Most modern gel batteries will recharge from 5V or less with a suitable reconditioning charger. acid type should be replaced if they fall below 6V as the plates are prone to damage even if the bike is not being used... I would advise to check the voltage, if it is lower than 10V replace it and purchase a conditioning charger for the battery if you are going to leave it standing for long periods of time...The battery you remove you could try to charge and if successful keep as a spare for that one time you need to test something you're working on...
