# Battery Ampere-hour rating vs Battery Amps (not an experienced user)

I have almost no experience whatsoever with the technicals of electronics so this is probably a very easy question for someone who does.

Do you know if a battery with a 200Ah rating can put out 200A for one hour or are there limitations? According to this website's third paragraph (Battery Ratings - Chapter 11 - Batteries And Power Systems), you can.

For example, an average automotive battery might have a capacity of about 70 amp-hours, specified at a current of 3.5 amps. This means that the amount of time this battery could continuously supply a current of 3.5 amps to a load would be 20 hours (70 amp-hours / 3.5 amps). But let’s suppose that a lower-resistance load were connected to that battery, drawing 70 amps continuously. Our amp-hour equation tells us that the battery should hold out for exactly 1 hour (70 amp-hours / 70 amps), but this might not be true in real life. With higher currents, the battery will dissipate more heat across its internal resistance, which has the effect of altering the chemical reactions taking place within. Chances are, the battery would fully discharge some time before the calculated time of 1 hour under this greater load.

But wouldn't that mean you could hook up a 200Ah battery and ask it to put out 12000A for one minute or 720000A for a second? That seems very unrealistic, lol. I'm trying to find the proper kind of off grid battery that can power a microwave through a 3000 watt power inverter. The microwave needs 1800 watts and the battery needs to be 12 volts so that would mean I need about 150 amps, I'm wondering if a battery with a 200Ah rating could do it?

• If you're off-grid (or even if you're not) it's vastly more efficient to cook things by burning something - wood, coal, gas, petrol, woodland creatures... as there's only one conversion going on (fuel -> heat) rather than something -> 12vDC -> mains AC -> microwaves -> heat as no conversion is ever 100% efficient. – John U Jul 25 '14 at 10:35
• Information only: !!! The following is from a deleted post (not by me). While it relates to LiPo and not lead acid it provides some good guidelines which may be of some value. Too good to throw away: ||| Post said: There are test methods for this and useful tables, but I only have the one for LiPo batteries to give you an idea of what's involved. maac.ca/docs/2013/… – Russell McMahon Sep 4 '14 at 8:20

Your hunch that batteries have a current limitation is correct. In general, it's hard to tell the current rating [A] from capacity [A·h]. You have to look it up in the datasheet. A lot depends on the design of the battery.

For example: coin cells with 500mAh capacity may have only 3mA max current.
Another (opposite) example: automotive starter battery with 40Ah capacity may have 500A max current.

Lead-acid batteries are interesting in this respect, because there are two distinct types.

1. Starter lead-acid batteries are designed specifically to deliver high peak current for a short period of time. Deep discharge, however, dramatically shortens the life of a starter battery. So, it's not suited for routine operation at high depths of discharge. Your typical starter battery in the automobile works at very shallow depth of discharge.
2. Deep cycle lead-acid batteries are designed (as name suggests) to discharge further. But they can not provide as much instantaneous current.
Here's an example datasheet for a deep cycle battery. Have a look at the nominal capacity on p.1. Notice that capacity depends on discharge current (i.e. the rate of discharge).

-

Depth of Discharge    Starter Battery    Deep-cycle Battery

100%                  12–15 cycles       150–200 cycles

50%                   100–120 cycles     400–500 cycles

30%                   130–150 cycles     1,000 and more cycles


(Source.)

p.s. If you want to read-up, here's an excellent web site on batteries - Battery University.

• I'd add that even the highest quality deep-cycle batteries suffer when discharged badly, even if they tolerate it better than most. – John U Jul 25 '14 at 10:28

No, not necessarily.

Amp-hours tells you how much energy is in the battery, the amps tells you how quickly that energy can be delivered (For the most part, for a fixed voltage).

Your suspicion is well founded, because the Amp-hour doesn't tell the story on the batteries wattage capability.

Car batteries post their peak amperage because that is important when starting a vehicle. For your purposes, you'll want to find a deep cycle battery. Significantly discharging a car battery pretty much ruins it. Deep cycle batteries are designed to be discharged to a much greater degree, down to as low as 20%, and those are rated such that they can deliver the rated current continuously.

You may be able to find a suitable battery, depending on how long you are going to run your microwave. A decent 20+ amp rated deep cycle should still be able to unload 150 amps for a minute or two. Don't run the battery full blast non-stop though, it will overheat. Shop around for batteries for boats, RVs, etc to find good batteries.

• Correction: deep cycle batteries can't generally be fully discharged either. However, where a 'normal' battery can only be discharged down to 75% or so, deep cycle batteries can go as low at 50% to even 25% without damaging the battery. Make sure you check the specs of the battery. Some background on Wikipedia. – RJR Jul 25 '14 at 1:47
• @RJR: You are correct. I must have always unknowningly had spiffy battery supervisors that protected the batteries whenever I've dealt with them. – whatsisname Jul 25 '14 at 1:57

In the answers above no mention was made regarding battery temperature environment. If the battery will be outside, possibly in a camper, to be used at temperatures below freezing, for a cup of hot coffee, cocoa, etc., the capacity drops dramatically. At 20 below F a car battery will deliver a fraction of its 77 degree capacity. At 40 below F is will be down to a mere blip. Auto service calls to jump start a dead battery in -40 degree F temperatures years ago would start with an 18V jump & if that did not work would go to 24V just to obtain a decent cranking speed to start the car. All this is dependent on the overall condition of the battery in the beginning. Older heavily used batteries will deliver still less capacity. http://www.batteryuniversity.com/learn/article/discharging_at_high_and_low_temperatures and http://alternative-energy.6pie.com/batteries/temperature-effects-on-batteries.php provide some good information on the topic.

The best explanation I have read or know of is as follows, and makes the assumption that you do not want to run your battery to less then 50%, as deep cycle batteries do not last long is you drain them to more then 50%.

So the quick answer for you is as follows: Take the amp hour rating of your battery and half it. (200ah/2= 100ah) This means your required load can't exceed 100A.
Your required load is 150A. (1800w/12v= 150A)

So without doing to much fancy maths but assuming a linear relationship, the quick answer is that a 200ah battery will not last long supplying 1800w load at 12v.

Working backwards, you would need a minimum of a 300ah battery to run your 1800w load. (150A x 2)

• Kinda depends on how abusive you want to be, or how often you want to replace the battery - they are related. A much better (more expensive at first, cheaper long-term) rule of thumb is to use the 20 hour rate, so you'd want about 3000 Ah of capacity. – Ecnerwal Jun 16 '16 at 21:18

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