A few questions/observations on li-ion balancing:
Thus far I have seen a few methods of cell balancing.
1. Actual charging of low cells. An isolated charger is connected to the low cell and the cell is charged until it is closer in charge level to the rest of the cells. I have only seen this done manually.
2. Resistive divider. A resistive divider is built so that each tap point corresponds to one cell junction. This will (slowly) balance all cells by taking more or less current from the higher or lower cells. Wastes power constantly, but Maxwell recommends this as a cheap and easy way to balance their ultracaps.
3. Resistive divider + op amp. Same as above but with op amp buffers. Can source/sink much more current, and quiescent power drain is much lower.
4. Shunt balancers. A 4.2 volt (or similar) shunt on each cell that will shunt current around the cell when maximum voltage is reached. As far as I can tell this is the most common li-ion balancer. Some problems include heat generation during balancing and requirements for fairly high accuracy on each shunt.
5. Flying capacitor balancer. A capacitor is switched between two cells (or all cells) transferring charge from higher voltage batteries to lower voltage batteries.
6. (the big hammer) Individual charging of each cell with individual voltage regulation. Basically one charger per series cell.
Any other schemes out there? Am I correct in assuming that option 4 is the most common?
Also, I have noticed that some of the lipo packs I've bought from BatterySpace have a warning that you should let the pack sit for 30 minutes after charging to allow the balancing function to work. As far as I can tell, they use shunt balancers. Why would a shunt balancer work better after the charger has been disconnected? I would think that 1) it would be most useful near charge termination (when you are pushing some cells over 4.2 volts) and 2) that most chargers terminate charge by essentially disconnecting anyway.
Thus far I have seen a few methods of cell balancing.
1. Actual charging of low cells. An isolated charger is connected to the low cell and the cell is charged until it is closer in charge level to the rest of the cells. I have only seen this done manually.
2. Resistive divider. A resistive divider is built so that each tap point corresponds to one cell junction. This will (slowly) balance all cells by taking more or less current from the higher or lower cells. Wastes power constantly, but Maxwell recommends this as a cheap and easy way to balance their ultracaps.
3. Resistive divider + op amp. Same as above but with op amp buffers. Can source/sink much more current, and quiescent power drain is much lower.
4. Shunt balancers. A 4.2 volt (or similar) shunt on each cell that will shunt current around the cell when maximum voltage is reached. As far as I can tell this is the most common li-ion balancer. Some problems include heat generation during balancing and requirements for fairly high accuracy on each shunt.
5. Flying capacitor balancer. A capacitor is switched between two cells (or all cells) transferring charge from higher voltage batteries to lower voltage batteries.
6. (the big hammer) Individual charging of each cell with individual voltage regulation. Basically one charger per series cell.
Any other schemes out there? Am I correct in assuming that option 4 is the most common?
Also, I have noticed that some of the lipo packs I've bought from BatterySpace have a warning that you should let the pack sit for 30 minutes after charging to allow the balancing function to work. As far as I can tell, they use shunt balancers. Why would a shunt balancer work better after the charger has been disconnected? I would think that 1) it would be most useful near charge termination (when you are pushing some cells over 4.2 volts) and 2) that most chargers terminate charge by essentially disconnecting anyway.
