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Author Topic: A few questions about batteries and charging  (Read 1474 times)

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Offline z.s.tar.gzTopic starter

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A few questions about batteries and charging
« on: December 11, 2009, 12:06:22 PM »
1. How do batteries actually charge? Can I just apply a voltage and wait, or do I have to apply a greater voltage than the batteries output?

2. I've seen batteries rated like '10C' and '2C', what does that actually mean? It seems to be related to charge rate, but that's just a guess.

3. Is there anything in particular I need to have for a charging circuit, or can I just hook up some wires and let it go?

I'm working with LiFe Phosphate batteries if it matters.
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Offline waltr

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Re: A few questions about batteries and charging
« Reply #1 on: December 11, 2009, 02:07:02 PM »
I'm working with LiFe Phosphate batteries if it matters.

YES, it matters greatly as the chemistry requires specific conditions to charge the battery safely and fully.

Here is a good tutorial on Lead Acid batteries to answer some of your questions.
http://www.batterystuff.com/tutorials_index.html

Once you understand these then google the other battery chemistry you are interested in for detailed information on charging and safety. A good place to go is the manufacturer's data sheets and App Notes. Panasonic does have good info on the batteries they sell, others also do.

Short answers to your questions:
1. The applied voltage must be higher the the batteries rest voltage to have current flow from the charger to the battery. This is only part of the charge requirements.
2. The C is the battery's capacity in AHr or mAHr. So a 10C discharge on a 1AHr battery means you can draw 20Amps. A 2C charge means you can charge at 2Amps.
3. NEVER just hook up wires and let it go. Use a properly designed charge circuit. (or monitor the charging and manually act as the charge controller).

Offline z.s.tar.gzTopic starter

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Re: A few questions about batteries and charging
« Reply #2 on: December 11, 2009, 03:18:16 PM »
Ok, what if I just had an input of 9V regulated and a power monitoring IC for my microcontroller? Could I make the microcontroller work as the regulator?

What I mean is: Can just a microcontroller and a steady input be used to charge the batteries? Or are there other things I need?
« Last Edit: December 11, 2009, 03:20:32 PM by z.s.tar.gz »
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Offline Soeren

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Re: A few questions about batteries and charging
« Reply #3 on: December 11, 2009, 08:42:47 PM »
Hi,

Just wanna add to the "C": You might see something like C2 (instead of 2C) and when the number is on the right side of the letter C, you should read an invisible division sign like C/2 meaning, for the academic 1Ah battery, a rate of 500mA.

If you value your health and your home, I'd say don't try to make a lithium charger yourself, judging from the questions you ask, you just aren't ready for it and lithium can quickly become bad news, when a charge (or discharge) is not strictly controlled!

Either switch to a nickel based chemistry, or buy a good quality charger - you wouldn't give a toddler a fork to play with either and for the same reason; there's too high a chance for a sudden interruption of the good life.
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 billhowl

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Re: A few questions about batteries and charging
« Reply #4 on: December 11, 2009, 08:50:59 PM »
Please read this book
Batteries in a Portable World - A Handbook on rechargeable Batteries for Non-Engineers
http://www.buchmann.ca/toc.asp
specifically on Proper Charge Methods

"C" is the capacity, is it 10C means max 10x capacity, for eg. 2Ah battery means max current supply is 20Amps.

Offline z.s.tar.gzTopic starter

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Re: A few questions about batteries and charging
« Reply #5 on: December 11, 2009, 09:05:42 PM »
That's why I'm using LiFe, so I don't burn down my house.
And from what I can tell, chargers don't seem that complicated.
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Offline billhowl

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Re: A few questions about batteries and charging
« Reply #6 on: December 11, 2009, 10:34:27 PM »
The charge time of all Li-ion batteries, when charged at a 1C initial current, is about 3 hours. Full charge is attained after the voltage has reached the upper voltage threshold and the current has dropped and leveled off at about 3 percent of the nominal charge current.


In stage 1, batteries charged at a 1C initial current until the cell voltage limits is reached (for Li Ion & Li polymer is 4.2V, LiFePO4 is 3.9V), once the voltage threshold is reached at the end of stage 1. The charge level at this point is about 70 percent. Stage 2, batteries charged at constant voltage of cell voltage limits, charge terminate when current dropped and leveled off at about 3 percent of the rated current.

so charging Lithium batteries you need constant current/constant voltage and you need to monitor the batteries voltage and current during charge.

Read more on this ATMEGA8 Microcontroller control lithium charger
http://www.seattlerobotics.org/encoder/200210/lithiumion.htm

Offline z.s.tar.gzTopic starter

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Re: A few questions about batteries and charging
« Reply #7 on: December 11, 2009, 10:36:51 PM »
If only I could find a though-hole IC for LiFe...

Also: Thank you very much billhowl, as after reading your post it all 'clicked' together.
« Last Edit: December 11, 2009, 10:43:40 PM by z.s.tar.gz »
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