Introduction: Easy Test of Battery Amp-Hours Capacity

Picture of Easy Test of Battery Amp-Hours Capacity

How many amp-hours of capacity does your battery really have?
Here's how to test the capacity of a 12 volt battery with an inverter, a lightbulb, and an electric clock. This can be pretty important to know. Will your battery last long enough to show a feature film at your guerrilla drive-in theater? Will your marker light stay on all night on your boat? I first saw this trick in the magazine Mother Earth News

That "deep cycle" sticker on this battery doesn't mean anything. Internally, the plates just aren't the right shape to get long life from deep cycles and still put out enough current to start a car. To make a real 12 volt deep cycle battery, take two 6 volt T105 golf cart batteries and tie them together.

Please comment with more warnings. It's late and other people are really good at battery warnings.

Step 1: Battery Tester

Picture of Battery Tester

You've probably seen your mechanic use this type of battery tester. It's got a voltmeter for checking battery open circuit voltage. It also has a switch that turns on a very low-resistance dummy load. You use that to test how much short-circuit current the battery can put out.

A meter like this is really handy if you're going to mess around with 6 and 12 volt batteries much.
You can buy one from harborfreight etc. with the money you get by scrapping a dead battery or two.

You can do both these functions with a handheld voltmeter and the dummy load of your choice.
A length of haywire would do nicely. But the store-bought meter probably ends up being cheaper than the haywire substitute. For one thing it's got a cage around the dummy load, so you don't get burned when it glows red.

Step 2: Voltage Test

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You can't tell anything for sure about a battery until it's charged.
If it's not fully charged hook it to a battery charger until it is charged. I usually put them on a 2 amp trickle charger for a day or two. A 200 amp-hour golf cart battery might need a few days to fully charge that way.

A fully charged "12 volt" lead-acid battery is about 12.6 volts. (w'pedia)
While charging you need to drive it at about 13.5 to 14 volts to make the current flow in. The battery will float high for a bit after charging, so voltage isn't great as an indicator of state-of-charge. That's why people check the cell electolyte density with a hydrometer. That IS a good indicator of state-of-charge.

Step 3: "cold Cranking" Current Test

Picture of "cold Cranking" Current Test

Push the button on your battery tester. It shorts the battery out through a low ohm resistor.
This battery is putting out 400 amps. No wonder batteries make good stick and wirefeed welders.

This is the important test for a car's starting battery. It's made to deliver high currents but not for very long.

This is a good test to use while scavenging for batteries. If a battery doesn't pass, it might just need charging. But if it does pass it's probably a fine battery.
Some batteries pass but have low capacity. That's why we need the capacity test.

Step 4: Set Up the Capacity Test Rig

Picture of Set Up the Capacity Test Rig

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've encountered this feature if you've played with inverters much.

The plug-in electric clock tells us how long it takes to discharge the battery that far.
The hands stop moving when the inverter turns off. We'll start the clock at 12:00:00, some time later when the light turns off, look at the time on the clock to see how long it took. An old 24hour electric clock with numbered flaps for a display would be perfect for this. Or an hour-meter from a stationary engine. Or a timeclock from a factory.

This 60 watt lightbulb makes the inverter draw about 5 amps from the battery.
Watts = Volts * Amps.
60 watts = 12 volts * 5 Amps.
5 Amps = 60Watts/ 12Volts

If you want to be totally sure, check the current with an ammeter.

Step 5: Size the Load to the Battery

Picture of Size the Load to the Battery

4 hours have gone by, the battery has put out 20 amp-hours and is going strong. This battery should be good for 80AH or so. It will take 16 hours to discharge at this rate.
If I'd used two of those lightbulbs the test would go twice as fast.
I'll have to look at the clock again in just under 12 hours to avoid uncertainty about what the reading means.

Look up how much current your battery was made to deliver. Don't overload it.
There's a relationship between cycle depth, rate of discharge, and lifetime of the battery.
Battery manufacturers can provide amazing amounts of data on this. It can be pretty confusing until you wrap your head around it.

Let me summarize it for you.
Flooded lead-acid 6 volt golfcart batteries such as the T105 are the cheapest way to get watt-hours from a battery. Except for free scrap batteries that is.
Some day soon Lithium Iron Phosphate batteries will catch up. They charge faster and are much lighter, so for applications such as laptops and some electric cars they make sense now.

One final thing: After the test, charge that battery up and keep it charged! Sitting around in the discharged state causes sulfation and loss of capacity in lead-acid batteries.


fatgnat (author)2015-07-02

I'm thinking that some information is incorrect. For example you are saying 60 watts at 12 volts equals 5 amps. Since you are using an inverter, it will be 60 watts at 120 volts which would be .5 amps. Isn't that correct?

mrsheen01 (author)fatgnat2016-02-03

It is correct to use 12 volts and not 110 volts, because the capacity is given with reference to 12 volts.

badzmorando (author)fatgnat2015-11-01

why focus only on the light bulbs...try considering also the stand by loss of your do this measure the amps without the load ...

richpo10 (author)2013-04-15

Nice work, I was wondering on what type of surface did you preform the test? If you left your battery on a concrete floor the concrete will discharge a car battery just siting there with no load. This could possibly have given you a false discharge reading. That's why retailers have there batteries stored on racks to prevent discharge.

badzmorando (author)richpo102015-11-01

It is a you still believe on this ?

Norman Fraser (author)richpo102015-10-11

an old wives tale, re concrete floor.

mampoer (author)richpo102013-08-15

Sorry richpo10, that is just plain wrong. Check here and here and here

badzmorando (author)2015-11-01

Nice work man! did you consider the stand by loss of your inverter while doing this experiment?

MeanUncleBob (author)2015-09-21

A deep cycle battery is not for starting engines and an engine starting battery is not for deep discharge.

seerixprojex (author)2014-01-21

Ok, there's a lot of talk about doing some pretty crazy stuff using the information you provided in your instructable. How one gets from doing a safe test on a battery's amp-hour capacity to trying to weld using a car battery I'm not sure. But me, I just want to thank you for the information. I really don't care what my battery's capacity is but in the process you answered some other questions I had about batteries. Good job.

hopekiller30 (author)2014-01-05

I thought it is supposed to be how long a battery can put out 1 amp for *blank* hours otherwise if u put a 10 amp hour battery to use 10 amps it may be dead within 45min (not exact amount of time but u get it)

yellowrobat (author)2012-05-22

The method is efficient~But need so many tools?

Emperor Dane (author)2008-12-27

Thats a wonderful idea, its so simple. i started building a nickel iron battery pack for back up power during blackouts, and this is perfect to see whats' actually being stored.

rgbarnes (author)Emperor Dane2012-05-03

Tell us more about your nickel Iron battery. I know Edison invented them and that many are still in use today, years later. How did you build yours? I am interested in doing this myself.


jamesjamesjames (author)2012-04-16

Is there a way to test the amp hours without an inverter? i use an SLA battery to power a car amp & speakers (portable, not whilst in the car) and fear i may have discharged the battery too much and damaged it. Any advice?

garrry (author)2011-02-24

I set up a battery powered PA system some while back that used a 12v battery and an inverter.

Since I had available a number of 'second hand' batteries I needed a way to test the capacity of each one to see which would last the longest.

I set up a rig the same as described in this article, except I used a 120 watt light bulb (my load was a 100 watt amplifier that drew in neighborhood of 1 amp when playing music through speakers, so 120 watt seemed appropriate).

I was primarily interested in a relative comparison between batteries rather than quantitative, so the choice of load or efficiency was not so important as long as the same setup was used each time. (120 watt bulb shortened the length of time of each test so was a bit more convenient.).

The elapsed time shown on the clock is written, along with the date, on the top each battery. ( I re-test the batteries every so often to see how they hold up in actual use and have found that they do 'degrade' over time, sometimes significantly.)

I use new batteries now, but still like to test them using this method as a way to reinforce any notion I might have that it is indeed time to replace them.

The setup for this method of testing battery capacity is simple, intuitive and certainly can provide useful information as to the capacity of a battery.

ShippingGuy (author)2010-07-03

I Agree with joinaqd - Great Job.

awang8 (author)2008-12-28

A car battery is 200 amps? Wouldn't that fry the roof off?

Yerboogieman (author)awang82010-02-23

No. If is were, it would probably fry the hood off.

grimgroper (author)Yerboogieman2010-04-30

this one time.. we were out moto riding and this guy dislocated most of the fingers on his hand so we took him to the hospital... i ended up fetching his ute from his house so he could drive home after, the genuine good bloke i am... okay so its off the beaten track, bit bumpy here and there. over the first bump, i hear all his tools jump in the back, bugger, okay ill fix it when i get there....

roll up to the hospital, he come running out arms waving, yelling, etc. 

turns out that the his diy job on his froffy fridge in the back wasnt so cold anymore... something from one of the tool racks had married the two terminals of the car battery shorting all ~200amps, and caught a nasty fire inside the steel canopy... right next to 40L of fuel i might add.. 


diluded000 (author)awang82008-12-28

Yeah, some jumper cables with one end hooked to your work piece and the other connected to a welding rod will make an workable welding arc from what I hear. I asked the guy at AirGas what welding rod would be best for welding off a 12v battery,and he fixed me up with some 3/16" rod. Haven't tried it yet. Mostly use the OA torch or MIG.

Dr.Bill (author)diluded0002009-02-25

Good way to blow-up a battery. Ever see one go?

bassbindevil (author)diluded0002009-01-02

A welding arc is typically around 20 volts, so I'd guess you'd need two car batteries in series to have a chance of welding. Something to add a little resistance and inductance in series should help, like winding a bunch of heavy wire around a few pieces of rebar. Polarity makes a difference too; try it both ways and see which works. 6011 or 6013 are about the most common electrode types for steel. You'll need a proper welding mask if you don't want to go blind, although in a pinch you could just buy the dark filter glass and improvise the rest out of duct tape and cardboard. Frankly, though, you're probably better off rejigging a car alternator, since you can adjust the output voltage/current using a rheostat or variable DC supply to the field terminal on the alternator. Google alternator and welder, and something will come up.

awang8 (author)bassbindevil2009-01-02

A welding arc is typically 20 volts? Cool, you can boost your car battery voltage up 8 volts.

ReCreate (author)awang82009-01-30

12v batteries actually hold up to 15 volts

bassbindevil (author)ReCreate2009-01-31

Well, not really. You'll only see 15V (more likely 13.8 to 14.4) if an external charger is charging the battery. Take the charger away, apply a real load, and it'll drop almost immediately to about 12V.

ReCreate (author)bassbindevil2009-02-01

no it would drop to near 13 Volts or so

awang8 (author)ReCreate2009-01-30

Well, it takes 15v to charge one...

bassbindevil (author)awang82009-01-02

Well, no, it won't boost the battery voltage if you only have one 12V battery. Also, I'm pretty sure you can't get a proper welding arc with only 12V, which is why two or more batteries in series are needed. There would be sparks, but I suspect it'll be impossible to get a stable arc.

awang8 (author)bassbindevil2009-01-04


jd570a (author)2009-04-20

I am a heavy equipment mechanic (retired) with almost 30 years in the field. I must say that this is concise and very well written. I have known a few mechanics and one factory rep that could have benefited from reading this. A few points that I would make are: A good hydrometer will sometimes show a weak cell before a load tester will. I personally prefer the ones that have the glass floating hydrometer inside the sampling tube. Much more accurate than the dial type. If you can find them, the old carbon pile load testers are great for checking batteries as well as the charging and starting systems. Deep cycle batteries are rated in A/H (amp hour). The deep cycle batteries that I worked with started at 225 amp hour. The small aerial work platforms would use 4 of these. Some of the larger man-lifts used 420 amp hour batteries, usually 8 per machine. Regular batteries are rated in CCA (cold cranking amps). Most of the batteries in heavy trucks and equipment start at around 600 CCA. The larger engines (excavators, graders, loaders etc) can use 1000 CCA and higher. On deep cycle batteries we would checked the water level, charged them a least 16 hours and then test each cell with a hydrometer. Load testers give false readings on deep cycle batteries. I know a lot of batteries are changed out due to a low charge. There are a few battery shops that will slow charge these batteries until that have "desulfated the plates" and will sell them as good used batteries. I am going to make your amp hour tester and give it a go. I think I'll take a pass on welding with a battery though. All in all, my hats off to you for a very good Instructable.

rangerman2889 (author)2009-02-07

I'm sure you already know but for anyone else using this method on a larger scale I'll say this anyways. Testing with an ammeter is always your best bet not only because the light bulb might vary slightly from their ratings and the fact that the clock is plugged in, but also due to losses inside the inverter. All in all I'd have to say kudos on another great instructable!

ReCreate (author)2009-01-30

so how long would a 10 amp one work with something that uses near 100 watts?

Derin (author)ReCreate2009-02-06

It would work for about an hour.

ReCreate (author)ReCreate2009-01-30

like a tv or a computer?

Da_Fudge (author)2009-02-02


joinaqd (author)2009-01-10

awesome job and simple..(gave you 5 star)

carpespasm (author)2008-12-28

Wouldn't you need to also factor in the efficiency of the inverter for this kind of test? I'm not familiar with how efficient that kind is, but it could lead you to a lower amp-hour conclusion than is true if you've got an inefficient one.

CAR_RAMROD (author)carpespasm2008-12-28

agreed. The setup is simple enough for most applications but the efficiency of the inverter is a big factor. I think at best they are 80%.

carpespasm (author)CAR_RAMROD2008-12-28

that's not too bad then. You'd just add another 20% to whatever your result is. I guess it's just something to keep in mind while testing. Just using the unadjusted reading from the clock would probably be a good idea anyway to keep some room for error in whatever you're using it for anyway if you're dealing with a used battery that's (assumably) lost some of it's capacity already to give it some slack while it further ages.

thinkdunson (author)carpespasm2009-01-03

actually, you would add 25%. if you add 20% of 80%, you're only adding back 16% of the original 100.

kill-a-watt (author)carpespasm2008-12-28

reread step 4 "..If you want to be totally sure, check the current with an ammeter." my meter goes to 10 amps easy, although you can only measure on that scale for a few seconds at a time. Still, take a quick sample of the current to begin with and perhaps a few more to see if it creeps up or down with battery voltage. like someone else said, it's DC so the math is easy.

scafool (author)2008-12-28

Very good, I was surprise at how low cost the basic battery tester was too. Would it be wise to actually measure the amps drawn with a meter instead of relying on the lightbulb's claimed wattage value, or are the bulb manufacturers's tolerances close enough for any real world measurements? Either way, it is a good way to get an idea of how healthy an older battery is and I will be using it.

thinkdunson (author)scafool2009-01-03

when you work with electricity and electronics, you learn to never trust what something is supposed to be. always measure it before making calculations. never assume something is working properly. you'll get weird results and not know why. plus, measuring the current (before the inverter to keep all your values in dc) will ensure that you didn't make a mistake and will take into account all the variables that you may have overlooked. (like the current that the clock and the inverter draw).

btw, using uncalibrated test equipment can be worse than just relying on rated specs. ...but you can only work with what you got. you can make sure your multimeter is in the ballpark (which is good enough for hobby work) by measuring known sources like house voltage for the ac setting, a new AA battery for dc (should be around 1.6v), and a resistor for the resistance setting (just compare your reading to the color code and take into account the tolerance). if your measurements aren't what you expect, then you're out of calibration. i once bought a multimeter from radio shack just to have an extra laying around. i tested it on house power and it said 80VAC. yeah, i measured it with my fluke and it read 120. so i returned it and bought an extra fluke instead.

LarryB (author)2009-01-02

Good idea and info.
As for the efficiency of the inverter, it will vary greatly depending on the watt age rating of the inverter versus the load wattage. I.E. a 100 watt inverter with 80 or 100 watts will typically be more efficient than a 400 watt one with the same load. Some large inverters will be above 94% efficient, but these are typically in commercial UPS units.

It is fairly easy to build a small shunt for up to say 20-30 amps. #10AWG wire is almost exactly .001 ohms per foot (actually 1.02') , and #20AWG is 0.01 ohms for the same length, so make two voltage tap points a foot apart (hey your meter will be plus or minus 2% anyway) and a little further out make two connections into your circuit. That way the connection resistance to the current flow doesn't contribute to the voltage drop. You measure the voltage across that 1 foot section and calculate current by ohms law. I could do an Instructable showing the process, if folks want more info. For #20 wire shunt 1mV of drop for each 0.1 amps of current flow. For #10 wire 1mV is at 1 amp of current. #10 in free air will handle 30 amps, no problem. Larger wire can be used if you want a high current shunt, up to about 100 amps, try - 1.02' of 1/0 = 0.0001 ohm or 10A=1mV

The battery discharge curves aren't linear as someone else noted. In telecom use common 48 volt strings are rated at an 8 hour discharge, if discharged over 4 hours we estimate we get 80% of the 8 hour rating, at 3 hours we only get 73% of the 8 hour rating.

A lot of manufacturers will have constant current discharge curves, or constant wattage charts on their websites. Most electronic loads are close to constant wattage, few loads would be constant current since voltage declines with discharge, and a pure resistive load will draw less current at the lower voltage.

Have fun, be safe!

THURGOBOB (author)2009-01-02

You meant hydrometer in step 2 not hygrometer. One checks specific gravity (hydro) the other checks humidity.

TimAnderson (author)THURGOBOB2009-01-02

Thanks! Fixed!

geeklord (author)2009-01-01

400amps???? Thats a lot, i geuss....

seamountie (author)2009-01-01

If you are using this to either check to the manufactures specs, or to see how much life is left in your deep cycle, make sure you compare apples to apples. Manufactures calculate their AHr rating using a time to full discharge rating. Full discharge is taken as being 10.5 V. There are usually two or three AHr ratings given, the two most common are 5 hour and 20 hour rates. Compare the closest time. Unfortunately the relationship is not linear, so try and jigger you results around to match the times that the manufacturer gives.

You could also adapt your load to match expected results. For instance if your 20 hour rating is 90 AHr... 90 AHr/20 Hr=4.5 A . 4.5 A * 12 V = 54 VA or Watts. That is the load to you need to use to meet the manufactures specs. Your inverter SHOULD cut out at 10.5 V, or full discharge. So for our hypothetical battery, hook in 2 x 20w + 1 x 15w for 55w draw. Run the clock till it stops. It should be about 20 Hours for a top notch battery. Use the math given in the Instructable to get you actual AHr rating for the battery in front of you.

wwlaveck (author)2009-01-01

Speaking of batteries. I use Fios internet. After about a year the back up battery system started to beep every few minutes indicating that I needed to replace the battery. Verizon was useless, all they offered was to replace the battery at a jacked up price. I was getting ready to order a battery via the internet. But before I did I decided to try using my car battery charger. I set it on 2amp for six to eight hours. This totally took care of the problem. It appears that my back up system did not adequately charge the battery. My system use a PX12072 battery.

About This Instructable




Bio: Tim Anderson is the author of the "Heirloom Technology" column in Make Magazine. He is co-founder of, manufacturers of "3D Printer" output ... More »
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