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Old Nov 29, 2004, 07:30 PM
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Dan Baldwin's Avatar
United States, CA, Norwalk
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Simple 3 cell LIPOLY charger

I have been asked to post a 3 cell version of the charger I posted a while back, so here goes. This charger has a couple of advantages over the LM317 type chargers. It has a lower minimum voltage drop, and it can be used at higher currents if an appropriate heat sink is used. The TIP120 pass transistor is rated at 5 amps.


QTY__MOUSER PN______DESCRIPTION_______________________PRICE
1___511-TIP120_______TIP120 DARLINGTON POWER TRANSISTOR_.58 EA
1___511-TL431ACZ____ADJUSTABLE SHUNT REGULATOR _________.28
2___625-MPS2222A____SMALL SIGNAL TRANSISTOR_____________.06
2___531-PT10MH-200__200 OHM POT_________________________.40
2___660-CF1/4L101J___100 OHM RESISTORS___________________.05
1___660-CF1/4L331J___330 OHM RESISTORS___________________.05
1___660-CF1/4L471J___470 OHM RESISTORS___________________.05
1___660-CF1/4L681J___680 OHM RESISTORS___________________.05
1___660-CF1/4L222J___2.2K OHM RESISTORS__________________.05
1___660-CF1/4L332J___3.3K OHM RESISTORS__________________.05
1___604-L1543SGC____GREEN LED___________________________.12



The current sense resistor could be one of the following values, or a switch could be used to switch between different resistors for different charge currents. Any combination of resistors can be hooked in parallel- just add the currents for each of the parallel resistors from the chart below.
MOUSER PN____DESCRIPTION________CHARGE CURRENT
280-CR5-2.0___2.0 OHM RESISTOR_____325 MA
280-CR5-1.3___1.3 OHM RESISTOR_____500 MA
280-CR5-0.91__.91 OHM RESISTOR_____700 MA
280-CR5-0.75__.75 OHM RESISTOR_____870 MA
280-CR5-0.62__.62 OHM RESISTOR_____1000 MA
280-CR5-0.56__.56 OHM RESISTOR_____1200 MA
280-CR5-0.43__.43 OHM RESISTOR_____1500 MA
280-CR5-0.33__.33 OHM RESISTOR_____2000 MA
280-CR5-0.27__.27 OHM RESISTOR_____2410 MA
280-CR5-0.22__.22 OHM RESISTOR_____3000 MA

A heat sink will have to be used. The size of this heat sink will depend on how much charge current will be used, and how high the power supply voltage being used is. If a 340 ma lipoly pack is to be charged at 325 MA, a small screw on heat sink could be used, but if a pack is going to be charged at 2000 ma (2 amps) a large heat sink, and possibly a fan, will have to be used. A larger heat sink will also be needed if higher input voltage is used.


Dan
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Old Nov 30, 2004, 10:12 AM
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Dan, Lets start off with how helpful you have been to all. Thank you so much. This 3cell charger looks great. Having built the SH charger already, I had inquired to you about the 3cell version. With SH's design having a 4 voltage drop you recommend yours with what seems to be around a 2volt drop. Great. On to the question at hand. Is it my understanding that this charger has a fixed voltge output? Compared with SH's with the adj pot.?
Also I have a wall plug dc transformer rated at 16v 900ma, 20v unloaded, and am wanting to build your charger for a 3cell 1320 or 1500ma pack. Will this 900ma transformer increase my charging time? Or in another word would there be any reason for me to put anything higher than 870ma current sense resistor?
Once again thanks for all the help and please keep up the great work.
Jon
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Old Nov 30, 2004, 10:26 AM
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Dan Baldwin's Avatar
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Jon
This charger is similar to Scott Henion's second charger in that it has a pot, but the pot is strictly for calibration, not for changing between cell counts. It would be possibly to add a switch to select between 2 or 3 cells.

Your 16 volt 900 MA wall transformer should work fine, but you will have to limit the charge current to 900 MA or less, so yes, it will make your 1320 and 1500 mahr battery packs take longer than they need to. The 1500's will probably take well over 2 hours to charge. There would be no reason to set the charger up for anything higher than 870 MA, and the wall transformer may get hot at that current. You can always change the current sense resistor, or add a switch to select charging currents if you find a power supply that will put out more current.


Dan
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Old Nov 30, 2004, 10:55 AM
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Thanks for the reply Dan. Also I've noticed the use of 2 pots, one being to adjust voltage the other for turning the led off or adj the ma output? Thanks in advance
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Last edited by mrfliboy; Nov 30, 2004 at 10:56 AM. Reason: typo
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Old Nov 30, 2004, 11:03 AM
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The second pot is for the LED. The LED doesn't have a hard turn on and turn off, instead it starts to get dimmer as the battery aproaches full charge and charge current goes down, and will eventually go out. If I have plenty of time, I wait until the LED goes out completely to cram every electron I can into the battery pack, but if I'm in a hurry, I pull it off the charger as soon as the LED starts to dim. I'll post details of how it is set up when I post calibration information later today.

Dan
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Old Nov 30, 2004, 12:29 PM
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I was planning on posting instructions on calibrating the charger using a precision adjustable shunt regulator, but It would probably be better in most cases to just use the precision shunt regulator (.25%) along with some .1% resistors in the circuit and eliminate the voltage calibration pot. The full charge voltage on this charger would be about 8.35 volts for a 2 cell pack, and 12.55 volts for a 3 cell pack. The precision regulator (TL1431ACZ) costs more than the TL431 it replaces, and the 3 precision resistors cost MUCH more than 5% resistors, making the charger cost about $3.00 more, but probably well worth the difference in cost. If all of the tolerances of the precision reference and the resistors were off to their maximum allowable tolerance, the voltage for 3 cells could be as low as 12.50 volts, or as high as 12.598 volts, which seems like an acceptable range. If someone wanted to run different voltages, such as deliberately running lower than 4.2 volts per cell to extend the life of their batteries, they could still use the original circuit and calibrate it, or change the values of the precision resistors in this schematic.

Dan


EDIT

The LED circuit shown in this schematic will not work properly when switching between 2 cells and 3 cells. The schematic shown on page 3, post #36 corrects this problem. This schematic will be replaced in a few days.
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Last edited by Dan Baldwin; Dec 13, 2004 at 06:19 PM. Reason: Changes to schematic shown in later post
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Old Nov 30, 2004, 12:45 PM
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soren's Avatar
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Hi

Electronics are fun I know that, but when you only need less that 2A you can make a charger with one simple single L200 IC.
Look at my homepage: http://users.cybercity.dk/~ccc16084/ and follow the "electronic" link, and then the "LIPO" link.
There you can find a diagram for a L200 charger with a LED that show the charging status, and a simple switch mode powersupply that make it possible to charge 3 LIPO cells from a 12V battery .
....I agree that this charger only can give 2A....

In addition to the many L200 chargers that I have, I must build a "Dan Baldwin charger" for the bigger packs.
Nice work Dan.


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Last edited by soren; Nov 30, 2004 at 12:54 PM.
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Old Nov 30, 2004, 12:53 PM
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What a great idea Dan. Problem being I really enjoyed messing with the RS 1k pot from the SH design charger. NOT!!! (Finally replacing it with a 15 turn pot) The fact you've made it quite versatile is fantastic. As the parts come in I'll try and keep you posted on the results. Thanks again.
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Old Nov 30, 2004, 02:29 PM
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rance's Avatar
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Dan,

This is GREAT! Thanks for providing this to the group. So could you tell me the switched resistor value for a single cell? TIA.

Rance
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Old Nov 30, 2004, 03:24 PM
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Quote:
Originally Posted by rance
Dan,

This is GREAT! Thanks for providing this to the group. So could you tell me the switched resistor value for a single cell? TIA.

Rance
If you put an 806 ohm resistor (66-RC55-D-806) across the 4.02K resistor instead of the 5.62K, it would give you a voltage of 4.18 volts, just about perfect to charge 1 cell. If you wanted to build a fixed 1 cell charger, you could substiture a 768 ohm resistor (66-RC55-D-768) for the 4.02K, and 1.13K (66-RC55-D-1.13K)for the 1.0K.

Dan
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Old Nov 30, 2004, 11:47 PM
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Hey Dan: I have a question. I don't have a 15VDC PS, but I have the makings for one. Got a full wave bridge, and a 5800 uF electrolytic cap. These are from a 36VDC PS for an old spot welding mahine - the control part, that is. Used the Tx for the PS for my foam cutter (puts out 27VAC or so). Anyhow, I figure I should be able to hook these up to a 120/12V Tx and use them. Will the ripple affect things? How much current can I draw with that cap if the ripple is a problem? If you give me a thumbs up on this, the charger and 3 of your balancers will all go into the same case - one stop charging!

Guess that means I owe you 4 thumbs up. Gotta collect me some more body parts....

Brad
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Old Dec 01, 2004, 09:22 AM
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Brad

Power supply ripple won't cause any problems other than to reduce the average charge rate if the bottom of the ripples is below the minimum neccesary voltage. A 12 volt transformer is a little low, and it will probably end up limiting the maximum charge current. How much current are you looking for?

All Electronics has a 20 volt regulated power supply that is rated at 3 amps for $14.00
http://www.allelectronics.com/cgi-bi...231&type=store
That's a little high, and would result in more heat, so you would need a larger heat sink, and probably a fan, but it would work.

Of course if you only want to charge 1 cell or 2 cell packs, your 12 volt power supply would work great.

No hurry on getting me the thumbs. I have 10 of them.

Dan
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Old Dec 01, 2004, 10:19 AM
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All electronics also has a 15 volt 2.4 amp AC wall transformer for $4.25. You could put that together with your bridge and cap to make a good PS for the charger.
http://www.allelectronics.com/cgi-bi...152&type=store

Dan
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Old Dec 01, 2004, 11:24 AM
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Dan: 12V * 1.414 = ~17VDC. Don't forget that the cap will be smoothing off peak voltage, not RMS.

Brad
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Old Dec 01, 2004, 02:30 PM
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Quote:
Originally Posted by arx_n_sparx
Dan: 12V * 1.414 = ~17VDC. Don't forget that the cap will be smoothing off peak voltage, not RMS.

Brad
Absolutely correct. The voltage from your supply would be almost 17 volts with no load, but the voltage at the peaks starts to sag as the load increases due to the internal impedance of the transformer. Remember that if you have a transformer that is rated at 1 amp, and you are pulling 1 amp from the power supply, the current flow at the voltage peak will be MUCH higher than that. If you are using a transformer that is rated at a much higher current than you are using, then you can depend on getting more than the nominal RMS rating of the transformer, but it will be below the calculated peak voltage.

But you can give it a try if you like.

Dan
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