# Battery capacity: Says 12V18Ah/20Hr - how do I interpret that?

I have two sealed lead-acid batteries in parallel that have that inscription on the sides.

They power multiple 10W (Actually, 7 on the meter) PIR lights all night long, as required.

That inscription has me confused.

The first part is obvious, 12 Volt, 18 Ah. But why does it say 20 Hr?

My guess is that it is telling me I shouldn't take more than 18/20 of an amp hour, and it will last 20 hours.

So, assuming 5 of my lights turn on, at 7 Watts Each, that would be 35 Watts, divided by 12V would be about 3 Amps.

Going strictly by the 18Ah rating I would expect 6 hours at that rate. (In reality, I never draw that much for that long. It is for young'uns to go to the bathroom at night and things like that.

The 20 Hr thing makes me think that it is a warning that battery damage might occur if I try to use all of those 18 Ah in less than 20 hours.

But clearly my concern is peak power load times. It is possible for all of the lights to be on for an hour or so if the girls get up and work in the kitchen at night.

I believe my best way to interpret this is to keep it on a 2 Amp AC-powered trickle charger at all times, instead of just running the trickle charger off the inverter during daytime when the sun is out.

Agree? Disagree? I haven't used sealed batteries before, so don't know if the constant trickling might exceed that limit, also.

• I would guess the time given is the time it was tested for. That is, the battery was discharged for twenty hours and found to have 18 Ah of capacity at that discharge rate. Battery capacity is a function of discharge rate; in general, you get a larger amp-hour measurement for slower discharge rates/longer times. It's not usually significant, but sometimes it can be. Commented Apr 24, 2017 at 3:08
• @Felthry try discharging one of those batteries in 20 minutes & you'll find that the discharge rate has a huge impact on the total power output. Commented Apr 24, 2017 at 3:10
• Sure, but I wouldn't call that a usual situation. Commented Apr 24, 2017 at 3:13
• @Felthry maybe not, but 2hrs would be considered useful by many engineers, depending on the project, but would likely reduce the total output power considerably, maybe to even less than 10Ah, depending on the internal resistance of the batteries. -- CORRECTION -- less than 120Wh, sorry...the current output would likely still be higher than 10AH, but with the voltage drop, the output power wouldn't be nearly as high. Commented Apr 24, 2017 at 3:19