Engine Battery Trickle Charger

I have a Peugot Boxer 1.9 and have sometimes had difficulty starting it when unused for a while.

I have used a trickle charger - but this year, when I went to use the van, the battery was dead.

I had it checked and am told the battery is fine and just needs to be run. Also, that the trickle charger I was using was of little use, if any. Can anyone recommend a suitable trickle charger?

Thank you