Inductive charging help

I am working on a senior design project at the University of Pittsburgh where we are building a spherical robot that requires inductive charging for the internal motors and circuitry. We have spec’d out 9V inductive chargers to use and a battery, but we are stuck on how to have the battery connected to both the charging circuit and the motherboard at the same time since we cannot access the circuitry to unplug the battery from the charging circuit and plug it back into the motherboard when it is finished charging.

Do you know of any hardware we could use like maybe a switch of some sort that would allow them both to be connected and remotely toggle between charging and powering the motherboard?

Hi Leah, thanks for your post. We should be able to help you with that. Sounds like a toggle switch would be a great choice. What kind of voltage and current ratings are you dealing with? Is the battery ever going to be powering the motherboard and need to be charged at the same time or will it be a one or the other type of scenario?

Thanks for the help! It will not need to be charged and running at the same time, but I won’t be able to access any of the circuitry after setting it up, so it can’t be a mechanical switch that is manually pressed I don’t think.

A magnet within the charging apparatus could be used to activate a reed switch internal to the 'bot, which could then control some sort of power switching/routing function such as a DPDT relay. No mechanical contact required.

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There are many ways to tackle this, depending on the architecture of your system.

Does your internal charging system have any sort of outputs which indicate that it is in charging mode? If so, you could simply take that signal and use it to turn off the power to the motherboard via a relay or solid state relay (SSR).

Does your sphere have to be lined up in a specific orientation to align internal and external coils for charging? If so, you could strategically place a magnet near this interface and place a magnetic sensor, such as a hall sensor or reed switch directly opposite the magnet, internally, which would be triggered when placed in close proximity. The signal from this would be used to turn off power to the mother board with a relay or SSR.

Another possibility would be to place a current sensor between the battery and the charging circuit to detect current flow in that path (making sure it does not sense current going out of the battery to the motherboard) and then using the signal from that to turn off power to the motherboard.

Still another option would be to monitor the voltage coming from the battery charger to the battery and use this to trigger the output relay. To do this, one would have to place a diode between the charging circuit and the battery and place a pull-down resistor (10k would probably work) between the anode and the negative side of the battery so that a significant voltage is measured only when the charger is on. One problem with this approach is that there would be a voltage drop across the diode, which would reduce the voltage applied to the battery.

If the sphere will be subject to lots of bumps and jostling (pretty likely, I would surmise) then using mechanical devices are typically less reliable. In your case, I would lean toward semiconductor-based devices such as Hall sensors and solid state switches over reed switches and mechanical relays.

One thing you will need to determine is how much current your system will draw. You will have to keep that in mind when specifying your relay, as you will want to leave yourself some headroom for current surges and such, which are common in motor control applications.

Here are some categories in which to look for options:

Hall sensors

DC capable Solid State Relays (note that the higher current rated SSRs require heat sinking, which could be a problem in your enclosed space)

Current sensing:
Current sensing op-amps
More current sensing IC’s

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