0
\$\begingroup\$

I would like to test max discharge rates for 1S (200 mAh) LiPo's. But I can see three ways of doing it:

  1. Using a charger, discharge at highest rate and see how high the battery goes.

  2. Using a resistor (0.25 Ohm) and measuring the voltage and amperage. (0.25 Ohm would generate 17.4 amps, which is a bit higher than the maximum expected amperage).

  3. Using a motor and normal load, and measuring voltage and amperage.

What are the differences between these methods? Will they all expose the max discharge rate?

I would like to use method #1 since it is the simplest.

Any relevant links or references would be appreciated.

EDIT: From the comments I see that we need a distinction between burst and sustained discharge rates. For my use case, I am most interested in the burst rate.

Perhaps a good approach would be to connect it to a 0.25 Ohm resistor for 5 sec and measure the result...?

\$\endgroup\$
  • \$\begingroup\$ For a 200 mAh battery, 0.25 Ohms is not a discharge test. It is a short circuit. If you are planning on having this battery supply 15 A for more than a few ms, you need a new plan. Regarding discharge testing, how you do it depends on what you are trying to learn from the process. So, what are you trying to learn? Why discharge the battery at all? \$\endgroup\$ – mkeith Jun 21 '18 at 23:15
  • \$\begingroup\$ Note that 17.4 / 0.2 = 87C. Is your LiPo rated to deliver 87C? I don't think so. \$\endgroup\$ – mkeith Jun 21 '18 at 23:17
  • \$\begingroup\$ Haha, I thought someone would comment on 17.4A. This battery was tested at 52C: sites.google.com/view/rchobbylocker/1s-lipo-testing/… . It's rated at 50C burst. \$\endgroup\$ – B Seven Jun 21 '18 at 23:25
  • 1
    \$\begingroup\$ Bursts lasting seconds can be simulated using resistors. You can use a 1 Ohm resistor as your baseline load, then connect another resistor in parlallel for several seconds. Just use a Y connector and manually plug the load in. You may get a spark, but it should not be too bad. Bursts lasting ms need to be simulated with RC or electronic switching. A function generator connected to a FET switch is a pretty easy way to do pulse testing of any power supply, including a battery. \$\endgroup\$ – mkeith Jun 21 '18 at 23:59
  • 1
    \$\begingroup\$ Lithium batteries in general do not have stellar life span to begin with. It is severely affected by irreversible chemical reactions under high discharge rates, further accelerated at high temperatures (due to same high rates). Properly specified burst ratings also have stated repetition time, meaning you can't disconnect battery for a second and expect it to survive another burst right away. Finally, 4A in your comment is not the same as 17A in your question. \$\endgroup\$ – Maple Jun 22 '18 at 0:27
0
\$\begingroup\$

You cannot test for "max discharge rate" using any of these methods, unless you have hundreds of cells you can afford to ruin. The max discharge rate is a parameter that was determined by cell manufacturer based on specified reliability and number of charge-discharge cycles. It can be determined only by mass testing and statistical processing/analysis. Typical discharge method is to use a certified "electronic" load with constant current settings, all after manufacturer's specified charging process.

\$\endgroup\$
  • \$\begingroup\$ I can afford to ruin 1 of each battery ($3)! See sites.google.com/view/rchobbylocker/1s-lipo-testing and scroll to bottom for Testing Methods and Notes. \$\endgroup\$ – B Seven Jun 22 '18 at 0:22
  • \$\begingroup\$ @BSeven, the max discharge rate is a statistical parameter. To determine it, you need to define the number of expected cycles for your battery. Therefore, technically you need to get a statistically-representative number of cells (typically about 30), and run each batch at ever increased discharge rate. You will need to take new batches with every new current for the expected number of cycles. This takes a lot of time. But if you plan to use your battery just 10-20 times, you can load it to whatever current you wish, just to watch for cell warm-up due to dissipation on internal impedance. \$\endgroup\$ – Ale..chenski Jun 22 '18 at 0:41
  • \$\begingroup\$ @BSeven, and don't do your extreme testing on room carpet or your computer desk. Use fire-retardant enclosures. \$\endgroup\$ – Ale..chenski Jun 22 '18 at 2:52
0
\$\begingroup\$

I am trying to learn which batteries provide the most power using a motor at high load. (The battery might last about 3 min, so it would provide approx. 4A.)

So, what you basically trying to test is not "max discharge rates", but an internal battery resistance. There are methods to measure this but none of them includes torturing the cell under maximum rated burst current.

Connecting the battery to the motor is an indirect but quite useful method to measure internal resistance, or rather compare performance of different cells. But once again, why do it under extreme conditions that will not happen (or will not happen often) during actual operation?

With this in mind, your method #3 is actually the best for the test goal you've specified in your comment, not for the one in the title.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.