Cordless drill battery packs die after about 18 months no matter what I do Repair

Two things in your description are killing them, and neither is the cold.

First, living on the charger. Most tool chargers stop charging but keep the pack sitting at full, and lithium cells age fastest when they are held at full charge and warm. Second, light use means those packs sit at 100 percent for weeks between jobs. Store them at roughly half charge instead, take them off the charger once the light goes green, and top them up the night before you need them.

The unheated garage is fine, cold storage is actually good for calendar ageing. What you must not do is charge a pack that is below freezing, so bring them inside for an hour first. Do that and you should see three to five years instead of eighteen months.

Before you bin the next one, open it and put a multimeter on the cells. A pack that dies after four screws is almost always one cell group that has dropped out of balance, not five bad ones. If you see four groups at 3.6 volts and one at 2.1, that is the whole fault.

Rebuilding with new cells is doable but you need a spot welder, or a friend with one. Soldering directly to cell tops cooks them and it is genuinely a fire risk, so do not be tempted. If you are not set up for it, at least you will know whether the pack was salvageable before you spend on a new one.

Honest answer from the trade side: packs are consumables and everyone budgets for them. Eighteen months is short, so the storage advice above is worth doing, but do not expect a decade. Buy the bare tools and put the money into good packs, and keep the dead ones for the recycling bin at the tool shop rather than the bin at home.