What could cause the battery in a smoke detector to heat up

In our area, local codes made at least one CO detectors mandatory in all condos and rental units. I have had to replace dozens of standard smoke detectors with combo units in hard wired/battery back-up systems. Most every manufacturer makes a CO/Smoke unit that will plug in directly to the existing system. A hard wired replacement is far better than a stand alone battery operated unit, as it will alarm all units in the system if there is a problem.

Having the system monitored or not monitored makes absolutely no difference on how the system works inside the house. All monitored systems will work fine in the house even if the monitoring system is off.

I take it the detector is mounted in the ceiling, and a cut-in 3-O or 4-O (plastic) boxes have a 2 hour fire rating. If this is not enough then you can use metal ceiling octagon box (2 hour fire rating) and use something like a 3M Fire Barrier Moldable Putty Pads. Most companies that deal with fire related caulks and putty make these. A lot of states require these in fire walls and ceilings, especially in multi-family units.

These pads are intumescent, which means that it swells as a result of heat exposure, thus increasing in volume, and decreasing in density. Wikipedia.

If I were you I would take it a step further and install a 120V with a battery back-up smoke/carbon monoxide combo units. You can also pay more for the detector and get either optical or ionization styles. Ionization detectors are more sensitive to flaming fires than optical detectors, while optical detectors are more sensitive to smouldering fires. Wikipedia again. Electrical supply stores should have all this or Amazon. I suggested 3M only because that is what I am used to.

Also, since the electrical boxes are fire rated, you might just use a fire caulk around the box. Caulks are intumescent also.