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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.
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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Signing UpStep 1Battery 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.
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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Ron
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.
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..
moral to this story MAKE SURE YOU INSULATE YOUR TERMINALS!!!!
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.
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!
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.