badcheese
08-19-2009, 07:57 PM
Has anybody looked into these guys? They offer a 48V 10Ah LiFePO4 pack with BMS for US$349, which is not much more than ebikes.ca charges for a 48V 8Ah NiCd pack (US$330).
http://www.evcomponents.com/ProductDetails.asp?ProductCode=H4810
It looks like a pretty economical choice, and that pack weighs 5.3kg vs. 8.0kg for the NiCd pack. Any thoughts?
Locutus
08-19-2009, 10:04 PM
Two points: Capacity and depth of discharge.
By capacity I mean the C rating, which is a standard for measureing battery performance. The higher the C rate, the more continuous amperage you can draw from the battery. The information you've linked to does not list the C rating for those lithium packs. The Nicads offered by ebikes.ca have a 5C rating and I would be surprised if the lithiums even come close to that.
As for depth of discharge, The lithiums are rated for 2000 cycles at 70% DoD. Go past 70% DoD and you risk permanent damage to the battery. So your useful number of amp hours is more like 7Ah (70% of 10Ah).
Nicads on the other hand can be discharged flat (100%) without damage, and without the need for a BMS. I have the 8 Ah 48V nicad pack from ebikes.ca, and can attest that I've drawn the full 8 Ah from it on some of my rides, and have even gone past that, coaxing nearly 9 Ah out of it on occasion (at lower power levels of course).
If space and weight are critical factors for you, lithium batteries will save on both. But you'll probably get better performance and range from the 8Ah nicads than you would from the 10 Ah lithiums. Just my opinion.
You can read ebikes.ca's technical information on batteries here:
http://www.ebikes.ca/batteries.shtml
TheKid
08-19-2009, 11:50 PM
Actually, the C rating is there, but it's stated as max. discharge rate. The max discharge rate is 60a, so it's rated at a max of 6C. (60a/10ah = 6) What hasn't been mentioned is that if you constantly exceed a discharge rate above 2C, the lifespan of the batteries is shortened.
The batteries in the link you provided shows they are cylindrical, which are inferior to prismatic LiFePo4 cells. Also, the quality of the BMS has a lot to do with how LiFePo4 batteries perform. If the BMS is of inferior quality, the cells will not charge evenly. I would check out other forums to see if anyone has tested these batteries. endlessphere.com is a good place to start.
I have a Golden Motor 36v 12ah LiFePo4 battery, which is OK, but I had some tests done after reading about testing that was done on their 10ah battery on Endless Sphere. The guy who tested mine said the BMS doesn't work correctly. After charging and partially discharging several times to make sure the battery was broken in, it was found that the cells do not charge evenly. Since this occurs with various cells at various times, it was determined that the BMS is probably at fault. The battery works fine, and I get plenty of miles per charge, but it will probably have a shorter life than it should.
On the other hand, I have a Ping 36v 20ah LiFePo4 battery pack that tested perfectly. It's rated at 2C max. It's on my DR with the Currie 24v motor running at 36v, which draws an average of 25a. So far, it hasn't drawn more than 32a, so it's safe to assume the lifespan will not be shortened.
Ping also sells a 48v 10ah battery rated at 2C. What you really need to know is the average draw of your motor and controller. If it's more than 20a, then you should go with a higher C rating, or preferably, a 15 or 20 ah LiFePo4 battery. If that's too expensive, then I'd go with the NiCads that Locutus is using. His batteries are high quality, which IMHO is better than taking a chance with inferior lithium cells or BMS systems. Also weight wise, it would be about the same as higher ah lithium packs.
http://www.pingbattery.com/servlet/the-13/48V-10AH-V2.5-LiFePO4/Detail