Charging a leisure battery on my tipping trailer?

[Aprisa]

I'm building a 3 tonne tipping trailer and it will use a 12v power pack on-board to rune the hydraulics for tipping.

I've looked at kits on the web that are for split charging but am a bit confused as to what is needed to charge the
trailer battery whilst the trailer is connected to the car so that it is kept topped up in between uses but not over-charged?

I'm running 13 pin electrics so will have the feed but is it simply a case of fitting the relay between this and the battery?

Thanks