A small battery charger should be sufficient to get you started unless there's a short in the 12v battery or somewhere in the 12v system. All the 12v battery has to do is run the computers and energize the HV power relays.

Once you get it running it's important to keep it in the "run" position for at least 30 minutes to make sure the 12v battery has a chance to charge.

Of course you probably know the FEH doesn't have an alternator...