Battery current tester?

Hi guys,

I was trying to search for existing topics about testing current trough batteries for testing purposes, and could not find it anymore. I know i have seen some test videos in the past about this, but could not find them anymore.
I would like to know if there is any way to build a test platform to test batteries packages to simulate variable currents to test the limits for our battery packs? Just to check at what current batteries start to heat or how much time a specific battery pack lasts until it reaches a specific voltage?


Are you talking about Watt Meter?
Look up 150A Watt Meter Power Analyser


No. I have a wattmeter analyzer. My problem is that i would like to simulate the current when foil engine is under water. It’s not easy for me to make a water test since i have no fresh water big surface near-by… i have the atlantic ocean, but probably not the best test conditions… i can only make dry tests and the current reaches only 5A. Under water 80cms… this value will be much higher…
So far u could only do fail tests… the batteries don’t supply enough current flow to be able to plan… after 5 seconds the batteries low tension alarms start beeping and no real engine force is obtained. I did these tests with two 6S5P in series to get 12S5P… now i build 2new battery packs and the intention is to have 2 6S5P in paralel connected to the other 2 6S5P in series to have in the end 12S10P but would like to be able to make some tests before the real water tests.

Its rather though to simulate the real conditions. To crate a pool big enough is not practical, and even then the motor would be static, not the same as when foiling in open water. Sharing what setup You have now might help, maybe the issue is not only the battery but other parts?

Today I am ko due to second vaccine dosis… fever and pain all over, but i will take some pictures of my build and post them when i feel better.
I know i have some issues with my build because when i ordered the batteries i didn’t use proper batteries and i have 3 different suppliers between all the packs… not only that. I have two battery packs 6S4P and 2 6S5P which can cause some issues between the different packages. I want to build new battery packs 6S12P with equal batteries to avoid issues but i am not sure how long such packs will last…


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You could wind up bailing wire and get to the length where the resistance is right to cause about 100-150Amps. Might need to have it in water to cool it off because you’ll be generating >5kw of heat!

Thanks Bryce,

But do you think i could do it in a way to increase resistance progressively? I am afraid with such high currents that the batteries will get hot really fast.

Start with a really long wire, and curl into loops that don’t touch each other.


Measure resistance so you are in the ball park to have something like 20A in there. R = V/I 50/20 = 2.5 ohm

Then start taping/tieing the loops together to electrically skip loops so the resistance is decreased to the current amount you’d like.

Also don’t zap yourself if you are doing this under water :smiley:

No idea how much wire this would take. Internet quick searching says it would take like 200ft of 20awg copper wire to get the right resistance. Steelwire is probably much shorter


Hi, one other possibility is that you attach an airplane propeller about 50 cm (maybe up to 70 cm) in diameter to the engine. Perhaps a modeler in your area can lend you a propeller. However, very strict safety precautions must be observed

  • the propeller must be attached to the engine very securely
  • the motor must be attached to a solid structure

In this test it is very easy to check the high amps …no water is spraying, just a lot of wind:-)


It is involve high power output to fully test a battery ( so money motor …)
But most of time , if you can split your battery ( like lipo 4s or 6s) or run a few row of cell ( 18650) you can use light bulb as discharge with a watt meter or run a outrunner in water with a boat prop in static against pool wall or duct

Wattmeter , a 80100 , boat prop , a wall , water , good cooling for the esc : bench test

Just built a battery from Molicel INR 21700 P42A. You can get the capacity from the datasheet. If I charge the pack to 4.2V and discharge it to 3v (under load), I get a bit over 90% of the nominal capacity.
12S8P, 4Ah x 3.6V x 12 x 8 = 1382Wh nominal capacity, Metr log shows 1280Wh.
Not sure if it ist worth the effort to buil a variable load up to 4000W just to check the capacity and current of your battery. If you use brand quality cells, you can rely on data sheets and tests to calculate how long the battery will last. What also helps is a metr log, you can see your max current, total capacity used as well as Wh/km which gives you an idea of the efficiency of your setup.

If you want to “dry” test the current sag, one relative easy way would be to use an inverter. Only the sky is the limit what you can connect to a 230 V inverter…hair dryer, coffee maker, heater, you name it:)

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I see. So, the idea since water is 800 times more dense than air is to make more air resistance to simulate engine working under water… thanks for your suggestion.

Hi Tom. Another great suggestion. For now i only have a 12v inverter but i guess i can use the sbec to have the 12v and make this test. Only limitation i see would be the inverter itself. The one i have is 300 w only, so maybe it will not be able to handle an hair dryer.

What you need is dummy load, I always wanted to
make something similar, just with much higher load.

Motor to motor version:

Resistors in water:

Car battery load tester:

Great idea, there you go:


Hi, here another very simple vesrion, you can increase the load in 150W increments and get some light too :grinning:. Each bulb just above 1 euros, so very safe and not expensive, with 20 bulbs plenty of load?


Hahaha I’ve done that with 6 bulbs, way brightener then you think and a ton of heat!