Earth X

Sean, you can definitely use a jump pack to power your refueling pump.  Basically, it's a small 12 volt battery, that just happens to be able to supply a lot of Amps for its size.  Anything you could do with a 12-volt battery can be done with a jump pack.  Electrical devices don't know how the electricity is created (lead-acid, LiFePo, etc.), they only see volts and watts (thus Amps) of electrons flowing through the wires.  That said, some battery technologies are more "capable" than others, with the Lithium batteries able to store and deliver a LOT move wattage than our old-tech lead-acid batteries, even in a much smaller package.

I have an early 12V Earth-X (LiFePo battery) jump pack purchased several years ago (Christmas present for my wife and myself), and have used it to jump-start my HD pickup truck's diesel engine (which has TWO very large lead-acid batteries), my diesel-engined John Deere tractor (a single very large lead-acid battery), a gasoline powered zero-turn mower and motorcycle (small lead-acid batteries), and more than a few cars.  In fact, I just used it last night (twice) to start my daughter's SUV whose battery will no longer take a charge.  And after using it twice, the jump pack's "charge state" indicator lights still shows 5 out of 5 lights for charge state - it's still well over 80% charged!  

I've also used it to recharge my iPhone and iPad (along with several friends' devices) while traveling.  I know some folks up in Alaska and Idaho who have "no electrical system Cubs" (with electric starters, but no generator or alternator so they meet the "no electrical system" exception for ADS-B and Transponder requirements).  They use an Earth-X jump-pack as their SOLE (one-and-only) source of electrical power to start their airplanes, run any on-board avionics and radios (admittedly sparse in those simple Cubs), as well as to charge their devices, power their fuel trasfer pumps, etc.  And they do this while flying in the back-country for a week or more at a time, in winter, where if it didn't perform, they might be stranded in the cold for a long time.  I think they're a bit nuts, but they've been doing it for years and their Earth-X jump pac has never stranded them yet...

As for "maintenance charging" of the Jump Pack...  Probably THE reason LiFePo jump packs are so popular is that you can charge them up, and throw them in your trunk to be ignored for months at a time, and when you need them, they still have the "juice" to start your car.  Aside from being MUCH lighter in weight, and often smaller in size, one of the KEY advantages of LiFePo batteries over lead-acid types is the ability of the LiFePo batteries to hold a charge for a long time while not being used.  Their self-discharge rate is almost non-existent...  If you have no parasitic loads on your airplane's electrical system (such as a 12-volt powered clock that is always running), it would take many months for a LiFePo battery to self-discharge to the point it would not start your airplane, unlike the old lead-acid types that could go completely dead in a couple of months...  

And also they're a lot safer because they don't leak explosive hydrogen gas into the engine compartment!

Li-ion batteries including LFP have only one downside in airplanes which is that the chemistry is more fragile and so to protect the battery the BMS will disconnect the cells to protect them from damage. That perfectly well protects the cells so their fragility is not a problem but it means that a failing generator causing voltage spikes or overvoltage will cause the BMS to isolate the battery and then the overvoltage can go straight to your avionics. A lead acid battery doesn't have a computer to isolate it so if there's a problem with the generator then the battery will absorb the abuse which will damage the battery but save the avionics.

But all the benefits of LFP outweigh the very minor downsides.

Sean,

 I see no reason for a "maintenance" charge. Lithium and LFP have a very low self discharge. I have LFP cells I used in a project and left lying around for three years and when checked were still at 96% of full charge. Perhaps scientifically not an ideal thing to do but no problems.

I have never done any external charging on my Earth X.

As James says they are very reliable and robust. Just use them and abuse them and they will be fine.

I even put 4 cells together (Headway 38120's) and made a "12v" lead acid replacement myself, no BMS no nothing just 4 cells in series. It worked well for years and was still working well when the vehicle was sold..never charged it never did anything.

The local "HAMS" guys do this for remote radio work and again simple and works well.

It seems that I can use any suitable "jump pack", to boost my Earth X battery/ start my engine. I can also use it to power my re fueling pump.

Glen doubts the need to top up my Earth X between opps which is good news indeed.

My aircraft is about to start ground runs etc prior to first flight - many starts & short runs, which may result in the Earth X battery getting a bit low. I have a very fancy lithium dedicated, mains supply, battery charger (goes through many phases & even performs a diagnostic check) which I will not be able to use, unless I go to the airfield with a generator. The Earth X is in a rather inaccessible location to easy removal to charge at home not an option. Can I charge the Earth X by leaving the jump pack connected to it for an extended time ?

If you have a 12 volt outlet, you could plug your jump pack into it, such that it connects the jump pack battery and the airframe battery in parallel. Over time the jump pack should charge the airframe battery until the voltages in both batteries are equal. You would have to experiment to see how long that would take. The lower the voltage of the airframe battery (compared to the jump pack battery) the more current would flow. The current would then taper until the voltages are equal. Everything should use appropriately sized wire, appropriately rated plugs and receptacles, and be properly fused. If the voltage of the airframe battery is higher than the jump pack battery, then the process would be reversed.