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Paralleling does not cause premature failure!
Neilchristophers,
Not true. When the cells are connected together with cable they will be at the same voltage in all conditions. As they are charged the charger will impose the voltage and the batteries will accept the charge dependent on their capacity and charge state. If they are exactly matched (an impossibility, I know, but just for the sake of argument) they will accept the same current, if the charger can deliver 30 amps, both will have 15 amps going in. As the charger reaches its upper voltage limit (boost or float charge) the current will taper off in both batteries until the current counterbalances the self discharge of the batteries (full charge).
The same happens if the batteries are not matched. The difference occurs in the amount of current each battery accepts curing the charge cycle. If one battery has twice the capacity of the other (through wear-out, or even if two batteries of different capacities are used), and using our 30 amp charger, the larger batter will accept twice the current of the smaller one (20 amps for the larger, 10 amps for the smaller). The same taper will occur as the final voltage is reached and both batteries will reach full charge at about the same time.
In the discharge mode, a similar thing happens. As you put a load on a battery, the internal resistance causes the output of the battery to drop immediately. This drop is small so if you try and measure it you will need a voltmeter to several decimal places, preferably measuring 1/100 of a volt @ 12volt range (this would be called a 5 and ½ digit meter). The important point is that this voltage drop allows the current of parrelled batteries to be shared in spite of their relative capacities! Taking out two examples the exactly matched batteries will exactly split the load current equally, and our mismatched batteries will have the current from the larger one at twice that of the smaller one.
This equates to the load currents of parrelled batteries as having the same relative current based on their capacity. Both batteries will discharge at the same rate (but with different currents) and have the same state of charge at the same time, and reach the end charge state (weather it’s 50%, 25%, 100%, or what ever) at the same time!
So, parrelled batteries charge at the same time and discharge to the same relative state of charge at the same time. Each batter will do the same relative work based on its own capacity. If one fails faster then the others, it would have failed in the same time if exposed to the same duty even if used independently. Paralleling batteries does NOT cause them to fail prematurely.
BTW, this conclusion is based on many, many scientific papers I have been using in the course of my work, and that I have reviewed while and after I have been posting on this thread. I will make them available to any who ask.
There are two types of fools...
One says this is old, and therefore good..
The other says this is new, and therefore better...