Ok so left the keys in my truck with the key turned on, truck died. I jumped it ran around town for 25 minuets to charge them back up. When I connect my battery minder that it mounted to the battery it lights up like it's charging. Then some time later it switches to solid red polarity reversed light. It has done it three times. Batteries are siting good volts wise per my gauges. What am I missing.

 

I had a deep cycle battery I ran way low with a winch. Battery minder kept telling me there was a problem (did same as yours), just took me awhile to believe it: It'd show full volts if I stuck it on a dumb charger, but then it would go to 8 or lower. I think the cells had a short.

One or both may be toast....

 

I'm reading it off of my SCT. It's sitting Around 11 cranking then back to 12ish and as soon as the glow plug cycle is off it goes to 13.7 13.8. Would it still have those readings if it one was dead.

 

test them after unhooking the negative cable from both. Test together won't give a good test.

 

That's a bigass Diesel battery, the alternator ain't the way to charge a battery back up noway nohow.

Neither is battery maintainer, it would take a week to charge it back up at that rate. It will choke on that. Starting batteries can be permanently damaged by a complete discharge, they don't like being drained below 11 or 12 volts. Get a real charger with some juice (10 amps or so) behind it and let it cook for a day or two and then have it load tested. May have a reversed cell.

 

Ok but how does a cell reverse? Doesn't it have to be charged the wrong way?

 

This is what I would do. Put one on at a time and see which one is bad.

I always thought it was better to charge them up slowly.

Also, I too am curious what a reversed cell is.

 

10 amps IS a slow charge. The idea generally is to provide as much current as the battery will draw, though no more than about 10 or 20% of the Ampere Hour capacity (Not CCA, Ah capacity). Fast charging will get you going (50 amp) in a hurry but may warp plates. The little tenders and wall warts simply can't supply enough current to charge a depleted battery, it would take a week so the smarter ones just default to the float charge to avoid excessive time in bulk stage charging and outgassing all the electrolyte. Gassing during charging is good but it has to be minimized to avoid loss of electrolyte.

Cell reversal can happen when an automotive start battery is drawn down to 11 volts or below, they can be permanently damaged when this happens. That's why deep cycle marine batteries are used for trolling motors and other similar applications because they are able to be drawn down repeatedly and take a recharge.

A start battery used in a deep cycle application may be ruined in just a half dozen cycles. They don't "like" to be discharged down much at all. The battery in those situations will often appear to take a charge, but it will normally read (after charging) about 10.5 volts, this is a diagnostic tell, subtract one cell voltage from six and what do you get? One more reason not to let a battery go dead.

The tenders and smart micro-maintainers are great, I have several, but they only really work to maintain an already fully 100% charged battery in good shape. Battery _chargers_ themselves need to be "sized" to the battery they are charging to be effective and efficient and keep the battery healthy.

 

Very good post ^^^^ right there!

 

Thanks. Charging them back separately then will test and see what I have.

 

No, its a pointless waste of time to charge them with a little battery tender. Put a charger on it and crank it up.

 

As Tedster stated, battery tenders are in no way designed to charge a dead or near dead battery. I have never seen a battery reverse polarity but I have seen them lose their polarity and the only way to bring them back, if they can be, is with a massive hit. I have had some luck hitting them with approximately 50 amps or more. When doing this don't be standing over the battery.