# Is there a device/battery that can be charged while it's being used?

You'll have to excuse my sheer lack of knowledge, I'm sure this must be among the most basic of questions posted on here.

I have a dynamo (motor), which is powering an LED (for argument sake).

If the dynamo stops spinning, the LED's no longer powered.

Is it possible to power the LED with batteries that are currently being charged? So, if the dynamo stops the LED has a bit of power left in the batteries to carry on using until the dynamo starts spinning again?

Worth noting, these are all very small, low-powered devices I'm talking about. Nothing of scale.

Thanks very much

• The answer depends on the voltage and currents involved, could you provide this information? Nov 16 '17 at 16:15
• either you are generating more energy than you are using and you have some to spare to "charge" the battery, or you are not generating as much as you are using and you are pulling energy from the battery. it is pretty simple the current with respect to the battery is going one way or the other, cant go both ways at the same time, but that doesnt mean you cant be generating energy charging the battery and using the load at the same time we see this with our phones, laptops, all kinds of devices. whether or not the battery is being charged depends on the load and the charger. Nov 16 '17 at 16:18

At the level of the actual battery cells, the answer is no: "Charging" means current is flowing through the cells in one direction, and "discharging" means that the current is flowing in the opposite direction. You can't have current flowing in both directions at the same time.

At a higher level, the answer probably is yes: It is possible to build a power supply that

• takes "bulk power" from an external source,
• provides regulated power to a load,
• uses the external source to charge its internal batteries and supply the load when the external source is available,
• uses its internal batteries to supply the load when the external source is not available.

Some "USB power bank" devices will operate that way. Unfortunately, not all of them will do that, and they rarely advertise the feature on the package. You just have to buy one and try it out.

Is there a device/battery that can be charged while it's being used?

Yes. Automotive electrical systems work in this way.

simulate this circuit – Schematic created using CircuitLab

Figure 1. The alternator can charge the battery and run a load simultaneously. Note that the lamp is being run from the alternator and not from the battery.

Note that the problem becomes a little complex if the power source is trying to control the charge. In the configuration of Figure 1 it has no means of knowing how much charge is going to the battery and how much to the load.