This could get expensive......
Li-ion cells, are prevented from charging when below a certain voltage level, because going below this level damages the battery further.
You need to keep the battery at about mid charge when not using the battery, checking with a simple digital meter, about once a month..
Mid charge is about when the "nominal" voltage is measured at the battery connector.
A possible fix is a bike shop which has a so called "Zer0 Volt Charger" for the battery voltage that you have.
I only know one company that sells them, and they sell only for 36Volt batteries, no other.....
The special charger costs around US$100 , but are only from Sweden.
Also, I don't know what the charger to battery connector is that you have, so a different might need to be soldered on, to suit! The ones from Sweden have 5.5mm barrel and either a 2.1mm or 2.55mm pin. The 2.1mm will not fit a 2.55mm Socket, but the other way round, MIGHT work. It will look something like this:-
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Assuming that the battery "accepts" a charge from the Zer0 Volt Charger eventually, then once the minimum voltage is exceeded, the special charger then simply charges like any other charger....
This is a well known problem of Li-ion batteries and I must have written about it here probably a dozen times.....I am also not alone.
If the battery "recovers", it will have lost some capacity that over the next 5-10 charges it might recover, but do not expect it to be quite as good as it was....
An experiment I did this year with a new battery of mine with Panasonic cells (one of the best), showed only a loss of 1 volt over 4-5 months.
This is far less than most lose over that time, so if your battery needs re-celling, make sure that you 100% get the Panasonic replacements!! Let them show you all the cells first! And tell them that you want to see inside before the battery is sealed up!!!
Apparently there are some companies that use low quality batteries, but charge the same as high quality.
I have never needed a re-celling, so I am the wrong person to ask, but many here have had that done and can give good, honest advice.....
You might mention here your battery voltage and the number of cells, all in a special number, usually printed on the battery somewhere, with the nominal voltage first!
I hope this helps.
Andy