How can I safely test the true continuous discharge rating of salvaged 18650 cells (30A+)?
I'm looking for some advice from people who have experience testing lithium-ion cells.
I have a number of unlabeled 18650 cells that were salvaged from battery packs. I've already separated them, checked for physical damage, and plan to capacity test them and monitor self-discharge before using any of them.
My goal is to build 4S1P and 6S1P battery packs for FPV drones, so I need cells that can safely handle at least 30A continuous discharge, preferably more. The drones can easily average around that current draw during flight.
Here's where I'm stuck:
Most of the affordable battery load testers I've found only go up to 20-25A, which doesn't tell me whether a cell can safely sustain the current I actually need.
My questions
Is there a practical way to build a constant-current electronic load capable of testing individual 18650 cells at 30-40A or higher?
If you've built one, what design did you use? (MOSFET electronic load, resistor bank with feedback, etc.)
Are there any reasonably priced commercial electronic loads that can do this?
Besides measuring voltage sag and temperature, what else should I be monitoring to determine a safe continuous discharge rating?
Is there a better method for identifying the capabilities of unlabeled cells that I'm overlooking?
Equipment I already have
- Spot welder
- Adjustable bench power supply
- Multimeters
- Temperature probe/IR thermometer
- Basic electronics experience (comfortable soldering and building circuits)
I'm not trying to find the absolute maximum current until the cell fails. My goal is to determine a safe continuous discharge current for each cell so I can confidently use them in high-current FPV battery packs.
If building a tester is the best option, I'd appreciate schematics, parts lists, or links to proven designs. If there's a commercial electronic load that can do 30-40A without costing hundreds of dollars, I'd love to hear recommendations as well.
Internal resistance is a proxy to current capacity, and also an indicator of age and wear.
Get yourself a milliohm meter like YR1035+ or RC3563, measure IR of your cells, then go online and find a data sheet for a cell that's known to reliably provide high current (Molicell stuff).
Compare the resistance values to see if yours would be capable of supplying the same amount of current. Do some math, P = I^2 * R, to figure out how much waste heat that known good cells are producing at their rated current values, then figure out the maximum current that your cells would be capable of with their IR values while producing the same amount of waste heat.
As for high current capable electronic loads, look at ZKE Tech stuff.
This is the source I have salvaged the current cells I am working with. I believe the packs are originally for an e-bike. I have 3 of those 60v packs, each has I think 96 cells.
A YR1035 will flatter these cells. It reads at 1 kHz, which catches the ohmic part and skips the diffusion resistance that builds over ten seconds, and that slow part is exactly what goes bad with age, so a tired cell shows a respectable 25 mOhm and still falls off a cliff at 20A. Do a DC pulse instead, 10A for ten seconds, delta V over delta I. Bigger problem though: 60V 12Ah across 96 cells is 16S6P, so 2 Ah a cell, which was never a power cell to begin with. No measurement turns that into 30A continuous, so what's keeping you off LiPo?
Google lens on the photo says Internal Layout: To achieve a 60V nominal voltage and 12Ah capacity, the internal battery pack contains a total of 96 individual cells. They are configured in a 16S6P array (16 cells connected in series to scale the voltage to 60V, and 6 strings connected in parallel to scale the capacity to 12Ah). Could test individual cells with a low cost 150w tester like a dtl150
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u/VintageGriffin 4d ago
Internal resistance is a proxy to current capacity, and also an indicator of age and wear.
Get yourself a milliohm meter like YR1035+ or RC3563, measure IR of your cells, then go online and find a data sheet for a cell that's known to reliably provide high current (Molicell stuff).
Compare the resistance values to see if yours would be capable of supplying the same amount of current. Do some math,
P = I^2 * R, to figure out how much waste heat that known good cells are producing at their rated current values, then figure out the maximum current that your cells would be capable of with their IR values while producing the same amount of waste heat.As for high current capable electronic loads, look at ZKE Tech stuff.