The BMS protects the battery from overcharging at the wall outlet.
The ESC/VESC protects the battery from excessive discharge.
The battery is unprotected from excessive regen current but there are advantages.
There are two different downhill regenerative current risks:
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Overcharged (voltage) due to decending a hill on a full battery. This is never wise but less experienced riders often fall into this trap.
a) If the BMS disconnects the ESC then you have no brakes on a hill.
b) If the BMS can’t disconnect the ESC then you keep your brakes but your battery is likely fried at the bottom of the hill. There are practical limits. There is only so long that you can overcharge a battery before the damage is so severe that you lose your brakes anyway. But this type of wiring can give you a few minutes of lifeline if you are foolish enough to decend a steep slope on a full battery. The battery cells can be destroyed in seconds. But the downhill regen current can keep the VESC powered – which gives you brakes – as long as the damaged battery can still act as a resistor. If the circuit is broken then you finally have no brakes. -
Excessive regeneratve current charging rate or high temperature. This can happen on long, steep slopes even when the battery is at a relatively low state of charge. For example if you climb a long steep hill and have 25% battery level on at the top, you may not make it down the same hill if the slope is steep. This happens becasue regen current is much more rapid than the charging bricks. So the down hill regen can be too rapid for a small battery to absorb. However for an extremely large battery there is no risk because the regent current is distributed across so many cells.
a) If the BMS is wired to disconnect the battery, you lose your brakes on a steep slope.
b) If the BMS can’t disconnect the battery and the battery is small then you have a bit more time to brake but eventually the battery may pop.
c) If the BMS can’t disconnect the ESC and the battery is very large then there is no risk of damage and you keep your brakes.
Between the two methods, I prefer bypassing the BMS for operation. I would rather fry a battery than find myself in a down hill free fall.
This is another area where the eskate manufacturers tell you alot less than is known by some customers. I was pretty oblivious to everything in this particular post until I went riding with friends on Mont Royal (Royal Mountain). The local riders with custom build boards and Lacroix had no problem decending. I also had no problem with my quadrupal battery L3-x. But my friend (riding a Mellow Board) lost power shortly after leaving the top of the hill. He lost it at 55 kph and broke his femur. He lost power with less than 50% charge so presumably battery temperature triggered the BMS to pull the plug on the Mellow board ESC.
Incidentally, the L3-x was factory wired to the battery in the same manner as the mountain climbing boards. The L3-x BMS cannot disconnect the power to the ESC. That wireing and the four batteries both contributed to my safe decent. If I had tried to decend that hill on only a single battery I would have likely fried the battery but I probably would have made it to the bottom without losing brakes.