When replacing batteries/cells in a golf cart, flashlight, backup power supply, or other device that uses series-connected storage elements, it’s highly recommended to:
A) Replace all the cells/batteries at the same time—do not mix new and used (or old unused)
B) Use identical replacements—do not mix brands/models/sizes
C) For rechargeable storage systems, ensure that each cell/battery is fully and individually charged prior to installation.
The reason for this has to do with a concept known as cell balancing. In the common case of a storage system made up of series-connected batteries or cells, unless (expensive and inconvenient) special provisions are made there’s only one path for current to flow, through each cell in turn. It’s a lot like a train: if the wheels fall off one train car, the entire train is affected.
In this sort of arrangement, maximum performance and longevity are achieved when all the storage elements are matched in both their capacity and state of charge. Age and use cause loss of capacity, so mixing new with used or unused old cells results in a capacity mismatch, hence (A) above. Similarly, using identical replacements minimizes initial capacity variations due to manufacturing differences, as well as differing rates of wear that cause capacity differences to develop over time, leading to suggestion (B). Finally, since there’s no convenient way to fix a state-of-charge imbalance in series-connected cells, it’s best to fix that before they’re connected… See (C).
So what happens if one doesn’t do these things? First, the useful capacity of the entire system is limited by that of the lowest-capacity cell in the case of a capacity imbalance, and reduced by the amount of charge difference between the strongest and weakest cells in the case of a state-of-charge difference.
Second, there’s a risk of cell damage occurring that becomes more probable and more consequential as the number of cells in the system and/or initial capacity or state of charge imbalance is increased. As the weakest cell in a series-connected system is depleted, the output voltage of the assembly begins to fall sharply. If its contribution to the output of the whole is significant (such as in a 2-cell whatever) it’s possible for an under-voltage detection mechanism to sense this, and shut things off (after a disappointingly short period of operation) before things get too out of hand. The image below illustrates the concept, assuming typical alkaline batteries and a 0.9V/cell cutoff point for an imaginary under-voltage detection circuit.
On the other hand, if the weak cell’s contribution to the whole is relatively small, its individual (precipitous) drop in output voltage often won’t cause a sufficient reduction in the output of the whole to cause the application to shut down. In this case, the remaining healthy cells can continue to power the application past the point where the weak cell is completely discharged, and the voltage across the weak cell actually changes sign. If one is fortunate, all that will happen is that the weak cell will be irreparably damaged and the assembly will become useless, with no further drama. The unfortunate may find this situation to result in the release of caustic, corrosive, toxic, and/or flammable materials, cell rupture due to internal pressure buildup, spewing of flame, and perhaps a combination of these and other inconveniences… The image below illustrates this, again assuming alkaline batteries and a 0.9v/cell cutoff point.
The foregoing applies to both single-use and rechargeable storage systems. Rechargeable systems however, give you the chance to damage your brand-new batteries before you ever really even use them. If you simply install them in series and plug in a charger for the whole stack, a state-of-charge imbalance can result in over-charge damage to the strongest element in the string; it’s the same state-of-charge and one-among-many sort of problems described above, just operating in the other direction. It’s an easy problem to have in the context of something like an electric golf cart—in which case that new battery that just got over-charged probably wasn’t cheap…