Instructables

Easy Test of Battery Amp-Hours Capacity

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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.
 
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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.
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geeklord5 years ago
400amps???? Thats a lot, i geuss....
seamountie5 years ago
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.
wwlaveck5 years ago
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.
gentry5 years ago
Does anyone know of a source of T105 or T120 batteries for less than like $120 per battery? (Even that isn't a real price because it isn't local to the SF bay area and doesn't include shipping, and requires T105/T120 cores).
Check with local golf courses, who typically have a surplus of 'old' batteries in them. Most of them can be used again and again--the chargers they were using at the golf course just suck, and they won't cycle them properly.
Sandisk1duo5 years ago
wow, i never know about Lithium Iron Phosphate batteries, cool!
i dont think i even have one of the regular bulbs in my house anywhere anymore. all we have is florescent.
Scott_Tx5 years ago
I always wanted to make something like this on a smaller scale for testing my AA batteries.
Seems like a christmas tree light and a battery operated clock might do the trick, but You'd have to be careful that the clock ran correctly with the diminished voltage the light would feed it. The advantage of this is that there's a higher margin of error since the batteries are much higher amp-hour packs than a single AA. You could always test it though and see how it works though. I'm not too familiar with battery powered clocks work, if they have a cutoff voltage, or how adding the bulb to the line would affect the whole thing.
woah, nice... fav'd
diluded0005 years ago
The harder way to get a really good measurement would go something like this: 1) get a high side DC shunt resistor that can handle a couple hundred watts and put it between the battery and the load.
2) Get your 5$ Harbor Freight DMM and measure the battery voltage and the voltage across your sense resistor every 15 minutes or so. (say during commercials).
3a) Since this is DC, the math is easy. For each measurement you know V (voltage) and I (current). Since I=V/R, and R=shunt resistance, the shunt voltage can be divided by the rated resistance of the shunt to yield the current supplied by the battery.
3b) The instantaneous power in Watts is the battery voltage times the sense resistor current.
3c) To see what the discharge curve looks like plot the power as a function of time and connect the dots.
4) The area under your curve is the Watt-hours or Joules for the battery. I like this a little better that amp-hours because it considers the decline in voltage as the battery discharges. But you can just use the current measurements and get Amp-hours instead of Joules. Or divide the Joules by the average battery voltage to get Amp-hours. Or cheat, and divide the Joules by 12 V to get Amp-hours. They will all be pretty close.

If you like, do the math and plots with your favorite spreadsheet application.

Depending on the load, a bulb and inverter as shown, you may get different shaped curves. The bulb and inverter may heat up and vary the power consumption a little bit, but this is pretty much a fixed load. The load applied by an electric car would be all over the place as you accelerate, coast, stop, etc. But on average this is a good approximation. For a real electric car the passenger (or laptop with A/D converter) could measure the current draw of the motor under different conditions to compute a real average power consumption. So say the real average power consumption is 500W: just connect up five 100W bulbs for a bench test like this.

some notes:
http://en.wikipedia.org/wiki/Shunt_(electrical)
http://www.mouser.com/Search/Refine.aspx?N=254577&Keyword=shunt&FS=True

Uncle Kudzu5 years ago
useful info as always, Tim! are you saying that in general two 6 volts are better than a 12 volt when it comes to deep cycle batteries?
TimAnderson (author)  Uncle Kudzu5 years ago
I can't find the electric car book where I saw the charts, but when you look at cost and discharge cycle life vs. depth of discharge, those 6 volt golf cart batteries are the cheapest watt-hours. Forklift batteries and other big industrial batteries have great cycle life, but when you chain them up to 96 volts or whatever your motor/controller needs, they'll be massive. They also tend to be tall and cost a lot when new. Scavenge them if you can for your off-grid house.
NightFire5 years ago
A quick comment about deep cycle batteries and regular car batteries. A deep cycle battery was never designed to crank a car, it is designed to supply a constant current until dead and then get recharged. Deep Cycle batteries are found in RVs, boats, golf carts and similar items, as well as a regular battery. A regular car battery is designed to crank a car over, and nothing else, the alternator was designed to run those things and keep the battery topped off. It should never be fully discharged. Fully draining a regular car battery will damage it.
LinuxH4x0r5 years ago
Brilliantly simple! Would you recommend t105s for an electric car?
TimAnderson (author)  LinuxH4x0r5 years ago
The "build your own electric car" books at your local public library go into this question in great detail. The short answer is yes. Most of the cars at the Alameda electric car club are powered by these batteries. T105s, T120s, any 6 volt golf cart battery of about that size. Add range by increasing the proportion of the car's weight that is batteries. 2/3 of the weight should be batteries for most people. If you need rapid charge or light weight there are lithiums that make sense. They cost more than lead batteries, but less than gas over the life of the car, even at today's weird low fuel cost. Look at http://www.evalbum.com/
Thanks
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