Winter is approaching when many horses are going to be kept in stables for the winter. As a result, the electric fencing is not going to be needed for the next few months. Over winter maintenance of energisers that will not be in use needs to be considered.

For both mains or 12 volt battery energisers these may simply be disconnected and safely stored. The battery must also be safely stored and charge maintained as explained in this link of how cold weather may affect a battery..

Electrical batteries are simply storage vessels that are capable of holding an electrical charge and then discharging it into a load. Some batteries are capable of producing an electrical current from their base components as soon as they are assembled. These batteries are called primary batteries, and they are typically disposed of once the charge has been depleted.

12 re-chargeable batteries fit into a different category of electrical battery that can be charged, discharged, and recharged again and again. These secondary batteries utilize a reversible chemical reaction that differs from one type of rechargeable battery to another. These batteries need to be routinely charged to maintain their chemical balance. 

Batteries become un-serviceable when they are left unused for extended periods of time.

If a battery is left in a partially discharged state for an extended period, sulfation occurs which can dramatically reduce the life of the battery and is difficult to reverse. Sulfation is a buildup of lead sulfate crystals and is the number one cause of early battery failure in lead-acid batteries. Sulfation occurs when a battery is deprived of a full charge, it builds up and remains on battery plates. When too much sulfation occurs, it can impede the chemical to electrical conversion. Newer lithium-ion (LiOn), nickel-metal hydride (NiMH) and nickel-cadmium (NiCd) are not affected by Sulfation.

For a solar powered battery units - Remove the fence and earth leads from the energiser and leave it in the sun. The battery will be topped up as and when required. A battery will drain over time whether in use or not and it is not best practice to allow one to drain completely. Batteries don't like to be fully discharged, so it's best to charge before they get to zero. If drained completely, there is the chance it will stop accepting charge again, but in most cases, full discharges will just shorten the life of the battery if done on a regular basis.

If you have a position where the light source will supply sufficient energy (not very much is required) then that will achieve the same objective.