I have two old laptop batteries that still work and they have lithium ion battery cells within their casing. I would like to re-use these battery cells for my remote controlled car. Both of the laptop battery packs say that they are 10.8v. If I wanted to increase the amp-hour capacity, could I put both battery banks in parallel? Is it safe to do so?
It is very risky using old laptop battery packs, because if one cell of the 3 cells in either pack fails it will not allow proper charging and if the packs are in parallel, that would damage both packs.
In any case you need protection circuitry on each cell to prevent overcharging or over discharging the cells, or they will be destroyed in the first usage.
Some people wire LiPo cells in parallel, if they are new, matched in capacity and at the same state of charge, but there is always a risk.