On 04/26/2012 03:47 PM, Klaus Dietrich wrote:
> I'm just back from the AERO in Friedrichshafen and besides the Rotax
> 912is, I was impressed by the new LiFePO4 starter batteries on offer;
The main problem with these batteries is that they require a totally
different way of charging. A lead-acid battery can be connected in
parallel with the alternator, and the lead-acid battery will just take
enough current to get recharged, and stops automatically. It won't get
any simpeler than that.
A LiFePO4 needs an overvoltage to initiate charging, strategically
tapered off to stop charging, and each individual cell needs to be
carefully monitored and balanced. It takes a lot of complicated
electronics, something we tend to avoid in aircraft. (You might look on
some wiki's on this subject to see what is needed to recharge these
babies properly).
The implication of all this is that you can not just connect the battery
in parallel to the alternator. In real life this means that you need to
run the ship entirely off the alternator, and the battery is connected
to the bus, not parallel, but via its complicated charger. If the
alternator momentarily needs some support from the battery it won't be
able to get it. Sure you can switch over to battery power, but you can't
use the alternator and the battery at the same time. Also, if I remember
correctly, the cell voltage of a LiFePO4 is 3.2 Volts. So you need to
operate at 12.8, which is quite low, or at 16 Volts which is a tad too
high. Normal lead-acid batteries parallel to the alternator operate at
13.7 to 14.5 Volts, and this is, not coincidentally, the voltage your
avionics is designed for.
LiFePO4 might become an interesting alternative once aircraft rated
chargers are developed, and when avionics is adapted to operate on the
new voltages. This is just not the time to do it, unless you want to be
the guinea pig.
Just my 2 cents.
Frans
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