Other Parts Discussed in Thread: LM74700DDFEVM
A client contacted us regarding the protection of the charging port for their e-scooters and other PEV’s. It seems that some customers are measuring the pack voltage via the port’s two pins and short-circuiting the battery pack, damaging the port pins.
They have a 10A fuse that blows when the port is shorted but it doesn’t do enough to prevent damage to the pins and the PEV needs to be brought in for fuse replacement which is quite inconvenient.
The packs are 10S and 12S li-ion, 8Ah to 32Ah. There is no BMS between the charging port and the battery pack, just the standard fuse. Some of their customers also wire up their own higher current chargers to speed up charging but accidentally reverse the polarity.
We were considering using the LM74700-Q1 to provide input short-circuit and reversed charger polarity protection.
The anode side of the LM74700-Q1 would be connected to the charging port connector. The cathode side would be connected to a the battery pack. The standard fuse would still be used between the LM74700-Q1 and the battery pack. Is the LM74700-Q1 appropriate for this application?
We also have a low profile requirement. Is the large 220uF electrolytic cap on the cathode side of the LM74700DDFEVM circuit recommended for an application like this? The wiring to the battery pack will typically be short, a few centimeters or less. Maximum length will be about 25cm. Can we just use ceramic caps? If yes, what would your recommendation be for the cathode-side cap value? We have no holdup requirement, just cathode-side spike suppression. The datasheet lists 1uF min/47uF typical so we were considering laying out pads for two paralleled caps, 10-22uF each, and testing their effectiveness.
Some chargers must see the pack voltage before charging to begin. Unfortunately this means that the LM74700-Q1 would need to be enabled at all times. Our client is grudgingly willing to accept the additional quiescent current but we were wondering if it is acceptable to connect the EN pin to the CATHODE pin, using the battery pack’s voltage to keep it enabled?
We were considering just using a high value resistor across the MOSFET (with EN connected to ANODE) to provide pack voltage for the charger to detect but weren’t sure if that would work for all chargers or interfere with the LM74700-Q1’s operation.
Regards, John M.