Ah yes 18650s- magic things. I have done the same- salvaged cells from laptop batteries. Normally just one in a bank go US and the rest are OK.
Model airoplanes me too again, but not electric: sailplane and diesel. Haven't done aything for years but do watch the men catching the thermals with their motor assisted sail planes. We are by the coast with good breeze and plenty of hills which helps.
The best thing I ever saw was at an RAF base in Singapore: B17 bomber replica about four feet long, complete with four glow plug motors which were a job to balance. The aircraft wasn't a kit but built to scale from scratch. I miss the smell of dope and diesel fuel.
We also have a model car track in the area. They have a meet at least once a week in the summer.The speeds they get are amazing- must see what batteries they use, fast charge/discharge high current types I think.
I've heard about LiPro but never investigated further. Presumably they are high current types, may be like the model car boys use.
I've also played about with lead acid batteris too. Made a portable PSU for testing equipment in EMC chambers. Thats when I learn't about deep discharge lead acid batteries. First I just used a standard car (auto) battery from the garage down the road. I was very surprised when it didn't last very long! After all, if they can do 100s of amps to start a car they shoul be able to supply 10 amps or so, no trouble. At one time, before all this battery technology took off, lead acid was the only way for a decent amount of power.
When money was tight and welders expensive, I was going to make a welder with a stack of car batteries, say six, but a friend gave me a pro welder that was his Dad's so that project never took off. The welder I was given was a monster and although it had taps for different currents none were low enough for the sort of stuff I was making. Say you wanted to weld a chassis seam- there would be a big flash and and a big hole. The guy down the rad used to borrow it to make anchors for his boat out of old angle iron- he got on Ok with it. I plan to get a TIG welder with AC so you can weld ally.
On EOT there was a thread about charging batteries. Can't find it now, but one member suggested using a welder as the power source for charging a battery array- good idea I thought. You can get an 60A arc welder dirt cheap in England, low as £40 UK, if you shop around.
I had a 50A 4V transformer- scrap from some earth continuity test gear- and wanted to up the voltage for a battery charger. So I had this idea to make it resonant. I tried various capacitors across it. Nothing happened untill one capacitor hit the spot- scared me to death. There was this almighty hum, the transformer vibrated the whole bench and it started smelling right away. I instantly turned off and forgot about that idea.