I think that running two SLA batteries in series to get 24V and using a
switching charger to keep them topped up would solve the problem. The high
load is not continuous as I understand from the op. The charger would run
continuously from the 14V source. 150W converters are not so easy to build
in one-offs. If you haven't done this before you will need to read some
books to be able to do it right. Lacking all the special measures (like
anti-saturation and inrush control and such), unless the converter parts
are hugely overdimensioned there will be subtle troubles that will plague
it.

Peter

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