48V 20AH Headway Pack $620 FOB

[recumbent]

Looking at the discharge curves for the 38120 cells it seems quite clear that they can deliver reasonably high currents for sustained periods with only modest amounts of cell heating. For example, a complete discharge into a 0.1 ohm load gives a total cell temperature increase of about 17 deg C, with the cell discharging over about 20 mins. .

Today it was 32-C as most summer days, if I was using Headway cells on my bike for half an hour it would increase to 50-C (120-F) ? That's hot.
For example: the LIFPO4 cells i got at ebikes.ca... after an hour of solid riding today didn't heat-up at all.
I've seen it mentioned before, on other test that Headway cells had heat issues, but that was half a year ago, maybe they addressed the problem. I would look more carefully around.

 

[Jeremy Harris]

Interestingly, that's pretty much exactly the same temperature as the Headway test data - the starting temperature for their testing was 32 deg C, the 17 deg C rise was right at the end of the discharge cycle, some 20 minutes or so after the start..

From this, it seems that these cells are fine even at close to 50 deg C.

Bear in mind that this was a continuous discharge test at around 30 amps, which for a 48V, 10Ah ebike pack equates to a continuous power demand of over 1400 watts, which seems pretty high. My guess is that most ebikes have an average power demand of a lot less than this, so I very much doubt that cell heating will be a problem in practice.

Jeremy

 

[olaf-lampe]

Today it was 32-C as most summer days, if I was using Headway cells on my bike for half an hour it would increase to 50-C (120-F) ?

@Jeremy
This was not a statement, but a question. He just repeated, what the datasheet says.

I agree 100%, that these temperatures will not occure in a usual ebike.

-Olaf

 

[eP]

Lets look again for 23120P test results and specification:
http://www.endless-sphere.com/forums/viewtopic.php?f=14&t=3374&st=0&sk=t&sd=a

Oem specs
3.2V
8.0Ah
300g = 85.3Wh/kg
25.6Wh/cell
80A continuous
160 peak
2.8mohm

Lets look at discharge graph:

and Doctorbass comment:

Please note that my CBA analyzer is not perfect (...from now.. ) and that at 1A the voltage drop between the direct battery tab and the volt reading is 5mV so in the graph you should read 5mV higher than the volt value (offset error) and that at 32A this offset is 170mV. that mean that as the current is high as the error increase and that at 32A the mean voltage should be 2.95V instead of 2.78V

So at 4C discharge rate we can see over 300mV lower voltage than at 1/8C rate.
It seems the cell has 10 mohm at least internal resistance which causing substantial heating yet at 4C rate.
It is hard to belive such cell could safely work at 10C discharge rate continuous at over 6 times more heat dissipation.

10 mohm if far more than 2.8 mohm within spec.

So my question is: are their energy cells behave the same way ?
Is anybody tested them so far ?

 

[Jeremy Harris]

The 38120P cells, the 8Ah ones whose discharge data you've given, aren't currently available, as far as I am aware. The cells that are available are the 38120L cells, which have a lower maximum and continuous discharge rating (5C continuous, 10C maximum pulse, which gives 50 amps and 100 amps respectively) and a slightly greater capacity (10Ah).

It seems that, following on from this comment from the ZEVA test, or, perhaps, by reaching the same conclusion, Headway opted to concentrate on producing the higher capacity 38120L cells:

"Discharge curves between the P and L variants were remarkably similar, I tend to wonder just how much more powerful the P cell is.. Based on these results, it seems like the L variant makes more sense. I would be interested to take them to the next step, and build a pack for a commuter vehicle from 38120Ls sometime."

The 38120L discharge curves are in the files section of the group buy Yahoo group, at a range of different discharge rates. All of the test data seems to have been done at the same fairly high ambient temperature, around 30 to 32 deg C. The greatest temperature rise was the one I mentioned above, of 17 deg C after a complete discharge into a 0.1 ohm load.

Jeremy