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Old Jun 17, 2004, 03:56 AM
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sharpsburg,ga
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Best homemade lipo charger design

I have seen a number of homemade lipo chargers and was hopeing to get some opinions on which one seems to be the most reliable, failsafe, fastest.
So far I have built the Scott Henion designed one, used it for about six months with great success. I have plans for the charger that incorporates two LM317's, one that uses a L200. I'm sure there are others.

Any opinions or facts appreciated.

Paul
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Old Jun 17, 2004, 07:29 AM
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South of Detroit
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Hi Paul,

Well, I have 5 rules for charging a LiIon battery

1.) Limit voltage to 4.2 volts per cell,

2.) Limit current to 1C or below. Charge a 500 ma hour battery at 500 ma MAX. (Or manufacturer specks)

3.) If the battery voltage is below 3 volts per cell, charge at C/10 until the voltage is above 3 volts per cell but below 4.2 volts per cell. (Lithium Ion Batteries should NOT be discharged below 3 volts per cell.)

4.) Do not charge a battery if the ambient temperature is above 45 degrees C or below 0 degrees C. (Or manufacturer’s specs).

5.) Charge in a safe location. (A damaged Lithium Ion Battery may burn violently.)



All the homemade charges you listed all follow rules 1 & 2 but none of them follow rules 3, 4 &5.

It is the users responsibility to enforce these Rules. Some commercial charger may enforce rule #3.

Jay
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Old Jun 17, 2004, 03:05 PM
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Blacksburg, VA 24060 USA
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Happy Hobit is correct on all points. That said, I built a charger using the L200 chip, and it works fine. It's simpler than the 2 LM317 design.

As for HH's Point 3: I check my pack with a voltmeter to determine the state of charge before charging. If it ever got below 3 volts per cell (it never has), I'd do the C/10 charge. Re Point 4: ambient temp check is not hard to do. Point 5: I charge my packs outside on the driveway.

Jim R.
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Old Jun 17, 2004, 03:07 PM
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Good idea. Not in the sun I hope.
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Old Jun 17, 2004, 03:40 PM
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Happy Hobit might also have added a 6th point: a simple charger can't determine the state of imbalance among cells in a pack. Would charging at C/10 reduce this imbalance?
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Old Jun 17, 2004, 05:23 PM
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South of Detroit
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Hi Jim,

(I could add this as a rule but it’s getting kind of wordy)

6.) If there is a voltage imbalance in a series pack, the only safe thing to do is parallel charge the cells. Series charging an ‘Imbalanced Pack’ will result in overcharging the ‘good’ cell and LiIon cells don’t handle this well.

Here’s an interesting article I got the graph below from.

http://www.planetanalog.com/printabl...cleID=12802557


When NiCad and NiMh cells are fully charged they dump the additional current as heat.
LiIon cells just overcharge. See fig2.

Jay
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Old Jun 18, 2004, 01:31 PM
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So based on the links various charger descriptions, most if not all of the current DIY LiPo chargers are of the simplest design, slowest to charge, Current Limited + Constant voltage types that taper-off the charging current steadily and well below the battery's allowable charging current as the battery's voltage approaches it's finishing voltage......This " reduced and tapered-off charge current before 4.2 volts per cell" effect is probably due to the rather high ohm value, fixed, series connected resistor that is calculated to determine the charger's max charging current ..It probably doesn't harm anything, but there's got to be less total charge ma's inputed to the battery over same 2 hrs when compared to chargers that automaticly reduce their internal impedance to a much lower value and remain at a true CC (like C1) till the 4.2 v is reached........ kw
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Old Jun 18, 2004, 03:01 PM
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Hi

The L200 works fine.
Take a look for a diagram at my homepage:
http://users.cybercity.dk/~ccc16084/
follow the "electronic" link

Soren
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Old Jun 18, 2004, 05:04 PM
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The L200 by itself wired-up in the CV and self-current limiting 2 amp applications without any < 2 amp output series resistor added does work great....but in the smaller LiPo battery size, say the 340 ma for instance, the required added current limiting series resistor of over 1 ohm is going to make keeping the charging current anywhere near 340 ma even at 4.0 cell volts impossible to achieve....The initial 340 ma charging current at 4.00 cell volts is probably going to have dropped to about 200 ma, not exactly following the full 340 ma CC charge current all the way to 4.20 cell volts charging curve ......... kw
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Old Jun 18, 2004, 06:48 PM
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Hi KillerWatt,

The article was interesting, but not particularly well written.

The writer of the article tried to describe different charging techniques but to my knowledge there is only One CORRECT way to charge a LiIon (LiPoly) battery. (See rules above)

I can’t speak for the L200 regulator, although I think it works the same, but the charging curve for the Dual LM317 is below.

The charger outputs MAX current (1C) (Rule #1) until the voltage reaches 4.2 volts. Then rule #2 kicks in.

The battery is not truly at 4.2 volts now. It’s probably at 4.1 or 4.0, but in order to pump in 1C current the charger voltage would have to be higher.

The current starts dropping now as the battery fills up.

This is the only way I know to charge LiIon cells.

The most accurate description would be ‘Current Limited / Voltage Limited’.

The battery has a natural internal resistance (ESR) not something added


P.S. Here’s Part 2 of the article. http://www.planetanalog.com/printabl...cleID=12802558

P.P.S. Based on the article writer the battery is fully charged in 60 min. (Charge current drops below .1C)

Jay
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Old Jun 18, 2004, 11:27 PM
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Edgewood, NM
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What about this:

http://www.rcgroups.com/forums/showt...ight=zagisrule

My design uses computer-controlled switching to determine state of pack, then delivers a CC charge, switching to CV at the manufacturer's suggested switch point. Turns off CV charge when the battery peak is reached.

I have gone no farther with the design yet, no need to here and not enough time to spend.

For some reason nobody really had any interest in the unit, while both I and my friend ("Roger Doger" on the forum) use the chargers and find them to work just like they should.

Here is a picture of my buddy's unit, up and running.



-Matt
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Old Jun 19, 2004, 10:09 AM
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Hi Matt,

I read your ‘Superb DIY Lithium Charger’ thread and it’s a nice design, but other that shutting off at end of cycle, there’s not much new.

And not everyone feels comfortable with microcontrollers. There’s a pretty steep learning curve. Learning a programming language (even basic), learning microcontroller architecture and then flashing the chip. (I’ve been playing with micros for 30 years and I sometimes forget this to.)

A few suggestions.

Get rid of the LM317 and the LM350 and use PWM to control the voltage and current.

Check-out the ‘LM1577 Step Up Voltage Regulator’. It can handle 3 amps and step-up 12 volts to 20+ volts.

Add a ‘Low Voltage Pre charge’. If the battery voltage is below 3 volts per cell the charger starts at C/10 until the battery voltage is above 3 volts.


Jay
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Old Jun 19, 2004, 02:07 PM
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Quote:
Originally Posted by Happy Hobit
Hi Matt,
Check-out the ‘LM1577 Step Up Voltage Regulator’. It can handle 3 amps and step-up 12 volts to 20+ volts.
Jay
Use the LM2587 step up voltage regulator - it can handle up to 5A - the current in the regulator is not the samte as the output current...

Soren
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Old Jun 19, 2004, 10:11 PM
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That was my plans for the new design, using MOSFET's to switch PWM to control voltage and current. I am trying to figure out how to do a reliable auto-cell count, but have not come up with anything yet. The start-up trickle charge would be easy...

Step up might be easier because I have found some great transformers that could do 40V at several amps from 12V...perhaps too much voltage? Because if the PIC or MOSFET's were to fail that would start a big fire

Wait....current can be controlled in with a feedback loop consisting of shunt resistor and ADC. If the PIC senses less than requested current, simply increase duty cycle a small amount until the current requested = the current delivered. If current is too high, then the charger lowers duty cycle to maintain requested current.

Constant voltage would just sense voltage output with ADC and adjust duty cycle accordingly, to deliver the voltage requested.

Hahaha! I have it figured out..we just need some active feedback and then we don't have to worry about math and we will have a spot-on output! Bam!

LCD is very easy to interface, so that can be included easy. Would a charger such as this sell in kit form? So people don't have to worry about the learning curve associated with MC's? Or just pre-programmed PIC's and PCB's?




-Matt
....thinking now....
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Old Jun 19, 2004, 10:18 PM
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BTW: Soren, are you that same "Soren" on diyaudio.com? I knew I saw that name somewhere else! RC Aircraft and Audio...the two best hobbies on the face of the earth!




-Matt
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