One of the recent advancements in batteries is the new LiFePO4 packs. You can get a two sell pack that has 3.3 volts per pack (nominal) and a 2100mah pack weighs in at quite a bit less than an equal capacity NiCd or NiMh pack and are safer to use than the LiPo's. They are more expensive, but have defininte advantages... and some disadvantages.


One major disadvantage is that the discharge curve is very flat.. good because the voltage is constant. But at the point of exhaustion, the curve takes a very fast and deliberate decay, and this happens very quickly. So, one has to be very careful with them.

A voltwatch or Volt Spy probably would not be very good because of this "plot knee" where the exhaustion point is reached very quickly. The charge level with such devices may show just fine, even with a loaded voltmeter, but could decay in flight to below minimum during a typical 10 minute flight. Not good. So, use with caution.

I was not aware of this really sharp discharge curve knee (if you don't know what I am talking about, look at the plot below. Note how quickly the voltage drops from usable to the uh-oh point. This is not an actual voltage plot as measured, but just a plot that shows that the usable vs. un-usable capacity can quickly catch up with you with LiFePO4 packs. They are very good, but users must be aware of this type of discharge curve. The first plot is an estimate of what a discharge curve would look like. The second one is an actual discharge curve. Note that they are pretty similar, and that the point where the batter is usable to where it is not usable is very close.


CGr.