Continuing from FJJ notes:
In the case of “36v” batteries: Charging voltages for different lithium batteries depends on the individual cells chemistry of Lithium ion used in the battery. So in the case of LiMnCo, cells are nominally 3.7volts per cell, and are charged to 4.2 volts per cell. Batteries with this type of cell chemistry are made using 10 cells in series giving nominal voltage of 37v and a charge voltage of 42v.
LiFePo4 cells have a nominal voltage of 3.2-3.3v and are charged to 3.65v per cell. Because the cell voltage of LiFePo4 is lower, batteries are made with 12 cells in series; in this case, these batteries have nominal voltage of 39v and a charge voltage of 43.8v.
Of course “36v” batteries are often made with more than 10/12 series cells, because the individual cell capacity Ah rating is usually too low for pedelelec /ebike use, so more cells are added in parallel.
For LiMnCo; 20,30,40 cells i.e 10s2p,10s3p,10s4p etc. And LiFePo4 batteries 24,36,48 cells i.e. 12s2p,12s3p,12s4p etc. Paralleling cells does not increase the voltage but increases the batteries capacity Ah rating.
Commercial lithium batteries are made with built in Battery Management Systems BMS. Depending on the BMS used by different battery manufacturers, there could be differences in the functionality of the BMS; but basically they control the max voltage per cell/parallel cells (balance cell voltages), max voltage of the battery High Voltage Cutoff HVC and the Low Voltage Cutoff LVC for the cells and or battery.
Back to chargers; although the battery has a built in voltage management system the charger also has to have the correct values for the particular battery type. Charges need to have an output voltage which equates to the HVC of the battery. Lithium battery charges usually have a constant voltage/ constant current cv/cc output. As the battery approaches HVC the current demand from the charger will decrease towards zero amps. But chargers can also differ in the way they work at or near the fully charged battery state. Some shut down when the current demand is near to zero. But as the BMS does its job of balancing cell/parallel cell voltages, more current may be required and the charger will continue to supply a constant voltage until cell balancing has been achieved.
Lithium batteries should not be left on trickle charge for long periods. That is unless the charger is capable of isolating itself automatically from the battery; which is unusual with most of the commercial chargers supplied with batteries. It is therefore important to keep an eye on the length of time the battery is left charging.
Chris
