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Old Sep 01, 2004, 03:50 PM
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Dan Baldwin's Avatar
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another simple lipoly charger

The circuit depicted here is for experimental purposes only. Any person using the circuit, or any part of the circuit depicted here does so at their own risk.

That said, this is a fixed 2 cell, fixed 1.5 amp lipo charger. The pots are trim pots for fine tuning only. It is designed to work from a 12 volt battery. It could be modified for 1 cell, or for a different current by changing the value of components. I have been using these chargers for almost a year with no problems. The pass transistor(TIP120) will generate a fair amount of heat, so it will have to be mounted on a heat sink. The voltage is adjusted to 8.4 volts open circuit. The current should be adjusted with a load such as a 1 ohm resistor in series with the ammeter, across the battery output. The led pot is adjusted so that the led is just turned off when the output is open. The led doesn't have a clean turn off, rather it gets dimmer when the lipos are almost charged, and then goes out.

Dan
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Old Sep 01, 2004, 03:53 PM
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I'll be posting a schematic for a simple balancing circuit shortly

Dan
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Old Sep 05, 2004, 03:06 AM
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You seem to like those 431 shunt regulators, Dan.
Looks like a nice charger. You could probably fit several in one box pretty cheap to charge all the batteries at once.

Dowd
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Old Sep 05, 2004, 03:35 AM
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Dan, What would need to chhange for this to be a 1 cell charger with variable output from say .5 up to 5 amps?

I would like to build 6 of them and run them from seperate power supplies.

Larry
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Old Sep 06, 2004, 05:40 PM
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Dowd;
You're absolutely right. I seem to have an addiction to the TL431 chips. The damn thing seems to find it's way into everything I design. I've looked, and I can't find a support group for that. Anyone know of a TL431 anonymous group? I have two of these chargers in one case with a single heat sink and fan, and I have it set up with switchable current sense resistors (.5 ohm in this schematic) so I can select between .8 amp (1 ohm), 1.5 amp (.5 ohm), and 3 amp (.25 ohm) charge rate.

Larry;
The only things that would need to be changed would be the 1.2k resistor, and the 4.7k resistor, and probably the 680 ohm resistor. Let me board it up and confirm the values, and I'll post the values you need. Remember, this charger has a drop out of about 2 volts. I have been playing with the low drop out design, but I've had some problems keeping the FET from oscillating when it's being used in a linear circuit. I'll get it ironed out.

You'll have to excuse me now. I have to get my TL431 fix.

Dan
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Old Sep 06, 2004, 07:45 PM
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The calculated values for the resistors to charge 1 cell would be to change the 4.7k to 1.8k, and change the 1.2k to 290 ohm. Sorry, I hadn't read the part about variable current, .5 amp to 5 amp. There isn't a good way to do that with this design other than by changing the current sense resistors as I mentioned above. You'll have to wait for the next design that uses a couple of op amps. It should be ready by next weekend. I saw another part mentioned in another thread that should work well for a single cell charger running from a 5 volt supply, but it also is designed for a fixed charge current using a current sense resistor. You can see a schematic at
http://focus.ti.com/lit/ds/symlink/bq2057c.pdf

Dan
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Old Sep 06, 2004, 09:09 PM
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Thanks Dan!

My goal is to make several "balancing" chargers.

I have decided that I dont want to use balancers on my existing packs because they all have JST conectors wired to each cell as per Tanic packs. It would be too easy to plug the wrong balancer lead into the wrong tap.

So I want to make 3 (for now, 9 eventually) chargers that have independent power sources. I'll probably go with cheep power supplies from All Electronics or something similar. They run from $3.00 to $ 6.00 each. I want to keep cost and complexity to a minimum.

My theory is that a pack does not need to be balanced on each charge. I generally fly each pack 2 or three times and re-charge at the field as I go.

These chargers will be to use at home. I can do a "balancing charge" every few cycles to keep everything in good shape. The chargers dont need hi outpout necessarily, but my packs vary from 850 mahr to over 3000 as they are now, so some variable amp output would be very nice inorder to keep charge times reasonable on the big packs but not kill the small ones.

Thanks!

Larry
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Old Sep 07, 2004, 10:07 AM
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Larry
You can use the balancers the same way that you would use the chargers; instead of wiring all the balancers in series as the diagram shows, make separate balancers with JST connectors on each one. You just plug a balancer into each tap, and then hook it up to your charger as usual. I think that would be a whole lot easier than building separate chargers for each cell, and it would be just as effective.

I will still post the LDO charger as soon as I get all the gremlins exorcised.

Dan
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Old Sep 07, 2004, 11:51 AM
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If I may ask an impertinent question, is this circuit as simple as an L200 chip circuit? That chip will handle 1.5 amps with a heat sink. I've built a charger with one and like it.

Jim R
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Old Sep 07, 2004, 12:13 PM
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Quote:
Originally Posted by Dan Baldwin
Larry
You can use the balancers the same way that you would use the chargers; instead of wiring all the balancers in series as the diagram shows, make separate balancers with JST connectors on each one. You just plug a balancer into each tap, and then hook it up to your charger as usual. I think that would be a whole lot easier than building separate chargers for each cell, and it would be just as effective.

I will still post the LDO charger as soon as I get all the gremlins exorcised.

Dan
What a simple solution!!

That eliminates all the problems!

Why didnt anyone mention this before?

Very well done Dan!

Thanks!

I'm off to build a bunch of balancers!

Larry
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Old Sep 07, 2004, 04:03 PM
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Quote:
Originally Posted by JRuggiero
If I may ask an impertinent question, is this circuit as simple as an L200 chip circuit? That chip will handle 1.5 amps with a heat sink. I've built a charger with one and like it.

Jim R
Jim R;

No, this circuit is not as simple as the L200 based chargers. When I first designed and built this charger I hadn't heard of that chip. The only advantage that my charger might have is that it could handle higher current with a change of the current sense resistor and a large enough heat sink. I'm using one at 3 amps. I want to do some playing with the L200 one of these days, but I don't think I'll need to post a charger design based on it. Others have done a good job of that.

Dan
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Old Sep 07, 2004, 07:59 PM
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Dan,

3 Amps charging is a good enough reason for me.

Jim R
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Old May 19, 2005, 07:43 PM
myk
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Dan,

I'm currently building your lipo balancers and found this thread while looking at your 3-cell charger design. I am interested in making a dedicated 2-cell charger, like the one depicted here with the adjustable voltage pot and the current sense resistors. This circuit looks to be somewhat different from the 3-cell charger and the 2-3 cell charger and I am wondering if you have updates to this charger circuit? Also do you have resistor values for the charge current on this design?

Much thanks, I really appreciate you putting these great designs on the forum. At times I wish I would have went with EE instead of ME oh well that was a long time ago.

Mike
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Old May 20, 2005, 04:25 AM
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An acknowledged specialist has some interesting comments on the care and feeding of liPo batteries, in connection with balancers.
(Note : Li-Ion charge program safely ends the charge of unknown state batteries at 4.1V)
Well worth reading and heeding the advice given !
http://www.schulze-elektronik-gmbh.de/index_uk.htm
"Li-Po maintenance : Hints for the use of isl-chargers in combination with "balancers" "
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Old May 20, 2005, 11:24 AM
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Quote:
Originally Posted by myk
Dan,

I'm currently building your lipo balancers and found this thread while looking at your 3-cell charger design. I am interested in making a dedicated 2-cell charger, like the one depicted here with the adjustable voltage pot and the current sense resistors. This circuit looks to be somewhat different from the 3-cell charger and the 2-3 cell charger and I am wondering if you have updates to this charger circuit? Also do you have resistor values for the charge current on this design?

Much thanks, I really appreciate you putting these great designs on the forum. At times I wish I would have went with EE instead of ME oh well that was a long time ago.

Mike
The dedicated 2 cell chargers I am using are this circuit exactly as is. The current is adjustable over a fairly small range, and that pot is only there so I could get 1.5 amps charge current using a readily available current sense resistor.
Here are some ranges for different current sense resistors

.25 ohms-2.8 amps to 4 amps
.33 ohms-2.1 amps to 3 amps
.5 ohms-1.4 amps to 2.1 amps
.75 ohms-.93 amps to 1.4 amps
1 ohm-.7 amp to 1 amp
1.5 ohms -.47 amps to .7 amps
2 ohms-.35 amps to .5 amps

The only chargers I have built using this circuit are based on a .5 ohm current sense resistor, so I can't guarantee that the circuit will work well at higher currents. I do suggest that you put a 1K resistor across the LED to make it's turn on/off a little cleaner.

I also suggest setting the charge voltage for 8.2 volts (4.1 volts/cell )because it should extend the life of your batteries with minimal loss of capacity, and because most common DVMs aren't very accurate, and 4.1 volts should be safe even if your meter is a little out of calibration.

Dan
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