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Vice president in charge of charging and discharging

January 31, 2014

3200. That's how many of these little monsters we will have installed by the end. 18650 cells. Just like the ones used in the battery pack on the Tesla model S with its 300 mile range. Except we will only have about 350 lbs of em, not 900. And ours have been painstakingly extracted from used laptop batteries sourced on ebay. Labour intensive but not too technical, the kind of thing on could do on the couch whilst watching TV, if one is ok with punctured fingertips and the occasional bit of zinc battery strap material stuck in your foot. This is particularily loved by one's significant other. So once done scattering shrapnel all over my living room it was time to start prepping these little cells chargewise. The key instrument in this procedure is a powerlab 8 battery charger / discharger. Originally created for the RC helicopter community, they will do just about anything you want to do to a battery pack. I set up a little fixture comprised of 10 cheap ass 18650 cell holders straight outta Hong Kong screwed to a plank and connected in parallel to the powerlab's outputs. So the plan involves bottom balancing all 3200 of these cells. Bottom balancing involves discharging every cell to its minimum operating voltage, in this case 2.75 volts, to avoid damage in the future caused by overdischarging the pack. If some of the cells are set at too low a starting voltage point they can actually have their polarity reversed by the time the other cells discharge to their minimum voltage. This tends to be the end of their service life. So with a few custom presets in the powerlab created with their software and uploaded with their interface cable we have achieved this. Along the way I have done capacity tests on sample batches of each type of cells we have extracted fom the laptop batteries. They are made by a variety of manufacurers including Sanyo, LG, Samsung and Panasonic. On average they test at about half the capacity of the new super badass Panasonic cells used by Tesla ut at about 4% of the price can't complain. Of the 2600 rated mah capacity my average cell is yeilding about 1800 mah when discharged from a full 4.2v to our minimum 2.75v. So hopefully this pack will give us about 60 miles of range realistically of the 80 mile theoretical range. We shall see. On the bright side its only about 35 miles from the old man's place to mine so I think we're in good shape. Or at least will be. Battery module #1 with 1600 bottom balanced cells should go together tomorrow. Ready for a capacity test once I get a charger for the car, and a meter to tell me how many Ah it absorbs. hopefully 168v and as close to 80 Ah as possible. We will soon find out.

 

Chris

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