Or rather, how can I tell what the maximum current is I can draw from my batteries?
For example: I have a 10 x 1.2v NiMH 10Ah battery pack. I want to drive some SST-90s at 9A using Der W.'s driver. Would this work OK and what's the calculation please?
And if I use a 10Ah Lead acid battery (at 12v), how do I tell if this will be OK with the same SST-90s and 9A driver?
Apologies for the noob question... :fail:
I'll expand fully because I was unclear. An SST-90 has a forward voltage of about 3.9v at 9 amps. If you have a 1.2v battery, you'll need a 9 amp boost driver to get to 3.9v. With a 100% efficient driver, this will take 3.25 times the current from the battery as goes through the SST90 (3.9v/1.2v), or 30 amps.
So with your 1.2v battery pack, you'll need at least 30 amps (driver efficiency is optimistically 85%, so 37.5 amps) to power this. And you'll have to buy a 9amp boost driver.
Most rechargeables are happy at 2C discharges - twice their amp-hour capacity in amps. If this battery is a 10 amp-hour pack made of 10 cells in parallel, then you could draw 20 amps at 1.2v safely. But at 1.2v you need 37 amps; the 20 amp load will give you about 5 amps at the emitter (20 amps times 85% efficiency divided by (3.9/1.2)v). I'm calculating based on watt-hours delivered, so amps*volts become the conversion.
Putting the batteries in series helps the voltage, but then you're trying to draw a lot of current. If you rip the battery pack open and rebuild it in series, you have 1 amp-hour at 10.8v. This will have to deliver 9A * (3.9v/10.8) = 3.25 amps - nearly twice the safe discharge level. And you'll need a buck driver rated at 9 amps (step down voltage).
If Vbatt is closer to Vled, you'll have higher efficiency. You basically need more battery for this to be safe. Lead-acid batteries have dramatically less available power when you put high loads on them. Converting 12v to 3.9v through (again, a 9 amp buck driver) lets you only draw about 3 amps from the battery, but that's not counting efficiency. This would work, but be heavy with low power density. Try to follow the 2C rule if you can.
