charge a truck or deep cell battery with a RMH?

Hello all!

Once upon a time, my family bought an expensive solar setup so that we could have stable power while we were off-grid. However, frequently we would have stretches of time where there just wasn't enough sun to charge the batteries, and the fridge would start to spoil. This happened over and over in a particularly cloudy season.

I'm wondering if there is a way to generate electricity from a Rocket Mass Heater. If we were running a RMH or other rockety device on a regular basis, it would be great to use a little of that energy to be trickle charging some kind of truck or deep cell battery at the same time. Ideally, when my system is running low and the forecast is cloudy, i could have some batteries all juiced up and ready to swap out.

I'm not particularly interested in a boom-squish situation, although it seems clear that a teapot steaming (is that unpressurized? partially?) could probably turn a wheel and generate a trickle charge, perhaps too tiny to be useful. I would have to vent the steam outside, it is already way too humid where I am! I'm not sure this is a practical approach.

It looks like you can buy this device that can give you 45 watts using a thermoelectric generator, which seems less dangerous, but it's pretty pricey. Its existence is encouraging:
https://www.youtube.com/watch?v=7O3FlujxXCw

I don't really know anything about thermoelectric generators except having read the Wikipedia page. My question for you folks is: could we build a thermoelectric generator around the barrel of a rocket mass heater? Wrap it in special wires?

TEG relies on a heat gradient, not just being generally hot. You need to have hot side - in contact with a wood stove, for example, and a cold side - careful cooling with fans or something similar. I suspect that the heat from being in the hottest part of a rocket stove (around the barrel) would damage the TEG components. I don't think this is something that could be easily homemade. An off the shelf version is probably the best bet. They are inherently low energy conversion efficiency. You won't be charging an electric car from one, but a few LED lights is viable.

If you're inclined to tinker, I'd say get a TEG module and find a way to run a liquid cooling setup on the cold side. A trickle of water, maybe gravity fed from a rain barrel, would do the job. The hot side of many commercially available hobby-grade TEGs would handle placement on the top of a RMH barrel as long as you paid attention to the heat shielding of the wires.