I see balancer ICs for series-connected lithium ion cells everywhere. I have not seen one for when cells are wired in parallel. If there is nothing available, I'd like advice as to how to design my own parallel balancer.
My application will consume about 0.25C of one cell, and I plan to use four cells connected in parallel to lengthen device battery life. I can't find cells that are impedance-matched that aren't in premade packs (requiring them to be disassembled to get access to the individual cells - which is probably dangerous anyway). Space and size constrains of my device require two pairs of cells to be placed end to end, but each pair will not be directly next to one another. Basically I'm saying that a premade parallel-wired pack will not work for me and that I need four individual cells (which will not be matched). I'd also like to have the option of charging the device via USB or some other lower voltage supply, which is why I'd like to use a parallel cell configuration rather than a series configuration.
My initial idea was to measure the voltage and current from each cell during device operation and charging (via current sense resistors on each cell). If a cell's voltage differs from the others' by a certain amount, a FET will be turned off cutting off that particular cell for a short time (until its voltage is back within range after the other cells are discharged from the device a little or charged a little more). Since my application requires less than 1C of one cell, I could even turn off three of the four cells if needed. In a similar fashion, if the direction of current flow into a particular cell is reversed from the others (indicating that it may be getting charged by the others) and if the magnitude of that current is over a certain threshold, it could be shut off temporarily.
Is that a good idea? I've read that you should only balance cells on the charge cycle (not when being discharged) and when they're above like 80% capacity, is that correct? In that case I could disable my balancer until charging and above 80%. Is it a better idea to scrap this plan and change everything to a series-wired configuration? I already have evaluation boards (charger / fuel gauge / DC/DC converter) for a parallel config and would need to purchase all new boards if I needed to switch.
Thanks!
