RC Car - But I've Heard Other Advice On Discharging Batteries

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Question About Your "discharge" Advice

Comments:Pro,

You stated in your reply to a question about discharging batteries called "How Should I Discharge My Cells?", that the absolute best way to discharge a pack was to run it in your car until it died. I have heard that is the worst way to discharge a pack. I have spoken with a number of racers and they have said that when you discharge a pack that way, the cells discharge as a "pack" so the car dies gradually as the weaker cells go first, the stronger cells continue to put out energy, but when this happens the polarity is reversed in the weak cells, and it damages them. I have been told that the best way to discharge packs is to discharge each cell individually, not as an entire unit.

Let me know what you think, and why so many people have different opinions.

I, like the rest of the people who write you are just trying to figure out the best way to treat my batteries.

Thanks
Daniel

Response: Daniel,

This is a touchy subject. There are as many methods to discharge a battery as there are opinions in this world.

Your source is correct, the safest (and to some the best) way to discharge a battery pack is to discharge each cell individually. The main point I was trying to make in the article I wrote that you read was that the rate of discharge to drain the battery should be the same as it is in the car. This is best done by draining the battery in the car. Eventually the car will have a battery that is so low that it is just creeping along. This is the point in which the battery should be discharged in a tray that does each cell individually.

It is possible to reverse a cell when the battery is discharged as a pack as opposed to individually but this is rare. It happens in the way that you already mentioned, weaker cells discharge before the stronger ones and then obtain a reverse charge from the stronger cells. This only happens when the "match" in your batteries is so bad that it is probably time to throw them out anyway. By "match" I don't mean that it happens to any battery that is not "matched" with Team numbers on it. The "match" of the pack is poor when a single cell or group of cells are so different from the performance of the rest of the pack that this cell reversal scenario could happen. Any battery that is in reasonable shape should not run the risk of cell reversal when discharged as a pack.

Up until about 6 years ago all racers used to discharge their race batteries as a pack. There were no manufacturers that made individual cell discharge trays. That is where the original ten, 1157 signal light bulb trick came from.

In any case, you and your source are correct in that it is good practice to discharge the battery as single cells but this is not the only way of doing it. I think you may have misunderstood the context in which I was giving the advice in my last writing that you read.

The basic idea remains the same, you want to discharge the battery at the same rate that it is discharged in the car. What you do with it to completely drain it after is up to you, an individual cell discharger is a good idea. I do however not recommend that you use an individual cell discharger to drain the pack when there is a good, say 30% of the current remaining in the battery. Individual cell discharge trays discharge at a much lower rate than 20 amps (which is a rough estimate of the average discharge rate of an RC car during a 4 minute race).

I appreciate the question,

Let's take off a few tenths per lap,

"PRO"
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