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grunf
PostPosted: Feb 26, 2005 - 12:54 PM
Newbie


Joined: Feb 26, 2005
Posts: 13
Location: Budisava, Serbia

Hi,

I was thinking of building a acid lead battery charger for my car battery(12V, 48Ah). Of course I will use AVR uC(probably 4433). It's still a concept but I need some suggestion from someone who actually built one. The dilemma is wheater to use a simple constant current source(with bjt and resistor) and monitoring circuit or to use a switcher(step up) like in Atmel'a app note 450(I think).It need to be robust, reliable and universal. With one switch one could select appropriate charging current(10% of battery's capacity) so that a charger could be used to charge batteries with different capacity as well.
I've browsed the forum looking for some solutions but came up with nothing.
Could anyone provide a good link to some battery theory sites? I'm interesting in a design ideas and designs that work. AVR software is not my primary concern.A good and robust charger circuit that works well
is more important.

Thanks!
 
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tschultz
PostPosted: Feb 26, 2005 - 02:45 PM
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Joined: Apr 12, 2004
Posts: 57
Location: Toronto, ON, Canada

Charging lead acid with constant current requires very close monitoring of the battery voltage. The battery is fairly sensitive to overcharge, this will stress the anode/cathode interface too much. You can safely charge this way as a stage 1 charge, then switch to current limited, constant voltage charge as a stage 2 charge. This way the constant current is a way to get lots of energy in the battery at the beginning, then switch to CV charge to prevent overcharging. The actual parameters are determined by the battery specs. Usually you are safe with a 1/10C charge rate.

Another method that can be used with Lead Acid is pulse charging. This method applies a current pulse to the battery, usually 1.5X the max average current. Then the current is removed, you wait a short time, measure the battery voltage. If the battery voltage is less than the float target then you apply another charge pulse. This method has the advantage of being lower cost and provides a fairly fast charge. A micro is usually required for this type of charge control. The charge/wait pulses are often sync'd with the 50/60Hz AC power. Active voltage/current regulation of the charge pulse is not normally required as the regulation is handled with the duty cycle. End of charge indication is normally done be counting the number of pulses that ar skipped, the pulse count never goes to zero because of self-discharge in the battery itself.
 
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ignoramus
PostPosted: Feb 26, 2005 - 03:09 PM
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Joined: Aug 04, 2002
Posts: 1943
Location: Brisbane, Australia

UNITRODE make a chipspecifcally aimed at the charging problem.

Search their site for app notes..theyhave a lot of good oil.
 
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grunf
PostPosted: Feb 26, 2005 - 03:37 PM
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Joined: Feb 26, 2005
Posts: 13
Location: Budisava, Serbia

[quote="tschultz"

Another method that can be used with Lead Acid is pulse charging.

Do you have any schematics regarding this type of charging?
 
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ekh
PostPosted: Feb 28, 2005 - 12:25 AM
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Joined: Sep 11, 2001
Posts: 189
Location: Denmark

I have seen Atmel has an application note and code for several battery types.
Be aware of the diagram, there has been threads reporting problems. But maybe the code can be usefull.

Erik
 
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tpappano
PostPosted: Feb 28, 2005 - 05:35 AM
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Joined: Dec 01, 2003
Posts: 2626


Needing to incorporate 12 volt 7 AH lead-acid battery management into a new product,
I looked at various dedicated battery charger chips. They all seemed to require
a lot of parts to operate, and seemed too expensive. I decided to use a Tiny
26 to do the job, using high speed pwm to implement a switching regulator,
while doing all the voltage and current profiling and timing tasks. Works great,
uses little real estate, and I can change it's personality by simply loading a new program.

Tom Pappano
Tulsa, Oklahoma
 
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smileymicros
PostPosted: Feb 28, 2005 - 03:38 PM
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Joined: Nov 17, 2004
Posts: 6496
Location: Great Smokey Mountains.

tpappano wrote:
Needing to incorporate 12 volt 7 AH lead-acid battery management into a new product,
I looked at various dedicated battery charger chips. They all seemed to require
a lot of parts to operate, and seemed too expensive. I decided to use a Tiny
26 to do the job, using high speed pwm to implement a switching regulator,
while doing all the voltage and current profiling and timing tasks. Works great,
uses little real estate, and I can change it's personality by simply loading a new program.

Tom Pappano
Tulsa, Oklahoma



Yum, that tastes great. Where can I get the recipe?

Smiley

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tpappano
PostPosted: Mar 01, 2005 - 10:13 PM
Raving lunatic


Joined: Dec 01, 2003
Posts: 2626


Quote:
Yum, that tastes great. Where can I get the recipe?


I can't get too specific because it is a "revenue" system, but
basically, I'm doing a 125 khz buck regulator consisting of a
LM2725 fet driver (that provides shoot-through protection)
driving a dual n-channel SO-8 fet as a synchronous rectifier.
Current monitoring is in the battery high side using 0.1%
resistors to translate the shunt voltages down to the adc range.
The precision resistors are needed to provide a reasonable
accuracy for current measurement. Additionally, at startup,
bridge balance and adc gain offset is tested and compensated.
The charging profile is per battery manufacturer specs, fast charging
at 1.5 amps until about 14.7 volts, then dropping back to .5 amps
and float charging at 13.7 volts. Timers provide limits on the
maximum time spent at each level. An external precision reference
is used for the adc so that I don't have to tweak anything. The
fets run negligible temp rise, in fact, the only thing that gets warm
at all is the inductor, due to it's resistance.

Tom Pappano
Tulsa, Oklahoma
 
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abcminiuser
PostPosted: Mar 01, 2005 - 11:01 PM
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Joined: Jan 23, 2004
Posts: 10215
Location: Melbourne, Australia

Silicon Chip (Australian Magazine) had a two-part feature article in the last couple of months that details a very advanced lead-acid battery charger with cycle times and temperature compensation. The whole thing was driven by a ( Evil or Very Mad ) PIC, however I have no doubts you could change the program to an ( Very Happy ) AVR.

I think their website is www.siliconchip.com.au. Have fun!
- Dean Twisted Evil

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