Hi
I looked at the simplest option of a series inductor shunted with a reverse biased diode.
Basic simulation showed I needed a 1mH inductor to keep the in-rush current below 70Amps on a 12V battery fed system. The shunt diode does very little, with only about 5 amps flowing when it conducts.
In addition, even with the diode shunt, the L/C combination showed a relatively slow harmonic decay.
This was fixed in the model by adding a series resistor to damp the oscillation. By itself, adding a resistor was sufficient to reduce the in-rush current below 70A (the battery trip current). Adding a resistor would be simple but inefficient.
Installing an 1mH inductor that doesn't saturate at 70A is not an option. Too big, too heavy and too expensive. An active in-rush limiter is the most obvious solution.
I found this paper
http://www.mosaic-industries.com/embedded-systems/_media/pdfs/application-notes/motorola-an1542-active-inrush-current-limiter.pdf that describes simple circuits that do what I want. I don't need overcurrent protection because the LiFePO4 battery includes a crowbar circuit as part of the internal BMS. I already have a standard fuse as well. I like the idea of the PTC thermally linked to the transistor but if the transistor is rated to take more power than the battery protection, I wouldn't need the PTC.
What I don't currently have is a battery fuel gauge so combining the in-rush current limiter with a fuel gauge on the same PCB would make sense.
Dazz