Simple CCCV Transmitter Charger/Maintainer

[SamoaFlyer]

I was getting tired of waiting a few days for my transmitters to charge using the stock chargers after upgrading to high capacity Nimh packs. Also, trying to keep track as to who needs a charge before flying was taxing my limited brain-power. I have enough other things to think about.

What I want is a charger that can handle multiple (I have four) transmitters, have a reasonable charge time, and be able to be left connected all the time so that I can just grab one and go fly on a moments notice without being concerned about over-charge or lack of charge.

So, a new charger was in order. It turned out to be quite simple even considering that a constant-current, constant voltage (CCCV) type was indicated to meet the requirements. Two active devices and a small handful of parts does the trick.

Specs:
Charge current: 200mA, divided as needed among the transmitters.
Float voltage: 1.40V per cell, 11.2V for the typical 8-cell pack.
Float current: 1-10mA depending on battery type.
Input source: 12Vdc power cube, 500mA.

For my usage, seldom do I need to put a significant charge into more than one at a time so the full 200mAh is automatically available for that one.

A LM317 (usually used as a voltage regulator) is used here as a 200mA current source with the current determined by the 6R resistor. R=1.2/I. It’s spec’d up to 1A with proper heat sinking. This is followed by a rather crude, but simple, shunt voltage regulator consisting of a N-channel, TO-220 case FET and a pot. This uses the FET threshold voltage as a reference point so it’s very dependent on the individual FET, hence the pot to adjust it. As I say, crude, but adequate for this task.

Since I like to see what’s going on, I included a small, 250mA ammeter. A single LED just doesn’t do it for me. Diodes provide isolation between the transmitters. Don’t forget about reversed polarity for JR transmitters.

I’ve found that most 12V power cubes will put out 15-16Vdc at half their rated current which works out fine here since the minimum input voltage needed is 13.0V.

Using common values, the 6R, .05W resistor can be made from two 12R, 0.25W resistors in parallel. Almost any TO-220 case N FET can be used. Small clip-on heat sinks are necessary on both devices. Since so few parts are involved, I simply space-wired them on the back of the meter. A PC board would be nice but not worth the trouble in this case.

I’ve run this charger 24/7 for about six months without a hitch. Sure is nice not to have to worry about transmitter charging – just grab one and go. Now, I can think about more important things when I get ready to fly – like, where did I put the beer opener.

One interesting thing I noticed is the difference in float current between battery brands. Cheap Tenergy 2000mAh packs float at 8-10mA while the more pricey Eneloops are at <1mA at 1.4V/cell.

[rich smith]

A little confused as CCCV chargers are generally for lipo and peak or trickle for NiMh.


I was getting tired of waiting a few days for my transmitters to charge using the stock chargers after upgrading to high capacity Nimh packs. Also, trying to keep track as to who needs a charge before flying was taxing my limited brain-power. I have enough other things to think about.

What I want is a charger that can handle multiple (I have four) transmitters, have a reasonable charge time, and be able to be left connected all the time so that I can just grab one and go fly on a moments notice without being concerned about over-charge or lack of charge.

So, a new charger was in order. It turned out to be quite simple even considering that a constant-current, constant voltage (CCCV) type was indicated to meet the requirements. Two active devices and a small handful of parts does the trick.

Specs:
Charge current: 200mA, divided as needed among the transmitters.
Float voltage: 1.40V per cell, 11.2V for the typical 8-cell pack.
Float current: 1-10mA depending on battery type.
Input source: 12Vdc power cube, 500mA.

For my usage, seldom do I need to put a significant charge into more than one at a time so the full 200mAh is automatically available for that one.

A LM317 (usually used as a voltage regulator) is used here as a 200mA current source with the current determined by the 6R resistor. R=1.2/I. It’s spec’d up to 1A with proper heat sinking. This is followed by a rather crude, but simple, shunt voltage regulator consisting of a N-channel, TO-220 case FET and a pot. This uses the FET threshold voltage as a reference point so it’s very dependent on the individual FET, hence the pot to adjust it. As I say, crude, but adequate for this task.

Since I like to see what’s going on, I included a small, 250mA ammeter. A single LED just doesn’t do it for me. Diodes provide isolation between the transmitters. Don’t forget about reversed polarity for JR transmitters.

I’ve found that most 12V power cubes will put out 15-16Vdc at half their rated current which works out fine here since the minimum input voltage needed is 13.0V.

Using common values, the 6R, .05W resistor can be made from two 12R, 0.25W resistors in parallel. Almost any TO-220 case N FET can be used. Small clip-on heat sinks are necessary on both devices. Since so few parts are involved, I simply space-wired them on the back of the meter. A PC board would be nice but not worth the trouble in this case.

I’ve run this charger 24/7 for about six months without a hitch. Sure is nice not to have to worry about transmitter charging – just grab one and go. Now, I can think about more important things when I get ready to fly – like, where did I put the beer opener.

One interesting thing I noticed is the difference in float current between battery brands. Cheap Tenergy 2000mAh packs float at 8-10mA while the more pricey Eneloops are at <1mA at 1.4V/cell.

[SamoaFlyer]

Rich,
Yes, indeed. One main objective here was to allow the charger to be connected 24/7 so the transmitter(s) are always ready to go. This requires the CV function at 1.4V/cell to keep from over-cooking the batteries. A stock 50-120mA charger (constant current only) will allow the voltage to rise to over 1,5V/cell and is not recommended to be left on all the time. At 1.4V the fully charged battery's current will drop to a trickle level of 1-10mA. This can be maintained indefinitely.

This voltage limit does increase the full charge time somewhat but is a small price to pay for the convenience of plugging it in and forgetting it.

Bill

[sntlewis]

What would I need to change to lower the voltage for a 4-5cell receiver pack?

[SamoaFlyer]

No problem, as long as the threshold voltage of the FET is lower than the desired output voltage it'll work as is. Logic level FETs would do well.

[rich smith]

I've always been curious about this idea of how to keep batteries charged but nobody's hired me to study this aspect yet so not an expert. I do know mfg say you can't just use fixed voltage or trickle charge as these shorten life. After peak detect good NiMh wall chargers sense voltage periodically and "pulse" charge if necessary.

Also note that what we call CV supplies must have current limit too or they will cook a battery in minutes. I.e. if you put 1.4v 100a supply across a fully discharged NiMh it will get very hot and lose some electrolyte. There is no such thing in the real world as a true CV supply.

Where did you get the idea for your method?

Rich,
Yes, indeed. One main objective here was to allow the charger to be connected 24/7 so the transmitter(s) are always ready to go. This requires the CV function at 1.4V/cell to keep from over-cooking the batteries. A stock 50-120mA charger (constant current only) will allow the voltage to rise to over 1,5V/cell and is not recommended to be left on all the time. At 1.4V the fully charged battery's current will drop to a trickle level of 1-10mA. This can be maintained indefinitely.

This voltage limit does increase the full charge time somewhat but is a small price to pay for the convenience of plugging it in and forgetting it.

Bill

[sntlewis]

So if my math is right....

1) reduce input voltage to around 7.50 vdc (to allow for drop through the lm327), thereby giving a Vout of around 6.25 Vdc

2) replace the 50K pot with a 30K pot as gate is 2-4 volts...

right?

Stan

[SamoaFlyer]

Rich,
Yes, I'm aware that some manufacturers don't recommend trickle charging Nimh but I think it depends somewhat on the definition of trickle charging. It's been my experience that low float currents of a few mA do no harm and in some cases actually improve capacity.

This design is not actually a CV charger. Most charge input to the battery is at constant-current as it should be for Nimh or NiCd. Part of the problem is the CCCV name. It's really a misnomer since neither current nor voltage is constant all the time.. Perhaps a better term for this technique would be a "voltage-limited current source".

I didn't get the idea from anywhere in particular. It's based on simple well-known battery charging algorithms.

Bill

[SamoaFlyer]

Stan,

Actually, you don't need to change anything except to adjust the pot as long as you use a logic-level FET, although reducing the input voltage would be a good idea to reduce dissipation in the LM317. The minimum supply voltage would be 1.4V x #cells +1.2V + 0.7V. For a 4-cell pack your 7.5V is the minimum. I'd actually go a little higher to allow for variable mains.

There's no need to change the pot value - it's quite non-critical. Anything from 1k to 500k is okay.

Bill

[sntlewis]

Is the IRF540 a logic-level FET?

[rich smith]

Looks like you know what's up. I suspect the idea may work for months even year or so compared to many years for stored or cycled cells. Fair trade to have TX ready all the time I guess.

PS My Hitec 3fm original cells in their 8th year. I just keep it plugged in all the time. I carry 8pack hybrids for backup but not had to use yet.

Rich,
Yes, I'm aware that some manufacturers don't recommend trickle charging Nimh but I think it depends somewhat on the definition of trickle charging. It's been my experience that low float currents of a few mA do no harm and in some cases actually improve capacity.

This design is not actually a CV charger. Most charge input to the battery is at constant-current as it should be for Nimh or NiCd. Part of the problem is the CCCV name. It's really a misnomer since neither current nor voltage is constant all the time.. Perhaps a better term for this technique would be a "voltage-limited current source".

I didn't get the idea from anywhere in particular. It's based on simple well-known battery charging algorithms.

Bill

[SamoaFlyer]

Stan.

Now that I think about it, most N FETs would probably work okay for your application. As long as the max Vgs threshold is less than about 4.0V, you should be okay. You do want a TO-220 case (or higher dissipation type) though to handle the heat with a small clip-on heatsink.

The IRF540 just happened to be handy, so I used it. It's Vgs threshold is 2.0 to 4.0V so it would probably be okay for 4-5 cells. No, it's not logic level.

Bill

[lazy-b]

Samoaflyer, Thanks for sharing your circuits........You just give me an idea of making a simple balancer, your circuits uses a Shunt-type Voltage regulator.....look like I can use your Shunt-type Voltage regulator as a Lipoly balancer, I am not sure if it will work, maybe will add a Resistor in series with the power Mosfet, just in-case if the Mosfet is fully ON it will limit its current flow.

I have build a simple Constant Current and Constant Voltage with very similar to Samoaflyer circuits, I use 2 pcs LM317.....the first stage is constant Current same circuit , its just a LM317 and a Resistor, the 2nd stage is a Constant Voltage, it consist of LM317 with a fixed resistor connected across Vout Terminal and Vadjs Terminal, a Variable resistor (5 Kilo Ohms) connected Vadjs and Ground.

I can adjust the Output Voltage from 1.25 to 15 Volts, I can use this to charge almost every kinds of Battery, its just a matter of setting the Voltage:

13.8 Volts to Charge a 12 Volts Lead Acid Battery, can leave it indefinete.
14.2 Volts to Charge a 12 Volts Lead Acid Battery, in Cycle Mode.
12.6 Volts to charge a 3 cell Lipo Battery
8.4 Volts to charge a 2 cell Lipo Battery
4.2 Volts to charge a single cell Lipo Battery

look like this is a simplest constant current/constant voltage charger, just need 2 pcs LM317, 2 pcs resistor and a Variable resistor.

But the one I construct is little complex, cause I make it adjustable Constant current and adjustable Constant Voltage, I have 2 Control Knob, One Knob is current adjust (from 0 to 1 amps) and the 2nd Knob is Voltage adjust (1.25 Volts to 20 Volts).

One dis-advantage of using this circuits , you need a higher voltage to drive it.....The minimum voltage is Output Voltage + 3 Volts (Voltage drop across the 1st LM317) + 3 Volts (Voltage drop across the 2nd LM317) + 1.25 Volts ( Voltage drop across the Resistor)

so, if you need a 15 Volts Output, you need 15+3+3+1.25= 22.25 Volts Voltage input......I think, I use a 24 Volts as Input.

been using this circuit for more than 15 years now and still working.

If I want to use it as a Normal slow charger, I just set the Voltage to Maximum and adjust the current to 50 milliamps.

Please check the attached ciruicts:
R1 = 1.25 / Max Amps
if you want a maximum of 1 amps, use R1=1.25
if you want a maximum of 0.5 amps, use R1=2.5 ohms
R2 = 240 Ohms
R3 = 5 Kilo Ohms

Just make sure Both LM317 is mounted on a Alumimum heatsink.

The maximum current for LM317 is 1.5 Amps, if you need much higher you can use LX8383A, this can provide is to 5 Amps (Peak 7.5 Amps) on the same TO-220 packages, and can operate very low voltage drop of 0.7 volts only, one dis-advantage of using LX8383A is that it can only operate up to 10 Volts Input Only.

Wow....its so much easier to share a picture now, I just draw that Circuit in a small piece of paper, take a digital Photo using my cheap cellphone camera and upload it.

Ellion