9.372 bazillion rural folks in the U.S. use/used 100 watt incandescent light bulbs in their well houses/covers (the high-tech rednecks put them on a thermostatic switch) to keep their wells from freezing. When it was announced these supposed energy wasters would no longer be made, there were people buying them by the case!
You could use a lower wattage bulb, just maybe more of them. Then you would have a backup or three. Whichever wattage you choose, you need to do some testing to see what temp you can maintain inside your battery box with which wattage bulbs. A 100 watt incandescent bulb produces about 300 BTU/hour. This is the least efficient solution, since you're heating air to in-turn heat the battery, but it is pretty simple.
A battery thermal wrap (most I've looked at are thermostatically controlled) runs at 50-100 watts, and may need to come off in warmer times of the year to prevent the battery from overheating. A little googling suggest these are the priciest solution.
A battery heating pad (cheaper ones require a thermostatic switch) sit under the battery and draw about the same 50-100 watts though there is no need to remove them from underneath during warmer times. Again, a little googling reveals there are less expensive, and there may be a manufacturer or two who can make a custom sized pad for you. Or you may look for RV tank heating pads, and maybe find one in a size to fit your battery system's footprint. Seems like the RV pads usually have a thermostat built in and can run on 12 volts (which doesn't help too much with your 48 volt system).
As to your question of why a pad or a wrap versus a simple lightbulb, I will guess people don't want a "redneck" solution, or worry the lightbulb as a heater may not have UL approval (our safety Nassis here in the U.S.A. for all you nice furren members), or don't want a bunch of room taken up by lightbulbs.
You might look at this person's post for more info on heating pads/wraps.
https://diysolarforum.com/threads/i...ent-bulb-for-heat-good-idea.14845/post-166025
If you're looking for approval, then you have my permission to do whatever you like (within the limits of the law).?