How to Store Batteries Long Term (or, should/could I make a larger battery bank)?

[Mark_K5LXP]

The battery with the higher state of charge will have a higher voltage

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.

Mark B.
Albuquerque, NM

 

[Gary RV_Wizard]

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

I think this is the important point in this particular discussion, and [in my opinion] that degree is tiny. The batteries are the same design from the same manufacturer and are the same age and (presumably) same new condition. The difference is that one set of 4 has a little more lead in each cell, yielding a mere 7% more amp-hours of capacity. That's almost trivial.

Probably the greater risk is a poor quality connection somewhere among those 8 total batteries. And finding room for an extra 4 GC2's without lengthy cabling may be a challenge too.

 

[Mark_K5LXP]

The difference is that one set of 4 has a little more lead in each cell, yielding a mere 7% more amp-hours of capacity. That's almost trivial.

I don't agree but it would be difficult to make a blanket go/no-go decision as results are relative. 7% at the outset doesn't sound like a lot and it will work OK for a "while". How many delivered Ah and cycles before it tips over depends on the application but I can personally attest to the fairly rapid demise of batteries once mismatch starts to set in. It can also happen in a single battery, where cell capacity diverges and ultimately a cell goes open or shorted (mitigated by proper absorption phase and periodic equalization). Capacity mismatch and diminished service life isn't as obvious in RV settings because typical use is so tepid, with trauma or neglect probably still being more likely cause of battricide than frequent or deep cycling and cell/battery mismatch. To start out with a mismatch is not unlike intentionally using the wrong air pressure in a tire by a little bit. Starting off it appears to work like nothing happened but the effects are cumulative and inescapable. In an off-grid application where cycles are daily and DoD can be deep, expect those effects to be more pronounced or dominant.

Mark B.
Albuquerque, NM