Load testing/discharging a custom battery pack

years and years ago in what seems like a whole other lifetime, i worked for a tech firm that made some of the most reliable and awesome remote power and data management equipment for data centers. They made these smart power switches that you could telnet into or make serial connections to and tell them to turn outlets on and off and read power draw from servers and other cool shit like that.

They had what they called HiPots to test them with (that’s a brand name for other test equipment too, so they were treating it like a generic term), and they were basically just giant aluminum racks of resistors hanging over the workstations. They would use them to load test the power strips we built.

I want something like that or maybe something better or more appropriate for discharging and load testing these packs i build. Running them down with motors on the bench is going to take forever and i’d like to have a way of testing them before i put them in a board and ride with them.

Is there something i could make myself out of a clever arrangement of resistors that wouldn’t burn my house down but would adequately drain the pack within a reasonable amount of time and let me test things like the BMS’s low voltage cutoff and shit like that?


use some lightbulbs? or get some massive capacitors, use the batteries to charge them, and then use them to charge the batteries?

R = V / I, my friend. And P = I^2 R. Choose your pack voltage and your discharge current (based on your limits there, and also the time you want to discharge over), calculate your resistor value, calculate the power rating for it, stick a voltmeter across your battery and away you go. You can parallel resistors to increase their power rating, but just make sure you work out the values you need to get your required series resistance.

To elaborate, say you have a 10s 10Ah pack thst does 20 C continuous. I = 200 A, V(max) = 42 V. Therefore R = 0.21 ohms. Power rating is P = 8.4 kW. Get 1000 10 W 220 Ohm resistors and stick them in parallel and you’ll have a 10 kW 0.22 Ohm resistor. Oh, and your battery will drain in 10 / 200 x 1hr = 3 minutes. That is fast.

Probably too fast, so maybe go with something more.conservative. I’d use half of the max continuous discharge rating as your guide, and make sure the resistor setup is very safe so that you don’t burn anything down/get electrocuted.

This is also bound to be expensive.


You want an electronic load, but they aren’t cheap (N3300). You can make shift jig with a bunch of power resistors.


slap these bad boys in parallel, it’s a safe way of draining 10Amps

2pcs in parallel is: 8//8 = 4 ohm

42V (worst case)/4ohm = 10.2Amps

10.2Amps * 42V = 441W

you will safely operate under the 1000W of capacity of this jig.


get 2 of these

you will drain 20A if you put them in parallel 10A if you use just one 5A if you put them in series. and you would be running in all cases under the max 1000W

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electrical math like this is an area i’m not strong in, so thanks for the formula. Can you define R, V, P and I just so i’m 100% clear on what those vars actually are? I’m assuming resistance, voltage, power, and I don’t know what I would be.

but if i build something i will definitely share of course.

Those look just like the things in the aluminum rails that they were calling HiPots at my old job. I’m checking them out now.

also i just saw a $3000 price tag on an n3300 load tester… yeah. not happening lol. That’s 2 X-Carves or half a steel hydraulic deck press.

See edits above. Oh yeah and V is voltage in Volts (V), P is power in Watts (W), I is current in Amperes (A), R is resistance in Ohms.

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yes… it seems so. But i think it will be worth it in the long run. It may be something i have to wait a bit to build, but its definitely happening.

I’d almost think it might be better to build a test setup similar to @onloop’s but with a “dyno” so to speak. Strap the board down with the wheel driving a drag wheel and then you can load test batteries, motors and ESCs all at the same time, probably for a lot cheaper. You’ll have more variables there though, so it might be hard to figure out what’s failing. The VESCs will provide pretty good diagnostics though. If you load some of these motors close to their max power draw, you’ll get these battery drained in no time.

It was actually @onloop 's dyno that gave me the idea, so i may just go this route if i can find a way to do it without consuming more garage space. It would be a purely mechanical solution at first, of course, and i’m not quite sure how i would put resistance on the wheel yet but i could maybe figure something out.

@longhairedboy Maybe you could rig up some drive system from the wheels to something like a small outboard boat motor & prop that’s mounted in a big bucket of water to provide resistance. It would be a consistent source of resistance if you could pull it off. Just a thought.

that sort of thing s along the lines of my thinking… i thought about a big wheel with a thick belt on it that would just drag on the wheel but that would make a lot of heat and wear out and probably make dust or grime as well. water as the force resister… hmmmmm

Water is great for heat absorption too (as if you didn’t know)! You could get all crazy and route a water cooling loop to the battery pack and motor to try and simulate the cooling effect from riding, or just keep temps down in general during testing.

don’t tempt me. I ran a water cooled pentium 4 dual core for a couple of years as a gaming rig. I still have the heat exchangers and pumps. next thing you know we’ve got water cooled battery packs, soft switches, and VESCs.

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Oh, I would NEVER try to tempt you. Besides, I don’t think you’ve got the skills or cajones to make it work anyway. :wink: :laughing: jk

And how cool would that be to run a water cooled system through some visible vein/tube system in the deck or enclosure and fill it with water dyed a crimson red color? Scarlet would have her own working circulatory system. And illuminate the whole system so it looks super sweet, while your at it.

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If you want to replicate cooling from riding, why not have a fan be the load? Just get a RC prop (which will obviously fit on a brushless motor) and have that be the load.

Build a bench test rig using a prop and a coulometer on a large motor. Cheapest way to get it done. Be careful though, you can easily pull enough wattage to ruin some gear.

This guy rigged up a thrust gauge but it isn’t needed for our use.

Oh yeah, almost forgot. This is very dangerous, it can kill you!


Used a simple online circuit design tool to come up with this.


It’s 2s2p 25ohm 100w resistors. Based on the calculations total drain should be 1.8a, and each resistor should be burning off a little under 20w or so; well under the 100w they are rated for. Interestingly at this low wattage they are still too hot to touch.

Running them in a 1s4p configuration would result in about an 8a current, and each resistor would be pushed to 80w. Given how hot they are now I’m not too keen on testing this. Might look around to see what else offers a suitable resistance for higher currents (and is able to sustain them) without costing too much.

I use the watt meter for charging, but it’s handy here. Looking to see if the capacity just before it (hopefully) gets shut down by the BMS matches what it should be. Speaking of which, the DieBieMS spits out everything the watt meter does but also includes individual cell voltages so it’s kind of a little redundant in this case.

You could put your cup off coffee on there so it doesn’t go cold or you might as well just boil the water for your coffee with it

Hmmm, might just get the multimeter out to probe the resistance of the kettle🤔.

We use electric kettles for boiling water round these parts; recently I was lead to believe this is quite the novelty in the US.