Electric Flight in Australia

 

Can I use LiPo with my brushed motor?

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Peter Pine
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Pottsville Beach NSW 2489
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I have a Le Crate cabin model that I built in the 1980s with a geared, brushed buggy motor and a brushed speed controller, formerly used with packs of NiCd or NiMH cells. I have inadvertently damaged some of my LiPo packs using this model.

It is true that LiPo packs can be damaged if they are drained too low - below 20% capacity. This does not render then unusable, but it does affect their power output, their capacity and their eventual life. They sometimes recover somewhat after being drained too low if they are charged again immediately.

In modern, brushless speed controllers there is a voltage cutoff which will cut power to the motor if the battery is drained too low. The motor can be reactivated briefly by lowering the throttle and opening it again allowing the flier to return home if necessary (or to hop over a barbed-wire fence!). The usual cut-off standard for LiPo packs is 3.0V per cell, but some fliers program their controllers to set a slightly higher cut-off level as a conservative measure.

You have several options if you want to convert to LiPo:

1. Add the LiPo pack (which will save an amazing amount of weight, give extra power and usually a much longer motor run). Fully charge the LiPo and then fly for a conservative amount of time, monitoring the flight time with a stop watch - let us say, 3 minutes. Land and check the capacity left in the pack. A Battery Sentry will give you this reading instantly, or some chargers tell you the capacity left when you connect the pack to recharge. In the absence of these facilities, simply recharge the pack and see how many mA go in to the pack to bring it back to full charge. You can then calculate how much of the pack's capacity you have used - say you have only used 1100 mA out of a 2100 mAh pack, so you have used 52%. You can then calculate mathematically that you can fly for 4.5 minutes and leave 20% capacity in the pack. On all future flights use countdown timer that warns you when 4.5 minutes is up - or you might be conservative and use 4 minutes as your marker - and land when the alarm sounds. Sounds complicated, but it works easily once you have established your parameter.

2. Obtain a brushed motor speed controller that is LiPo compatible and has a facility to program the motor cut-off level. I carry the Skyborne brand which have this facility - 35A for $79.

3. Change over to a brushless motor as well as LiPo - that means changing your whole drive train. Change the motor, speed controller and battery. Sounds a lot, but it is not so expensive these days, especially in low-power sports models. The speed controller is probably the most expensive item, and it is worth buying a good one! But I can assure you that you will be amazed with change to your model - it will be much lighter, the performance will be better and your flight time will be much longer! I was slow to make the change to LiPo and brushless, but now I will never go back!