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.

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    \$\begingroup\$ 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. \$\endgroup\$
    – Hearth
    Commented Apr 24, 2017 at 3:08
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    \$\begingroup\$ @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. \$\endgroup\$ Commented Apr 24, 2017 at 3:10
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    \$\begingroup\$ Sure, but I wouldn't call that a usual situation. \$\endgroup\$
    – Hearth
    Commented Apr 24, 2017 at 3:13
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    \$\begingroup\$ @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. \$\endgroup\$ Commented Apr 24, 2017 at 3:19

1 Answer 1


As @Felthry touched on in his comment, the 20hr part of your 1218Ah/20hr label tells you that the batteries' storage capacity was measured based on a 20-hour discharge (from fully charged, to fully discharged) rate.

All batteries have what's called "internal resistance" which means that the faster you charge/discharge the battery, the more energy is wasted as heat in a process very similar to ohmic heating of an ordinary resistor.

Because of this, battery capacities are often listed with a "discharge time" qualifier to give you an idea of what they're designed for; higher-end batteries, made for grid-storage applications (power substations & the like) will often come with a chart that shows you the total discharge capacity at several different output amperages (discharge rates), but with less expensive batteries, we're expected to "be glad we got the data at all."

With that said, as long as you're not drawing more than 1.8A from your batteries, you can do your calculations based on a 36Ahx12V=432Wh (watt-hours) total capacity for the two parallel batteries. However, if your current exceeds 1.8A, then you'll have to expect a bit less total output, as more energy will be wasted heating up the cells.

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    \$\begingroup\$ Furthermore, by convention, lead acid battery capacities are almost always based on a 20 hour discharge rate. If the time is not reported, you can assume it is probably 20 hours. But this is not true for lithium batteries. Also, it is much less important for lithium batteries because they are much less sensitive to discharge rate. \$\endgroup\$
    – user57037
    Commented Apr 24, 2017 at 3:40
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    \$\begingroup\$ @mkeith +1, accurate on all counts! \$\endgroup\$ Commented Apr 24, 2017 at 4:21
  • \$\begingroup\$ +1 for "be glad we got the data at all." and nice complete answer. \$\endgroup\$
    – Trevor_G
    Commented Apr 24, 2017 at 16:28

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