Portable charger?
 

My battery is buried and hard to change. Has anyone come up with a way to top them up between courses? Something small enough to just carry around with me.

I have and use a dynamite passport lipo 6s ac/dc charger. It works at home on 120 volts and on the rocks on 12 volt. It has been a great charger for the price and nearly 2 years old and still works like new. It also charges life batteries as well.

I use a small 12volt ~7ah gel cell battery. You can find them for ~$25 at Radio Shack, some LHS's (used for starting boxes for fuel RC), industrial supply (like Graingers) for use in exit lights.
Just charge it up @ home and use to power your DC input RC charger.

Yeah I stole the Batrey and charger from my sons old battery powered four wheeler works great

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I wonder if you could take one of those Turnigy or E-Flite mini-chargers and power it with a 3S lipo. Probably wouldn't last long, but doesn't have to. Just a quick partial top-up between runs.

I'm after something small and light enought to carry into the rocks.

Yes, but it's not a lot of capacity. A 7ah battery like I mentioned is the same as a 7000mah battery. I also don't mind using them as they are robust and don't tend to burn or blow up if dropped.
I use it to power my Venom ProCharger.

I use a small automotive booster box, but depending how many MAH the packs your charging are, you can use 3 and 4 cell car lipo packs, my buddy uses his 5000mah 4s 1/8 buggy packs to power his charger when he's out flying.

I've ran my charger on a 3s 5000 mah pack to charge at comps.