Many lithium battery packs contain an integrated battery management system (BMS) that controls charging, protects against overdischarge (low voltage), and protects against over current during discharge. "Duct tape", Ping, DeWalt and many others have a battery management system.
When attempting to put packs containing a BMS in series, there is a potential problem. If the BMS ever trips and goes open due to an overcurrent, the transistor switch in the BMS will be exposed to the full voltage of all the packs combined, possibly taking over the rated voltage for the part and causing destruction.
The way to avoid this problem is to place diodes across each pack's output terminals to keep the voltage from going higher than the BMS was designed for.
If one pack trips, the diode will conduct, allowing voltage from the other pack to pass to the controller. The tripped pack will not "see" more than its own voltage, plus the voltage drop in the diode.
For a typical 36v pack, you'd want to use something like a 50 volt, 50amp rated diode.
Since the diode is only working when a pack trips, low losses are not very important and you can use just about any diode with a high enough rating. Note the diodes are placed across the battery in the normally non-conducting direction.
BRICKTOP wrote:Thanks Fechter, thats awesome, its explained all my questions.
Ive just shrink wrapped my headway cells with 60mm heatshrink and next im thinking I may stick them all together with something, im thinking maybe silicone. My ebike project has had to take a back seat this last week but im keen to get on and ill make a post with some pics when its a little more completed so everyone can check out my creation
Thanks again to everyone who commented.
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