Mark_K5LXP
Well-known member
Generally true but batteries of different capacity will have different internal impedance, electrolyte diffusion rates and can have a (slightly) different SOC at a given point of a charge or discharge cylce at the same exact terminal voltage of others in the bank. The greater the currents during charge and discharge the further the divergence will be, and over time and cycles the effects of this will result in some batteries being overcharged and some undercharged. During bulk charge even a miniscule difference in internal impedance (and any cable/terminal mismatch) will result in very different currents going through individual batteries and hence their Ah restoration will be different at the end of bulk phase. During subsequent absorb phase the terminal voltage can be locked at 14.4V but there can be a range of SOC a number of percentage points different across the bank. If you stop absorb phase when the first battery hits 100%, the others will be undercharged. Conversely if you leave it in absorb phase until the last battery hits 100%, others will be overcharged. When a bank is new and with identical batteries these differences are small, under/overcharging is minor, and things go along pretty well as the set will degrade over time and cycles pretty much uniformly. Start out with batteries that are not matched and you're out of balance right out of the chute, and service life as a set will be reduced. This is true even of like batteries but of different age. Generally introducing a new battery will result in the new battery being "beaten down" by the older ones, and generally too the weaker ones will also suffer an earlier demise from the disparity in charge profile and discharge depth. Which is why the prevailing advice to replace batteries in sets hasn't changed in decades, unless you literally have a BMS on each individual cell the mismatch will ultimately do them in. It's of course a matter of degree, how quickly will this happen under a given set of operational parameters? No way to really guess or predict due to the number of variables but in most installations cost is a big factor, and most will want to create optimum conditions to net the greatest Ah throughput per dollar they can. If that's not a consideration then hook 'em up, run 'em and see how far you get. But I guarantee you won't make any new discoveries, mismatched sets degrade more quickly.The battery with the higher state of charge will have a higher voltage
Mark B.
Albuquerque, NM