Originally Posted by John Ridley
The only problem is that I'm using a lead-acid cell that's only 2.3 AH. I foolishly thought that dividing AH by amps would get me hours of runtime. Well, lead-acid cells taper their voltage off pretty badly as they discharge, so even with 1.6A draw and 2.3 AH, by 40 minutes the headlight is pretty orange.
That's a truth. You'd think a 4.5 AH SLA would give you twice the run time of a 2.3 AH. But when you put a heavy drain (20W) on a 2.3 AH battery, your run time was only 40 minutes, while the 4.5 AH will give you about 2 hours.
SLA's don't like a fast drain. By the same token, they don't like a fast charge. So a smaller battery will not tolerate the same charger that a larger battery can use quite safely.
In either case, whether draining a battery, or charging it, too fast, the battery's capacity begins to dwindle. Make sure you get a charger that doesn't charge the battery too quickly for it's capacity. Also be sure not to run your battery so low that the light goes yellow (at least, not too often.)
Fortunately, SLA's are cheap compared to NiCd's and others. If you ruin one, you can get another for the price of pizza (but don't do it--waste pizza money on a battery, that is!)