The most fool proof energy storage would be an ultra capacitor battery with balancers.
It's the BMS that makes lithium packs less reliable. Perfectly good sets of cells get ruined by a malfunctioning BMS. I wouldn't want to test that with ultra-capacitors...
"LEAF" is actually an acronym made up by Nissan, so the plural is "Leafs." As for voltmeters, the problem with installing one is that they are all made in China. Some are accurate, others wildly not. If you get one that tests good, it could at least provide a benchmark.
2013 "Brilliant Silver" SV with Premium and no QC, a 2009 Vectrix VX-1 W/18 Leaf modules, and 3 EZIP E-bicycles.
PLEASE don't PM me with Leaf questions. Just post in the topic that seems most appropriate.