Tommy L
100 kW
SLC Tortfeasor said:
Hello, then please update your profile and tell us where you are from.
I'm not a gramer cind of gi. I perfer math and tek stuff, but electical have an "R" in it
Like the Dogman said, but I will add this. Know what you are trying to achieve. Then build/design the pack
to do achieve your outcome. If most people could understand this, then it's not rocket science.
I dislike when people build a 2C max pack let's say 48v with 10ah and put a 40amp controller on it and hammer
the throttle at ever turn or take-off, then say that LiFePO4 sags. No kidding? Really?
The example above would require a 20amp max controller and maybe a 15amp controller depending on
what cells the pack was made from.
I've purchased different cells over the years. I test them to see what their capability is, and to me that means
a "C" rate with voltage in the nominal range of the cell. Example:
I like my LiFePO4 to run between 3.1 and 3.2v under load. So I test them and gradually increase the load until
I see results that keep the cell in that voltage range. Even if a vendor says 30C on a 10ah cell, sure it will do
300amps for 10 seconds (if that's their rating) but the voltage will be nearly non-existent. Electric motors
need require voltage to maintain a certain speed, so I design my pack to handle a certain load while maintaining
voltage. Simple really.
LiFePO4 if built this way has some advantages over other chemistries. It will maintain that 3.1 or 3.2v per cell
right up until if fall off the cliff and no more capacity is left in the cell. Some here on the forum think because
the charge rate of the cell is 3.65v that the cell sags to 3.1 or 3.2v under load. Well it doesn't. To fill the cell
adequately, one must apply a voltage higher than the nominal voltage to fill the cell to capacity. Just like a lead
battery needs 14. something vots, but then it will do work around 12v under reasonable load.
The RC Lico lovers think that those cells are great, but again those cells have characteristics too.
Start at 4.1v and finish at 3.6v. That is a 1/2 volt per cell drop from start to finish.
One of my packs that I built is 128v nominal with 40 cells(142v hot off the charger), but soon as I put it under load, and in
my application, it operates at 132v and down as low as 128v for the remainder of the capacity
until it falls off the cliff at 100% depth of discharge.
RC Lico with 32 cells would have a start voltage under load of around 132v but will finish at 115v.
My pack has 13-17 more volts to maintain my wheel rpm since wheel rpm is directly affect by the voltage of pack.
You cant judge remaining capacity by voltage with LiFePO4 as you maybe able to with RC lico.
But this is why I use a Watt meter.
Sure, you could probably add enough in parallel to the RC Lico so it the voltage drops very slowly over a given
distance of use. But again, build the pack for it's intended outcome are my best words of my knowledge gathered
thus far.
and welcome to the forum!Tommy L sends.....
