For the battery, actually -- doesn't matter, it's a big electrochemical capacitor. Indeed it has very much the Z ~ sqrt(1/F) diffusion response means that, attaching an LC resonator to one, tends to dampen the Q quite well.
You'll have some stray inductance due to the batteries, pack and connecting cable (under 1uH I would guess), and this has a lowpass against the SMPS input caps. Batteries don't radiate (any more than their physical size allows), so this will most likely suffice.
For testing from a bench supply, you may find it beneficial to add a CLC filter, so that you aren't measuring the ripple that would otherwise be reaching the battery. And this will manifest as common mode noise.
Common mode noise: effectively, both wires to the power supply have inductance, so half the ripple reaching the supply appears as ground loop voltage. Half of it dropped across the +, half across the -. If the power supply is earthed, and the scope is earthed, the minus voltage drop appears between these however, and you'll get a ground loop error, anywhere you probe in the circuit -- even if probing from ground to ground (clip the probe tip to the ground clip, and poke that at circuit ground)!
And if you're using a corded power supply, or making an offline supply, same thing applies of course, noise going up the power cable can manifest as errors when connected through to other things. So while you probably don't need to go for it in the battery case, this will be what you're shooting for, and what you'll see if it's bad, and also how to fix it (common mode chokes to break that ground loop!).
So for the filter, the negative resistance is still there, it's fine; I'm just saying it has less significance at higher frequencies (closer to Fsw). Though it's not entirely clear how much less. It would be best to determine by measurement -- set up an impedance measurement fixture (also known as a bias tee), and actually measure the input impedance of the SMPS itself.
For filter design, the lowest magnitude is the most significant; -25Ω in parallel with modest value resistances is still positive, but -0.45Ω dominates much more strongly.
Again, use a parallel damping R+C (somewhat oversized C*, matched R), you can dominate even that, and dampen the LC at the same time.
*Oh, also, C can be much bigger, no problem there. Electrolytics often need to be huge to get the ESR down where you need it, and as long as the extra capacitance doesn't cause other problems (like uh, inrush current I suppose, or of course bigger physical size, or higher cost), that's fine.
Personally, I never worry about negative input resistance, because it's mostly a low frequency parameter, and because my filters are designed for low impedances (the characteristic impedance of an LC is sqrt(L/C)) in the first place, and well damped on their own.
By the way, the meaning of impedance of a filter, is this: if you have a step change in current at the load, then expect its voltage to fluctuate by ΔV = ΔI*Z. This of course matters when a substantial negative resistance is in parallel (ΔV/ΔI is dominant and negative), but it's also important to operation of the SMPS itself (you don't want load changes to cause wide swings in supply voltage), and designing to the latter (for a moderate sized fluctuation, ΔV/V < 10%, say) covers the former.
Tim