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

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:

1. P_in > P_out: there is netto power going into the battery: charging
2. P_in < P_out: there is netto power going out of the battery: discharging
3. 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:

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).

• 1. See Olin's answer. 2. When Iinbat=Ioutbat, if you remove the battery what will happen? ie the battery has no role, as Olin states. The comparison with a semiconductor fails because the model is too different to be applicable. A better model is a pipe with say 10 l/s fluid flow into a node and 10l/s out of a node and a side tap at the node into a storage tank. In a steady state condition Iin = Iout and no current flows to or from the tank. (I said steady state as tank level needs to reflect the pressure at the node. Once stable then above situation applies). – Russell McMahon Apr 4 '14 at 14:00

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.