If you must (or want) to use solid core cable make sure that near to where you terminate it you put a couple of loops in the wire to give it some flexibility about half to one inch diameter should do.
Alternatively you could choose to terminate the solid core stuff and just use some flexible tails to complete? I know our engineers have done this where a main contractor has used SWA solid-core stuff as a generator output feed (to isolate the main cable run from the vibration as nbrown alluded to).
When I was moving my battery around in the van from under the sofa-bed to the rear storage I got lucky and a sparky mate bunged me a few metres of 5-core 4mm flexible (clear plastic sheath stuff with a silver reinforcing armouring lattice underneath, then a central insulation and 5-core individual mult-strand copper flexible cores - think its fire-retardant and all that as well. Its got a proper name in the sparky game but I cant recall what that is).
I used 3-cores for +ive (brown, black, grey) and the Blue and Green/Yellow wires for the Negatives. Terminated into 30A Choccy Block screwed to the wall, and then used one core for the battery charger input, and other for the heater circuit and also the DC Dis board. All seems to work alright!
I also blagged a redundant Computronic Controls 5A Intelligent panel charger out of a scrapped panel in work and installed that next to my Webasto - so on Hook-up outside the house the LB is getting maintained automatically. These are decent quality little chargers (not cheap to buy new either) and (as nbrown says) keeps the battery tip-top.
