Solar charging your DS

[dark sponge] decided to make his DS lite, solar powered. Or, at least charged via solar panels. He was able to find solar cells that were 60×60 mm, which means he could fit 4 of them on the outside of the DS. This gives him a total of 6V at 80 mA output. These panels charge the battery between uses. The DS has to be open and laying on its face for all 4 panels to be exposed, but this way of mounting them didn’t alter the pocket-ability of the unit. He says he’s been using it for a while and hasn’t had to plug it in yet, but we have concerns about wiring the panels directly to the battery. As [cyrozap] points out in the comments on the instructable, this is a Lithium Ion battery, shouldn’t there be some charging circuitry involved?