Oh by the way, if you're brave, and skilled, you can sometimes open laptop batteries and replace the 18650s yourself with cheap ones from eBay.

However, you really have to know what you're doing. Even with load-balancing, they all have to be charged to around the same voltage for safety, and opening the laptop battery in the first place usually means breaking it. Even if you get inside without hopelessly smashing its case, directly soldering to the metal cases of highly flammable lithium batteries is best left to people with the correct equipment and a safe place to do it.

I know what I'm doing - mostly. But I'd still never do this unless I had no choice and couldn't find a replacement. One day that time will come for my old netbook...