I want to add a 3rd battery to my existing 2 battery bank. I currently have 2, 200ah, 12 volt batteries that have been in my boat for a year during the refit. The boat has been consistently on shore power, the battery bank is complete and tested. Can I add a 3rd battery to the system OR when is it too late to add a 3rd battery to the system?
Most would probably say no, but if the 3rd battery is exactly the same model and capacity, I'll say that you can add it in parallel.
Just make sure that all are fully charged before you connect the 3rd one in parallel.
In case you are not using busbars, this is the perfect time to add them. Fuse each positive battery connection to the positive busbar. Make sure that all positive wires are the same length and all negative wire are also the same length. The length of the positive wires can be different than the length of the negative wires. The wire cross section of all positive and negative wires must be the same. I am talking here about the wires between each battery terminals and the positive, respectively negative busbars (including the fuses for the positive wires).
Hello @Gregbear…I assume you are considering another battery in parallel with the original two. It is always preferable to install the house bank so all batteries are of the same age with the same number of charge discharge cycles but this is to provide the most efficient solution. Batteries that have been installed for a year may not have the same characteristics as a brand new battery but what does that mean?
If you put a new battery in parallel on the charge cycle the new battery may take a little more current than the other two. During discharge the new battery may contribute a little more current than the other two. This difference in charge discharge current doesn’t make a huge difference to the experience of you, the user and may be of only academic interest. Over time the new battery may grow close to the characteristics of the original two and the batteries may eventually share the load evenly.
If these batteries are lithium they have very little degradation over time with shallow depth of discharge so I think it is fine to simply put the extra battery in.
thank you both for the responses. I will add another battery, same type and in parallel within the month.
