| |> Lets me offer this simple analogy for parallel batteries, because they |> are nothing more than storage reservoirs for electrical energy. |> Consider two buckets (A & B) of water sitting on the floor with a pipe |> connecting them at the bottom. If you fill Bucket A, the level in B |> will rise right along with it. Likewise, if you remove water from |> either, the level of the opposite will drop the same amount. (The |> height of both above the floor will be equal; the principle of the |> water level) |>
|> Note that one bucket *does not* drain the other. Why on Earth should |> it? Where would the energy go? |>
| | |> Connect the batteries in parallel (+ to +, - to -) and don't worry |> about it. |>
| |It is not good practice to directly connect batteries in parallel, as an |earlier response stated batteries do have internal resistance and unless the |batteries are exactly matched the higher charged battery will discharge to |the level of the lower one.
So what? Be an optimist, the battery with the lower terminal voltage has been *charged* to a high voltage. The total AH capacity remains the same except for the slight I^2R loss.
|If you don't believe it do the Kirchoff |analysis.
Oh please, I'm a retired EE, don't try to baffle me with big words.
|In emergency situations batteries are connected in parallel, such |as when jump starting a car, but as you will notice there is an ititial |spark and heavy current flow from the donor battery trying to charge the |dead one.
So what? *You* do a Kirchoff analysis and tell us why this surprises you. | |The analogy of the two water buckets doesn't hold water in this case because |you overlook the connection between the individual cells of the batteries.
That's why they're called "batteries", they are a collection of cells. I didn't overlook anything of the sort. But if this baffles you, change "batteries" to "single cells" and connect them in parallel. The charge/discharge analogy continues to "hold water."
|One cell can becomes high resistance, quite common, in which case the |capacity of that battery becomes virtually zero;
So what? If it has high internal series resistance, it becomes less of a voltage source and more of a current (limited) source. So it may not contribute to the load current, but it certainly doesn't detract from it either. The *high internal resistance* sure the hell isn't discharging anything, as was the misguided concern of the post to which I originally responded.
|or if a cell partially |shorts, due to buckling of the plates or sediment shorting the plates, in |which case the battery terminal voltage drops.
So what? A failure can occur in a stand alone battery too. I guess that means we shouldn't use batteries; they might fail afterall.
| |In professional installations the solution would be two batteries controlled |by a differential relay so when the first battery drops to its rated |discharge voltage it would be dropped from circuit and the second connected. |In this application the diode isolators would probably be the best |alternative.
There are millions of RVs running around (one of them parked outside my house) and who knows how many solar energy installations that are using many multiples of parallel connected batteries. Do failures ever occur? Sure, usually from abuse in the charge/discharge cycle, but *not* because there is something inherently wrong with parallel connections. See:
formatting link
stand by my earlier remarks.