I am Looking for a good field charging lipo of large capacity (e.g. >8000mah). Any recommendations? Cheers

Definitely get a 6S The higher voltage is more effecient in storing energy and charging your batteries.

16000mah 6S from HK is the best imo, charge over 20 packs of 1500mAh 4S without a problem and you are covered once the trend move to 6S as the charger doesnt have to boost up the voltage. Usually I parallel charge 11pcs at a time at 20A current.

You should get higher voltage than your target battery. So if you are charging 4S, get at least 5S for best efficiency. When on your source battery voltage drops, charger needs to raise it and you are losing some of the power.

I seen a few threads in other forums where they use gel batteries for field charging. Just wanted to mention it as they would be more safe and probably last longer than lipos...

The forums i remember are german but i could dig up some example seller links maybe if you're interested.

So far i'm using larger lipos too...

I'm using a 16Ah rated 12V AGM battery (sometimes confused with Gel cells). At 6A discharge rate (which is what I use when parallel charging 4 lipos), I can easily charge 8 to 12 1300mAh lipos at the field on a single charge.

The nice thing about AGM vs Gel is that AGM doesn't require a special charge profile (like Gel). The PB setting on my charger works just fine.

This is the one I use: https://www.amazon.com/Odyssey-PC680-Bat...B0002ILK6I
The size and weight are just about right for lugging around in a small tool box. (Another thing I like about it is that I can use it to more quickly parallel discharge my lipos into if I don't use them up.)

Note: This is a lead battery so it does weigh about 4 times a similar capacity lipo. However, at the cost and longevity, I'm OK lugging around a few extra pounds.