Nimh pack build and test

[tomv]

With lots of useful advice from this board I've now finished and tested my Nimh pack. I'll post some info that I hope will be useful for others doing the same.

Nimh is now the simplest and cheapest option for those not happy with SLA.

The Specs:
40 D Nimh cells with tabs
48V 10Ah
Weight: ~15lbs (~7kg)
Cost: $260 for cells + few bucks for wires, connectors, duct tape, etc.

The building process:
It took a few evenings. If it wasn't fun, it's probably worth getting a prebuilt pack at twice the cost. Ironically I think I would build my second pack in a few hours and it would be much better

What I would do differently:
Find some way to hold batteries together in a more robust way. Now it's all duct tape and mechanical strength is not confidence inspiring. I've dropped it from about 3ft and had to resolder 3 joints.

 

[tomv]

Here are the test results for the pack, running on Xlyte 408:

Test was on a mix of city streats (stop & go) and bike only path (fast, but somewhat hilly). I've ended test when voltage started sagging under 30V under load and 40V no load. End of power from batteries was pretty abrupt and obvious. It went from being able to deliver 500W to 200W in a few minutes.

Nimh Pack
Rated capcity: 48V x 10Ah = 480 rated Wh
Start V (no load): 56.37
End V (no load) : 40.45V
Min V (under load): 27.04V
Max Vsag: 13V (at 22A)
Total Capacity, Ah: 8.364
Total Capacity, Wh: 333.5
Average V under load Vout: 39.87V (calculated from above two)
Peak power Wp: 900
Distance: 22.18 miles
Average speed: 15.8 mph (didn't try to max it)
Max speed on level road: ~22 mph
Pack weight: 15 lbs.

For comparison, similar test for SLA pack. The route was different in that it was less bike path/more city, so lower avg speed and more stops/accelerations. Duration of the test is similar so I don't think either battery is looking better/worse because of test conditions

Rated capcity: 36V x 12Ah = 432 rated Wh
Start V (no load): 39.05
End V (no load) : 34.13V
Min V (under load): ??? didn't record
Max Vsag: ~5V (at 22A)
Total Capacity, Ah: 8.538
Total Capacity, Wh: 283.6
Distance: 15.5 miles
Average speed: 13.2 mph (didn't try to max it)
Max speed on level road: ~20 mph
Pack weight: 35 lbs.

Conclustion
Performance of 48V Nimh is similar to 36V SLA.
Nimh is less than half the weight and a bit more than 2X more expensive. Worth it in my opinion.

 

[NickF23]

nice work. One useful tip when pack building nimh ( i got this from the factory that made my packs) is to put some thin card between the cells. It you get a shortcircuit then the shrink around the cellls will eventually melt causing more shorts and a chain reaction.

 

[NickF23]

The 13 volt sag should also improve a bit with a few cycles. Keep us updated. I think this is the first written out report with those all-battery cells.

 

[fechter]

Nice looking pack 8)
The sag is pretty significant.

Like Nick says, it might improve with a few cycles.

Adding some copper braid over the top of the interconnects might help too. If you add up the length of all the tabs, it's a fair amount. The stuff is pretty thin, so it might be contributing quite a bit to the sag.