Leaving batteries over winter is a complex and controversial subject with several options. Here are my thoughts.
For many the simplest solution is just leave their batteries disconnected from any loads for 6 months and they are pleased if they are still at 12.5v when they return. What they don’t realise is the damage to the Ah capacity this has caused, because all batteries need some charging over the winter to compensate for their self-discharge. Even if the main isolator switch is shut off some loads may still be directly connected to the battery, like a bilge pump
or the memory on a car radio/CD player. Even these very small loads will accelerate the self discharge, especially over a long layup
Self-discharge is dependent on battery temperature and the age of the battery, AGMs and Gels self discharge only 2% a month, but Flooded Lead Acid, may be as much as 10% a month. Cold winters mean less self discharge.
After 6 months the effect of any self discharge means parts
of the battery plates will have permanently sulfated because the lead sulfate crystals that are part of normal discharge will have hardened. A 100Ah battery will no longer deliver 100Ah!
, even small ones, can easily keep the batteries full charged, but if left connected for 6 months when all loads on the boat are switched off they can “overcharge” and “boil” batteries even with a quality solar controller. This happens because every night solar provides no power and switches off, when they sun is up the controller switches on and goes straight into bulk mode. Because the batteries are almost 100% full they very quickly reach absorption which takes the battery up to the gassing voltage of about 14.4v. A battery is normally happy to sit at this voltage whilst it is charging as the gassing is very slight which helps to stir up the electrolyte and remove sulfation. But even smart controllers are pretty dumb, most don't know the battery is really almost fully charged so when it automatically kicks into absorption it may stay there for the pre-programmed time period of up to 3 hours before it drops to float. The battery won’t be taking any current when it is full but it’s the voltage that does the damage, especially if applied for three hours everyday for 6 months to already fully charged batteries!
Others leave their shorepower connected and maybe a light left on and they return to find the cable removed and the batteries flat, or to find a battery has a bad cell which has boiled dry and set fire to the boat. Better to have someone reliable connect shorepower once a month for 24 hours and cheque the batteries are all right.
So none of the these options are ideal!
The key message is all chargers, especially shore power
, should be programmable if they are being left over winter. Set the solar absorption voltage lower than normal at about 14.0v and set the absorption time to about 20 minutes. When it comes on its below the gassing voltage but will give a battery a bit of a boost to stir up the electrolyte and avoid stratification. Below 14.4v there will be no gassing at normal temperatures.
Shorepower chargers like a Victron can also be set to limit the current to a couple of amps to stop the batteries boiling if there is a shorted cell.