Need a resistance/load to test power tool batteries - Page 1
Forum section: electronics / projects

author: sac02
author_rank: Newbie
timestamp: September 12, 2017, 07:43:39 pm
content: Hi all,This is my first post here, forgive me if I sound a little lost. I'm a long time engineer/mechanic, but just a recent electronics tinker-er.I have a bunch of 18V Milwaukee and Makita battery packs that I need to evaluate their health. Mainly I want to know how they perform (current/voltage) when under at least a moderately high load.My first thought was something similar to how I test individual 18650 Li-Ion cells, with a basic resistive load and then compare voltage at rest and voltage/current under load. Then I looked at the price on 500+W resistors (it took me a while to even find them) and wondered if there was a less expensive method. I thought of maybe a fan mounted to a DC motor, but I think motors of that size are expensive too.Can anyone here help me brainstorm some cost-effective ideas on how to accomplish this?

author: DBecker
author_rank: Frequent Contributor
timestamp: September 12, 2017, 09:28:39 pm
content: Get a "carbon pile" automotive battery tester.The dial won't come calibrated, and the resistance isn't notably stable, but they work really well for testing devices at high currents.

author: ConKbot
author_rank: Super Contributor
timestamp: September 12, 2017, 10:17:18 pm
content: Mind the load you put on the makita batteries, if you send it into protection, the battery will no longer charge on a factory charger and throw an error light.I wanted to run a 24V linear actuator on a 18V makita battery, gave a quick test touching the wires to the terminals, and despite the motor being of similar size to what's in a tool, and the actuator being unloaded, the pack never charged again.Not sure if it can be reset if I crack it open, or what exactly the cause was (turn on current surge? Inductive spike shutting it off? )But yeah, that control box that took a makita battery never got built after that.

author: sac02
author_rank: Newbie
timestamp: September 13, 2017, 01:43:50 am
content: I have heard of carbon pile testers but do not own one. I see that 500A is common (and see that they are variable) but I do wonder if they will be able to dial in what is (for a 500A tool) a very low load. Good idea I will look into it more.Good observation on the protection circuit. Potentially overloaded, or maybe the pack was even smart enough to recognize it was being loaded without a tool attached? I know certain packs and tools can actually monitor the serial numbers of the attached tool/pack. Tool manufacturers will then use that info to refuse warranty claims since they can see a tool or battery pack was potentially used improperly.I wonder if I should wire up a tool as well with the load (whatever that may be) and lightly pull the trigger so that the battery "sees" a matching Makita tool at all times. Or, if worst came to worst, I could open the pack and attach leads to the battery cells directly, bypassing any protection circuitry.Another forum suggested electric stove top burner coils, but I just measured and they are 24ohm, so I would need too many coils in parallel to get the kind of current I want to test the batteries with (I'm thinking 15-20A).

author: SeanB
author_rank: Super Contributor
timestamp: September 13, 2017, 06:16:21 pm
content: how about just taking a regular 2 bar electric heater and using the resistive coils in there, just take the wound element out ( you will have to avoid the ones with carbon fibre elements, look for the wire wound ones) and double up the wire by cutting in the middle and doubling the coil, then stretch to fit the length of the quarts glass tube again. If not low enough then do it again to get 4 springs in the glass. A 110V heater will then be a 25v 15a load complete with a housing.