Re:Lithium jump packs

Living in a large city is becoming a motoring nightmare, luckily our 2006 Jimny Petrol Auto is ULEZ compliant at present, however for how much longer ? This is why we have been investigating converting both our Land Rover and Jimny to electric traction. As you have rightly said, the cost of new battery packs can write an electric vehicle off, but at least it provides a source of batteries, motors and controllers. The Toyota Prius batteries were an option, but being old technology of Ni-Cad and then NM-H were not practical. The Tesla Panasonic Lithium Ion batteries were a step up, the models S&X contained 7,104 of 18650 (18mmx 65mm) cells (85Kwh), and the later 2015 Panasonic Li-Ion batteries held 8,256 of 18650 cells (100Kwh). The next battery was the 21700 (21mm dia x 70mm long) and now Tesla are joint developing a new Li-Co-Ac 46800 cell, which packs even more power, however these will be extremely expensive and so not practical. Although not an easy task, it is possible to replace failed individual  18650 cells, so finding scrap batteries is practical, but at what cost ?

If one or two cells have failed the rest may well be on the way out, so it might not be worth it.

Plus you might need 'matched' cells, these battery packs are very high performance, have huge amounts of stored energy and can are designed for rapid charging at high voltage and current, they often require 'matched' cells to work correctly.

You would really need to know what you were doing if you went down this route, I wouldn't risk a 18650 battery you got off eBay for a vape if I was you.

I did my training with a company providing standby battery power systems, some of the pictures we saw and the kit we got back when it went wrong, cabinets blown apart, batteries melted, bits of circuit embedded in the walls, that sort of stuff. Not what I want sitting below my seat.

Robin

I've been using this one for a few months now. I run lots of old machines and so far I haven't found one it won't start. Cars, lawnmowers, motorbikes, forklifts, even a 6.6l tractor.
Portable, easy to use and get's plenty of starts from a charge. It's a game changer!
www.mobilesolarchargers.co.uk/shop/details/overland-plus

I've repaired battery packs on a smaller scale (model aircraft), as Robin said keeping the batteries balanced is the key. Any weak cell will have a hard time and eventually "fail" in that it goes flat before the others. It then attempts to charge in a reverse polarity as the others discharge, at this point the pack capacity is shot. Charging in series is the main issue as you should in theory charge each cell individually to the same voltage, ie balanced.
Various BMS circuits (Battery Monitoring System) attempt to do this by looking at each cell voltage, but eventually they can't maintain the balance.
When I have repaired a pack I've replaced the failed cell then individually charged / discharged each cell to get them all balanced before putting back into operation. Ok for a few cells, but try and do that on a Tesla pack would almost be impossible.
The other issue is how they are assembled. Your are going to need a battery tag welder to "spot weld" the cells back together, soldering may not be an option to maintain the pack footprint.
If you must it's better to take two old packs of the same manufacturer and make one good one out of the two. That way your cells will be better matched. Buying any cell from eBay is unlikely to work for very long unless you can find exactly the same cell (internal resistance and things of this nature).
Not saying it can't be done, but it's going to take a long time to complete and investment in the correct equipment.