Smartly Switching Device Charger

Hello All,

I am trying to charge several devices through USB output and trying to select between which device is being charged.

Basically what I need to figure out is that if I have three USB devices plugged in (A,B,C), how can I have my circuit detect when a device is full or empty and switch to the next one in the line. I am using an Arduino Nano to measure and output voltage stored in the batteries to an LCD and was thinking about setting a condition on the current measured in a device to a threshold level and then moving on to charge the next. However, this presents a problem because no two devices will have the same current at an 85% charged level. If anybody has some suggestions about how to solve this problem it'd be greatly appreciated.

The charge level of most batteries can not be reliably determined solely by the voltage level. Depending upon the battery type it usually takes some combination of current and voltage monitoring, and possibly battery temperature to determine when it is charged.

What I'm thinking about doing is building a differentiator circuit to help detect when a steep voltage increase occurs. The devices are going to be primarily Li-ion such as ipods, phones, etc. Battery temperature monitoring would be hard to monitor since the point is to create a "black box" which you can connect your devices to and which will priority charge those devices one at a time, so measuring temperature doesn't seem feasible. What kind of circuit would you reccommend that can measure both current and voltage or which would detect when a device is reaching capacity charging?

holyguacamole said:

What I'm thinking about doing is building a differentiator circuit to help detect when a steep voltage increase occurs. The devices are going to be primarily Li-ion such as ipods, phones, etc. Battery temperature monitoring would be hard to monitor since the point is to create a "black box" which you can connect your devices to and which will priority charge those devices one at a time, so measuring temperature doesn't seem feasible. What kind of circuit would you reccommend that can measure both current and voltage or which would detect when a device is reaching capacity charging?

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If you are charging these devices through their built-in chargers then you likely can not determine when they are charged from the voltage since the battery voltage is usually isolated from the charging terminals.

A better way may be to monitor current. There probably is a drop in charging current when the built-in chargers terminate the charging but I don't know if they all do that. You might try monitoring the current from some typical devices and see if it changes as they charge.

Thanks Carl, we will try to monitor the current by measuring the voltage drop across a resistor. There is a drop in charging current that we observe occurs when a device is nearing charging capacity, and we have also considered telling our Arduino to continue charging a device is its current is measured below a certain threshold even if it is 90% charged as it will not be draining much power from our batteries at that level. Thank you for your help.