Instant, off the shelf battery capacity measurement

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Joined: 12/01/2006
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I've come up with a bulletproof way to test battery capacity. It does only work for 12 volts, but since I use a serial / parallel jumper plug to my 48V pack for charging, it is easy to tap the same pack for 12 volts as well.

I take a conventional 12V inverter. The one I have is 400Watts capacity. I hook that to my test battery with heavy capacity cabling. I then plug a "Kill-a-Watt" power meter into the 120V output of the inverter. I then plug a 275Watt resistive heater into the Kill-a-Watt.

The Kill-a-Watt will then monitor and total the electrical power used by the heater. A 275 Watt load means 2.3 Amps @ 120 Volts, which requires that the inverter draw 27.5 amps from the 12V pack. (Of course, the inefficiency in the inverter means that the actual draw on the battery is a little greater (by this source, that would amount to 6% more.

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Re: Instant, off the shelf battery capacity measurement

Your method sounds a bit complexified by having that inverter involved. Earlier discussions: Load testing SLA Batteries?; Li-Ion cell testing for dummies?; Halogen Heater as Load Tester??

It can be as simple as a big resistor - as I showed in the 'Li-Ion cell testing' thread. And there are existing products available at auto parts stores everywhere for load testing 12v batteries, like these: Advanced Tool Design Model ATD-5495 125 Amp Fixed Load Tester and Battery Load Tester

If you're using the kill-a-watt for counting amp-hours, there's a simpler way. Both all-battery and batteryspace (see the links above) carry little power measuring widgets that connect in-line to a circuit and show volts, amps, can measure amp-watt-hours.

Another factoid is the official way to measure capacity is to draw from the battery at a fixed amps rate. That is, how many hours will the battery deliver 'n' amps. However batteries, especially lead-acid, vary the amps they deliver over time as the battery exhausts. This means the resistance of the load needs to decrease over time to keep the amps constant. Are you going to monitor the battery manually over the whole period of the discharge test? Again, all-battery and batteryspace (links above) both carry a computerized battery analyzer that's supposed to make this automatic.

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Re: Instant, off the shelf battery capacity measurement

Um, I didn't say "Load" testing. I said "capacity." The lnks you provided all seemed to allow a "stress test" of the maximum current the battery could deliver. Frankly, I don't care, as long as they can provide the current my 950Watt motor needs.

The question I would like to be able to answer is "How much electrical energy can I realistically draw from my set of batteries?" I'm looking for a repeatable, realistic, and safe way to capacity test my battery pack. The capacity measure I ended up with is Watt-hours. In my case, with a load of 270 Watts, I've got 660 Watt-hours in the pack if I draw the pack to 11 V per cell, well above the usual 10.5V end-of-load cutoff. After a year of use, this compares well to the capacity I got with the first test. The load also compares well to the cruise power needed for flat roads.

The inverter provides a way to monitor the battery voltage during discharge automatically. The one I've got starts a warning alarm at 10.8 and cuts off operation when voltage drops to 10.5.

I originally hoped to use my BrainDrain as the capacity monitor, but it apparently doesn't run on 12V. The Kill-a-Watt easily handles a 300 Watt load, and doesn't require a separate purchase.

As the voltage dropped, you are quite right that the amp load would increase to keep a constant power draw. This corresponds to a real life situation where I would still need to get back as the pack level dropped.

I got a picture of the setup, but my internet connection is being bad tonight. Perhaps tomorrow.

Mark

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Re: Instant, off the shelf battery capacity measurement

Another advantage of testing the pack in parallel rather than series is that weak batteries will not be drawn below the 10.5 line. Of course, identifying the weak cells is important, but for this test, we're measuring pack capacity, not pack balance. When I first tested the capacity of the pack, I used a 44V cutoff, but I worried that a weak battery might well have gotten pushed below the 10.5V line.

If the discharge is to the 11.00V level, you might then go on to individually discharge each battery to see which ones were strongest.

Mark

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Re: Instant, off the shelf battery capacity measurement

mf70 wrote:

Um, I didn't say "Load" testing. I said "capacity." The lnks you provided all seemed to allow a "stress test" of the maximum current the battery could deliver.

no, they were very much capacity.
the tests taking hours to do is the dead give away.

the DC watts up runs on 12v IIRC.
thats both Ah counter and Wh counter.

Matt

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Re: Instant, off the shelf battery capacity measurement

Here's the pix:

Meter is showing the Watt draw, about equal to level travel.

Mark

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