Li-ion voltage fluctuation

[zrx_s6]

I'm designing a portable power bank with a 4S Li-ion pack and my DC-DC buck converter is failing under load transients. So when the load intermittently kicks in, the pack's voltage sags dramatically, momentarily dropping the buck converter's input below its minimum dropout voltage. My synchronous buck is rated for 18V max, but the transient dips hit around 12.5V, while its dropout is 13.5V. This instability is disrupting both the regulation and my ability to trust the SOH readings.

Is this extreme transient sag common for high-C-rate Li-ion cells? Should I simply switch to a buck-boost topology to handle the wide input voltage range, or is there a power electronics solution I'm overlooking for better local decoupling and filtering right at the battery output to smooth that initial voltage kick?

Open to hear your advice! Thanks!

 

[OhmMy]

Voltage sag usually points to either not enough input capacitance or a high equivalent series resistance (ESR) in the cells. To tackle this, consider adding some bulk low-ESR capacitors at the input of your buck converter to help smooth out those transients. Going for a buck-boost setup might be overkill.

 

[Circuitsmith]

Don’t forget about the layout! If you have long traces or your grounding isn’t great, it can really make those transient dips worse. Placing a snubber close to the input could also be a good solution.

 

[Tovaren]

Also don't overlook about precharge circuitry or soft-start control. These features can really help manage those sudden bursts of power and keep your converter steady when there's a quick change in load. It’s all about making sure things run smoothly, so you don’t have to worry when the pressure's on.