[Wombii]

I did the planning for a NIMH, but because of the specific mix of electronics I ended up with, I went with a 2s Lipo pack (Standard r/c car hardshell pack, same size as the NIMH stick but rectangular). 7,4v 5000mAh, I think. I ran into some edge case issues because the NIMH pack will start out around 7,8 and discharge down towards 6v and give issues with some of my voltage regulators. The Lipo starts at 8,4 and stays above 7v during its useful range.

Will the solenoids work at 7.2v? I also hope you describe your plumbing in details, including where to find the tubing, valves and connection hardware if you're going that route, as I chickened out and went with tank -> pump -> monitor and an option for a second pump for filling the tank.

If you go with Lipo, there are many sizes to choose from, and even the hardshell racing packs come in a compact "shorty" version.

With regards to runtime, I haven't actually been running it enough to give a good answer. I'd guess 30-80 minutes of real playtime based on prior experience with other cars. I can easily do short test runs and long light/sound testing sessions several times in a week without charging.
Guesstimation factors: 3500 mAh NIMH would give me just under an hour of runtime in a Tamiya m-03 with some lights and with a CC-01.
My LEDs theoretically draw ~740mA. Arduinos + receiver + sound is roughly the same. I think the pump was around 2A last time I checked, but it drains the tank in a minute or so and won't affect runtime much.

If I were to plan it out all over again, I might consider going 3s for something closer to 12v to get some extra power to the 12v pump. That way I could also go with a smaller pump instead of running an oversized 12v pump on 7-8v. I'm also using a boost circuit to boost the battery voltage to 9v for some of the LEDs to allow wiring them in series where the space is extra tight and could avoid that with an extra battery cell.

I don't think I've uploaded a diagram for my power setup yet.
I've set it up so I can use two power sources: An R/C battery pack for everything or a USB port for low power lights only (display mode).
The battery wires split through switches into 3 main branches: pump (esc), drive (esc) and electronics. The drive ESC has an onboard BEC supplying steering + gear servo.
The electronics branch split into:
A: BEC (vBatt->6v for servo and lights).
B: Boost (vBatt->9.5v for high power LEDs)
C: vBatt into Arduino mega (onboard 5v regulator)
D: vBatt into Arduino nano (onboard 5v regulator)
E: sound (vBatt -> 3.3v for sound amplifier board and ESP32)

Most of the LEDs are on a separate voltage bus that I call vLED. This bus can be connected to either the BEC for 6v operation on battery or to USB power for 5v display mode through a switch.