C/3 too much charge for an SLA?

BatteryCharger · Nov 28, 2004

I want to buy a 1 amp automatic SLA charger for a few 17ah batteries I have. It would also be nice to use on my vector spotlight, which has a 3ah battery. I know SLAs are generally charged at around C/10. Would C/3 cook the battery?

mattheww50 · Nov 28, 2004

Actually you can charge most lead acid batteries at C if you are very careful. Not recommended, but 40-60 amp quick chargers used to be pretty common in service stations, and they could fully charge most automobile storage batteries in about an hour.

It is really going to revolve around what the trickle charge rate on the 1 amp charger is, and whether or not the 1 amp SLA charger is really designed exclusively for the larger cell, or really can sense what is going on. A trickle charge rate of several hundred Ma is probably OK for 17AH, but it is likely to cause problems for the 3Ah batteries.

BatteryCharger · Nov 29, 2004

The charger I want doesn't trickle charge, it just charges the battery at 1 amp and turns off. I see that panasonic says .4C is fine for their batteries.

RussH · Nov 29, 2004

Although the charger is rated 1 amp, it probably does not charge at that rate when the battery is in the last charging phase (after the bulk & adsorption stages, I forget what it's called). The C/3 rate should be OK for the bulk & adsorption stages up to around 85% state of charge. Then hopefully the charger tapers off to something like C/6 (500ma) or less, which is probably OK as long as you don't leave it on too long & boil water off.

My guess is that you will lose a small amount of water & shorten the battery life somewhat charging a 3ah battery at this rate. I used to charge my 4ah gell cells with a 2 amp wallwart in just this manner. I still got 5-6 years from these batteries which only last 7-8 years with the best, regulated charging methods.

BatteryCharger · Nov 29, 2004

That's fine if it doesn't last a full 500+ cycles...a new 3ah SLA is only like $8. /ubbthreads/images/graemlins/grin.gif

RussH · Dec 1, 2004

I'd try it. If you do think it's shortening the battery life too much you can make a current limiting circuit using an LM317 and resistor for $1, or $2 if you get fancy. Change the resistor size to adjust from about 25ma up to a maximum of 1500ma (LM317 limit). I did this a few months ago with one set up for 1amp, 500ma, and 350ma for testing LuxIIIs. I have another set up for 25,50 and 100ma for testing regular LEDs. The 100ma setting gets used for arrays of several LEDs, since most LEDs soon become DEDs (dark emitting diodes) at 100ma.