Li ion jump start pack as source for charger for field use
 

I had a " heavy" Marine battery that I used as a power source for my field charger. The battery died.
Instead of replacing with another heavy marine battery , I was thinking of replacing it with one of those Li Ion jump start battery packs. They are much lighter also cheaper then a heavy marine PB acid battery
I will only be using it to recharge 3S 22000mah or smaller packs . I think something with capacity of 40000 mah should be sufficient for several charges. Are there any negative to this approach?