Kensmith

My battery charger is automatic and stops charging at 12 volts. It does have a 2 amp slow charge and a 15 amp regular charge. If I used the 15 amp, it will shut down when the battery reaches 12 volts. If I use the 2 amp, it seems to continue charging (I think) to maintain a charge? Is there any way to get it to 12.5 volts other than with the alternator charging it?

67*427

If it REALLY is stopping at 12 volts, it's not doing the job. A fully charged 12 volt, lead acid battery has a resting voltage of 12.7 volts. It takes 13.6 - 14.7 to get it there, depending on the battery chemistry. 13.6 is the lowest I would want to go for a charger.

As for your question, if the 2 amp charger continues to charge, it's okay to use that although it will take a long time to bring the battery all the way up.

Might be time to buy a new charger....

Steve

Bill Irwin

Ken, make sure the battery charger clamps are shiny copper in color, use a metal file to clean up. Bill

Kensmith

It is a new charger. If I don't charge it at 15 amps and leave it at 2 amps, it will finally bring it up to 12.5 volts. But the quick charge only to 12.

67*427

Are you SURE your meter is working correctly? 12.5 is NOT fully charged. In fact, 12.5 for a battery at rest (not being charged) is only 90% charged. You need to get the battery to 13.6 - 14.7 (depending on chemistry) to get the battery fully charged.

Check the voltage on a running engine alternator and I think you will find voltages similar to what I mentioned above. I think either your meter is faulty or your charger is not working correctly.

Steve

Kensmith

I know the alternator will charge at about 14.5 but the battery will only maintain 12.5 from what I have always understood.

67*427

I'd do a little checking on your meter. If the battery is REALLY settling to 12.5 after charging (assuming it is fully charged) and there is NO load on it, the battery is not in very good shape. Here's a table of voltage VS. charge. This is for a typical lead/acid battery that has been sitting ~ 3 hours or more with NO load on it. You can see that 12.5 is only 90% charged.

14.5 is alright for charging voltage although just a little higher than I would like. As stated before, it takes 13.6 or so MINIMUM to force the conversion to fully charge a 12.6 volt battery.

State of Charge 12 Volt battery
100% = 12.7
90% = 12.5
80% = 12.42
70% = 12.32
60% = 12.20
50% = 12.06
40% = 11.9
30% = 11.75
20% = 11.58
10% = 11.31
0 = 10.5

Hope that helps,

Steve

Kensmith

Ok, 12.7 but my point was the amount of charge the battery will charge up to. I was hearing over that amount and always thought around 12.5. I understand the alternator puts out more but as I stated I was using a charger to try and get it up to full charge. Maybe I was not clear (it happens to me)

67*427

I hope we're not miscommunicating here but the point I was trying to make was that the battery doesn’t care what’s charging it. It takes 13.6 -14.7 or so to get it fully charged.

At the risk of overstating the obvious, to fully charge the battery, you need to get its terminal voltage to 13.6 -14.7 and hold it there until the current going into the battery drops very low. On a running alternator, this will be a couple of amps or so at most.

That’s part of the problem with cars that only have a voltmeter; with a big enough alternator (in terms of amps output), you will see the voltage get to the “proper” point but without knowing how much current is going into the battery, you have no way of knowing if it’s charged or not.

I hope that makes more sense….

Steve

toddalin

Two questions:

1. Did you stop to think that maybe this is a battery protection scheme purposely intended of the charger to keep people from accidently overcharging their battery? You can only go so far on fast charge and if you're not around tending things..., it just saved your battery by not letting you cook it for the next day. But the 2 amp portion is intended to act as a trickle charger and can safely be used to finish off that last 10% as it will take care of itself in trickle mode.

2. To what end do you need the battery so fully charged that it can't wait for trickle? If your battery was down and you want to fast charge it to start the car, 90% charge is fine and the car should start right up if you do get it to that point. Also, it could sit at 90% rather than 100% for quite some time and still work fine. The difference in retention time is not that much.

Kensmith

RE: #1, yes I do believe the charger turns off at 12 volts to keep from overcharging and if switcher to 2 amp setting, it does continue to charge. I think that is what I was trying to say. Not sure how high the voltage charge will go at 2 amps since it is such a low rate. Maybe my idea of 12.5 volt maximum was told to me some time ago by a Optima sales rep back when I had my 32 coupe. He said never let it get under 12 volts but try and maintain 12.5, but in any case, never overcharge a battery.

toddalin

The final battery voltage has little to do with the rate. It has to do with the charge voltage. If the charge voltage is not high enough, the battery will never get a full charge. Essentially, the charger "pushes" electrons into the battery. So the force of this push (charger voltage) must be greater than what it is pushing against (highest battery voltage). Hence, the charge voltage is usually 1-2 volt greater than what the battery could push back with at full charge. If the charge rate is exceptionally slow, but the voltage is there, the battery will eventually come up to full charge if it is in good condition. Trickle chargers are supposed to sense when the battery is fully charged and adjust themselves/turn themselves off automatically.

As a simple test, put a voltmeter across the charger's terminals (without the battery connected) to see indeed it exceeds 14 volts in either/both modes.

Kensmith

That's a good idea.

stellar

If charging an optima battery it is a good idea to only charge to 12.5 volts. To finish (top off) optima battery, charge with a conventional battery connected to it. connect optima and lead acid battery together pos to pos and neg to neg, then connect your charger and charge both together. I think you will see the voltage go higher. Do not allow to charge over 14 to 14.5 volts.