More on LiPo batteries from Andre Kilian

Hi,

As more of the hi-rate lipos (15-20C) packs are becoming available in SA, just note that dropping the voltage below 2.7V per cell, even momentarily under load, can permanently damage your cells. You can thus have a situation, for example, where you only use 1450mAh from an 1800mAh pack, but damage your batteries in the process ("fat pack" syndrome - normally fatal!).

It is thus not only important to stay within the capacity limits of your batteries, but also above the minimum voltage limit. This is normally not a problem when using a good quality speed controller which shuts down the motor if the voltage drops too low (Lipo-cut off) but may become a problem when using more than 3S lipo packs (normally with a separate receiver pack), or when the cells become imbalanced.

Also note that we are having the same race with Lipo ratings (e.g. 8C, 10C, 15C and now 22C) that we observed with the capacity rating of the Nicad/NiMh batteries. Some battery manufacturers are claiming very high C-ratings for their batteries (some are honest enough to admit that they are peak ratings and not continuous ratings) but watch for that voltage drop under peak (burst) loads! If the voltage drops to below 2.7V per cell (i.e. 8.1V for a 3S pack) you can kiss your batteries goodbye...

Don't count on the prop unloading in the air to reduce the current, as other factors such as the motor heating up may also play a role. - I have measured 46A in flight with my Acrow Wot, whereas the static current draw was about 40A (15% increase!). If anybody is interested in actual flight data or needs evidence, send me a message and I will mail you the graphs with in-flight data and/or a photo of my collection of "fat packs".

The bottom line is be wary of the "el cheapo" brushless controllers and hi-rate cells that are now flooding the market, as a bargain may turn out to be very expensive. By all means, use these cells and controllers (some offer good value for money), but be aware of their limitations and the fact that the harder you push your lipos, the shorter their operational life will be.

My advice is to measure the current on your setup, and stay within 80% of the continuous current rating, and if you are using more than a 15C pack or more than a 3S configuration, ensure that you buy batteries with pigtails so that you can check the individual cell voltage for imbalance.

Regards,

Andre Kilian