What is acceptable internal resistance per cell on lipos and what is too much and should be disposed of. This has probably been discussed but I havent found it.
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RE: Battery IR
The answer depends a lot on how you use the battery, and what capacity it is. If you are using it in models that require bursts of high current, or a steady high power draw, 50 milliOhms are probably too much, but if it is in a lightly-loaded trainer, that much may not be a problem. If, however, one of several cells in a battery is higher than the rest, the battery is failing. Also, small mAh batteries have higher IR than very large batteries; I have 1000 mAh batteries with over 100mOhm per cell, while my fairly new 4000 mAh batteries measure less than 10. Drawing 10C from the smaller battery is only 10A, while it is 40A from the larger, and there is less resistance to let the resulting heat out in the smaller battery. What you really need to do is keep a chart of your batteries, and, when the IR of one begins to rise significantly, you will know it is not long for this world.Originally posted by farmflyer -
RE: Battery IR
It totally depends on the size (capacity) of the battery cells. Larger cells always have less resistance. Smaller cells have a lot more resistance. It also depends on the state of charge when you measure IR. A fully charged cell (4.2V) will show lower IR than a cell at a storage charge (3.85V). The key is to measure the IR in each battery when it is new and record it. Periodically measure it as the battery ages. As IR goes up, the C rating essentially falls and eventually the IR will be too high to power your model. It will seem as if you hit LVC after a very short flight but the battery may measure 3.95V per cells when you land. The IR is too high at this point.
What makes the IR go up? Age, number of uses, how you store the batteries, how you use the batteries. If you land and measure your battery at 3.8V per cell a few min later, you are doing things properly and you battery will last a long time. If you land at 3.7V per cell or less, you will raise IR and kill the cells early. If you hit LVC on a regular basis, your voltage is way too low and you will kill the battery after a few flights. I've seen people land and measure the battery at 3.5V per cell or less and that pack will have a very short life - sometimes 1 flight if you really run it down too far. If you store your batteries at 3.8-3.9V per cell, you are doing it right. If you store above or below this voltage (especially below), you will see the cells age faster.
The bottom line is to record the initial IR per cell, and track it as each battery pack ages.Comment
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RE: Battery IR
Thanks for the info guys. I haven't been checking them like this but I will start. I must be doing ok since I have several batteries up to 2 years old with dozens of flights. I also have been starting new planes with shorter flight times and working up trying to keep end flight voltage 3.8 or more.Comment
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RE: Battery IR
If you are going to compare IR readings, you also have to control for temperature; the IR drops rapidly with increasing temperature, so just a couple degrees F can give very different values. According to the maker of my IR meter, best readings are done around 72-77F.Comment
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RE: Battery IR
Yes, good point. Temp does make a difference.Comment
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