It usually happens that one battery or cell will fail before the others in your array. Assuming you can buy an identical battery what happens to this battery when placed in an existing array of older batteries is that the new battery will perform to the level of the weakest or least performing battery in the string while being much more active chemically.
Mixing old and new batteries is never a good idea. Batteries age over time, when tied together in a single battery bank the batteries act as and are treated as a single battery resource. The limit of this resource is the oldest, weakest battery within the array. Batteries within a battery bank should all be identical. At minimum the batteries should be of the same construction and rating.
Where does the energy go? It is absorbed within the other batteries as resistance/heat.
What does this do to the new battery when charging? The new battery accepts charges faster then gets overcharged while the others catch up.
What happens to the new battery while discharging? It contributes no more to the battery array output but because it is more active chemically it will discharge deeper than the others.
Read about Depth of Discharge (DOD) http://www.STRATEnerGY.ca/cycling_batteries.htm
So in summary what is the bottom line? A new cell will extend the life of the battery array for a period of time at the expense of wearing itself out much faster than any others in the array. Yours is a cost/payback decision as this perfectly good battery will perform no better than the weakest link in your battery array at the expense of wearing this one out very quickly.
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