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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

WARNING
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.

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

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.

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<p>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?</p>
<p>It is correct to use 12 volts and not 110 volts, because the capacity is given with reference to 12 volts.</p>
<p>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 ...</p>
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.
<p>It is a myth.....do you still believe on this ?</p>
<p>an old wives tale, re concrete floor.</p>
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
<p>Nice work man! did you consider the stand by loss of your inverter while doing this experiment? </p>
<p>A deep cycle battery is not for starting engines and an engine starting battery is not for deep discharge.</p>
<p>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.</p>
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)
The method is efficient~But need so many tools?
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.
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. <br> <br>Ron
Is there a way to test the amp hours without an inverter? i use an SLA battery to power a car amp &amp; speakers (portable, not whilst in the car) and fear i may have discharged the battery too much and damaged it. Any advice?
I set up a battery powered PA system some while back that used a 12v battery and an inverter.<br><br>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. <br><br>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).<br><br>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.).<br><br>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.) <br><br>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.<br><br>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.
I Agree with joinaqd - Great Job.
A car battery is 200 amps? Wouldn't that fry the roof off?
No. If is were, it would probably fry the hood off.
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.... <br /> <br /> roll up to the hospital, he come running out arms waving, yelling, etc.&nbsp;<br /> <br /> 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..&nbsp;<br /> <br /> moral to this story MAKE SURE YOU INSULATE YOUR TERMINALS!!!!<br />
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.
Good way to blow-up a battery. Ever see one go?
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.
A welding arc is typically 20 volts? Cool, you can boost your car battery voltage up 8 volts.
12v batteries actually hold up to 15 volts
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.
no it would drop to near 13 Volts or so
Well, it takes 15v to charge one...
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.
Oh...
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.
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!
so how long would a 10 amp one work with something that uses near 100 watts?
It would work for about an hour.
like a tv or a computer?
Awesome!!!
awesome job dude...smart and simple..(gave you 5 star)
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.
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%.
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.
actually, you would add 25%. if you add 20% of 80%, you're only adding back 16% of the original 100.
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.

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Bio: 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 ... More »
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