USB charging port for a small robot

[Zunaid Vania]

Perhaps I'm searching online for the wrong phrases, but I can't seem to find much info online about adding a USB charging port to a small robot.

The robot runs on 4xAAA batteries and the primary goal is to enable my young son to plug it into the USB port in the wall to charge overnight, avoiding the need for adapters in electrical outlets.

Ideally I'll want a circuit that avoids overcharging the robot, that works whether the robot is on or off, and that doesn't double the size of the small bot; however, I don't want to get picky since I don't even have a starting point.

Any advice or pointers would be most welcome.

Thanks

[David Murphy]

There are quite a larger number of charger chips that support usb charging for cell phones and processors. They basically expect a single lithium battery ( or 2 or 3 in parallel). you could look at the TI bq24190 for an example of one of these chips.

If you go to usb.org, you can look for the bc1.2 spec. This describes the usb charging scheme - what is expect of the adapter, current limits, etc.

In short though, you can identify a usb charger by the fact that it's d+ and d- lines are shorted. according to the bc1.2 spec, this condition means that you can draw 1.5A from the adapter. There is a difference between the bc1.2 and bc1.1 spec however, the bc1.1 spec only allowed 1A, so in practice, one must be cautions. Many products that support usb charging in fact ship with chargers that are capable of supplying 2A at 5V. These products often identify their own adapter by additional resistors places from d+ or d- or both, but there is no standard here, everyone does it differently. Some of the charger chips can detect the charger type on their own, for the ones that can not, there are stand alone chips for charger detection. In the case of phones/tablets, often the usb phy is manipulated in software to detect the various conditions.

From the charger perspective, some of the charger chips are completely autonomous, but they rely on a thermister in the battery pack to monitor over/under temperature conditions and assume the battery has it's own protection circuits in the case of failure. The TI devices are a good examples of this.

Cheers,

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[Ralph Gnauck]

This app note may be helpfull.

http://www.maximintegrated.com/app-notes/index.mvp/id/3241

 

Ralph

---

From: David Murphy <dfm...@gmail.com>
To: hbrob...@googlegroups.com
Sent: Thu, April 4, 2013 8:59:47 PM
Subject: Re: [HBRobotics] USB charging port for a small robot

[Zunaid Vania]

Thanks to David's  pointer, I was just looking through this one: http://www.maximintegrated.com/app-notes/index.mvp/id/3607

Very helpful - thanks!

[Bob Smith]

Zunaid Vania wrote:
> The robot runs on 4xAAA batteries and the primary goal is to enable my young son to plug it into the USB port in the wall to charge overnight, avoiding the need for
> adapters in electrical outlets.

Could you replace the existing battery pack
with one of these?

http://www.frys.com/product/6558383;jsessionid=63qeVzS1NGyl7mtOT7KNzg__.node4?site=sr:SEARCH:MAIN_RSLT_PG
http://www.frys.com/product/7222013;jsessionid=63qeVzS1NGyl7mtOT7KNzg__.node4?site=sr:SEARCH:MAIN_RSLT_PG

[Zunaid Vania]

Bob, that might be worth considering on a new design, but probably not on the current bot (though I don't see dimensions listed).

Since I searched for the actual charging ICs, I've come across many options, including several designed for the NiMH batteries I already have.

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[Andy Jang]

Hi Zunaid,

Saw this on Sparkfun site: https://www.sparkfun.com/products/10217.

But is uses LiPo instead of NiMH.

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[Ted Larson]

Or….you could just use something like this….comes with a LiPo battery, and a little USB charging board you can mount to your robot.

 

https://www.sparkfun.com/products/9876

 

-Ted

 

 

From: hbrob...@googlegroups.com [mailto:hbrob...@googlegroups.com] On Behalf Of Zunaid Vania
Sent: Thursday, April 04, 2013 9:46 PM
To: HomeBrew Robotics Club
Subject: Re: [HBRobotics] USB charging port for a small robot

 

Bob, that might be worth considering on a new design, but probably not on the current bot (though I don't see dimensions listed).

 

Since I searched for the actual charging ICs, I've come across many options, including several designed for the NiMH batteries I already have.

 

 

 

On Thu, Apr 4, 2013 at 9:25 PM, Bob Smith <bsm...@linuxtoys.org> wrote:

Zunaid Vania wrote:

The robot runs on 4xAAA batteries and the primary goal is to enable my young son to plug it into the USB port in the wall to charge overnight, avoiding the need for
adapters in electrical outlets.

 

Could you replace the existing battery pack
with one of these?

http://www.frys.com/product/6558383;jsessionid=63qeVzS1NGyl7mtOT7KNzg__.node4?site=sr:SEARCH:MAIN_RSLT_PG
http://www.frys.com/product/7222013;jsessionid=63qeVzS1NGyl7mtOT7KNzg__.node4?site=sr:SEARCH:MAIN_RSLT_PG

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[Ted Larson]

Andy,

 

Most of these single-chip USB charging solutions are designed to charge a single LiIon/LiPoly cell.  Which is a bit of a problem, because one cell is always 3.7V.  So, when you have a 6 or 9V motor….this can be a problem.

HOWEVER, there is a solution!  Yay!  Put one of these in your robot for generating the motor voltage:  http://www.pololu.com/catalog/product/791

 

It is a boost converter.  It steps the voltage up from 3.7V to an adjustable voltage up to 9V, and is good to almost 2A (which is questionable for other than bursts).  If your motors draw lots when they are running, then this may not be a solution that will work.  However, this is exactly what Pololu does in their little 3pi robot.  It runs on 3xAAA cells.  They boost it up to 9V for driving the motors, and the voltage stays nice and constant no matter what the battery voltage is.

 

I don’t know if you saw the Oddwerx Smartphone robot we designed and brought around to the meetings a few times, but this type of solution is also what we did there.  We had 4 hobby servo’s that needed 6V to run.  We had one little Li-Ion cell in there to power it.  We ran the logic at 3V and then boosted the voltage for the servos to 6V using a dc/dc converter.  In all the abuse those robots received, with the servos sometimes up to 2A, we never blew up a step-up supply.  I was pretty surprised at how robust the solution was.  It allowed us to make the robot really small and compact because it only had once cell in there.

 

Hope this helps,

 

-Ted

 

 

From: Andy Jang [mailto:aj48...@gmail.com]
Sent: Thursday, April 04, 2013 10:51 PM
To: t...@larsonland.com
Subject: Fwd: [HBRobotics] USB charging port for a small robot

 

Ted,

This is a pretty good solution for a robot. Do you know if there is a

7.2 v version because my motor uses about 5-6v?