Thermal runaway can be a problem so if you are using Lithium batteries just make sure it’s a quality product. There are lots of safe lithium batteries out there.
I removed the back up battery from my boat and now carry a small and very light NOCO product. It has saved me a few times when I drained my deep cycle battery working on engine issues. Fraction of the weight of carrying a lead acid cranking battery.
https://no.co/gb40Also a back-up phone charger for safety.
Now, MH370 had skids of densely packed lithium ion mobile phone batteries in the belly, this is likely what caused the disaster.