I scored a bunch (like a dozen) of used Makita batteries (LXT 18V) because our work converted over to a different brand.
I’m trying to figure out a way to test these packs and see which ones are good, which ones are OK, and which ones are no good.
For individual cells, I have some properly sized power resistors, and would measure voltage/current and then calculate internal resistance. But I’m not quite sure if (or how) I can do the same for a battery pack.
Can anyone help me figure out how to test these batteries, other than getting a cordless tool, taping down the switch, and seeing how long it runs? (note: I don’t want to do that, both for the time it will take, and the heat/stress if would put on the tool)
Would have to be a damn good hobby charger for power tools. The basic models top out at 1A discharge, less for higher voltages. For power tools you really want something that can mimic the high draw a tool would require.
I’ve used a Wattmeter and cobbled connectors together to use the actual tool but it’s pure jury-rig and a pain.
Flydiver, that was my concern - these batteries are designed (and need) to output dozens of amps. I’m not sure a hobby charger or my little 10W power resistors are up to that task.
Sure it could test capacity at 1A, but the battery’s output near stall (up around 50A on some tools?) is a totally different matter.
Im using diy connectors and put them into springs without battery disassembly.
The best way to load them with big current is making electronic load tool. Somebody are using set of 50-100W resistors. There is no budget hobby charges that can give big load, but I have found little cheat. Some hobby charger allows using batteries both on input and output, so load current is not restricted by internal resistors. This way you can see actual capacity with 10A load on input battery.
In most situations high current test is not necessary. You need to check them with any current in charge-discharge-charge test, measure charged voltage with high precise and put packs away for about 1 month. If pack capacity is better than 2000mah, and self-discharge does not exceed 0.2V, you can continue using it.
P.S. 50Amp is not truth. All one-cell series compatible (thin packs with 5 cells) tools have 10Amp motor and 8Amp fuse on board, have opened not so many bigger tools but can suppose that for 10-cell batteries numbers are two times bigger.
Not in normal use, but when leaning hard on a tool, the motor can definitely draw that kind of current at or near stall.
Good point - maybe just a discharge test and confirming pack self-discharge, is good enough. But I still kind of want to try and load test them, but maybe 5 or 10A is plenty.
If you want to verify the packs are good for resale, nothing short of a proper test with a properly sized DC load suffices, ideally 10 sec. on, 20 off pulsed discharge at above 15 amps (depending on the cells in the pack). Ideally with a gutted powertool that fits the packs, to access the connection points inside. For your own use, if you don’t want to spend $2000 on a DC load then there’s not much you can do.
Here is a picture (worth a thousand words)
Now go to this thread for an explanation.
Those are 2, 175W 3 ohm resistors.
For 18V packs I can use just 1 or both in parallel, for 40V packs I use both in series.
I got mine from a local retailer. They were pulls from electronic equipment and were about $4 each. A good price to be sure.
Even rated at 175W, they get quite hot. I use a portable fan to keep them cool.
You could no doubt find something on eBay. I would think that 4 of those 10 ohm, 100W resistors in parallel would work. I think they are meant to be mounted onto a heat sink, I don’t they could dissipate 100W each otherwise. The ones I use are laying on a heat sink just to keep them a little cooler.
When working with power resistors remember to check the working temperature. What you call hot is usual rather cool for a power resistor, a metal resistor may have a working temperature in the 200C to 300C range and that is mounted on a heatsink.
