Do I Have a Bad Cell?

[geoleo]

My sailboat has a Trojan bank of 8 6v marine 125 s Solar charger of 2-145s with a older xantrex regulator. Trojans about 4-5 years old. Recently have observed a voltage drop to about 12.8 --used to be higher. Are my batts going bad? Maybe it is just a cell somewhere? How do I check this? Can I leave the batts connected while I search by checking every cell with a hydrometer? Thanks

[sy_gilana]

Can do hygrometer wile connected. The bad one will normally be flanked by higher Sg ones. Then disconnect and measure voltage per batt, it should confirm Sg readings.
Pull the bad one asap it will ruin the others.

[tkeithlu]

If you get to the point of disconnecting them, you can also run a load test using an automotive tester. Cheap at Harbor Freight or similar; they are just a resistance wire like an electric heater and a voltmeter. If the cell is bad, holding the resistance across it for ten seconds or so will cause the voltage to drop quickly.

[boatbod]

Disconnect one terminal from all the batteries and measure each one. They should all be similar in the 6.3-6.4v range. If one is markedly less then you have a bad cell.

Problem is once you identify the bad cell you still need to replace ALL the batteries because mixing old and new in the same bank just drags the new ones down.

[smac999]

12.8v is a 100% full battery. Not sure what your question is

[geoleo]

HHHM I heard full batt flooded 12Volt is 13.2volts! or is that float charge volts??

[roverhi]

As smac999 says, 12.8 volts is the steady state charge of a good FLA battery. They will show a higher voltage just after the charger is disconnected but will drop to a steady state around 12.8 volts. As others have said, do a individual cell hydrometer check and a load check with a battery checker to see if there is a bad cell.

At 5 years, the batteries are nearing the end of their useful life. Would not replace a single battery if you find a bad cell. Time for a new bank of batteries if that is the case especially if you are going somewhere where optimum battery function is necessary.

Not that familiar with other types of batteries but those may hold a higher initial charge than FLA batteries.

[dlymn]

Put them on the charger for a few hours. Does one of the batteries or even a cell feel hotter than the others. Good sign of an irregular cell. Take turn off the charger disconnect leads and use a voltmeter to read each battery (record readings) After a few hours read again. Has one battery dropped voltage more than the others? Put the batteries under load for a while (fridge will do). Again disconnect leads and read voltage. These tests should identify a crook battery. Another way is to get your battery supplier to come on board and measure each battery with his load meter. If he finds a dead one, he'll probably have a replacement in his van.

[Cadence]

Originally Posted by tkeithlu

If you get to the point of disconnecting them, you can also run a load test using an automotive tester. Cheap at Harbor Freight or similar; they are just a resistance wire like an electric heater and a voltmeter. If the cell is bad, holding the resistance across it for ten seconds or so will cause the voltage to drop quickly.

Do batteries still have exposed cell connections that allow the use of a load tester per cell?

[Bleemus]

Originally Posted by Cadence

Do batteries still have exposed cell connections that allow the use of a load tester per cell?

Nope.

[sailcrazy]

Besides checking each 6v battery for voltage, pay attention to the watering rate. If one cell takes more (or less) water than the others, especially over repeated waterings, suspect something is not quite right. I watch the individual watering rate, individual battery voltage, and the amount of corrosion/build-up on each post. When something does not match the rest.....somethings likely up.

[Cadence]

Originally Posted by Bleemus

Nope.

I didn't think so. It would be nice to be able to use a load tester as someone suggested above. I think that is long gone, for the good of bad?

[boatbod]

Load testers are for the most part consigned to museums these days. Conductance testers can tell you far more with way less amperage draw. We use a Midtronics EXP-1000HD here at the yard and have found it to be a pretty reliable indicator of future battery failure.

[Cheechako]

I see no problem with 12.8 volts. In fact, fully charged Interstates in my little power boat, when brand new never exceeded 12.5 v after sitting over night disconnected. Both were usually identical at 12.46 volts.
Charge your batteries up, disconnect them and measure voltage the next am. If one is real low it is shorted/bad cell.

[tkeithlu]

Hey, Boatbod - Thanks for the update on load testing. I'm a victim of learning my mechanics in the 1960s, and didn't know electronics had entered that area. What scares me is diesels becoming electronic - leaving the possibility of a blackbox failure that no at-sea mechanic can repair. Fortunately for me, I run an old Navy 4-53 main and a Universal M-25 genset.