Cold temperatures slow the self discharge of batteries, so they can be fully charged and then disconnected,Fully charged batteries lead acid batteries will not freeze until about -90F. but 100% discharged batteries can freeze at about 20f.
Even if freezing does not crack the battery casing the plates are likely compromised and perhaps shorted if freezing does occur. It can be dangerous trying to charge batteries that have frozen over the winter.
Cold temperatures do not Kill batteries, cold temps reveal weak batteries as they lose capacity and cranking amps when cold, the same time the engine requires more power to for the starter to turn.
If there is power in the barn, one can run a battery maintainer, but these, if they should go bad or be disconnected can actually drain the battery to dead as a doornail, and then they can freeze at temps above zero, and be destroyed.
If you can fully charge all the batteries, and remove the ground terminals on both house and engine battery, they should still be above 80% charged in 4 to 5 months when you reconnect them.
Much depends on the actual average battery temperature and health of the batteries, as to how much they self discharge.
If you can go in monthly and top charge them, they would be happier than if they just sat and slowly self discharged.
Lots of philosophy can enter this equation. Some swear by battery maintainers by battery tender jr or battery minder, when the area has reliable power. Others have found that the breaker tripped, or the extension cord got unplugged, or the maintainer failed then completely drained and allowed freezing temperatures to destroy the battery.
I think safest is to fully charge them, and disconnect the ground wires over the winter, assuming a cold climate. If one can go in every so often and top charge them, all the better.
The goal should be to keep them above 80% charged when in storage, and they should not have lost much capacity when stored at full charge. As long as they are fully charged, the colder the better in terms of self discharge.
Older batteries will have more self discharge than new ones, and batteries watered with contaminated water will also self discharge more.
The batteryminder maintainer, if this is the route chosen, at least has the option of buying and adding a remote battery temperature sensor so it can keep the correct float/maintenance voltage. other maintainers have an ambient air temp sensor which is not as good as a battery mounted sensor.
https://www.batteryminders.com/1510-12-v...h-warranty
how much better the batteries will be after a winter of being on a temperature compensating battery maintenance charger, vs fully charged and disconnected is highly variable, depending on battery condition when disconnected and actual battery temperature and the length of time that passes.
Will paying 70$ for a maintainer instead of putting that 70$ towards new batteries sooner, be worth it?
Do note there are cheaper wall wart style 'maintainers' sold by harbor freight and others. These would be OK for short periods in mild winter, but their voltage is poorly regulated, and these, if they do fail, are known to then drain battery dead as a doornail
If you do go the maintainer route on house batteries, then the engine battery is out of the loop. You can run a single+ red wire to it to keep it at near the same voltage as the house batteries, or bridge the big contacts on the isolating solenoid. it really depends on how they are isolated now from engine battery with engine off.
without power in the barn and perhaps even with it, I would likely insure the batteries are top charged, then disconnect the ground cables from all the batteries.
But
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