[Mr V]

Yes it should work fine. The DC-DC converter should monitor the charge current and voltage and no overcharge the battery. It is not like the FRC battery is that much smaller than the tiny battery that the Insight is equipped with.

The bigger issue I see it connecting it to the car which is equipped to connect to a battery with automotive post connections. For the ground you could probably get away with leaving the factory cable dangling and make a new one but for the positive depending on where it goes it may be difficult to substitute something else. You really don't want to cut the end off of a $50 battery cable to save $30~$40 on a replacement battery. Then add in the cost of the new ground cable and you are likely creating a false economy and it would be cheaper in the long run to replace it with the proper Group 51 battery.