I deal with 3 pairs of 6V(36V) in my ancient Golf Cart and I can assure you it is not a good idea to mix new and old batteries. 1) the new batteries will only charge up the capacity of the old batteries 2) instead of having to deal with replacing the batteries every 5-7 years you will likely have to deal with them every 2 years as the old batteries will need replacement, and the new will become the old. 3) It will play havoc with your battery charger as the old batteries will trigger a we are full signal before the new ones are full. As the old ones start failing the charging maladies will increase, they will have the charger cycling at mixed signals. There is one thing that will work for what you would like to do but will require your attention and work on your part. That is to create two separate battery banks, one old batteries and the other new batteries. You could make use of a 1,2,Both,Off Switch to isolate the two battery banks from each other. You could alternate using one bank for one outing and the other bank for the next outing or you could choose to faster age the new bank by using it more than the old bank. The benefits are you would have more capacity than what you have now, not exactly double as we do not know the capacity left on your old bank but you could deplete one bank and switch to the other when needed. Also having two separate banks gives you redundancy should one bank fail you still have the other as backup. 6v banks are great but if you loose one battery you have essentially lost both. The downside is you will not have full access to one whole battery bank at once. If you combine the banks by switching them to Both the current will spill over from the new bank to the old bank trying to charge it beyond its capacity until an equilibrium is reached at the old battery capacity. Over a period of time this will reduce the capacity and the useful life of the new batteries.
