Sunday, September 1, 2013

Top-Balancing the Pack

I did not do exactly "nothing" on the Jeep over the year.  In May/June I top-balanced the pack.  Balancing and monitoring of lithium battery packs is a hotly debated topic in DIY electric vehicle circles.  There are those who advocate what is called "bottom balancing" - discharge all the cells to a given voltage level (frequently, 2.7V) and then charge the whole pack in series until the first cell hits a "full" voltage level (often, 3.5V or 3.6V).  You record the overall pack voltage at this point, which becomes the "full" point.  Not all the cells are full, but they theoretically will discharge to the same low point, which is why this is called bottom balancing.  Bottom balancing advocates say that they can run safely without a battery monitoring system - they check the cell voltages occasionally - but really that just means that the *owner* has become the BMS.

Top balancing, as you might imagine, is the opposite of bottom balancing.  All of the cells are charged to the same "full" voltage level, and then monitored to make sure that no cell either goes too low during discharge, or too high during recharge.  With 96 cells, this requires automated assistance - the BMS.  In my case, I'm using the MiniBMS from Clean Power Auto.  It is simple, relatively inexpensive, and conceptually robust.  But it will work much better if the cells are all fully charged to the same level before hooking everything up.

To do this, I used a 25A nominal battery charger, with an adjustable voltage level.  Although the battery manual suggests that "full" is when the charge level hits 3.6V, I chose 3.5V to give a slight margin of error.  There are almost no amp-hours between 3.5V and 3.6V.  I hooked four cells up in parallel to charge - this fit well with my schedule, since it took about 18-20 hours or so for this 400AH megacell to fully charge, and I could just go out every morning and swap out the cells.  This went on for 96/4 = 24 days.  Here is one example, showing the target voltage set to 3.5V, and the charger putting out 13.7 amps:

The cells you see cropped to the right of the picture are from another project - the Toad Car - which I used in the 2013 St. Patrick's Day Parade in Fort Collins.  I used it to get some experience with lithium cells, and with the MiniBMS.  Those are 40AH cells, paralleled to get 80AH, and then put in series to get 25.6 volts.  Here is the Toad Car in the parade:

The Toad Car, and the City Council race, were another huge time investment which delayed resumption of work on the ElectroJeep.

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