Limiting cap. inrush current when assembling Li-ion BMS + Charger board - Page 1
Forum section: electronics / projects

author: tinfever
author_rank: Regular Contributor
timestamp: July 15, 2025, 06:16:38 pm
content: I'm working on a 6-cell Li-ion battery management system with integrated USB Type-C charging, using a BQ25756 buck-boost charge controller. The BMS/charger will be soldered to the pack directly. The BQ25756 requires 160uF of output capacitance, connected directly to the battery. I'm concerned that when this device is assembled and the battery pack is connected to the board, the inrush current of the battery into these output capacitors will cause sparking and might cause damaging voltage spikes. Is there a standard way to mitigate this during initial assembly?I could add an NFET with high-side driver between the charge circuit output and the battery, but I'd rather not add extra circuit that is only ever used once. I guess this would fix potential issues with output cap leakage current draining the battery though. (Tantalum polymer caps would have 700uA max leakage current.) Since only a handful of these will be made, I could just manually pre-charge the input caps with a resistor. Or maybe have the boards assembled with a resistor in the path and parallel solder jumper pads, then just assemble and solder the jumper?

author: Siwastaja
author_rank: Super Contributor
timestamp: July 15, 2025, 06:29:09 pm
content: It's also possible that the inrush isn't bad enough to matter. 160µF for 20-ish V isn't that much; it's going to have ESR. And today cells are often shipped at 20-30% SoC (instead of earlier 50%) which means lower voltage and higher ESR from the battery side too.Inrush is made worse if your circuit immediately starts operating and consuming from the capacitor - then assembler can see repeated sparking instead of a single spark they experience if the circuit is in low-power sleep state and capacitor remains charged.

author: temperance
author_rank: Super Contributor
timestamp: July 16, 2025, 12:51:28 pm
content: QuoteYou can add some zener or TVS diodes across the capacitors, rated such that the overvoltage spike is within the limits of the IC.

author: David Hess
author_rank: Super Contributor
timestamp: July 16, 2025, 07:12:36 pm
content: Quote from: tinfever on July 15, 2025, 06:16:38 pmThe complete schematic in the datasheet shows the 160 microfarads of capacitance divided into two 80 microfarad banks on either side of each current shunt, and talks about a combination of electrolytic and ceramic capacitors for better high frequency performance.QuoteI see three connections from the BQ25756 to the battery, and they have absolute maximum ratings of 85 volts. These connections also have some combination of RC filtering. TVS diodes could be placed across these lines close to the BQ25756.With only 6 cells for 24 volts, I doubt there is anything to worry about. The voltage spike and ringing depends on the series inductance, which will be low because of the high current connection and short length, and the Q of the capacitors which will be swamped by the low Q of the electrolytics and low Q of the battery.QuoteI like that solution. There really should be a fuse placed close to the battery and this could be a good place for it. Have the resistor installed on the board, attach the battery, and then install the fuse.I sure would not do anything more complex because reliability would likely be lower.

author: tinfever
author_rank: Regular Contributor
timestamp: July 16, 2025, 07:50:08 pm
content: Thank you all for the advice! That makes sense that the inrush current probably won't hurt anything in this case, even if it did cause a voltage spike up to 50-60V. I'll probably include the resistor with jumper because it's so easy, but otherwise just make sure all the inputs can handle any potential voltage spike.

author: temperance
author_rank: Super Contributor
timestamp: July 17, 2025, 12:12:42 am
content: QuoteI think it is a good idea to calculate the damping factor of the network and add damping as required. Eventually you might need electrolytic capacitors with known resistors in series.

author: David Hess
author_rank: Super Contributor
timestamp: July 17, 2025, 04:06:23 am
content: Quote from: temperance on July 17, 2025, 12:12:42 amThe problem is a real one associated with the high Q of ceramic capacitors. Intel had a problem with it where a MOSFET was used to switch the programming voltage for their Flash memory. The high Q of the ceramic capacitor and wire leading to the Flash memory chip would ring by several volts out of 12.5 volts or something, destroying the Flash memory which had a programming voltage not much lower than its absolute maximum rating.It also crops up with small transformer powered AC adapters where the size of the filtering capacitors is small because high ripple is acceptable.One of the solutions in both cases is to add enough bulk capacitance, which will be low Q anyway, to absorb the energy stored in the high Q LC circuit. A more analytical approach is to calculate or meausre the impedance of the transmission line used for power distribution and AC terminate it with an electrolytic capacitor with the proper ESR, which is why in old designs a bulk capacitor is included among a bunch of ceramic decoupling capacitors on each board.Here we have a lot of leeway because the absolute maximum voltage is so high compared to the operating voltage, and the line lengths are so short.

author: temperance
author_rank: Super Contributor
timestamp: July 22, 2025, 11:41:05 am
content: QuoteThe problem is very real. In any circuit containing only ceramics as decoupling, you have to investigate if the damping is high enough.A real world example:Let's say you have a circuit and all decoupling is done with ceramic capacitors. Any device sinking or sourcing current from the supply rail with some high di/dt can trigger the LC circuit (A high speed RS485 interface, for example). The same can happen when performing EFT burst tests. Some high di/dt current will make it into the supply rail anyhow and trigger the LC circuit.Long ago when decoupling was done with aluminium or tantalum capacitor, the circuit Q was very low. But those 10 µF ceramics while providing excellent decoupling should be used with some care. But some damping is easily added by placing some low quality low Q capacitor of reasonable value.