LiFePO4 batteries - performance issues running 6.6V?

[colmo]

As a returnee to RC and a newcomer to racing, my research to catch up with latest technology included the new lithium-based cells - both LiPo and nano-phosphate/LiFe(PO4).

The advantages of LiFe over LiPo look good - potentially twice as many cycles, higher charge and discharge rates, more robust and without the safety caveats LiPos come with.

However, the standard LiFe cell suitable for RC car racing, the M1 made by A123 Racing (and now sole owners of Enerland), have a standard voltage of 3.3V nominal. This doesn't quite fit the standard 7.2-7.4V voltage, the closest you can get being 6.6V in a 2S pack.

My question, then, is what performance impacts will this lower voltage have on the performance of a modern brushless motor in a modern RC car? As the voltage is lower, the current drawn must be higher to obtain the same power (in practice, this would be done by gearing up to compensate for low voltage) - are modern ESCs sufficiently over-specified to cope with the extra current?

I refer to this article; it indicates that in low voltage situations, the higher current drawn is not in itself a problem, unless it goes above the upper limit the motor is capable of. As brushless motors can be geared more aggressively than brushed, does this place a lower ceiling for gearing if using lower voltage cells, or risk damage and/or shorter lifespan from the motor?

Torque is also lost with lower voltage (a 10% voltage drop, which is roughly the difference we are considering here, would reduce the starting torque, pull-up torque, and pullout torque to 81% of the full voltage values) - is this as big a loss on the track as it is on paper?

[sosidge]

Bear in mind that for the last couple of years touring cars have been running 5 NiMH cells in modified class, just 6V nominal voltage. Performance has not been a problem, drivers are running a slightly hotter motor wind than they would have done with 6 cells, the cars (albeit running at a lighter weight limit) are faster than they were despite less voltage.

If you are in a modified class, 6.6V in itself won't be a problem, just drop a wind.

In spec classes the 6.6V pack will never compete unless an "equivalency" is introduced - although equivalency rules never work, one setup always has a benefit depending on the track.

Unfortunately I don't know enough about the LiFePO4 cells to comment on how they can handle high current draw. I know that they are more robust than LiPo cells, which is why Tamiya have made a pack of them which is a direct fit in a normal car - but the capacity of that cell is only 2200mAh, which doesn't compare very well with the 5000mAh+ available from LiPo at present.

[colmo]

Thanks for that.

Originally Posted by sosidge

Unfortunately I don't know enough about the LiFePO4 cells to comment on how they can handle high current draw. I know that they are more robust than LiPo cells, which is why Tamiya have made a pack of them which is a direct fit in a normal car - but the capacity of that cell is only 2200mAh, which doesn't compare very well with the 5000mAh+ available from LiPo at present.

I can help there - they appear to handle high draw very comfortably - indicated as 30C continuous here - and a standard 2S2P (6.6V 4600mah) pack weighs in at 330g, more than a comparable Lipo but less than a NiMh. Depending on class, it may be possible to run a 2S1P pack, weighing 170-odd grams, and recharge it nearly completely between runs (can handle 8-10 Amp charge rate).

Under load, each cell drops a little voltage, but deliver a steady 3v until almost completely discharged.

[sosidge]

Unfortunately the larger A123 cells build up into a slightly larger pack than a traditional 7.2V or LiPo equivalent, so you may have trouble fitting them neatly in a car.

It seems Tamiya have built there pack with the smaller A123 cell, hence it's small capacity.

In a parallel universe I'm sure A123 would be an obvious choice for RC cars, unfortunately the cell dimensions and nominal voltage mean it will probably stay on the fringes, although I expect Tamiya will create a class for their own pack in some of their championships.

[colmo]

I've checked the dimensions of the 4-cell pack (2S2P or 4S1P) and it is slightly too large on every dimension than BRCA's standard LiPo limits. Length is only a millimetre or so, width a little more, but thickness is the worst - this shouldn't be an issue in most 1/10th buggies or TCs, but I know it's an issue with some trucks with enclosed battery trays.

[SlowOne]

No need to worry about the voltage - we use 4.8v in both 12th and GT10 on BL motors without any concerns. Bedworth Club have run 4.8v TCs using Bl, again with not problems.

Just because you're a but less than a volt short of the LiPo does not mean you'll be much slower. A track is 10% straight, and 90% corners and links that aren't always full throttle, and where a good chassis counts for more than pure power.

Give it a whirl, it's closer to the future than LiPo is! HTH

[mark christopher]

Originally Posted by SlowOne

No need to worry about the voltage - we use 4.8v in both 12th and GT10 on BL motors without any concerns. Bedworth Club have run 4.8v TCs using Bl, again with not problems.

Just because you're a but less than a volt short of the LiPo does not mean you'll be much slower. A track is 10% straight, and 90% corners and links that aren't always full throttle, and where a good chassis counts for more than pure power.

Give it a whirl, it's closer to the future than LiPo is! HTH

very true, Lipo is here and in use now, it the present!!