Battery Question

Your detectors will eventually drain a battery. Smoke, CO, and propane. But 4 days seems pretty quick.

If it’s a new lead-acid battery, check the fluid level in EACH cell. The dealer may have missed filling one or didn’t fill it completely. If it’s a maintenance free battery, I would put it on a charger for a few hours to see if it will charge up and then test it again after a day (not connected) to be sure it’s holding the charge. If the voltage drops below 12 volts, I would return it to the dealer for a replacement.

IMHO, you don’t have to disconnect the battery between trips unless you have a habit of leaving the lights on, or the refrigerator on, or some other device like a charger for cell phone/computer battery. If you have enough of those devices turned on, they will discharge your battery. In that case – disconnect it.

It's possible it wasn't fully charged when installed but more likely you have something draining it. It's better to disconnect the negative side when not in use if you don't have someway of applying power on a constant basis to keep it charged.