6v battery charger circuit problems

crutschow

Joined Mar 14, 2008
34,286
Okay, starting from scratch using an LM311 voltage comparator ...:)
Go for it.
But the disadvantage of an LM311 is that it has an open-collector output so it requires the addition of a small value pull-up resistor to generate a voltage high enough to turn off the top LED.
 

Thread Starter

Gary Durst

Joined Aug 9, 2016
18
I switched the circuit around using the new schematic with the existing lm741 and the led are working correctly but still cannot get a chargING current of more then 15mA.

I then switched out the lm741 for a LM358 and again, the led's swap positions while adjusting the 10k pot. The charging current is still low at 15mA. It seems like the tip122 isn't working, so I swapped it out for another thinking maybe it was damaged. After swapping it out nothing changed. I get 15mA of current regardless of the output of the LM358 and led's.

The one thing that I didn't do is connect the opamp to another supply of v+. In the schematic it is showing 12v feeding the lm317 but in reality I have 20v coming from a solar panel and I wasn't sure if I could connect the opamp to the output of the solar panels 20v.

The battrry that I am trying to charge is showing is showing 6.45v. Is this the problem with the circuit or am I missing something?
 

AlbertHall

Joined Jun 4, 2014
12,345
Make sure you have the TIP122 pinout correct. Looking at the face with the type number and with the legs downward, from left to right the pins are base, collector, emitter.
If this is correct can you measure the voltage between emitter and collector, and between emitter and base, when the charge LED is lit.
 

Thread Starter

Gary Durst

Joined Aug 9, 2016
18
Emitter to collector voltage is .46v
Emitter to base is .35v

These readings are taken with the green led lit and battery charging
 

AlbertHall

Joined Jun 4, 2014
12,345
So the base emitter voltage is way too low to turn on the TIP122 so there is something wrong with the circuit between the base of the transistor and the output of the op-amp - there should be just a 10k resistor. Check the connections here, and check the value of the resistor.

As the transistor is connected across the 470 Ohm trickle charge resistor, there must be 0.46V across that resistor which gives a current of about 1mA not 15mA (confused?)

Also the output of the '723 (via the diode) seems to be only 0.46V higher than the battery voltage. In the charging condition what is the output voltage of the '723?
 

Thread Starter

Gary Durst

Joined Aug 9, 2016
18
I have the lm350 output set to 7.28v after the diodes drop.

I have a 10k resistor at the output of the opamp and the base of the transistor. The voltage at the opamp output is 5.79v when left open. If I connect the output to the tip122 the voltage is .37v to ground. Emitter to base is .35v and emitter to collector is .77v. Thsee readings are a little different because it is hooked up to a power supply now but the chargING current is still at 1.8mA regardless of output of opamp.

I have swapped the tip122 out for another but the results are the same. I have checked the wiring and connections numerous times. Any help would be greatly appreciated
 

Thread Starter

Gary Durst

Joined Aug 9, 2016
18
The battery that I am trying to charge has a voltage of 6.5v without a load. I am trying to bring the battery's voltage up to 7.2v on the charging circuit, then switch to float mode
 

Thread Starter

Gary Durst

Joined Aug 9, 2016
18
Albert, good catch..... I had the 10k and the 10k resistors in opposite positions. Know that I switched the resistors to the correct locations, the led's will no longer swap their illuminations according to the opamp output.
 

AlbertHall

Joined Jun 4, 2014
12,345
You say the LED's no longer switch. Which one is lit? Do they change brightness?
What voltages do you get on the op-amp output as you adjust the pot?
 

Thread Starter

Gary Durst

Joined Aug 9, 2016
18
Albert,

Before connecting the output of the opamp to the base of the transistor, the led's swap positions according to the position of the 10k pot. As long as the inverting input is less then the noniverting the green led is lit. As soon as I touch the output of the opamp to the base of the tip122, the red led lights up.

Changing the value of the inverting input through the 10k pot doesn't effect the led's anymore. But, if I measure the current of the battery I am trying to charge it looks like the tip122 is working. The current goes from 15mA when charging down to 1mA will in float mode. The main charging current still seems low and the indicating leds's no longer function properly.
 

Thread Starter

Gary Durst

Joined Aug 9, 2016
18
Yes, I am talking about the output of the opamp via the 10k resister, to the base of the transistor. When i swapped out the lm741 to a lm358 I got the 100k and the 10k resistors flip flopped. The actual resistance of the resistor going to the base of the tip122 is 9.2k
 

AlbertHall

Joined Jun 4, 2014
12,345
In that case something isn't wired correctly. How is it constructed - breadboard, stripboard, etc?
Could you give us some pictures of the construction to see if we can spot anything, please.
 

crutschow

Joined Mar 14, 2008
34,286
After some further simulations of the circuit I realized that it can't really operate correctly because the trigger voltages were being affected by the current through the trickle charge resistor (R7).
I thus changed the circuit so that the reference trigger points are measured directly across the battery.
This required moving the zener reference from ground to the battery plus supply, connecting the bottom of the pot to the battery minus terminal, and reversing the inputs to the op amp comparator.
The LTspice simulation results are shown below.
With the values and pot set points shown, the circuit goes from charge (≈550mA) to trickle charge (≈5mA) at about a battery voltage of 6.9V [V(B,C)] and turns the charge back on at about 6.1V, for a hysteresis of about 0.5V.

Please examine the schematic carefully to see all the circuit changes that have been made including part values.
If you miss any, the circuit likely still will not work properly.

upload_2016-8-26_0-6-49.png
 

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Thread Starter

Gary Durst

Joined Aug 9, 2016
18
I have changed the circuit around to the latest that was recommended. The led's are indicating charge and float but the charging current is still low at ..040mA while charging. If I test without the battery I get a little ovet 1a of current directly across the outputs going to the battery.

Is the battery's resistance effecting the circuit?
 

crutschow

Joined Mar 14, 2008
34,286
I have changed the circuit around to the latest that was recommended. The led's are indicating charge and float but the charging current is still low at ..040mA while charging. If I test without the battery I get a little ovet 1a of current directly across the outputs going to the battery.

Is the battery's resistance effecting the circuit?
No, the battery resistance should not have that much of an effect unless the battery is totally dead.

Did you change the circuit to exactly as shown in post #37?
If so, what is the output voltage of the LM317 and how did you adjust pot U2?
 

Thread Starter

Gary Durst

Joined Aug 9, 2016
18
The output voltage of the lm317 is set at 7.2v after the diode.

I took R6, 200 feedback resistor out of the circuit and adjust U2, until the led flip flopped their illumination. Then I checked the voltages of the inverting and noniverting inputs and adjusted U2 so that the noniverting input of the opamp was a higher then the inverting input. Installed the R6 resistor back into the circuit, verified the green led was still lit.
 
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