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Jul 05, 2003, 02:09 PM
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Electron:

I don't seem to be able to obtain much more than a partial charge on 3 cells. Starting with a battery which indicates a little over 10 volts, the current starts at about 1 amp, falling rapidly to about 200 Ma over about 30 seconds.

It then trails off to less than 50 ma in about a minute. Battery terminal voltage climbs to, and matches the 12.5 volts set on the charger.

Discharging the battery into a 10 ohm, 10 watt resistor mounted on a heatsink at about 1.5 amps takes only a couple of minutes to discharge it back down to around 10 volts again.

I'm puzzled, as my converted Motorola cell phone charger takes much longer to do 2 cells and will deliver about 600 ma for quite some time before the charge rate begins to decrease, eventually stopping when it reaches 8.4 volts.

I'm wondering if I shouldn't try a different power supply to feed this?

Lee
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Jul 05, 2003, 03:42 PM
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electron_head's Avatar
Hi Lee,

"After you have built it check that the output voltage is no greater than 4.2 volts per cell. Then set your multimeter up to read amps and short out the outputs with the probes to make sure the current is correct. "

Can you tell me what the Amp reading is after this test?
Could you tell me what current seting resistor you are using?


Arent you trying to charge a 3S2P pack of 1250mAH cells? This will require 2.5 Amps at 1C. The max current a LM317 is rated for is 1.5A

I assume your using some kind of Lab power supply where you can crank up the voltage and also adjust the current.

Also are the LM317s getting warm as they should be quite hot for the first hour then drop off to stone cold after that. I hope you aren't excedeing the 1.5 amp limit and they are going out on overload.

Thanks
Jul 05, 2003, 03:59 PM
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electron_head's Avatar
Sorry reading back on the thread I am now assuming you are using 2 3cell 1250mAH Etecs and paralleling them up after they are charged?

If it is a lab supply you have and you are happy satisfied the circuit is limiting the amps and the voltage never excedes 12.6 on the output side turn the amp adjust on the Lab supply to full as this is being limited on circuit anyway and then keep turning the voltage up until the ampmeter on the Lab Supply reachs the max you have set on the circuit. Note the voltage. I would go for around 20 volt input. What is the max current your supply can deliver at this voltage?

Hope we can get this sorted soon.
Last edited by electron_head; Jul 05, 2003 at 04:02 PM.
Jul 05, 2003, 04:34 PM
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electron_head's Avatar
Quote:
Originally posted by Lee Smith

I don't seem to be able to obtain much more than a partial charge on 3 cells. Starting with a battery which indicates a little over 10 volts, the current starts at about 1 amp, falling rapidly to about 200 Ma over about 30 seconds.

It then trails off to less than 50 ma in about a minute. Battery terminal voltage climbs to, and matches the 12.5 volts set on the charger.
This is the bit that really puzzles me how could your cells terminal voltage rise to a fully charged state in just 1 min at less than 1 amp. This should be over 1 hour for only a 90% charge.
Jul 05, 2003, 05:18 PM
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electron_head's Avatar
Heres a couplle of pics showing testing

1.2 ohm resistor for 1.04A current

Connected like this the current reg will get hot very quickly but the input voltage should not alter the amperage reading (above the dropout voltage that is. And the Voltage regulator should remain cool.
Jul 05, 2003, 05:24 PM
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electron_head's Avatar
For a 2 Cell pack being charged to 8.4 volts
(96 x 8.4v) - 120 = 686 ohms

Using a trimpot is not good practice in my view.

Note the different connections to the multimeter.

The more I think about the problems you are having the more I think it must be the supply isnt putting out enough voltage or amps.

I'm putting these up for others as well as you Lee so don't feel like I'm treating you like you can't use a multimeter.
Jul 05, 2003, 06:26 PM
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JBowling's Avatar
I just built a 3 cell version, fired right up. I bread boarded it in 10 minutes... now that I know it worked well I'll build a permanent version. Very nice little circuit - it filled my needs very well.

I used a 2k pot for the voltage adjustment. I'll put in a 10-turn unit in the final model.

I'm using a laptop power supply rated at 4A at 19.7V. I have not tried to completely charge a pack yet, but my initial current draw was 1.1A with a 1 ohm resistor. I plugged in a 10 ohm to drop the charge rate to 200 ma measured, since I am charging smaller cells.

Thanks for the design!

Jason
Jul 05, 2003, 08:07 PM
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electron_head's Avatar
Thanks Jason

The small stuff is where this circuit will shine as then its well with the specs of the regulators.

It would be nice if some kind off fast finishing could be implemented as that second hour of charging is a bit annoying.
Jul 05, 2003, 08:25 PM
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Happy Hobit's Avatar
A small warning.

The LM317 datasheet specifies the ďMinimum Load Current to Maintain RegulationĒ to be typically 3.5ma but not more than 10ma.

My 4.2 volt charger worked on 2.3ma, but itís a good idea to test the output with no load.

I built a charger a few weeks ago, 4.2 volts @ 125ma, using two LM317ís but I used a 5k pot for voltage reg.. I had to add a 2.2k in parallel to get it to regulate with no load.

A 2k pot for a 12.6 volt supply would probably work fine, but a 5k pot on a 4.2 volt supply wonít.


Jay
Jul 05, 2003, 08:42 PM
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electron_head's Avatar
Thanks Happy Hobit the small load and the Low ESR capacitor are good points and should really be put on. I actually considered the battery to be load enough but if a pack is left on charge for extended periods some strange things could possibly happen.

I also see you have used a 1 watt for the current regulation 5 watt is overkill here do you think?

The thing that concerned me most when building this was the accuracy of my muiltimeter as the output has to be right.
Jul 05, 2003, 09:47 PM
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Happy Hobit's Avatar
Hi Electron_Head,

>I actually considered the battery to be load enough but if a pack is left on charge for extended periods some strange things could possibly happen.

A battery charged to 4.2 volts on a 4.2 power supply is no load (except battery leakage)

This would probably take a while because of the way the charger works and I canít see any way to make it faster with the voltage limit.


>I also see you have used a 1 watt for the current regulation

No itís only 1/4 watt.

The voltage drop across the current regulator resistor (10 ohms) is 1.25 volts. I = E / R, 1.25 / 10 = .125 amps

P=E*I, 1.25 volts *.125 amps = .156 watts.

A 5 watt resistor could handle 4 amps.

Like you said it makes a fine, simple, cheep, charger.

Jay
Jul 05, 2003, 09:52 PM
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electron_head's Avatar
Hi Jay,

Your starting to show up my lack of electrical knowlege now.

So heres the circuit with the mods as suggested by Jay. If you download a datasheet for the 317 or 350 you will see the reasons for each capacitor and the output load.

Can be omitted in most cases but much better to include for peace of mind as these batteries arent cheap.
Jul 06, 2003, 12:31 AM
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Happy Hobit's Avatar
You have a fine eye to pick up the capacitors I had included.

Iíd suggest a ceramic for the input cap, Tantalum and electrolytic donít usually come that small.

I bought a 5-pack of 10 ohm resistors from Radio Shack to build 2 chargers. A 125 ma, using 1 of the resistors, and a 500ma charger using the other 4 in parallel.

Hereís the corrected schematic for my 125 ma charger.

Jay : )
Jul 07, 2003, 01:00 AM
Registered User
Electron:

Was only able to steal a couple of minutes to devote to the charger this evening.

Amps with the terminals shorted is 1.69, so it is drawing too much current for the 317 it would seem. Ther wirewound resistor measures 1.68 ohms, and is a 5% tolerance.

Voltage adjust resistor is a 2K 10 turn pot. Using 122 ohms in the output to adjust of the voltage regualting 317. In the next day or two, I'll go back to the fixed value restors I started with at the beginning.

Power supply is only voltage variable, not current. Feeding 20 volts to the input of the charger. Should be good for about 3 amps.

Will keep you posted as I get the time to tinker further. The 3 cell battery behaviour while on charge has me puzzled though.

Lee
Jul 07, 2003, 01:36 AM
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electron_head's Avatar
Lee

Check very closely the current limiting part of your circuit.

With 1.68 ohm resistor there you should only be getting 0.74 Amps across the output when shorted.

Don't attempt any battery charging until after you have got it to pass the two tests in the pictures above (Voltage and Current)

For 12.6volts the voltage resistor is 1090 ohms or less (1K and 82 ohm resistors in series or 820 ohm and 270 ohm) would be close enogh to begin with. Or whatever you have to add up right . 120 ohms is for the other fixed resistor and this always remains at 120 ohms. Agian whatever the input voltage is the output should never excede 12.6 volts


Regards ,
E


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