Discharging a 30C LiPo at 30C!
Note: We did this, so you don’t have to! Seriously, do NOT try this at home! The results are very unpredictable, and you can even hear us scramble a little bit in the video. Something like this can turn into a very dangerous situation. Now that that is out of the way, on to the good stuff!
Each time we’ve done a Lipo shootout, the number one complaint we receive afterwards is “Why didn’t you test the batteries at their claimed C rating?”. Luckily, we aren’t a mag, so you asked for it and we grab our test gear and actually do it!
So what exactly what happens when you put a real 30C load on a battery pack that has 30C stamped across the top of it? Let’s find out. 🙂
We did not have optimal video equipment available the day of the test, but the vid captures all the nitty gritty. If you don’t know much about batteries, he’s the skinny- the battery dropped voltage extremely quickly, and it ended up ruined.
For those of you that do know a thing or two about batteries, here’s the details of the test.
The battery pack used was a name brand 3S 5000 mah 30C (150 amps) LiCoO2 Lipo that you can buy in most any good LHS. We aren’t going to list the name here because we didn’t do this test to try and discredit one brand. If anything, our “nameless” pack performed better than a typical sample does at it’s rated C discharge. Also, no company wants to see their label on a pack that is hell bent for destruction, so it was left off and replaced with a BigSquidRC sticker.
A 3S Lipo was chosen because it would result in maximum amp draw from the dischargers we were using. With BigSquidRC being “By Bashers for Bashers” we set up the test like any good basher would, using readily available components.
For discharging duties we searched into the 12v 1:1 automotive world…. Lead acid “battery testers” are easy to find at nearly any Pep Boys or Auto Zone, so that’s what we used. Ours came from Harbor Freight under the Chicago Power Tools name, part number 90636. These dischargers are claimed to do 100 amps at 12 volts DC, or 50 amps at 6 volts. These dischargers can not maintain a constant amp rate, the lower the voltage, the lower the amp rate.
We didn’t use any battery connectors, we soldered the battery wires directly to the wires coming out of the dischargers. We wanted to test the battery, not how little current most connectors can handle.
We used a BK Precision clamp meter 367 to see what the actual amp draw was during our tests.
An Ideal 61-340 was used to accurately measure battery voltage during the tests.
To our test results-
Test #1- Only one automotive discharger (not shown in video)
Test battery was fully charged on a Hyperion 720iNet3
Resting voltage of battery before test- 12.61 Volts
5 Seconds into test-
Conclusions from first test- Using only one automotive discharger yielded a highest discharge rate of 79.6 amps. Our test 5000 mah battery should have lasted roughly 3.75 minutes under that load. You would have thought a “30C” rated pack would have lasted longer than 28.8 seconds under this load, especially considering the discharge rate was only at 64.2 amps by the end of the test. In theory, our 30C rated pack should have lasted at least 3 minutes in this test with its voltage above 3 volts per cell, it did not, nor did it come even close, but the battery didn’t seem to get overly hot nor suffer any ill effects from this test.
Test #2- The BIG Test, Two Auto Dischargers (Shown in video)
Two Auto Discharger were wired in parallel, doubling the load of the first test
Test Battery was fully charged on an Hyperion 720iNet3
Rest voltage of battery before test- 12.69
2 Seconds Into Test-
Conclusion for Test #2- A 147.8 amp load was the highest we saw, at the 6.8 second mark it had dropped to 118 amps. The entire “official” test ended in 6.8 seconds when the pack hit 3 volts per cell, 9 volts total pack voltage. In short, we applied a slightly less than 30C discharge load, and got 6.8 seconds of runtime out of the battery. At that point the battery had not gotten too hot, and perhaps could have lived a normal life if we would have quit right there. But we are BigSquidRC, we had to keep going.
Now, a “30C” discharge rate should give you a runtime of 2 minutes, so we continued to apply the load no matter how low the voltage dropped. 9 seconds under the load had dropped the pack voltage to 4.9 volts (at 89 amp load), and that was far under the recommend minimum 3S pack voltage of 9 volts. About 1 minute under load caused massive internal cell venting (puffing), and at around 1:55 seconds under the load one of the cells ruptured and began to spew. Full thermal runaway set in later after we continue to discharge the pack even further.
To Boil It All Down- The pack we tested did not like having a 30C load applied. A 30C load caused it to drop voltage extremely quickly, and eventually lead to its own demise. Does that mean the “Brand XYZ” pack you bought would do the same? No, it may have performed much better than our test pack, but from my previous experience putting big loads on Lipo cells the pack we tested actually performed better than most. Many packs on the market would have burnt a tab the moment the load was applied, and many would have went into thermal runaway after the first 10 seconds of load being placed on it. The pack we tested lived through the “official” testing, the point where we simply took it down to 9 volts total pack voltage, and could probably (but not certainly) have been usable afterwards.
If you ask ten different battery companies what the “30C” on their pack means, the most common answer will be “It means our 5000 mah pack can handle 150 amps of discharge”. They typically don’t say how long it will do 150 amps, or even if the pack will live afterwards. Consumers tend to think the “30C” means you can discharge it at a 30C rate a hundred times and get a full 2 minutes above 3 volts per cell out of it. The people who engineer the cells will tell you “30C” means they have designed all the cell internals to pass that much current, once.
So have we proven anything here? I really think not, as a sample size of one doesn’t mean a hill of beans scientifically. But if anything I think our lil’ ole’ test just brings to light that most of the “C” ratings out there are very misunderstood, and effectively meaningless until there is an industry standard to truly define the term by.
In closing, I must note that very, VERY few hobbyists are looking to only drive for 2 minutes (except the drag, speed run, and pull’n crowds). If you are a basher the longer the runtime the better, as you are looking for the battery that gives you good performance for 20 minutes or more (a 3C load). If you are a racer type, you are looking for the pack with the highest voltage under load for 5 to 10 minutes (a 12C to 6C load). The priority should NOT be what a Lipo can do under a 30C (or 40C, 50C, etc) load, it should be what it can do under the load you will actually use it at.
YOUR Cub Reporter
Again: DO NOT TRY THIS AT HOME! OR AT YOUR LHS!