Found this on the net.

Jeff



"The idea of a trickle charger is that it can be left charging the battery continuously without fear of damage to the battery. To do this, the rate must be no more than 1/100th the power rating of the cell, and at least as high as the internal discharge rate of the cell. Most NiCd batteries will self discharge in about 3 months, or about 90 days. This means that the self discharge rate is about one percent per day, so we see that the "rule-of-thumb" 1/100th trickle charge rate was designed to offset the self discharge rate of NiCd batteries. This rule is used for all rechargeable batteries that I am familiar with. (I am not yet familiar with the newer rechargeable Alkaline batteries.) So, we need to design a recharger that can supply no more than 1/100th the current rating of the battery we wish to trickle charge. An important point is that neither liquid or gel cell lead acid batteries have this high a self discharge rate, and in fact some well designed NiCds don't either."