I recently got into micro RC cars for indoors and I am using a Thunder AC 6 charger. My batteries are tiny 2S that say 400mah and 3.0WH. I don't want to cause a fire. How should I charge these? What amps and ? I have the 2S part figured out but thats about it.

Charge Lipo packs at 1C rate, where C is the capacity of the pack. For your micro packs, use 0.4 amps.

Ok, I don't see anything that says "C" but I will throw it on at 0.4 amps and see what happens. Thank you

Simple, all LiPo's are rated to charge with 1C and thats a current number equal as the capacity. So 400mAh -> charge with 400mA
All LiPo's can go a bit over it, 1.5C (in your case: 1.5x 400mA = 600mA) is still safe to use.

2C and higher is at own risk or the manufacturers says it can be used.

OP's 400mAh pack charged at a 1C rate = move decimal point three spaces to the left of mAh capacity = .4 amp setting on charger.

If you don't see a charge rating on a LiPo, it's best to stick to a 1C charge rate for the safety and longevity of the cells.

To calculate the charge rate of a LiPo pack:

mAh (capacity) divided by (/) 1,000 = Amp hour of pack (Ah)
Amp hour (Ah) x 1C charge rate = charging amps to enter on charger

For example:

A 4,000mAh pack, that does not state a charge rate, revert to a 1C charge rate.
In you head, move the decimal point three spaces to the left ...
4,000mAh / 1,000 = 4.0 Ah
4.0 Ah x 1C charge rate = 4.0 Amps (set charger to 4 amps)
The 4,000mAh battery pack should be charged at 4.0 amps.

If a 4,000mAh pack is rated for a 2C charge rate, the 4,000mAh pack can be charged at 8.0 amps (4x2). If 4,000mAh pack is rated for a 3C charge rate, the 4,000mAh pack can be charged at 12 amps (4x3).