[PIC] Solar Battery Charger

[Okada]

Hi

This is the circuit I designed for charging 12V gel and Lead Acid batteries upto 12Ah. Do I have to make any changes to it ?

This is the Solar Panel I am going to use.

**broken link removed**

Circuit Attached.

While designing Inverters I found that Battery Charging current should be 10% of the Battery Ah rating.

For 6 Ah battery can I use 2.5A charging current ? 10% 0f 6 is 600 mA charging current. I don't want the batteries to explode.

Do I actually need the STTH1202 diode for blocking the reverse flow of voltage ? because PWM is used for charging and battery voltage is monitored at all times. If battery is fully charged then PWM duty reduces to 0 and this will turn OFF the mosfet. So all the 21V (open circuit voltage of solar Panel) appear accross mosfet drian and source but as no current flows there will be no heat dissipated in mosfet. Also when battery is fully charged the mosfet will be off and hence there will be no reverse flow of current to panel.

If I use diode then with 2.5A charging current 2.5W power is dissipated in diode. I want to save this power.

FDP8440 Rds(on) is 2.2 mOhms.

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I referred this circuit before designing mine.

https://electronicsforu.com/electronics-projects/hardware-diy/smart-solar-charger

He has one 5E 5W and 1n4007 is series with the battery.

I = V/R = 12V/5 = 2.4A

How does 1N4007 1A diode handle this current ?

He has used 1N4007 as reverse blocking diode but do I need a diode for this purpose in my circuit ?

 

[Okada]

The panel gives 17.18V @ full power
Current @ full power is 2.33A

Battery charging voltage required is 14.4V

so, pwm duty percentage = 14.4/17.18 = 83.8%

pwm duty varies in code between 0 and 255 (0 to 100%)

so 255 * 0.838 = 213.7 approx 214

So, I control this duty value to maintain constant charging voltage ?

Can I use this MBRD1035CTL Shottkey Power Rectifier ?

It has Vf = 0.47V

So, Pdiode = 2.33A (max panel current) * 0.47V = 1.0951W

Initially the mosfet will be off.
Panel voltage is measured and if it is >= 15.V then pwm duty is adjusted to get 13.8V across battery
Charging current is monitored continuously
As the battery charges the current decreases
If current reaches battery full value then pwm is turned off and charging is put on trickle charge mode in which the pwm is adjusted so that 14.4V apply across battery for 2 minutes once evry 2 hours.

Is this a good idea ?

I finally chose to use MBR2030CTLG diode. Its Vf = 0.52V

Pdiode = 2.33 * 0.52 = 1.2116W


Revised circuit is attached

ON/OFF switch is used to turn ON/OFF mosfet. Before removing battery mosfet can be turned ON


2KHz PWM is used.

 

[FvM]

The pwm duty cycle calculations don't apply to the given circuit which is no boost voltage converter. Instead you are chopping the solar panel output current. No problem if you don't need maximum efficiency.

The "panel voltage" measurement isn't of much use unless you synchronize the ADC with the pwm. Similarly the battery current is also pulsed.

There isn't much advantage of your circuit compared to the hysteretic relay on/off control in the post #1 link, provided the battery can stand the maximum current supplied by the solar panel. If you have current measurement + relays, you can even save the rectifier diode by disconnecting the panel in case of reverse current.

 

[Okada]

I am using Panel Voltage measurement because there will be 0.57V drop across diode and so if Panel voltage is more than 13.8 + 0.57V then only the mosfet gets turned on based on the battery voltage. If battery voltage is 12 or more volts then charger doesn't turn ON even if panel voltage is good. If battery voltage is 10.4 or less volts then after confirming that panel voltage is good the mosfet is turned ON. I want to maintain constant charging voltage of 13.8V during boost charge and 14.4V during trickly charge. How to do that using PWM. I don't want to use relay because it make noise even if frequency of operation is less. I will be charging 12V 6Ah to 12Ah batteries only.

During charging once in a while the mosfet is turned off and the battery voltage is measured and if it is 12V then charging is stopped. Is this method good ?

ADC is used to control the PWM duty.

This is the latest circuit. Just changed the resistor values.

In PIC there are two pins with CCP2 functionality. How to select RA5 for CCP2 ?

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In this circuit

https://electronicsforu.com/electronics-projects/hardware-diy/smart-solar-charger

I don't know what wattage Solar Panel the author used but as he has used 1N4007 1A diode in the charging circuit, I am sure the Panel wattage is low.

If panel max current is assumend to be 750 mA power dissipated in 5E 5W resistor during charging will be (0.75 * 0.75 * 5) = 2.8125 approx 3W.

I want to lessen this power loss.

In my circuit and also the circuit in above link there is much loss of power in LM7805 as it has to drop more than 10V.

In my circuit power loss is minimized in charging circuit.

In my circuit Higher wattage Panel can be used by just changing the ACS712-5A to ACS712-20/30A type.

If 12V 300W panel is used then panel current will be 25A and this can easily charge a 12V 200Ah battery.


V = IR

Only in the charging path battery has R

So, R is constant

If I is increased then V increases and if I is decreased by decreasing pwm duty then V also decreases. Right ?

So, I can control I and charging voltage by varying pwm duty ?

 

[Okada]

I am referring this document. I want to implement 3 Stage Charging (Page 1).

I am testing in Proteus without Battery connected. The "BATTERY VOLTAGE" remains always 0. Is it because of the PWM ? I have used a 1000 uF capacitor from diode cathode to ground.

 

[BradtheRad]

I am testing in Proteus without Battery connected. The "BATTERY VOLTAGE" remains always 0. Is it because of the PWM ?

If battery=0V then it might think it's a short circuit. So the IC is 'smart'. It does not turn on.

I'm guessing that the IC needs to see a minimum voltage (say 10V) before it decides it is hooked up to the proper type of battery. Only then will it turn on.

 

[Okada]

@BradTheRad

Which IC ?

I have only mosfet and diode in charging path. Problem is not with ACS712. Problem is with ADC "BATTERY VOLTAGE" measurement. Code is correct. Voltmeter across diode cathode and ground with battery disconnected shows 0V when mosfet is On with 2 KHz pwm. I am only testing in Proteus.

 

[smijesh]

@BradTheRad

Which IC ?

I have only mosfet and diode in charging path. Problem is not with ACS712. Problem is with ADC "BATTERY VOLTAGE" measurement. Code is correct. Voltmeter across diode cathode and ground with battery disconnected shows 0V when mosfet is On with 2 KHz pwm. I am only testing in Proteus.

So, I can control I and charging voltage by varying pwm duty ?

Yes you can vary the charging voltage by PWM control

The battery charged in two modes—boost and trickle. If battery voltage is greater than 12V the battery is charged in trickle mode, whereas if battery voltage is less than 12V it is charged in boost mode. If the voltage is less than UVLO (Set value in the micro - PIC 16F877A) the PWM output will shut down for protection. Check the ADC input value you may get the solution