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.

**Signing Up**

## Step 1: Battery Tester

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.

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.

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