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I purchased a 40v Greenworks lawnmower battery, and it's listed capacity (6,000 mah) doesn't seem to be accurate based on what I'd expect to get out of it when mowing a lawn.

How would you go about measuring the capacity of this battery as easily/cheaply as possible (e.g., with components [resistors, step-downs, load testers] I can purchase from Amazon, Lowes, HD, Ebay, or the like)? I'm having trouble following more generalized examples, don't want to start a fire, but also want to understand how to do it.

Thanks!

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  • \$\begingroup\$ This isn't a question that has an easy safe answer as suitable load resistors are tricky and power levels non-trivial. Realistically you may be best discussing your concderns with other users of the product, though you could see what monster RC folks have tried. A youtube video from someone foolish enough to teardown the 4Ah version found 20 18650's, eg, they're counting them at 2AH each which isn't absurd, presumably your 6AH either has 30 or they're claiming more from each cell, which gets into more questionable territory. If they didn't up the cell count, the 4AH might be a better buy. \$\endgroup\$ – Chris Stratton Dec 6 '20 at 5:43
  • \$\begingroup\$ what do you expect to get out of the battery when mowing a lawn? \$\endgroup\$ – jsotola Dec 6 '20 at 5:46
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    \$\begingroup\$ One test you could do would be to use it until whatever point the mower decides its used up, then charge it through an integrating mains plug meter like a "Kill-A-Watt". This won't be accurate as it will include inefficiency of the charger design and the balancing process, but it will be bounding - you can't get more lawn mowing work out of a battery than you put electrical energy into the mains plug of the charger, at least on any ongoing basis. \$\endgroup\$ – Chris Stratton Dec 6 '20 at 6:47
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Get four car headlamp bulbs in series. Charge the battery then connect the headlamps and use a multimeter to measure the current. Measure the time the lamps are on. This will give you current and time - ie Ah.

Note that car headlamps get very hot! Be sure to mount them in a way that they won't cause a fire.

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  • \$\begingroup\$ Tail lights rated around 12W rather than 50 or 100W bulbs might be a better bet. That should be around 1A and take up to 6 hours. \$\endgroup\$ – user_1818839 Dec 6 '20 at 12:07
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The best way to test it is with a real load, ie. in the mower while mowing a lawn. To do this you need a wattmeter that measures Ah, eg. RC Watt Meter DC 60V/100A Power Analyzer...

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...and some way to insert it into the battery circuit. The simplest way is to wire it into the mower, perhaps using high current battery plugs such as these XT60 Connectors.

Then you can see how much battery capacity is required to mow your lawn. Record the Ah used in each mowing session until the battery runs out, and then you have its usable capacity (which will almost certainly be considerably less than the rated capacity).

Though you could build your own wattmeter, commercial units like the one above are so cheap that it probably isn't worth the hassle. As well as recording capacity they also show the peak current draw, minimum voltage, maximum power and watt-hours used (the most important figure).

I recently replaced the 24 V SLA battery in my mower with a 6 cell Lipo, and used this method to find out how much energy my mower uses. The information it provided gave me a lot more confidence than the 3 'idiot' lights on my old battery.

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