I got this cool little Dagu I-Racer from SparkFun a few months back (https://www.sparkfun.com/products/11162). I had a ton of fun playing with it, including using a Windows Phone app created by Jeff Albrecht to drive it (http://www.rodaw.com/dagu-i-racer-windows-phone-8-2/).
Anyhow, I put it away and it sat in the box for a couple of months. When I took it back out, it wouldn’t recharge. I contacted SparkFun Tech Support (firstname.lastname@example.org) and they told me to try and "jump start" it by applying 5v to the battery terminals (by passing the charging circuit). Once you have the lights blinking you can then plug in the regular charger and get it charging again. I’ve done this successfully a few times now (more than I’d like), but it works!