Wall Wart = 12V 500 ma
SLA Battery = 12V 12 ah

Does anyone have a calculation I could use to figure out how long I need to charge the battery to get a full charge?

Or a close estimate?

Thanks.

 

A 12 volt wall wart shouldn't float above 13.8 volts, especially with any kind of load so you should be able to safely leave it connected indefinitely. At 12AHs and 500mas 24 hours is a worst case scenario, and any 'sane' battery is going to be less because no one ever fully discharges a battery. Internal resistance can vary dramatically so it's hard to say exactly unless you test it.

__________________

 

An unregulated wall wart could output >20V when the load is light so you need to either use a 13.8V regulated supply or add a comparator circuit to cut-off the power when the voltage exceeds 13.8V.

__________________

I do not answer private messages asking for help because no one else can: benefit from advice I may give or correct me if I'm wrong.

Please ask on the open forum if you have a question and I'll be happy to help, if I know the answer.

 

Depending on how heavy the supply needs to be loaded a zener regulator would work well in this situation to prevent over voltage.

__________________

 

12 at 500ma means just that you may draw 500ma at 12v
12 @12ah means 12v @ 12amps per hour.
both are specification of power attainable for different devices. a transformer may have 5kva output but if you measure the output it has only 5 volts so what is the power.

 

A 12 Volt SLA battery will need a float charge of 13.8 volts (I think that's it) So while it may charge a 12 volt battery up from a very low charge state to a moderate one, it may never fully charge the battery, as it can't produce the required voltage. Test the wallwart voltage on a multimeter under no and very low load conditions. If it produces at least 13.8 volts no load then it will eventually fully charge a 12 volt lead acid pack, but figuring out exactly how long requires more information of the voltage/current graphs of the specific battery and wallwart.

__________________

 

A 12 Ah (the "C" rate then equals 12 A) battery probably takes 18 Ah to fully charge [at constant current], so this battery would be 100*12/18 = 67% efficient.
It's easier on the battery if you take >20 hours to charge.

 

What are saying that is I charge a battery for with more or less current it becomes les or more efficient. What a novel idea. AND 100% IS 12 A/HOUR THEN 18/12= 150% NOT 67% or maybe you have no idea.

 

No, I'm saying the battery service lifetime is longer if you slow-charge it. These cordless drills that charge the battery in 1/4 hr. are probably whacking the daylights out of the batteries.

I don't know how A-h in vs. A-h out varies with charge rate. As a first cut, as I increases (I^2)*R loss increases.