Has anyone tried running 2 smaller lithium batteries in parallel?

[Richard Hudgins]

DMXF,

I am going to assume that the chemistry that you are looking at using is LI-FePo4 as this is the normal for large scale cells and readily available.

The problem is cell balancing as you were told. The individual cells are 3.2 volts at full charge. They can be cycled down to 2.8 volts without damage. What you run into is during charge using normal 12 volt chargers (This assumes that you have 4 cells in series) is that it will not balance the cells. As you are charging the pack in reality one cell may be 3.0, another 2.9, another 2.6, and 2.8. They will come up in equal volt amounts.

In other words each one will gain the same amount of charge IE: .3V This will give you a cell at 3.3, (Definite No-No) one at 3.2, one at 2.9, and 3.1 giving a pack voltage of 12.5 which the charger will assume is full charge. but only one cell is at full charge. With one too high and the rest low.

As you cycle the battery this imbalance just gets worse until you fail a cell either from over charge or over depletion.

That is why you need a BMS for LI batteries. The system will individually charge each cell to the correct voltage.

There are suppliers out there that make 12V packs for automotive use that have internal BMS that will work for your application. Look under racing Li-FePo4 12V sport batteries.