Best way to keep chassis battery charged over winter?

I disconnect the engine battery. I also turn off the liability insurance for the winter so Incant drive it once a month. So I trot out the charger every couple of months and top up the battery. Temps go down to -40 here in northern Alberta, so I don't risk just leaving the battery all winter. I take the lawn mower battery into the house for the winter, no recharging, and they are almost fully charged in May when I need it.