Batteries

LiFePO4 Battery Management Design (In Progress)

The purpose is extremely simple, just don't over-charge or over-discharge any of the cells.  When a cell reaches the low voltage the ESC will stop operation.  To avoid over-charge, current is either shunted around a fully charged cell, or charge current is stopped, the cell is discharged slightly, then charging continues.  Since an off-the-shelf charger is being used there is no control over starting and stopping the charge current, so shunting must be used.

An ideal zener is the perfect solution, it would clamp the battery cell voltage at the high cutoff, conduct 0A until 3.2V and become a short thereafter.  An actual zener will conduct before its cutoff though as shown below.

The blue trace is the zener current which is non-trivial even at the low-voltage cutoff!

There are many off-the-shelf IC's that have a better voltage transfer characterisitc, a board was developed using TI's BQ76PL455A as shown below, it has not yet been assembled.

Shown below are the 26650 holders I picked up from Digikey along with some A123 cells I've had laying around.  I finally had the chance to mount them to some whiteboard and wire them up in series.  You can see from the voltage they are in need of a charge (3.3*8 = 26.4V)!