Easy Test of Battery Amp-Hours Capacity




About: Tim Anderson is the author of the "Heirloom Technology" column in Make Magazine. He is co-founder of www.zcorp.com, manufacturers of "3D Printer" output devices. His detailed drawings of traditional Pacific...

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.

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Step 1: 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

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

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

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

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.

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    69 Discussions


    4 years ago on Introduction

    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?

    3 replies

    Reply 4 years ago

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


    Reply 6 months ago

    Those details can be ignored except when u seek pricision


    Reply 3 years ago

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


    1 year ago

    all good untill you skipped most important part how to calculate the watthours and how to convert them to amphours, no information given


    Question 1 year ago on Introduction

    If battery have 12 volts it's good condition r bad


    1 year ago

    Thank you very mucjh for thsi best idea


    6 years ago on Step 5

    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.

    3 replies

    Reply 4 years ago

    It is a myth.....do you still believe on this ?


    Reply 6 years ago on Step 5

    Sorry richpo10, that is just plain wrong. Check here http://www.snopes.com/autos/techno/battery.asp and here http://www.autotips.com/battmyth.htm and here http://www.thebatteryterminal.com/TechTalk_Batteries_on_Concrete.htm


    4 years ago

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


    4 years ago on Introduction

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


    5 years ago on Introduction

    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.


    5 years ago on Step 5

    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)

    Emperor Dane

    10 years ago on Introduction

    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.

    1 reply
    rgbarnesEmperor Dane

    Reply 7 years ago on Introduction

    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.


    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?