I've designed a few boards that use LiPoly batteries with the Microchip MCP73833 charger chip. It seems to work well in singles, but I haven't been brave enough to run a few in series for some 12V motors.
My first thought was to have an MCP73833 for each LiPoly cell. The Vin sides would be in parallel and the Vout sides would be in series. However, as soon as I drew that out in my head, it became clear that it would end up all explody due to the common ground on one side and non-common round on the other.
I need to switch the whole system between parallel and series, grounds included. I could certainly do it with some relays or multi-pole switches, but automatic would be cool.
It's been on the back burner for a while, but I've got some pretty good small 12V motors that I want to use. The search for a non-explosive solution is back on.
Duane -- the LiFePo4 Batteries are used in Power Tools for Motor Drive (Will take more abuse than Li-Poly Battery) -- Something to think about for a mobile robotics device -- also they will take about 350C heat rather than 100-150C heat(failure point for LI-Poly)
I hadn't considered that type, but with a bit of looking I can see that they seem to have good availability and reasonable pricing. Not as dense as LiPoly, but still pretty good. I really like the fact that it's more robust. LiPoly make me nervous.
@Duane, I read an article on charging Lithium chemistries and it seems that current limiting to less than the maximum charge current while restricting the maximum open terminal voltage is all they need. This means that if you have a load cell in parallel with each cell that bleeds current to avoid charging above ~3.6V per cell (depends on exact chemistry) is the way to go. Then you can put the cells in series. The current will either flow through the bleed circuit (above the cell voltage) or charge the cell (below the cell voltage. When the cell voltage == the charge voltage no current can flow so you can't overcharge the battery.
I used a precision low current opamp and reference driving a power transistor across each cell that would bleed at 3.7V (for my chemistry) and then charged with an 11.1V 5A supply.
Check out the ams AS8506C autonomous balancer: passive or active mode up to 7 cells. Integrated w/ temp sensor +2 temp sensor inputs, all necessary analog functions & switches built in. No error prone code or micro needed. Only 3 resistors needed: one for discharge 2 for high and low thresholds. Balancing & Monitoring now simplified in the real analog world.
LithFire-X, LLC (dba LithSafe) has developed our patented methodology; 'SCIC': suppression through cooling, isolation and containment. We have developed methods for the safe testing transport and storage of lithium ion technologies.
I will be designing a charger for a series stack of LiFeO4, to reach a high voltage. I plan to make individual, Isolated chargers for each sub-pack. This is for my regular day job. However, I will share my general approach, so you all can have some fun.