# Inductive Charging

I've build a small three wheel robot with an Arduino Mega to drive the thing. I'm looking to keep this bot mobile as much as I can without needing to interact with it for charging etc.

For the moment I am running on a collection of normal AA batteries as the power source. I'd like to replace this with an appropriate rechargeable battery pack, and a charging circuit of some kind.

So I'm after two things.

1. Information on charging circuits. I feel like I am searching for the wrong thing, and am not coming up with anything useful.

2. Inductive charging. I really would like to be able to park the robot on a charging base, and leave it there, and have it just drive off the base when its done. Most articles I can find say to rip apart an electric toothbrush for this. But I would love to do it myself.

Inductive charging stations is a very neat idea, and one I hope you are able to pull off with your robot platform.

Although I have never built a inductive charger myself, I have put together several home-made, from scratch RFID type devices and what I have learned is that the key to getting good coupling between the primary coil of the charging station and the pickup coil on your robot is having a well tuned (high-Q) resonant network on the primary. I think you will be best off coupling this with a non-resonant pickup coil on the robot, that way the load introduced by the robot will have minimal impact on the resonance of your primary coil.

They have put together a basic system that should have all of the parts you need to provide power to charge your robot. On the base they use a two-coil setup with a primary driver coil being driven by a power MOSFET coupled to a resonant secondary coil. Since the secondary coil is electrically isolated from the driver they are able to preserve the resonance and improve the Q of the secondary system better than if they were just driving it directly.

On the receiver end they are using a non-resonant pickup coil (basically coiled wire) connected to a cascade multiplier which boosts the voltage picked up by the coil to a level they can use on their robots. I have built similar pickups for the RFID projects and they work surprisingly well with the right combination of Schottky diodes and capacitor values (ceramic work best here).

I think the challenge in your case would be to get the output voltage high enough to trickle charge whatever battery pack you are using. Since I don't know if you are using LiPo, NiMH, or even lead acid I couldn't say if this would work for sure. I imagine with enough tweaking you can easily get 4-5V out of this type of setup and that should be enough for LiPo or NiMH.

I worked at a company where we developed and eventually manufactured an electric toothbrush with inductive charging, so your comment about ripping apart a toothbrush caught my attention. It should be noted that most (all?) electronic toothbrush inductive charging systems are very inefficient. The are trickle charging the batteries. They just need to charge the batteries enough to power a led charging indicator and run the brush about four minutes a day. The circuitry is generally quite simple, especially on the toothbrush side which consists of little more than a full wave rectifier, a coil, and a ferrite. Often a led is used as one of the diodes of the rectifier so it does double duty as a charging indicator. I doubt there is anything in the toothbrush that you couldn't easily put together yourself. The charging bases are slightly more complex, but still pretty simple oscillating circuits, often not very well optimized. Chinese companies often reuse charger designs with little sensitivity to maximizing efficiency.

If you do get the urge to take apart a toothbrush, keep in mind taking apart most electric toothbrushes be harder than you expect. They are typically ultrasonically welded, so you'll need to cut it open. Pipe cutters are a good tool for this.

• +1 for pipe cutter, sounds a lot easier than a Dremel cutter wheel. Jul 5, 2011 at 20:18

Make a nice fat NiMH battery pack, use a MAX712 fast charge controller (http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1666/t/al). Not sure about inductive charging; imagine if you could program your robot to find its way home back to the charging base for a top-up! That'd be cool :)

Also came across this today, it may be of interest: http://www.cse.unsw.edu.au/~acra2005/proceedings/papers/emami.pdf http://ratslam.itee.uq.edu.au/

• Thats precisely the idea. I endeavour to enable the bot to determine its remaining charge, and to navigate its way back to the charging base then it needs a top up. Having an inductive charging method means that could be as simple as parking atop an inductive charging pad. Thanks for the suggestions there, I'll take a look at those controllers. Will be interesting to see if anyone can help me with the inductive side.
– Predominant
Apr 11, 2010 at 3:40
• I've marked your response as correct tronixstuff. Thanks for reponding :) I might see what else I can fond in inductive tech with a few more google searches.
– Predominant
Apr 12, 2010 at 13:59
• Cheers mate, looking forward to hearing how you go!
– user1307
Apr 12, 2010 at 23:42

Have a look here: http://www.instructables.com/id/Low-Power-Wireless-Charging/

Plenty of references too.

I've been researching something similar and came across this paper which might be of help

http://www.araa.asn.au/acra/acra2006/papers/paper_5_18.pdf

I have no experience with it, but at $\$\$9.50 it might be worth a shot.