Hey you guys! Is thread going to go on forever?
Let’s keep it going. My posting doesn’t go on forever. Really.*
I’m not so sure that we really have a difference of opinion in this thread at the profound level.
I think one camp is stating what CAN be done, and the other is simply stating what SHOULD be done. It’s usually pretty easy to figure out what CAN be done. What SHOULD be done can be more elusive and complicated to uncover.
What CAN be done is subject to the real-world conditions – exactly as you’ve illustrated, esc, using what is readily available. When it comes time to replace your batteries it’s useful to know what SHOULD be done, if it happens to be practicable under those new circumstances.
Now the technical bits.
Here’s what we do know about flooded lead acid batteries:
-overcharging a battery will damage it (= shorten its service life)
-undercharging a battery (chronically) will damage it (= shorten its service life)
-batteries are charged in 3 stages, known as bulk, absorption, and float.
-the charger (assume a “smart” charger, one which is not designed for charging more than a single battery, looks like that’s what’s being discussed here) adjusts the voltage and amperage to appropriate levels for these three stages based on what it measures from the battery (resistance and voltage, sometimes temperature)
-based on these readings a determination is made of the stage and state of charge and the voltage output of the charger is varied over a range from around 10.5 to over 15 volts, depending on the stage required and multiple other factors, which, let’s call, the “fingerprint” of the battery being charged.
The fingerprint includes many variable characteristics such as the level of discharge; the physical and chemical makeup of the lead plates (spongy lead, high/low antimony, high/low calcium, level of sulphation, positive plate degradation, plate size, thickness, surface area; presence of dendrites etc.); temperature; age; electrolyte volume and purity (no tap water, no acid, no snake oil!) etc. There are many more, but these readily come to mind.
All agreed so far? Should be, I think these are pretty well accepted maxims.
If two batteries are connected to the charger in parallel, the charger only sees one voltage. If one of them is at a higher level of discharge than the other, a net flow of electrons will happen before the charger is even connected.
Let’s look at the extreme and say one of the two batteries is at 12.0 V, and the other is at 12.7V. Connect them together and there will be a net flow of current from the 12.7V battery to the 12.0V battery, the rate at which will depend on the internal resistances of the batteries at that particular point in time. (The internal resistance of a battery is not a constant).
One is being drained, the other charged. Wait long enough and they will eventually become equal, as measured open circuit of course.
Connect the charger and it will measure the net voltage and the net internal resistance of the two batteries connected together. If the two batteries have identical “fingerprints” what the charger deduces will be pretty close to the actual conditions of the two batteries, and charging will be optimized.
If the two batteries are not identically matched, they will be either undercharged or overcharged, sometimes grossly so if they require different stages (bulk, absorption, float, or a level of these) of charging.
This also can happen to a 2V “cell” within a 12V battery itself. A single 2V cell within a battery can go “dead” because it was in some way out of match with the other five and ultimately failed. If you more closely check a “bad” 12V battery you will often discover that it is really only a single cell that is the cause of the failure.
I have a solar/wind battery bank which consists of 4 deep cycle batteries which are of consecutive serial numbers. This is canon in the industry because it is recognized that there can be variations even within a battery product cycle that are significant enough that they can affect longevity when connected in banks. This arrangement is particularly sensitive because banks are connected in series/parallel and are all normally charged (solar/wind/charger) and discharged (inverter, usually) together as a single unit.
I know first-hand of a situation where the 4 deep-cycle batteries used in a solar/wind setup were not from a consecutive production run and his system (all four were replaced at once) required replacement in less than one year. (Under full warrantee, BTW, the vendor realized his $500+ mistake and ate it). This failure is not an isolated occurrence either, others on this board have reported it as well.
If you want to charge two different batteries (and I define “different” as any two or more batteries which are not consecutively manufactured, although you might be slightly less fussy) and want to maximize the probability of long-term reliability, you will want to use a charger designed specifically for such use.
Or, you can use a mechanical battery isolating switch, or use intervening circuitry that will allow the charger to “read” the condition of each battery individually and apply the correct charging voltage which will bring it up to its fully-charged condition.
Or, you can choose not to maximize the probability of long-term reliability and just charge them together!
Similarly, you can choose to smoke cigarettes, which will minimize YOUR probability of long-term reliability.
But just because you CAN, doesn’t mean you SHOULD!
Best regards! - B.
* OK, OK . Sorry if I hogged more than my share of server bandwidth on this one.
hull ! 