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Author Topic: Batteries on arduino uno  (Read 2427 times)

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Offline jghostzTopic starter

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Batteries on arduino uno
« on: July 28, 2012, 07:27:54 PM »
Hello,
I am a newbie to robotics and am working on my first robot using a cheap rc H2 and infrared sensors for object detection.  I will be using an Arduino UNO for this project and don't know to much about it.  As for batteries I have both cells out of a Li-ion 7.4V 2000mAh battery and   2X Li-ion 3.7V 1500mAh cell phone batteries.  What I was needing to know is if i put both of the cells from the 7.4V and one of the cell phone batteries in a series could I just plug a 12V power source into the plug on the UNO to charge it or do I need to set a reference point within the UNO or do I need to build a circuit for charging or if I hook these batteries together if they are just going to explode and blow my robot to a pile of smoldering ash?

Offline Soeren

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Re: Batteries on arduino uno
« Reply #1 on: July 29, 2012, 11:27:51 AM »
Hi,

[...] or if I hook these batteries together if they are just going to explode and blow my robot to a pile of smoldering ash?
That one.

Never put cells of different capacity together - especially with lithium based cells.

Unless you know exactly what you do and every detail of the battery chemistry and its particular specs, do not attempt to just whip a charger together - especially with lithium based cells!

You might hate your home, but don't burn it down unless you really mean to ;D
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline Gertlex

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Re: Batteries on arduino uno
« Reply #2 on: July 29, 2012, 02:25:28 PM »
I will second what Soeren said.  Besides the mixing of cells being a bad idea, it's best practice to just disconnect the battery from ALL of your electronics before using a proper charger to charge it.  Whether you leave the battery on the bot/device, is another matter; I design my stuff so that batteries are easily removable for swapping/charging.

I'm sure there's ways to re-use cellphone batteries for these projects, but no one does that, really.  You're much, much better off to buy new LiPo batteries, a proper charger, and the correct battery monitor.
I

Offline jghostzTopic starter

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Re: Batteries on arduino uno
« Reply #3 on: July 29, 2012, 06:24:16 PM »
Thanks again guys that's why I ask on here before just trying it although the temptation was almost overwhelming I was hoping to make it re-chargeable without having to take batteries out and those are what I happened to have lying around.  The better question for this I guess would be do I really need all that power someone told me to get as much as the board could safely handle which is 12v but the only thing that I am running off of the UNO itself are the transistors for 2 h-briges (although I'm thinking about replacing the steering mechanism with a servo) and 2 IR proximity sensors and maybe an IR module on top  to make it act out preset commands via remote.  The whole project is going to be very light 3-4lbs max.  and the drive motor is pretty small I am using the 5V plug off of the UNO to power it but the original system was 4.5V.  Thanks again for the info and saving my house I'm not sure how my roommate would have handled that I'm already pushing my luck by having an HHO generator sitting on my bookshelf that is not even hooked up to power.  Had to explain that not only was I not making drugs but it was not going to blow the house up either.

Offline Soeren

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Re: Batteries on arduino uno
« Reply #4 on: July 30, 2012, 05:45:40 PM »
Hi,

[...] do I really need all that power someone told me to get as much as the board could safely handle which is 12v but the only thing that I am running off of the UNO itself are the transistors for 2 h-briges (although I'm thinking about replacing the steering mechanism with a servo) and 2 IR proximity sensors and maybe an IR module on top  to make it act out preset commands via remote.  The whole project is going to be very light 3-4lbs max.  and the drive motor is pretty small I am using the 5V plug off of the UNO to power it but the original system was 4.5V.  Thanks again for the info and saving my house I'm not sure how my roommate would have handled that
No, you are much better off sticking to only the 2 cell lithium (the 7.4V battery).

Your Arduino and the stuff you added all run on 5V, the Arduino has got an LDO regulator I assume, which means that it needs at least 6V (perhaps a little more) to stay in regulation.
If you feed it with 12V, there's an excess of 6V * [current draw] of heat to get rid of. If you add a servo, 1A of curernt draw is fairly realistic and makes it easy to calculate the power lost to 6V * 1A = 6W extra (there's the ~1V * 1A = 1W that you can't do nothing about) that you have to get rid of, needs a hefty heat sink on your regulator.
This extra 6V is doing nothing for you at all.
Your two cell lithium is 7.4 (probably around 8V right out of the charger), so the excess voltage is much smaller. at 7.4V there is a total of 2.4V * 1A = 2.4W to dissipate.
While the battery discharges, the voltage goes down towards ~6V, which is just perfect for a 5V LDO. Power dissipation goes down linearly with the voltage and at ~6V you have the minimum drop possible (and around 1W dissipated.
he above is all assuming a constant draw of 1A. In real use it will vary and most ofthe time it will be less, but the voltage/power relationship mentioned still holds.

On a slightly different note...You seem to confuse power (which is V*A and is termed Watt) with voltage and this tells me that you are not ready to build your own charger.
However, you'd do well in making a voltage detector (either implemented in your microcontroller via A/D-C, in an op-amp, or by one of the very simple to use voltage detector chips. This way the robot knows when the battery needs to be recharged and that's very important, as if you discharge a lithium too far, you ruin it (with the risk of having your very own personal fireball).

You can easily charge the battery while it's powering your electronics, but you're up and running much faster with a spare to swap in.

If you want it to charge on a pod of sorts, connect a reliable charger to a base with a couple of bronze leaf springs for contact with a couple of contacts on the 'bot and make arrangements for it to hit the right spot - a contact pressure of 10..15oz should work with the weight of your 'bot and make reliable contact.

Another way is a connector that you have to plug into the 'bot - I like to use 3-prong XLR's as used in pro audio (4 prong versions for when I want feedback), as they can handle the current well and are locked together, so they won't accidentally fall out (unfortunately, the cheap versions are crap and the good ones are pretty expensive compared to other connectors).
Even a Phono (RCA/Cinch) plug and socket can be used, or obviously, a DC connector, but he latter is the most un-standardized connector, so it may be hard to find good matching male and female connectors).

By charging this way you loose the protection that thermo-detectors give in good lithium chargers and you need to make sure that your charger can work reliably without it (or you can add one to the battery yourself if you know how to connect it to the charger)


[...] Had to explain that not only was I not making drugs but it was not going to blow the house up either.
And that's when your room mate rolled over and dozed off out of boredom - you have to keep a certain amount of mystique ;D
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

 


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