I had my deep cycle batteries sit all winter prior to buying the boat (and who knows how long before). I had them charged with a less than satisfactory charger last season (before switching to smart charge) and this winter they were on a single small solar battery (which probably did not make enough difference).
In may I charged them one more time with old charger, and it appeared that they are nearly dead - low voltage, not enough power. Then I did two things:
1) added sulphur removing liquid
2) used smart charger to charge them properly (it took a day from where the old charger thought it was done).
The result - batteries are holding 70-80% charge (relative to what they should) and work great.
The lesson here is - sometimes "dead" battery isn't.