Convert SMPS into a Solar Charger

The post explains how to convert smps into a solar charger circuit. The method will result in an extremely efficient and fast solar charging of the connected battery.

SMPS Solar Chargers

SMPSs have become very common nowadays and we find them being used in the form of AC to DC adapters wherever needed. The best example is our cell phone chargers which are actually compact SMPS 5V chargers.

Solar charger devices are also becoming popular nowadays and folks are constantly in look out for options in the form of solar chargers having the most efficient charging response.

Solar panels or PV devices are normally utilized for charging lead acid batteries which tends to take relatively long hours for getting fully charged, besides when the sunlight conditions are bad things start getting even more sluggish.

For tackling the above condition or rather for enabling quicker charging from solar panels, special MPPT based soar chargers have been developed which effectively monitor the solar panel maximum power point levels and generate the most efficient charging conditions for the connected battery.

In this article although we won't be discussing an ideal MPPT, yet the discussed method will give you an opportunity to acquire the most efficient way of charging your battery through a solar panel.

As proposed in one of my previous articles discussing understanding solar mppt solar chargers, a switch mode based power supply (SMPS) is probably the best option for making it work as a solar charger circuit, so here we will learn how to make an smps based solar charger circuit at home.

Making an SMPS can be  quite complex and might require considerable amount of time and knowledge for the implementations, so here rather we will focus on how to convert a ready made smps into an effective solar charger circuit quickly.

For this you will require the following materials, assuming the battery to be charged is 12V rated:

A ready-made  120V or 220V to 12V SMPS unit having current rating equal to 1/5th of the battery AH which is to be charged.

A few Solar Panels whose total open circuit voltage equals around 100V.

Connecting wires.

Converting SMPS into a Solar Charger Circuit.

As we all know a normal  mains  SMPS are mostly rated with minimum of 85V to 100V input in order to provide the specified output DC, let's assume it to be 12V, meaning for acquiring 12V it must be supplied with a minimum of 100V at the input.

Keeping the above issue in mind, we must select a solar panel which may be able to produce approximately 100V for making the procured SMPS work.

Since PV panels with such high voltage might not be available, we may opt for many low voltage solar panels connected in series for generating the above voltage.

For instance you can go for 3nos. of 30V solar panels and connect them in series to get 90V from it, which might just do the job.

The above input supplied to the procured SMPS would generate the required 12V which may be directly attached to the battery for charging it efficiently.

However a 12V supply might not charge a 12V battery we need at least 14V for it, so that's not a big issue, the required voltage can be easily tweaked and set by adjusting the output voltage of the SMPS manually, the procedures can be learned in this article which explains how to modify an SMPS circuit.

That's it, you have just now converted a ready made SMPS unit into an efficient solar charger circuit that might generate results equivalent to MPPT charger circuits for you.

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Pradeep Gb said…
dear sir i have 110v solar module with 0.98amps currents need to do battery charge with solar pannels so please guide me to build the circuit for this. and please if possible can you send me the details to my mail id
Swagatam said…
dear pradeep, since the output of your solar panel is 110V, you can easily configure it with any ready made 24V/1amp SMPS unit. these units will work right from 85V to 285V of input range.

the output from the smps then can be simply used for charging the desired battery after passing it through a LM317 circuit stage.
Raja Banerjee said…
most robust and easily available 12 V smps is the CPU smps which comes at around Rs. 400/- with a whopping output of 20 amps on 12 v rail. and works on minimum input of 115 V.

If I connect 12 X 12 V. 10 watt panel in series it will give 12 X 20 = 240 V at the Higher Side (open circuit voltage = 20 V /panel) and work till 115 / 12 = 9.58 V per panel O/P.
1. how to manipulate the CPU SMPS to give 14V O/P
2. how to control the charging current as per the batt. req
3. how good will be the the O/P to charge a 12 V /150 Ah batt
Swagatam said…
Hello, here are the answers to your questions:

1) There's no easy way of doing this, you'll have to open the smps and look for the resistor near the output of the main ferrite transformer which decides the output voltage level of the unit, and tweak its value until the output adjusts to about 14V, if you see a preset that would make the job much simpler.

2) For current limiting you could probably employ the idea explained here:

3) For charging a 150AH, the 20amp output could be used directly without a current limiter in the middle, but you will need to switch off the supply as soon as the battery gets fully charged at 14.4V.
Raja Banerjee said…
hello again!!
after much googling and you-tubing I have finally succeeded two of my old cpu smps to give 15v output and tested with a 12v -100 watt automotive lamp. now is their any way to monitor the 110v to 240 v dc coming from panel and switch some parallel LM 338 for charging current control according to the panel voltage?? (as u used LM3914 for 12 v monitoring) .... may be converting 220 v dc to 12v unregulated dc and then use LM3914?? Thanks.
Swagatam said…
Hello, I don't think the panel voltage needs to be monitored, because the SMPS would be able to produce the set constant voltage regardless of the input variations, provided the input does not drop below 90V.

However just for a visual indication the LM3914 circuit can be used, powered through the SMPS, the sensing voltage could be acquired through a voltage divider network and fed across the sensing input of the IC.

The voltage divider resistors could be calculated using any online "voltage divider calculator" software
Raja Banerjee said…
Thanks for the reply. But as the panel wattage drops with the voltage drop, don't we need to control the charging current so that the panel don't get over loaded?? suppose at any instance the 12 series panel giving 120 volt, that means per panel voltage is 120/12=10v. at 10v the panel is able to generate a certain amount of current and when the series voltage is say about 204v the voltage per panel becomes 204/12=17v. again at 17 volt the panel generates a higher amount of current.


The current remains the same, it is the voltage produced that differs the wattage produced?? please give a clear idea
Swagatam said…
The smps should be having an in built current controller stage, which could be set up manually by opening it and tweaking the relevant components, an LM338 regulator will dissipate a significant amount of power so it may not be recommended.

The best idea would be to select the load specs such that it works optimally even at the lowest panel voltage....this will ensure an all round efficient working of the load.

I am not able to figure out any easier alternative....
Raja Banerjee said…
but what if we clamp LM338 at 3 amps and use multiple in parallel? As it is rated @ 5 amps, will it dissipate heat at 3 amps?
What happens to the output of a solar panel when incident light comes down? Is the voltage and current both goes down or the current remains same, only the voltage that goes down with the decreasing incident light??
Swagatam said…
Still it will dissipate a lot of heat, that's the drawback with linear ICs.
Both, amp and voltage will get affected with decreasing light on the solar panel.
VIJAY AJ said…
Sir.I have a doubt??? In morning the solar panel output voltage should be high at this time smps helps us to making the constant voltage by increasing the current or making only the constant voltage without increase in current???
Swagatam said…
Vijay, it'll produce a constant current as per its specifications, it'll not increase current because SMPS is not an MPPT

but you can modify the secondary winding of its trafo and force it to produce more current..
Disha Karnataki said…
We are planning for building a charger which could charge a 12V 110Ah smps battery using solar panel of 21.7V open circuit voltage ,5.79A and 110W power. As we know that smp utilize wet batteries and the charging cycle is different from dry batteries.But would it be economical and possible to charge a 110Ah smps battery with above mentioned solar specifications?
Swagatam said…
110AH is quite high, so I don't think the above method will work...instead you will need to connect quite a few numbers panels in parallel for increasing the current input....and then use an MPPT kind of charger for getting maximum efficiency from the panel.
if MPPT is not feasible you can try using a buck converter for the same with an auto cut off at full charge
Furqan Khokhar said…
can this supply work with the dc input ???
Unknown said…
For an SMPS the input waveform is not relevant. DC works even great. Keep in mind that an SMPS rectifies the AC Voltage to DC and after that it produces the output voltage as marked on the label of the SMPS.
Sheraz Ahmad said…
Hi Sir! dear sir i want to modify cell phone charger by using it as 12V to 5V SMPS based buck converter because if we use 7805 regulator for 5v it get hot. that's why i want to use cell phone charger AC-input 220v-100 to 5V DC as buck converter converting 12V dc to 5V how can i do it?
Swag said…
Hi Sheraz, you can try modifying the primary side of the transformer by reducing its number of turns to 12 and see if that helps.
Sheraz said…
sir is there any need to change the snubber network or PWM block resistors values or it my be kept with no change?
Swag said…
you can keep them as is initially, since here we are reducing the voltage so probably no harm will be done even if something is missed!
Sheraz said…
sir and how many turn for auxiliary winding to use cell phone charger as 12v to 5v buck converter?
Swag said…
Sorry, calculating all the parameters can be difficult without seeing the circuit. I would recommend you trying the following design by reducing the primary turns to 12 or 15.

the 180V transil may not be required, and the opto can be also removed initially

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