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I am in the process of creating a somewhat hard military sci fi setting and was enamored by the idea of all-electric armored vehicles for a future war.

For this I decided to used Redox Flow Batteries: https://en.m.wikipedia.org/wiki/Flow_battery

I decided redox flow batteries were a good fit as it allowed the electrolyte of the batteries to be tranfered from one vehicle to another, meaning that armored vehicles could recharge themselves by filling up from a tanker vehicle that stored the electrolyte and recharged it using renewables or even portable nuclear sources.

This allows your "fuel" to be recycled between the tankers and the vehicles indefinitiely meaning that unless a tanker was destroyed or an accident happens and cause fuel leakage your forces could wove through wide swaths of enemy territory with minimal resupply.

They also have additional advantages of full discharge capability, very long cycle life, and have non-flammable electrolytes that don't explode like Li-ions.

Redox flow batteries are normally plagued with low power density issues however articles like the one below give me hope that future higher density electrolytes can be discovered:

https://www.sciencedaily.com/releases/2009/10/091012135506.htm

The above article claims they can now reach the equivalent density of Li-ions, so considering future technology, for the purposes of this answer assume maybe 0.5 times better energy density than current Li-ions.

Would this system work for the sustainment of an armored division on extended combat operations?

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  • $\begingroup$ No current battery tech comes close to the energy density of hydrocarbon fuels used today. What assumptions are you making regarding this in specific terms? $\endgroup$ – Rottweiler on market-day. Feb 3 at 1:59
  • $\begingroup$ @Tantalus' touch. I edited the question. You are correct in stating that the energy density would be lower so you'd have to store more fluid, however that being said, you also have to take into account the weight savings that this could bring such as simpler to no transmission, lack of engine or APU, no lubricants needed, and other small changes that really add up. Also technologies such as regenerative braking would allow the system to go further on a single charge than normal. $\endgroup$ – Efialtes Feb 3 at 3:36
  • $\begingroup$ There is actually a couple technologies that, at least theoretically, compete with petrol. There was a magnesium hydride sludge recently that potentially competes with petrol (it's technically still really inefficient, but the losses are at the refinery rather than in the engine like petrol). Then there's Vanadium Boride which (while hard to reprocess) nukes petrol with nearly 3x the Joules per Liter (and it works at 800'C so... red-hot glow FTW) $\endgroup$ – Samwise Feb 3 at 21:45
  • $\begingroup$ Maybe I'm dumb, but aren't your "Redox flow batteries" more commonly known as "fuel cells"? Fuel cells are great, low maintenance, high-efficiency power sources. They also have power density (power output per kg of total system) that is less than 1/100th that of the same fuel burned in an internal combustion motor. great for use on a spaceship, submarine or such as auxiliary power source. utterly useless as primary motive power for an armored fighting vehicle. $\endgroup$ – PcMan Feb 4 at 21:35
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It sounds good for a story.

Let us remember that this is for a fiction and so should be cool for the readers or engaging for the players. It reads to me like you have your head wrapped around this concept well enough to bring it to life in your story. The idea of pumping electrolytes, tanker "refuel" trucks, soldiers with siphon hoses, fuel dumps etc has plenty of precedent in real militaries since WW1, and you can update and freshen this by making it electrolyte and batteries.

I have to say that in the real world I would rather have a lumpy box as my battery because I am certain electrolyte would somehow manage to spill in my lap because it always does and then they wont let me in the club. A soldier can grab the battery from a damaged vehicle and run with it; difficult with electrolyte or diesel.

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  • $\begingroup$ I'd say that transferring electrolyte from one vehicle to another would require some sort of jerrycan or fuel pump with a gasket to seal over the top of the fuel tank so you don't spill as much in transit. The point of electrolyte is so that you don't have to bring nearly as much POL stuff in country to sustain your assault because you can just keep recycling used fuel. But yeah thanks for the insightful answer. $\endgroup$ – Efialtes Feb 3 at 3:45
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Military uses will want compact fuel sources. The less weight of fuel they carry, the more weight of armor or shells they can bring. They aren't going to care much about the environment, except in terms of whose it is.

Bringing along a separate tanker means having a vehicle that can be targeted for attack to immobilize the others. The act of transfer might expose personnel or reduce the ability of the vehicles to maneuver. Also, is the tanker armored? If not, it limits the situations the whole group can enter. But if it is, why can't the other armored vehicles power themselves?

Mostly, I don't understand why the "renewable source" or nuclear source that you describe isn't right on the vehicle that needs the energy, instead of a tanker.

Update for specifics: if the battery has '0.5 times better energy density than lithium' (lithium + 50%), then I see 220 Wh/kg or 265 Wh/kg for that figure. "Wh" is an awful unit, J/s * 3600 s, so let's call that 954,000 J/kg = 954 J/g ... I'll be charitable and round it up to 1 kJ/g, then add 50% to 1.5 kJ/g. By comparison, octane has a fuel value of 41 kJ/g, and I see some figures for gasoline a bit higher. The site that says 265 Wh/kg above says gasoline is 100 times more energy dense than (less efficient, more widely used) lithium batteries.

It seems like common sense: If you load up a gasoline car with all the extra gas jugs it can carry, in the trunk and back seat and passenger seat until the weight capacity is maxed out, you expect it to go a long way; but if you double the batteries aboard an electric car, you would wonder if it could move. Electric cars have many advantages, especially since at lower fuel capacity the lack of engine weight makes a difference, but carrying troops a long way across hostile terrain in a barren war zone when resupply can't be guaranteed? I doubt it.

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  • $\begingroup$ Much of this would have been better as a (couple of) comment(s) under the OP's post than as an answer. As such flagging for a mod to convert it. Where did the "nuclear fuel source" come in, why do you bring that up? Yes protons are found in the nucleus of an atom, but every cell in your body has proton pumps, your answer obfuscates the issues. $\endgroup$ – Rottweiler on market-day. Feb 3 at 1:18
  • $\begingroup$ Our current fleets of armored vehicles already require tanker fleets, I'm simply adapting it to the use of electric vehicles. The reason the power pack isn't on the vehicle is because it's likely to be too bulky to bring along and would need a seperate truck. $\endgroup$ – Efialtes Feb 3 at 1:21
  • $\begingroup$ @Efialtes Redox flow batteries have energy density at least two orders of magnitude lower than hydrocarbon fuels. We'll need a fleet of tankers just to keep one tank running. $\endgroup$ – Alexander Feb 3 at 1:29
  • $\begingroup$ @Alexander I addressed that in the question. Redbox flow batteries are improving and have already begun to reach parity with li-ion at least in lab conditions, so I'm making an optimistic estimate that they could advance. $\endgroup$ – Efialtes Feb 3 at 1:33
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    $\begingroup$ @Efialtes "have already begun to reach parity with li-ion at least in lab conditions" - sorry, I don't see that in the question. You can add that energy density assumption to the question. However, this edit may effectively invalidate Mike Serfas' answer. $\endgroup$ – Alexander Feb 3 at 1:51

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