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Author Topic: Battery selection advice  (Read 821 times)

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Offline ConscriptedTopic starter

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Battery selection advice
« on: January 30, 2011, 02:54:19 PM »
Good day all. I'm building a small line following robot (about the size of a pack of playing cards). I've got all of the pieces I need to build the mechanics. The last consideration I need to make before I start building is the power source. I'm using an ATTINY85 as the brain. I'm using 3 sensors and will run the whole thing off of a 3.3 vdc regulator. The motors are rated to stall at 260 mA although I doubt they will be anywhere near stall during normal operations. I've seen people use coin batteries on small projects but I'm not sure if they would be best for my application.

What would you use?

Thanks
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Offline waltr

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Re: Battery selection advice
« Reply #1 on: January 30, 2011, 05:30:52 PM »
My choice would be a single LiPo battery of about 900mA-Hr. A Buck-boost converter (LTC3440) for the electronics and run the motors directly from the battery.
The flat shape of LiPo batteries are ideal for fitting the most power into a small space.

SparkFun has a 900mAHr LiPo that is 0.23x1.16x2.0" (5.84x29.5x51mm) and weights 18.5g (0.65oz).
http://www.sparkfun.com/products/341

Offline Soeren

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Re: Battery selection advice
« Reply #2 on: January 30, 2011, 05:53:35 PM »
Hi,

I picked up some flat Li-Ion cells rated 650mAh, made as replacements for the Nintendo DS.
I don't know if the DS is on the way out, or the dealer had just over stocked, but I got them for around $2.50 a pop.
I guess you could find similar offers in several web shops.

Given the motors stall current, I'd estimate a run time of a couple of hours with such a 600mAh cell.
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 ConscriptedTopic starter

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Re: Battery selection advice
« Reply #3 on: January 30, 2011, 09:55:00 PM »
Hi,

I picked up some flat Li-Ion cells rated 650mAh, made as replacements for the Nintendo DS.
I don't know if the DS is on the way out, or the dealer had just over stocked, but I got them for around $2.50 a pop.
I guess you could find similar offers in several web shops.

Given the motors stall current, I'd estimate a run time of a couple of hours with such a 600mAh cell.



I don't see a price quite that low but it should work. What is the most frugal way to charge a single cell li-ion battery? Also is a single sell enough to power THIS? I read the data sheet but found it confusing.

Thanks.
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Offline galannthegreat

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Re: Battery selection advice
« Reply #4 on: January 30, 2011, 11:18:54 PM »
I'd recommend you go with a charging circuit that Sparkfun already sells, as I personally wouldn't trust anything but an appropriate charger/charging circuit for Li-ion chemistries. Also since the regulator you have chosen there will work with a 3.7V power source, as the regulator is Low Dropout, meaning you can put close to its output voltage and still have regulation.

http://www.sparkfun.com/products/10161

« Last Edit: January 30, 2011, 11:21:08 PM by galannthegreat »
Kurt

Offline Soeren

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Re: Battery selection advice
« Reply #5 on: January 31, 2011, 09:36:32 PM »
Hi,

What is the most frugal way to charge a single cell li-ion battery?

A current limited voltage supply - Can be build with eg. an LM317 and a bit of "glue".


Also is a single sell enough to power THIS? I read the data sheet but found it confusing.

Not really.
Read page 6.

Output Voltage (@ Iout = 0 to 800 mA and Vin = 4.75 to 10 V) 3.235 to 3.365 V

Vd Dropout Voltage
IO = 100 mA 1.0V (typ)  1.1V (max)
IO = 500 mA 1.05V (typ)  1.15V (max)
IO = 800 mA 1.10V (typ)  1.2V (max)

That means that if you need 3.3V out at 500mA, you need 4.45V minimum in (worst case)
And your Lithium battery will go down to 2.5V..3V over its discharge life - remember to have an automatic switch off before it goes too low!

You need something like a buck/boost, SEPIC or Cuck regulator to pull it off. This will also be the most lean and green way to go.
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 Soeren

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Re: Battery selection advice
« Reply #6 on: January 31, 2011, 09:46:06 PM »
Hi,

I'd recommend you go with a charging circuit that Sparkfun already sells, as I personally wouldn't trust anything but an appropriate charger/charging circuit for Li-ion chemistries.
But you find it OK to use a LiPo charger for a Li-Ion cell ???


Also since the regulator you have chosen there will work with a 3.7V power source, as the regulator is Low Dropout, meaning you can put close to its output voltage and still have regulation.
There's LDO's and there's LDO's...
This one has a huge drop out of more than 1V, so it won't even do when fresh out of a charger - and no battery cells stay at their fully charged voltage for long.
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 galannthegreat

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Re: Battery selection advice
« Reply #7 on: February 01, 2011, 12:34:16 AM »
Hi,

I'd recommend you go with a charging circuit that Sparkfun already sells, as I personally wouldn't trust anything but an appropriate charger/charging circuit for Li-ion chemistries.
But you find it OK to use a LiPo charger for a Li-Ion cell ???


Also since the regulator you have chosen there will work with a 3.7V power source, as the regulator is Low Dropout, meaning you can put close to its output voltage and still have regulation.
There's LDO's and there's LDO's...
This one has a huge drop out of more than 1V, so it won't even do when fresh out of a charger - and no battery cells stay at their fully charged voltage for long.

Whoops, got me on that one, I meant Li-poly. You can probably negate what I said earlier, but with those regulators I haven't had issue with 3.7V cells in my experiences, but that was with light loads so it would change with heavier loads (obviously).
Kurt

 


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