Battery reconditioning is most popular on lead acid batteries. A good deep cycle lead acid battery can cost upwards of 0 and the parts to recondition it can be had for less than . Quite simply that means that by reconditioning one lead acid battery you'll be saving money. Some savvy people have turned it into a side business and earn money by battery reconditioning.
In order to understand why lead acids need to be reconditioned we need to take a look at what happens during a charge/discharge cycle. As the battery is charged and discharged little sulfate crystals build up on the lead plates.
Reconditioned Tool
Over time these little crystals build up and start interfering with how much power the battery can hold. Over time the charge level will go down and the battery will appear 'dead'. It's not really dead though it's just got a buildup of these sulfate crystals.
Battery reconditioners for lead acids are called desulfators or nanopulsers. Basically what they do is send little pulses into your battery which eventually break up all those little crystals and make your battery work again.
Reconditioning a lead acid battery is a little time consuming as it can take up to 2 weeks to fully desulfate it, but since it only costs (and only in power for each one after) it's well worth the cost.
There are several places online where you can learn how to recondition batteries. A quick internet search will turn up several guides. I highly recommend you use a guide because there are some tricky parts that you need to follow exactly or the process may not work.
I highly recommend reconditioning your batteries. It just doesn't make sense not to. Not only are you saving money, but you're also helping the environment by keep that battery from a landfill a little bit longer.