Charging SLA battery?

[LEDUser]

What kind of circuit do I need to charge a sealed Lead acid battery? it's a 6V 4.5AH. It came from one of those 1 Mil CP spot lights. they wired something backwards and a resistor caught fire /ubbthreads/images/graemlins/crazy.gif /ubbthreads/images/graemlins/icon23.gif . So I took the battery and the light

 

[gwbaltzell]

While there are better ways, typical these are charged at 7.25 - 7.46 V while limiting the current to 0.4 of the the rated capacity (1.8 A in your case). Or float charged at 6.8 - 6.9 V. BTW note that capacity is based on 20 hour discharge, real world capacity will be a lot less. For longest life recharge after each use or at least once a month if not used.

 

[RussH]

I burned up the charging circuit on one of those myself. I've also fixed a number of others since people try to run the light on them sooner or later. I just use an LM317 set up for constant current of around 500ma. I would use 350ma or less for those who are prone to leave them on the charger. The chip is about 45 cents ($1.79 at RS?)and about 5 cents for the resistor. If you can stand the expense of two resistors, you could also set it up for voltage regulation at around 6.8v. The current regulator charges faster, but the voltage regulator is slightly better for long terms (weeks) on the charger. Look it up with Google or on the National Semiconductor site. I wouldn't worry about a heat sink at 500ma or below, or with the voltage regulator. You will of course need the wall wart to supply power to the circuit. If you don't have the one that came with it, just get one 500ma or higher at 6v. -RussH

 

[LEDUser]

The one that came with it was 9V 800MA but the DC jack was wired backward so the resistor saw a short.

 

[RussH]

Was that the DC jack in the light? I have found a number of wallwarts with the polarity backwards. IIRC, the 'marked' (white stripe, ribs) wire is normally supposed to be the negative. I try to always use center positive, so I check the cheap surplus wallwarts that I buy for $2-3 and change the connections on the plug when necessary. I do ocassionally run across some equipment that has a jack wired center negative, and either I can't get into it, or there really is a reason it needs to be that way.