Sometime back I ordered a Clansman Hand Generator (please perform internet search on these as I do not want to bias you on whom to order it from). It generates 24 Volt DC (open voltage) at moderate speed cranking and a maximum of 30V (open voltage) when cranking at the maximal speed that I can still manage relatively comfortably. Please see YouTube videos for further details on this crank generator.
So how could you use these 24V to charge a portable battery? Just connect two small 12V batteries in series... and you have a heavy duty (yes - that does also imply fairly heavy weight).