Quote:
Originally Posted by bwilson4web
You're going after a 12V replacement and I'm going after a 48V eBike replacement for a lead-acid battery. In my case, this is my first thoughts:
Risky areas to resolve: - series shunt regulator - I've specified a 3.6V Zener but there is also a 3.3V that might be a better part. Bench testing will help resolve it. Also, I need to SPICE the Zener and current limiting resistor with both parts. From DOAX's charts, it looks like the 3.3V might be a better part because the current limiting resistor gives a slight, over-voltage to achieve peak charge.
- MOSFET + Schottky relay - I may have the wrong symbol for the MOSFET and it isn't clear if I'll need a reverse biased Schottky to pass regen current into the string. I've also shown a directly driven gate and this might be a case where reverse bias of the MOSFET puts the gate and attached microcontroller at risk. My fall-back is a relay but "UGH!"
- 30A vs 50A - the eBike has a 0.5 hp motor which should be ~15A sustained but I haven't measured the peak current draw.
I haven't seen a series, shunt regulator like this before but it is an obvious solution. Using ordinary Zeners means I need to model the thermal effects. In theory, as the part warms up, the Zener threshold voltage should go down which increases the draw from the peak cells towards a normalized voltage for all cells. In other words, helping to level-charge the peak cells. The key will be a collective heat-sink for the 15 Zeners.
I have not show thermistors for cell-strings. If I can have one for each of the LC6803s, it should be fairly straight forward. I did go with Linear instead of the TI part because I only need two instead of three chips and it has a 1/4W, built-in, per-cell, MOSFET. After all, what is 'time' to a computer and it may be this low, rate might be enough for cell balancing and not require the series shunt.
Bob Wilson |
I'm no electrical engineer (far from it in fact), but I'm a bit confused why you are using the zener diodes? The LTC6803 has mosfets inside it to activate shunting to each cell at a much more precise voltage than what those zeners can do.
edit: I read the datasheet a bit more (hadn't read much at all) and see what you are doing.