) seems to be discharging. What would be a good way to find the problem?

I can guess either 1 of 2 things:

- it's not being charged... alternator problem.

- it's discharging when not in use... a current leak of some sort.

Any suggestions?

Dennis,
If the battery is a wet cell are all fluid level of the cells even or is one cell consistently in need of water? That might indicate a bad cell.

After that you could try to measure if there is a small current drain with the battery switch off. You'll need an ammeter. Are there any accesories that are wired directly to the battery, bypassing the battery switch like an active bilge pump?


With the engine running you ought to see 13.5 or more volts across the battery.

Good luck,

Rob

Are there 1 or 2 batteries and can each be isolated by a cutout switch? If so, is the problem with one or both batteries.

Quick check on the alternator -
1. Look at battery voltage with engine off but switches in normal position for running.
2. Start the engine - look at battery voltage again. It should be at least 1/2 volt higher if the battery is charging.

Simple discharge check - with everything off, remove the battery terminal. Connect a millameter (like on a VOM) between the removed terminal and the battery. See if there is leakage current. Anything more than a couple of milliamps and you should investigate where the juice is flowing. Note that corroded terminals are potential leakage patch to ground and should be cleaned.

Basic battery stuff - if not maintenance free type, make sure the cells are all full with DI water. Get a hydrometer and measure the specific gravity in each cell. A bad cell will have a low specific gravity. Uniform low specific gravities are a discharged battery. If the battery can't be charged, it may just be past its prime. Try a battery charger to see if it takes a charge.

Finally - did it start discharging after some system "upgrade"? Perhaps there is a connection that should be isolated but is not connected properly.

I'm told both batteries are new, so fluid levels shouldn't be a problem.

Can you use an ohm meter across the "house" side of things to look for a current leak?

How about the diodes on the alternator? Is that a likely source for failure to charge?

Tnx muchly.

Dennis - Even in a new battery it does not hurt to check fluid levels. You can use an ohmmeter, but may not tell the real story on leakage currents. There may be diode voltage drops that only pass current if the potential is greater than about 1 volt. Some ohm meters don't use that much potential to drive them, and good ones use a differential current bridge with very low potential. That is why I recommend using a milliameter with the terminal off the battery looking at what the leakage currents are. It's best to start at the battery terminal to measure and work away from the terminal to isolate things checking for leakage currents.

It could be bad diodes, or possibly the voltage regulator. The voltage check engine off and engine running should show that and is a fast test to run. Diodes can be removed and checked for front & back resistance. Using an ohmmeter that is built for testing diodes (at least 1V source) you should have high resistance in one direction and low resistance the opposite direction.