Lithium batteries are the lightest and most energy dense batteries commercially available. They do not need to be recharged immediately after use - but you should recharge after every use so you will start with a full charge. Like a cell phone battery, they perform better and for longer if you keep them charged. They are much lighter, environmentally friendly, and safe. If they will not be used for a month or more, make sure they are fully charged before storage. They should be charged once a month during storage.
SLA stands for sealed lead acid and are the most economical. Our batteries use a AGM technology that means that there is nothing to spill and the batteries can be operated in any position. These batteries will hold a charge for weeks between charges. You should always recharge SLA batteries as soon as you can after riding. They will last much longer if they are always recharged immediately after use.
NIHM are Nickel-Metal-Hydride batteries. These batteries are lighter and will last longer than SLA types. They will lose charge much quicker than SLA batteries and so they need to be recharged more often. They cost about twice as much as SLA types. For a 36 volt system, there are typically 30 batteries strung together. The main advantages are the reduced weight and longer life.
In general, the less you run any of these batteries down before recharge, the longer they will last.