Can batteries be charged and discharged at the same time? And do they wear down faster when doing so?

Question

Imagine batteries connected to a charge controller and a load at the same time. When the load asks for power, and the charge controller delivers power, there are three possible situations:

P_in > P_out: there is netto power going into the battery: charging
P_in < P_out: there is netto power going out of the battery: discharging
P_in = P_out: battery keeps unchanged

The power that the battery loses/gains is |P_in - P_out|, but in the system as a whole there's flowing a lot more current than that. Does all the current go through the batteries in this system? Or is there no current flow through the battery in situation 3 (such that the current is flowing directly from the charge controller to the load).

Do the batteries wear down in situation 3? Or are they just completely unused then?

EDIT: I think my question is different than the so-called duplicate, since it's more about the physical working of the battery. I made a basic diagram about my question as asked by Olin Lathrop: enter image description here

So the question boils down to: are the light green and pink currents present (comparable to a dynamic equilibrium state in a PN-junction at rest due to drift and diffusion, when there's also a lot of electrical current flowing throught the component but no net measurable current), which would make the battery wear down? And if they're not, why? Is "path of least resistance" a sufficient reasoning to state this?

Also I am not completely satisfied with the answer given in the other topic, since he tells it depends on the kind of charge controller. If the charge controller delivers e.g. 24W at 24V (so 1A) and the load asks 12W, why wouldn't that current go to the load? Or would the charge controller then deliver only .5A due to the applied resistance (the load), and neglegt the battery? And what will happen with the rest of the energy that the charge controller receives from its source (which is thus 24W-12W=12W)? Will it be dissipated somewhere?

I am talking about a system where charge controller, battery and load are always connected (a stand-alone solar electric system).

Olin Lathrop · Answer 1 · 2014-04-03T15:43:43.587

5

Your confusion is not about batteries but a basic issue with current.

No, a battery can't be charged and discharged at the same time. If a battery is connected to a charger delivering 1 A and a load drawing 3 A, then the battery will be discharged at 2 A. There is no simultaneous charging and discharging going on.

Draw out the circuit and follow the currents. You can conceptualize the above example as 1 A charging the battery and 3 A discharging it, but the battery sees the sum. Again, draw a diagram and it should be more clear. Handwaving makes everything difficult to understand.

Added:

Your diagram should make it clear. You can't have current going thru the battery both directions simultaneously. You can think of it conceptually as being two currents, but the battery will see the sum of the pink and green currents in your diagram.

edited Apr 03 '14 at 15:43

answered Apr 02 '14 at 17:45

Olin Lathrop

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My confusion is about the battery. I know how the diagram would look, but I was just wondering whether the current flows through the chemicals in the battery, such that the battery will wear down. So as you say it, if the current going in and current going out is the same, there's no current going through the battery, and it doesn't wear down? And thus, the current is flowing directly from the charge controller to the load, without passing the battery? – BNJMNDDNN Apr 02 '14 at 18:26
Yes, if "Current going in"="Current going out" then it goes directly from controller to the load. No current goes into or out of the battery in that case. – horta Apr 02 '14 at 18:28
@BNJ: This really has nothing to do with batteries. You could ask the same question replacing the battery with a resistor. Again, if you actually drew out a diagram, this sum of currents thing would be alot more obvious. – Olin Lathrop Apr 02 '14 at 19:12
http://prntscr.com/36fvqw here's the diagram. The question thus boils down to: are the light green and pink currents present (comparable to a dynamic equilibrium state in a PN-junction at rest due to drift and diffusion, when there's also a lot of electrical current flowing but no net measurable current) or not? I can guess they're not, based on choice of path of least resistance, but I don't know enough about the working of batteries and charge controllers to make this short-cut conclusion. – BNJMNDDNN Apr 02 '14 at 19:56
Oops, never mind, I didn't see that it was a duplicate... – bitsmack Apr 02 '14 at 21:41
You said (before you edited your comment) to omit the currents going back from the load to the battery, and from the battery to the charge controller, why? Should a current not always go in a cycle? – BNJMNDDNN Apr 03 '14 at 15:13

Can batteries be charged and discharged at the same time? And do they wear down faster when doing so?

1 Answers1

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