Hello Fred
I plan on having a anderson power pole modular connecter at easy reach in
flight that will allow a quick emergency connection to a fuel pump.
If used in parallel with a lead acid for starting, i think the majority will be
used for the starter. I have a amp meter and will measure.The starter when
cranking is going to be drawing very high amps. When not cranking, and motor
stopped, if i parallel a 14.4V 12 cell NiMh pack with a 12V lead acid, if the
lead acid is charged, it will bearly be trying to charge the lead acid. If the
lead acid is flat, it will try to charge it. I only plan on connecting the pack
for starting, if i think it could use a bit more RPMs at that start, like when
things are really cold out, or doing a hot engine start. The NiMh would only be
connected to the lead acid for a few seconds before starter engaged. Think of
it as jumping a car with another car. If the car being jumped has a full
charged battery, it will not take any charge from the car doing the jumping. If
you try and start the car being jumped, then it will pull down the battery with
the higher voltage first, or the car doing the jumping because it has the
alternator, or generator pumping float ~13.7Vs or so. So by using 14.4V NiMh it
will be like a jumper car in your pocket that you can use at will.
If NiMh battery was completly flat, you could peak charge it at 2C in bout 30
minutes. For a start helper i suppose you would never use even 1/4C. Most
likely less than that. If you peaked it at 1C, chances it would be fully
charged in 5 to 15 minutes. BTW C is the capacity of the cell. In the case of a
2600mA NiMh, that is 2.6 amps. Anyway for a backup battery for fuel pump, i
think it sufficient if it could eeck out a half hour for most of my flying. If
I make some long over hostile terratory flights, could either bring another
pack or 2. Weight for 3 packs is under 5 pounds.
As far as a fuse, for starting I would not use one. I will make it a point to
never leave the pack connected in the starter port for more than a few seconds.
That port will only be used for starting.
Internal resistance of batteries is what drops voltage when drawing amps.
Alkalines have great capacity but very high internal resistance. they can burn
a small load for a long time, but can not dump capacity very fast. a lead acid
for its, weight has a much higher internal resistance compared to New
Generation NiMhs. You must be very careful whan quoting, or being quoted on
capacity. The only number that matters is the capacity at load you are using.
So for instance if a Sub C alkaline had a rated capacity of 5,000mAs, and a
NiMh has a rating of 2,600mAs, those numbers are moot if you are drawing 3
amps, and under a 3 amp load the alkaline can only put out bearly 1,000mAs, and
drop alot of voltage, and the 2,600ma cells can put out a genuine 2,600mas.
The aux. fuel pump port will be fused.
Ron Parigoris
Fred Fillinger wrote:
>
> Ronald J. Parigoris wrote:
>
> > Thanks for the info. ((the pump is 3A for planning purposes).
> >
> > Do you know gross amp draw when you are starting?
>
> The starter relay is max 75A, but whatever greater current pulses caused
> by compression strokes I don't know. The voltage across a lead acid
> battery drops significantly under high current draw; on a NiMH it does
> not. In parallel it's going to appear to it as a partial dead short,
> drawing very high current. My Makita cordless drill has a circuit
> breaker to prevent the abuse if you stall the motor. Can you then rely
> on this battery as emergency backup for the fuel pump? It seems at
> least there will be insufficient time in flight to recharge it.
>
> Fred F.
>
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