LM317 battery charger - full indicator not working

Thread Starter

testuserabcdef

Joined Jul 12, 2016
127

I'm trying to make a charger that can charge two cordless phone batteries.
Each battery is 3.6V NiCD 600mAh.

I used the schematic shown below with these exceptions:

1. The 470uF capacitor is replaced with 22uF.
2. The power source is 12 VDC wall adapter. (everything to the left of the capacitor is replaced with the power supply conector).
3. Both RV1 and R1 replaced with a single 1.8K (1 watt) resistor
4. R2 is replaced with a 330 ohm (1 watt) resistor
5. I used a TIP31 power NPN for Q1.
6. I added a 220 ohm (1 watt) resistor in series with the FULL led to prevent it from blowing up.
7. I made R5 51 ohm (1 watt) resistor to make the charge faster.
8. D5 is changed to 1N4001
9. I tried 1n5232 then in the other test, I tried 1n5235 zener diode.

When I tested things, nothing wanted to blow up, and I had the circuit running for 4 1/2 hours straight and the FULL LED never lit up with the 1n5235 diode.
I did however add a 10K resistor between base of the NPN and common ground. It helped the situation a bit but now the indication is bright for full charge or half-bright light for not charged.

Is there an alternate simplified circuit I can use to detect when two NiCD batteries (7.2V) are fully charged?
 

Willen

Joined Nov 13, 2015
333
Your R1 and R2 (1.8k+330R) makes 8.1V output. Series diodes D5 drops 0.6V which means you have 7.5V output.

You tested with two diodes with 5v6 and then 6v8. Which means circuit stops charging at zener voltage or 7.2V battery just gets 6.8V peak charging voltage and it's no full charging. Here you have two battery pack and voltage is 7.2 (in series). So you need almost 9V (as I read 1.2V cell needs 1.5V charging). I guess you need to add B-E junction (of Q1) voltage drop of 0.6V and fullcharge LED voltage drop of approx 1.8V in output voltage.

Zener should be connected after the current limiter resistor (you need to monitor real battery voltage so), exactly from battery terminal.

Overall circuit seems not good to me.
 
Last edited:

dl324

Joined Mar 30, 2015
16,845
I agree with the circuit not being suitable. There is so much wrong with the modifications..

It won't charge NiCd batteries using any of the approved methods. You'd be better off using constant voltage and monitoring charge current to determine charge level. Unfortunately, that won't handle shorted cells.

Connecting batteries in series is for convenience, but is not optimal for either discharging or charging. Stacking two 3.6V (3 cells) in series to charge is a mistake.
 

Willen

Joined Nov 13, 2015
333
Stacking two 3.6V (3 cells) in series to charge is a mistake.
By reading this sentence, probably he will try by connecting two battery in 'parallel'. :) He should be clear that connecting two battery in parallel is a huge mistake, isn't it?
 

spinnaker

Joined Oct 29, 2009
7,830
And You can't just charge NiCAD batteries like this. Charging batteries is an exact science. You just don't apply a charge and expect it to do so both efficiently and safely. If you don't burn your house down with this circuit you are likely to ruin your batteries. Buy a proper charger for your batteries.


There are some fairly sophisticated chips to perform charging functions.

http://www.linear.com/product/LTC4011


Check out this link to learn more about batteries.

http://batteryuniversity.com/
 

Willen

Joined Nov 13, 2015
333
I assume the 7.2V series made by factory, not by you. If you made them series, please do not charge in such way. So for the factory made 7.2v battery charging, you can try this DIY circuit with modification if you cannot get advanced charging chips:
- set the 8.5v output voltage (across + battery and - battery terminal with no battery connected). I think it's safe charging voltage for 7.2V battery pack. Correct me if I am making mistake.
- set same 8.5v voltage (across two red test points after TL431 regulator with no battery connected).
- R4 is a current limiting resistor. It has 0.6v drop so you can calculate its charging current easily as this: V/R. So if you used 1 ohm resistor you will get 600mA charging current.
-When battery gets 8.5v (as you had set) the LED says fully charged. And do not worry, charging will be almost off automatically. But it's Better to remove battery after full charged indication though.
 

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dl324

Joined Mar 30, 2015
16,845
For your edification:
1. The 470uF capacitor is replaced with 22uF.
2. The power source is 12 VDC wall adapter. (everything to the left of the capacitor is replaced with the power supply conector).
3. Both RV1 and R1 replaced with a single 1.8K (1 watt) resistor
These changes are okay. But resistor could be 1/4W or 1/8W.
4. R2 is replaced with a 330 ohm (1 watt) resistor
This resistor can be 1/4W or 1/8W. But this resistor value gives a program current less than the 10mA minimum required for the regulator to regulate voltage. LM317 often work with half that, but it's not guaranteed.
5. I used a TIP31 power NPN for Q1.
A small signal transistor would be more appropriate; e.g. 2N3904.
6. I added a 220 ohm (1 watt) resistor in series with the FULL led to prevent it from blowing up.
Current is already limited. Adding a resistor increases voltage required for FULL indication.
7. I made R5 51 ohm (1 watt) resistor to make the charge faster.
8. D5 is changed to 1N4001
OK, but resistor could be stressed with low battery voltage.
9. I tried 1n5232 then in the other test, I tried 1n5235 zener diode.
5.6V and 6.8V zeners, respectively and not interchangeable.
 

Thread Starter

testuserabcdef

Joined Jul 12, 2016
127
But resistor could be 1/4W or 1/8W...A small signal transistor would be more appropriate; e.g. 2N3904.
Ok, I guess I can use a 2N3904 thanks to the base resistor being 1K.
As long as a resistor doesn't burn over the change, I just may make them 1/4 watt but since 1 watt resistors are very cheap online, I might continue with them.

This resistor (330 ohm) gives a program current less than the 10mA minimum required for the regulator to regulate voltage. LM317 often work with half that, but it's not guaranteed.
So I guess I must use 220 or 240 ohm? In that case I'll need to add extra resistors (instead of 1 1.8K due to limited part availability locally).


Current is already limited. Adding a resistor increases voltage required for FULL indication.
I'm trying to understand here. If a transistor is supposed to be a switch and the zener turns the transistor on when sufficient positive voltage enters the cathode of it, is my 220 ohm resistor I added in emitter being added to the 330 ohm resistor attached to the collector? and if so, that wouldn't be sufficient to turn the LED right off at 7+ VDC supply? or am I supposed to factor extra resistances and/or currents?

OK, but resistor could be stressed with low battery voltage.
I'm not sure what you meant by stressed, but I deliberately made it a 1 watt resistor so it won't blow up.


5.6V and 6.8V zeners, respectively and not interchangeable.
I understand. So I ran tests with each one separately to see if voltage was the major factor.
 

dl324

Joined Mar 30, 2015
16,845
Ok, I guess I can use a 2N3904 thanks to the base resistor being 1K.
As long as a resistor doesn't burn over the change, I just may make them 1/4 watt but since 1 watt resistors are very cheap online, I might continue with them.
Your choice, but it's like using a 2" brad to fasten a sheet of paper to a bulletin board.
So I guess I must use 220 or 240 ohm? In that case I'll need to add extra resistors (instead of 1 1.8K due to limited part availability locally).
Feel free to use what you can source. The current from the charging LED can be included in the minimum load calculation.
I'm trying to understand here. If a transistor is supposed to be a switch and the zener turns the transistor on when sufficient positive voltage enters the cathode of it, is my 220 ohm resistor I added in emitter being added to the 330 ohm resistor attached to the collector? and if so, that wouldn't be sufficient to turn the LED right off at 7+ VDC supply? or am I supposed to factor extra resistances and/or currents?
Without the resistor in series with the LED, the LED would start turning on when about 1.5V was across it, 0.7V across the BE junction, and 5.6V across the zener. That's a total of about 7.8V. The harder the transistor is driven, the closer the output of the regulator gets clamped to 1.25V. But this circuit will never get there.

I won't analyze further because the circuit is poorly designed. The voltage sense should be on the other side of the 50 ohm resistor so the battery voltage is being monitored. And the transistor/zener/LED charged indicator should be replaced with a comparator that disconnects the battery once it's charged.

If you limit charge current to 0.1C, this circuit might be salvageable without undue risk of fire. But you still shouldn't charge multiple battery packs in series.

I'm not sure what you meant by stressed, but I deliberately made it a 1 watt resistor so it won't blow up.
A short would cause the 50 ohm resistor to dissipate 1W. We normally derate components by 25-50% to be conservative.
 
Last edited:

Thread Starter

testuserabcdef

Joined Jul 12, 2016
127


I found this circuit on the web, and was wondering if it is more recommended than the transistor approach.
I'm thinking replacing the symbol with 12.5mA beside it with a 330 ohm resistor (or should I use 220 ohm to give batteries extra voltage? but then again, I don't want a fire.), and because I like using NPN transistors better, I'm thinking replacing the PNP with NPN and swapping the inputs of the opamp. and P1 + R3 with cap is replaced with 1.8K.

That reverse biased diode I'm thinking of making it a forward-biased LED because I want to use an NPN transistor.

Can this circuit be a much better battery charger? I think It follows someones advice of measuring the voltage directly from the battery.

Or should I replace a resistor in the voltage divider with a diode?
 

Dodgydave

Joined Jun 22, 2012
11,284
It will work with a better op amp like Lm358, the voltage is set by the preset on pin3, when the battery voltage rises the voltage at pin2 is bigger than pin3 and the output switches the transistor off, and charging finishes, the diode is to make sure the Pnp turns off.
 

ian field

Joined Oct 27, 2012
6,536

I'm trying to make a charger that can charge two cordless phone batteries.
Each battery is 3.6V NiCD 600mAh.

I used the schematic shown below with these exceptions:

1. The 470uF capacitor is replaced with 22uF.
2. The power source is 12 VDC wall adapter. (everything to the left of the capacitor is replaced with the power supply conector).
3. Both RV1 and R1 replaced with a single 1.8K (1 watt) resistor
4. R2 is replaced with a 330 ohm (1 watt) resistor
5. I used a TIP31 power NPN for Q1.
6. I added a 220 ohm (1 watt) resistor in series with the FULL led to prevent it from blowing up.
7. I made R5 51 ohm (1 watt) resistor to make the charge faster.
8. D5 is changed to 1N4001
9. I tried 1n5232 then in the other test, I tried 1n5235 zener diode.

When I tested things, nothing wanted to blow up, and I had the circuit running for 4 1/2 hours straight and the FULL LED never lit up with the 1n5235 diode.
I did however add a 10K resistor between base of the NPN and common ground. It helped the situation a bit but now the indication is bright for full charge or half-bright light for not charged.

Is there an alternate simplified circuit I can use to detect when two NiCD batteries (7.2V) are fully charged?
The BD139 probably doesn't have enough gain for that job - with only 240R from the battery voltage I can understand the use of a stout transistor, but not when its maximum current is also flowing through a LED.

Draw up a table of maximum values like collector current and voltage and calculate the maximum expected dissipation. You might get away with a BC107 or 547 that should have enough gain to get the job done.

The ratios of the resistors around the 317 are important - but you can probably revise them upwards to accommodate a low power transistor with more gain.
 

hp1729

Joined Nov 23, 2015
2,304
Design 322 b.PNG

I'm trying to make a charger that can charge two cordless phone batteries.
Each battery is 3.6V NiCD 600mAh.

I used the schematic shown below with these exceptions:

1. The 470uF capacitor is replaced with 22uF.
2. The power source is 12 VDC wall adapter. (everything to the left of the capacitor is replaced with the power supply conector).
3. Both RV1 and R1 replaced with a single 1.8K (1 watt) resistor
4. R2 is replaced with a 330 ohm (1 watt) resistor
5. I used a TIP31 power NPN for Q1.
6. I added a 220 ohm (1 watt) resistor in series with the FULL led to prevent it from blowing up.
7. I made R5 51 ohm (1 watt) resistor to make the charge faster.
8. D5 is changed to 1N4001
9. I tried 1n5232 then in the other test, I tried 1n5235 zener diode.

When I tested things, nothing wanted to blow up, and I had the circuit running for 4 1/2 hours straight and the FULL LED never lit up with the 1n5235 diode.
I did however add a 10K resistor between base of the NPN and common ground. It helped the situation a bit but now the indication is bright for full charge or half-bright light for not charged.

Is there an alternate simplified circuit I can use to detect when two NiCD batteries (7.2V) are fully charged?
Another approach. Designed for a lower voltage battery and can easily be modified to your needs ... Constant current battery charger. Battery voltage is monitored and when it gets to the proper voltage power is shut off.
Would this work for you?
 

ian field

Joined Oct 27, 2012
6,536
I agree with the circuit not being suitable. There is so much wrong with the modifications..

It won't charge NiCd batteries using any of the approved methods. You'd be better off using constant voltage and monitoring charge current to determine charge level. Unfortunately, that won't handle shorted cells.

Connecting batteries in series is for convenience, but is not optimal for either discharging or charging. Stacking two 3.6V (3 cells) in series to charge is a mistake.
For nickel chemistry - temperature sensing is the simplest way.

I use a bead thermistor in the resistor chain for a window comparator. That shifts a 2 transistor bistable which in turn controls a power management pass transistor. This arrangement makes a constant current circuit an extravagance - a current limiting resistor is sufficient.
View attachment 122155


Another approach. Designed for a lower voltage battery and can easily be modified to your needs ... Constant current battery charger. Battery voltage is monitored and when it gets to the proper voltage power is shut off.
Would this work for you?
 

ian field

Joined Oct 27, 2012
6,536
View attachment 122155


Another approach. Designed for a lower voltage battery and can easily be modified to your needs ... Constant current battery charger. Battery voltage is monitored and when it gets to the proper voltage power is shut off.
Would this work for you?
With nickel cells, the "proper voltage" isn't that simple.

The cells have a temperature coefficient that makes the voltage fall slightly when the cell starts heating - the cells start heating at full charge, the energy that *WAS* producing chemical change, starts producing heat. Maintaining a "proper voltage" will heat the cell even more, cause the cell voltage to fall even more and destroy the cell.

Commercial chargers (the ones that are any good) detect that voltage fall to terminate the charge cycle. The voltage change is much more subtle with Ni-Mh than with Ni-Cd. You can buy off the shelf charge management chips or programme a micro to detect the dip. For junk box DIY - temperature sensing is sooooooooooo much easier.
 

Thread Starter

testuserabcdef

Joined Jul 12, 2016
127
Also, what I want to eventually do is replace the charger source with an unreliable source such as a solar panel or a coil that can receive current (a.k.a. wireless power transmission)
 
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