Whilst we install lithium batteries, we see a lot of lead acid battery installations too - usually when we're parked up somewhere and we hear complaints from a nearby chair around the camp fire. There are principles of battery and solar connection that apply to any RV system.
When installing two lead acid batteries, they should be connected in parallel -positive to positive and negative to negative. The positive goes on one battery, the negative goes on the next battery. The two batteries should be linked with heavy cable - zero gauge is ideal.
While this is not the optimum way to build a battery, this will at least give you good life out of your two lead acid batteries.
If you have lithium batteries, however, this will at least them sort of to work together. But because of the extremely low internal resistance of lithium batteries, they should not be installed as multiple batteries in parallel. At this point, I think I need to clarify the difference between a battery and a CELL.
If you have three or more lead acid batteries, the above system will not work.
To make multiple lead acid batteries work together in parallel, there must be a common positive point and a common negative point. The cable from each battery to this common point must be the same length, as close to exactly as possible, because ANY difference in resistance between one battery and another will mean that the battery with the lowest resistance will do all the work.
This setup will share both the charging and the load across all the batteries.
All 12V batteries are composed of cells, and that includes lead acid batteries. However, lead acid, and increasingly many lithium batteries, are made up of cells and then enclosed in a box.
At T1 Lithium, we don't enclose our cells because there are advantages to keeping them out of a box. They are easy to keep balanced when the cells are accessible and if a cell should fail, it's fairly easy to replace just that one cell instead of the whole battery. When the cells are enclosed, it's not possible to read what the actual cell voltages are, and it's not a case of what you don't know won't hurt you.
The information below applies in principle to both lithium and lead acid technology, but the voltage figures are specific to lithium.
A battery is 4 cells connected in series for a 12 volt battery, or 8 cells in series for a 24 volt battery. A cell is 3.2 volts.
You can connect the cells together in parallel, to build a large capacity 3.2 volt cell from multiple cells, then join these together in series to make the 12 volt or 24 volt battery, and now you actually have one high-capacity lithium battery.
If you have purchased multiple 12 volt lithium batteries, in an attempt to get them to work together, you will need to connect each battery to a common positive and common negative point.
The cable from each battery to this common point is critical. The cleanliness of the connection to each terminal is also critical, because ANY difference in resistance between one battery and another will mean that the battery with the lowest resistance will do all the work.
Your best teacher is your last mistake - Ralph Nader