General Battery Chargers
Battery chargers have become more and more sophisticated and capable recently
Battery chargers can be of the early and cheap variety, simple bridge rectifier type, 4, 6 or 8 Amp with basic voltage smoothing but poor current control. These can do the job if monitored well and the battery is in good condition.
Modern (and more expensive) models are now capable of charging 'difficult' batteries without damage while giving a degree of condition feedback. Some will not only continue to safely trickle charge a battery but even have a plate cleaning mode.
Price therefore varies a lot - generally you get what you pay for.
£50 should buy you a good 6~8A intelligent charger
£150~£250 buys you a very heavy duty 20A intelligent charger
Occasionally, outlets like Lidls, Aldi or others willspecial offer a small 4~5A intelligent charger, for as little as £12~£15
Pictures of chargers
Decades old but still working a very basic 4A charger. This sort of current source could also be used for plating metal or de-rusting metal Bodywork and Glass - Rust - Elecrolytic de-rusting (whereas a modern multi-stage charger wouldn't work in those situations)
Not the latest model, but a good make - Sparkrite. Pretty functional and safe, this Sparkrite charges in stages and gives reasonable feedback as to whether the battery can accept a complete charge.
Charging batteries - Facts
Basic voltages to look for (a 12V battery will usually show 12.6V when fully charged)
A very well respected range of sophisticated chargers is produced by quality Swedish manufacturer CTEK. Typically one to suit a high capacity truck battery would be about £50 ~ £100