Numbers that I have seen for auto batteries are a self-discharge at between 1 and 5 mA. That is a VERY general number. The warmer the storage, the faster the chemical reaction so the faster the discharge. Storing batteries in cooler places is the best for long term storage. For the sake of discussion, I will use the higher number; 5mA. A typical auto battery might have between 45 and 75 AH storage capacity. There are 24 * 30 hrs in a month. So, worse case, a battery will self-discharge 3.6AH per month with nothing connected to it. In 6 months time that would be 21AH or ~1/2 the capacity of the smaller battery. That seems to be a fair number if the higher rate is used and we store the battery at room temp. The cooler the storage temperature the less the discharge. For winter storage, the self discharge rate is much less than the 5mA used for the above example.
All other things being equal, the higher the capacity of a battery, the faster the self-discharge rate. To look at it another way, one could use a figure of ~5% self discharge per month. So in 6 months, the battery has discharged to 70% (100% -(6 months x 5%)) of its capacity
if it was fully charged to start with.
As far as not disconnecting the batteries for long term storage, by all means, disconnect them. You will disconnect any system loads however small they may be. Even a 5mA load (1/2 the current for a typical LED) could double the total discharge. Many diode packs in modern alternators will have reverse leakage at 1-2mA. Add to that the small current draw of its internal electronic regulator. If a battery was originally sized to a specific engine with no fudge factor ( a technical term
) then I can see how long term storage could discharge enough to prevent starting form the slightly discharged battery. Add to that, some engines need to turn the engine over to pump fuel (no electric fuel pump) and you will loose a lot of the remaining battery capacity before fuel is in the carb. Electric fuel pumps are great for starting and engine after long periods of storage. Old tractors had a lever to manually activate the mechanical fuel pump before trying to start after long periods of non-use. 1950's OMC outboards had squeeze pumps in the fuel line for the same purpose.
If a battery is allowed to
fully discharge it is difficult to impossible to recover. Plus the electrolyte becomes water when fully discharged and can freeze if stored in an unheated area during winter months. A battery will not freeze if it is mostly charged.
Sorry for the long dissertation.
_________________
Chuck.
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