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  LIPO  compared to  NMH

   Lithium polymer cells are in the news. We are frequently told of the superb energy-to-weight ratio   (2 to 3 times that of nickel metal hydride)

   The equally-important property of power-to-weight ratio is rarely mentioned.

   From the table below you will see that power/weight varies over a wide range.

    
 

cell

   run time

  minutes

 max discharge rate

         "C"

  power/weight at max rate

  watts per gram

   energy/weight

  watt minutes per gram
nickel metal hydride Panasonic 2000 5 min 12C .53 W/g 2.65 WM/g
Kokam

lithium polymer

 560T 20 min 3C .34 W/g 6.70 WM/g
880T .35 W/g 7.10 WM/g
1020T .39 W/g 7.74 WM/g
3270T .37 W/g 7.38 WM/g

145T

12 min

5C

.50 W/g

5.97 WM/g
640HCT

.49 W/g

5.85 WM/g

1200HCT

.63 W/g

6.75 WM/g

700HCT

10 min

6C

.68 W/g

7.00 WM/g

1500HCT

7.5 min

8C

.86 W/g

6.50 WM/g

340SHCT

3 min

20C

1.8 W/g

5.49 WM/g

 

 With respect to energy/weight, all the LIPOs are much better than the NMH.

With respect to power/weight, LIPOs of 5C are about the same as NMH. Below 5C they are worse; above, they are better.                                                                                                                                                                                  For cells of 3C max discharge rate, the power/weight is about 70% of that of a good NMH cell. This makes them unattractive to me. OK for slow-fly long-duration (30 minute) models.

  For cells of 5C max discharge rate, the power/weight is generally similar to a good NMH. For the same battery weight you get the same power for 2.5 times as long.

  For cells of 6C max discharge rate, for the same battery weight as NMH, you get 28% more power for twice as long. Very useful.

  For cells of 8C max discharge rate, for the same battery weight as NMH, you get 62% more power for 1.5 times the run time. This gives the realistic option to reduce  battery weight. Most attractive.

  For cells of 20C max discharge rate, for the same battery weight as NMH, you can get 3.4 times the power for 60% of the run time. This doesn't interest me, but some folk will love it.

  In all the preceding, power can be reduced in exchange for longer run times; but the opposite trade (higher power for shorter time) is not an option.

  Please be aware that this is a "paper exercise" based on FMA data, not on my own tests. "Hands-on" tests by me will have to await a large reduction in cell prices.

  The FMA website contains lots of useful data on LIPOs. Click on store/Kokam USA for cell data, and on support/applications notes/AN000002 to download technical information. Address is www.fmadirect.com