It is the industry-best-practice, standard advice of battery retailers to recommend changing all the batteries in a bank at the same time and to have all of the batteries that make up a bank of the same brand, type, capacity and age. Despite the up-front cost of replacing a large bank all at the same time, that advice is based on sound underlying technical considerations. All batteries have life spans. Batteries of the same type that make up a bank are likely to have similar life spans based on the physical and chemical make up of the batteries and the profile of the service the bank has been called upon to perform. The useful life span of any particular battery depends on a lot on factors: operating temperature, vibration, suitability of charging and discharging profiles, number of charge/discharge cycles, average depth-of-discharge and the extent to which they have been fully re-charged and equalized over their service lifetimes.
Individual cells in batteries all age at slightly different rates. Plate surfaces become etched at slightly different rates. Plates sulfate at slightly different rates. The Specific Gravity of electrolyte changes at slightly different rates. Specific gravity is the most reliable indicator of battery state-of-health, but the construction of AGM and Gel batteries makes it impossible to measure SpGr. In the “equivalent circuit” of a battery, age-related variation in the Specific Gravity means the “internal resistance” of individual battery cells increases; some cells increase at higher rates than others.
Batteries wired into banks are functional sub-components of a “power distribution network.” Individual batteries are affected by whatever happens in the rest of the network. Regardless of age or state-of-health, a miscreant neighbor in the bank will affect the entire bank, almost always negatively. Plate and electrolyte variations between individual cells in batteries in a parallel network leads to circulating currents that hasten the self-discharge process and lead to unbalances in both load-sharing and charging currents. Within that battery bank network, the internal resistance of the individual battery cells and variations in the external resistance of the terminals, wiring crimps and conductors that interconnect the batteries affect the way large charging and load currents will be distributed among and between the members of the bank. Circulating currents that do flow within the bank are distributed unevenly within a bank. Even a bank that is physically disconnected from outside circuits will have internal circulating currents where more highly charged cells will try to “equalize” with cells that are less charged. All this leads to cell deterioration over time. A battery is considered “failed” when any cell in its construction is failed.
The size of a bank also affects the above phenomena. In a bank of two batteries in parallel, there are few interconnections. In a bank of 6 or 8 paralleled batteries, there are many interconnections to affect the distribution of circulating currents. In very large banks, many more interconnections again. In series/parallel banks – two 6V batteries in series to create a 12V battery, or two 12V batteries in series to create a 24V battery – are extremely dependent on the length of the jumpers that make up the series-connection. Very slight differences affect the resistance of the series-connection, which in turn affects the distribution of load and charging currents.
When any one battery in a bank reaches end-of-life, by definition, that means the other batteries in that bank *are not* new. Internal sulfation has already occurred to some extent in all of the batteries in that bank. Some amount of lead plate deterioration has happened in all of the batteries in that bank. When one battery in a bank fails, there is no way to know the extent of internal, microscopic deterioration that has been accumulated and incurred on the not-yet-failed neighboring kinsman. There is also no way to know if the others have one day of remaining service life or one year of remaining service life. There is no way to know if the others will fail in a graceful, peaceful manner or go out in a blaze of glory!
If a new battery is placed in an bank network of older batteries, the resulting bank may “work” for some period of time; or perhaps, not. A new battery has lower internal resistance than aged, older batteries. Adding a new battery – even of the same brand, construction and technical specifications – will cause unbalanced currents flow in the bank. That new battery will carry a disproportionally large share of the total load. So, it may actually hasten the failure of another, older peer in the bank.
If a remaining, unknown state, older battery in the bank suffers an internal short, it may take other batteries with it, including a new one added only recently and wasting that investment. Catastrophic battery failure can create heat, smoke and worse. If that same remaining, unknown state, older battery just dies a quiet and uneventful death at anchor one night, then maybe you’ll be able to start the engine, and maybe you’ll be able to get the boat to safe harbor, and maybe a local chandlery will be able to provide a replacement battery, and maybe that replacement will be at a cost you’ll feel OK about. Or, maybe, none of the above.
The only time I’d consider replacing just a single battery in a bank would be if I had very recently replaced the entire bank and subsequently had an “early life” failure – not more than 6 months – with one of the “new” batteries. If that failure was peaceful, and not glorious, I’d replace just that one battery in just that one case. In that one case, there would also be a warranty claim to ease the financial pain and cruising inconvenience.
Of course, this entire choice also depends on how the boat is used and the level of reliability the owner feels is necessary. A boat that is always less than 50 miles from home, always in sight of land, and also carries towing insurance, has one set of reliability considerations. A boat that cruises to remote regions, where banjo music and wolves baying at the moon in the distance can be heard in the isolated, remote anchorage, has – perhaps – a different set of reliability considerations.
My personal choice: to maximize reliability and peace-of-mind, I bite the bullet and replace all the batteries in a bank at the same time.