SLA charging off 12V, 15V, and other questions

KevinL

Flashlight Enthusiast
I've recently acquired a spotlight and it has a very simple, brain-dead charging system. There is a wallwart, with nominal 15V 500mA, plugged into the spot. Inside the spot, the electrical connections from the jack terminate directly into the batteries. So basically, the existing charger system is brain dead, it pumps 15V/500mA into the battery constantly. Also, it has what I believe is a 1W, 2.2 ohm resistor on the line leading to the positive jack of the battery. This spot can also be charged with the supplied car adapter.

Is it a bad thing to leave it on the wallwart all the time? It strikes me that this setup may overcharge the SLA. I suppose you can't expect much from a cheap cordless spot.

Circuit diagram: (forgive the crap drawing, you really don't want to know WHAT I used to cook up that diagram..really /ubbthreads/images/graemlins/grin.gif)

Also, instead of a car, I am planning to hook the charger up to a voltage-regulated 12V supply that pumps out exactly 12V. Would this be too little for the simplistic charging circuit to function? I suspect that whoever designed this setup got away with it because car power is really something in the order of 13+ volts, which is enough to charge the SLA.

eluminator

Flashlight Enthusiast
Your brain-dead charger will damage the battery if left on all the time.

I don't think 12v will charge the battery. A "float" charge of somewhere around 13.5 volts will do the trick, but it will take a while to charge.

You can buy automatic chargers that will charge at a higher rate with a voltage around 14.5 volts, and then switch to the float charge when the battery is fully charged.

eluminator

Flashlight Enthusiast
On further thought it really depends on the voltage the brain dead charger supplies to the battery. If it's in the "float charge" range, then it might be okay to leave it hooked up.

There seems to be disagreement of just what a proper float voltage is and how it affects the life of the battery. I've seen 13.5 to 13.8 volts for this. My automatic charger puts out 13.3 when it shifts to float mode, and that seems good to me. I don't leave the battery on the charger though. But it should be topped off every three months even if it isn't used. A lead-acid battery will slowly self-discharge, and a battery that sits around not fully charged will loose capacity permanently.

Brock

Flashaholic
Battery float voltage depends on the make of the cell. For instance I float my 12v 1000amp bank at 13.4v, but they bulk to 14.6 first. These are open lead acid, sealed I wouldn't bulk that that high.

Many wall wart chargers will significantly drop voltage under charge. So say that 15v 500mA charger left on for a week might only push the battery to 14.6v because of the loss in charging heat (and that resistor). Of course the trade is speed of charging vs life of the battery.

A good two stage or better yet a three stage charger is a wise investment for anyone using 12v batteries regularly.

KevinL

Flashlight Enthusiast
Thanks guys /ubbthreads/images/graemlins/smile.gif Should I measure the voltage across the terminals when the thing is charging?

But I think you guys are right that in the end, a proper charger is the only way to fly. It would be useful as a blackout light, because I can float charge it, and if the power goes out, load it up. At the moment, I play with it a little, let it charge a few hours, then turn it off. What's the self-discharge rate of SLAs like?

I'm just wondering how they think they can get away with it on car DC with that resistor.. oh well, guess that 'if it blows just buy a new one'.