Hi,
my 36V ebike battery (Bosch) is 6 years old and has covered 25000km over that period. It still functions perfectly but the range has obviously dropped a little bit since new and I guess the internal resistance of that battery is pretty high by now. I'd be interested to use a new 36V battery softpack with its own BMS in parallel with the Bosch battery to increase the range. This has already been done by many people on a german forum (www.pedelecforum.de) so I know this would work. There are different ways to achieve this but one way which is appealing for different reasons is to open the Bosch battery pack and connect the external battery between the Bosch BMS and the cells directly (without ideal diodes). Of course the two batteries would be connected together only after making sure they have the same voltage. The external battery would use the same cells (Samsung 29E) as the original one but would have a larger capacity (e.g. 20Ah vs 11Ah).
Now my question: since the original battery is pretty old and the extender pack would be brand new, I guess the internal resistance of the extender pack would be much lower than that of the original battery. So it is obvious that the extender pack would provide most of the current, which is perfectly fine. However what happens when the voltage in the two batteries tries to equilibrate if the internal resistance of the two packs is vastly different? In other words, is it safe to parallel two batteries with vastly different internal resistances e.g. a 6 years old pack with a brand new pack even if the batteries use the exact same cell type or is this unsafe to do?
Thanks for your insight!
EDIT: the question applies to both discharging and charging. The two batteries would be charged together too by connecting the original charger to the original battery. I don't really see a problem for charging as everything would be watched by the two BMS. Is that correct? However during discharging, if the extender pack has a slightly higher voltage at some point (quite unlikely I guess since it has a lower internal resistance), it would charge the original pack without going through the original BMS because it is connected between the BMS and the cells. On the other hand, if the Bosch pack has a higher voltage than the extender (e.g. after a load phase with most of the current being provided by the extender, it would charge the extender but would go through the extender's BMS so it should be safe...
my 36V ebike battery (Bosch) is 6 years old and has covered 25000km over that period. It still functions perfectly but the range has obviously dropped a little bit since new and I guess the internal resistance of that battery is pretty high by now. I'd be interested to use a new 36V battery softpack with its own BMS in parallel with the Bosch battery to increase the range. This has already been done by many people on a german forum (www.pedelecforum.de) so I know this would work. There are different ways to achieve this but one way which is appealing for different reasons is to open the Bosch battery pack and connect the external battery between the Bosch BMS and the cells directly (without ideal diodes). Of course the two batteries would be connected together only after making sure they have the same voltage. The external battery would use the same cells (Samsung 29E) as the original one but would have a larger capacity (e.g. 20Ah vs 11Ah).
Now my question: since the original battery is pretty old and the extender pack would be brand new, I guess the internal resistance of the extender pack would be much lower than that of the original battery. So it is obvious that the extender pack would provide most of the current, which is perfectly fine. However what happens when the voltage in the two batteries tries to equilibrate if the internal resistance of the two packs is vastly different? In other words, is it safe to parallel two batteries with vastly different internal resistances e.g. a 6 years old pack with a brand new pack even if the batteries use the exact same cell type or is this unsafe to do?
Thanks for your insight!
EDIT: the question applies to both discharging and charging. The two batteries would be charged together too by connecting the original charger to the original battery. I don't really see a problem for charging as everything would be watched by the two BMS. Is that correct? However during discharging, if the extender pack has a slightly higher voltage at some point (quite unlikely I guess since it has a lower internal resistance), it would charge the original pack without going through the original BMS because it is connected between the BMS and the cells. On the other hand, if the Bosch pack has a higher voltage than the extender (e.g. after a load phase with most of the current being provided by the extender, it would charge the extender but would go through the extender's BMS so it should be safe...