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I was participating in an emergency radio training and I was asked how long I could operate.

I'm using an old car battery, which was sometimes not capable of starting the car in winter, which is why I have it. It has a nominal capacity of 45Ah.

Is there a good way to find out its real remaining capacity except draining it by a known current and measuring time?

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There are many ways to do this. My favorite and I would assume you have these items in your shack. Most inverters have an automatic low-voltage disconnect feature. It makes a beeping sound and stops putting out 110AC when the battery voltage drops below 11 volts or so. You have probably encountered this feature while using your inverter. You connect a load to the inverter (light bulbs work great) and an electric clock (I start at 12:00) to determine how long it runs. When the inverter quits the clock will tell you how long it ran. If you have a 24 hour electric clock it would would be perfect for this however a 12 hour will work just as good. A 60 watt light bulb makes my inverter draw about 5 amps from the battery. Using your favorite math formula from OHM: Watts = Volts * Amps. 60 watts = 12 volts * 5 Amps. 5 Amps = 60 Watts / 12 Volts to be sure, check the current and voltage with your meter(s).

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    $\begingroup$ Fun! I found a step-by-step tutorial with pictures and additional notes about this technique here: instructables.com/AmpHourTest $\endgroup$ May 31 at 21:40
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    $\begingroup$ Thanks. Yes, I have an inverter and a light bulb. So I'll probably just go that way. $\endgroup$ Jun 1 at 6:30
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    $\begingroup$ I'd think one would want to stop discharging a lead-acid battery well before 11 V in order to not cause permanent damage. $\endgroup$
    – rclocher3
    Jun 1 at 16:20
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    $\begingroup$ It is generally stated you should never discharge your lead acid battery below 80% of its rated capacity. However this number appears to change at the whim of the person stating the numbers. Discharging it below this point or 10.5 volts can damage it. The manufacturers chose the 11V I didn't and what I find confirms they are OK. They want to sell units not destroy batteries. What is your statement 11V based on? $\endgroup$
    – Gil
    Jun 1 at 23:56
  • $\begingroup$ A deep cycle battery is much less subject to damage from discharging than a car battery is. They are designed to be regularly deeply discharged using most of its capacity, unlike a car battery. $\endgroup$
    – Mike Waters
    Jun 8 at 19:42
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Is there a good way to find out [a battery's] real remaining capacity except draining it by a known current and measuring time?

While I'm curious to this myself and would love to see other answers, my guess is that applying a known load and seeing how it responds is the most reliable way.

In fact a "battery load tester" is a fairly common tool used by auto mechanics when evaluating the health of a battery. You may be able to borrow a simple one from an auto parts store. It's essentially just a dummy load and meter and you'll still have to extrapolate from the voltage under load to an estimate of health/capacity.

Your parts store or auto mechanic may also have a fancier "battery analyzer" or "intelligent battery tester" that does an actual discharge test for you and can display a report when it's done.

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