Any one in the field of li-ion battery pack for long enough, will notice that there are 2 cells (2s), 3 cells(3s), 4 cells(4s), 6 cells(3s2p, 2s3p), 9 cells(3s3p), 12cells(3s4p, 4s3p) battery pack, is there any possible that we could have 5cells, 7 cells, 10 cells, 11 cells? 's' mean series, 'p' mean parellel.
By parellel up cell, one can expect to get more power surge from the cell, it simply mean more current can be supplied by the cells. By series it up, it simply mean more voltage. It is not very pratical to assemble 6 cell in series, it will produce up to 4.2x6=25.2V from the battery pack, which is way too high for high current flow power mosfet to operate at greatest efficiency, or there will be some compromise at the filter side of PWM.
Lenovo is the very first company that adopt 7 cell battery, manufacture by OEM of Sanyo, many people have no sense of what is OEM, OEM mean original equipment manufacturer, it simply mean Lenovo make an custom order from Sanyo, other manufacturer that clone the battery thus can not be called OEM, cause Lenove didn't make an order from them.
I have no idea how they call this type of battery pack, but I call it "hybrid battery archetecture". It is basically 2 pieces of battery of 3 cells and 4 cells combine in one. There are three reasons I can think of why they must do this.
- There are only room for 7 cells left for particular model (this is very unlikely).
- The bussiness machine company do not want other clone battery company follow their design, 6 cells design is easilly duplicated, 7 cell is harder and cost is higher (please note that the protection board use extra component(higher cost), it is 2 battery in one, remember?) What make it so dificuit to other clone manufacturer is that the two battery automatically shift at certain preset voltage level, from one to another. This reasons is very close to be truth, but also not very likely.
- This battery is best in cell balancing performance. In other word, the controller board will balance the cell in series better as too many cooks spoil the broth, because the current flow through series cells is same in all cell, there will not be a particular one higher than another. There is however, there are some trade off point, i.e. this battery must not be used in system that are power hungry, since it is only three or four cell in series at one time (parellel can suppy higher current remember?). This reason make those third party lo quality cell imposible to be used for this battery, because three or four cell in series make them not enough for the output, it will be very hot and tend to terminate discharge before it able to deliver 100% of it energy. In fact I guess third party lower quality will only output less than 80% of energy that it can store.
Picture above: There are one gas gauge(left, almost same price with one piece of Li-ion cell) and two analog front end voltage/current over limit protection. All component is doubled except the gas gauge which in charge communicate with system management. This is an engineering perfect battery pack, obviously, with some extra cost .
