Wednesday, December 29, 2010

Holiday Inn

Time seems to be a rare commodity lately.  I had to cover up El Gixxer for the winter to keep the other garage projects from contaminating the old boy.  A sad state of affairs.  Prior to my Winter break I had the 25 Headway cells installed and did a series of test rides to see how they faired.  To say I'm impressed is a gross understatement.  25 of these 16AH batteries are 50 lbs. lighter than the 17AH lead units they replaced, but put out  way more juice. 

Not wanting to damage the cells, I needed to keep my current draw under 10C (160 Amps).  I borrowed a clamp meter and did some riding to calibrate my wrist.  Turns out 150 Amps accelerates the bike pretty well, and maintaining the top speed of 48mph only takes about 50-60 Amps.  I started out at 5.0 miles, and worked my way up 10.2 miles on the last trip, which happens to be 0.2 miles longer than my round trip commute.  With the average voltage of the cells starting at 3.40 I finished at at average voltage of 3.24.  Plenty of juice to get to work and back.  But having spent all funding on batteries, I was out of cash to finish up the 12V wiring. 

Bummed I couldn't be legally on the road, I turned my attention to charging and monitoring voltages.  My plan was to use the Headway 72V charger (which charges to 86V) on the 25 cells without BMS.  By using 25 cells instead of the usual 24 cells, they are less likely to be overcharged. With 25 cells in the pack, the average charged cell is 3.42V, which is just about right, since there is very little capacity above 3.45V.  Some of the cells were noticeably higher, between 3.50 and 3.8V, and some were lower at 3.32 to 3.34.  I made some attempt to top balance, by draining a few of the highest ones with a small light bulb.  But every time I ran them through a discharge/charge cycle, they were out of balance again.  I kinda see the need for a BMS, but still think the safest route is to under-charge and under-discharge.

The only thing I wanted for Christmas was another set of Headway 40160S cells and Santa sure delivered. 

Next step is to build up my 2P25S pack of 50 cells and turn up the Amps.

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