Originally Posted by groundpounder
I disagree, just because some coaches are built with parallel batteries it is still bad practice (done for a price point). Yes it can be done, yes match new battery with new battery.
There are three parts to this, drawing power, charging and idle state. I have no issue while drawing power, the batteries can be mismatched and still operate safely as the power will be divided according to available charge.
The problem occurs during charging, yes each battery will absorb its own charge according to internal resistance of the chemical state of the cells. At some point the battery will reach its charged state partially reflected by the voltage. The other battery may not be at the same state. This sets the stage for either a overcharged or undercharged battery. An overcharged battery can be damaged and can get hot and cause it to lose water more so than the other battery. An undercharged battery is wasted.
At idle the batteries charge and discharge each other looking for middle ground, still you have a loss efficiency.
BTW the word "power" is usually referred in electrical circuits, "energy" covers everything. I could have used watts or work to be more specific.
Even batteries in series are not perfect because any cell can be different.
But when given a choice go with batteries in series is safer more efficient and may last longer.
Watt is power, work is energy. You store energy in a battery. You can't store power. Both terms are used in electrical circuits as well as any system doing work.
And you didn't disagree as much as change your original statement: "bad practice" is not the same as "dangerous".
Could the bad practice lead to one of the parallel batteries being overcharged? Sure, if the batteries were grossly mismatched. But you can do the same thing with batteries in series as well since all the batteries are experiencing the exact same charging current regardless of it's own voltage. So the good batteries can all be overcharged because a bad battery will not come up to voltage. In submarines where they use 128 lead acid cells in series they monitor each cell for voltage and specific gravity for this very reason. If a cell is not up to snuff it is jumpered around so it doesn't affect the rest of the cells. And in case you're wondering, each cell is bigger than a person, so each cell stores a lot of energy.
At least in parallel the charged battery with the highest voltage resists the flow of charging current so it is more difficult to overcharge it.