Car batteries paired with solar panels?

I have been doing research on solar panels and I even bought some panels of my own. I have been considering using a large array of used car batteries and perhaps replacing some of the liquid inside of the batteries, as I have found these videos online about DIY old car battery repair



Though many say car batteries cannot work with solar, I have also read experiences of people using them and only allowing the batteries to drop 10%, and to do so using a charge controller. I'm curious if anyone has thoughts about such a system, or knowledge on what charge controller/ components could help build this system.  I have read one of the best options to go would be a MPPT charge controller using an Arduino-based system. However I do not have a lot of experience with programming an Arduino, so I am looking at other options, however charge controllers are all over the place in types (and price). Please share your thoughts and experience. Thanks!

The car batteries cant have deep discharges.
But if you play with number of batteries and the total cost of those batteries, you may be able to get something working.
I have used car batteries when I had no money spare.

John C Daley wrote:The car batteries cant have deep discharges.
But if you play with number of batteries, total cost of those batteries you may be able to get something working.
I have used car batteries when I had no money spare.

Thanks for saying something, its good to know that it is possible. Tell me some about your set-up back when you used car batteries, what kind of panels did you use, and how many? What kind of charge controller did you use? Did you use AC or DC? I'm sure if you had no money to spare, you cut corners when you could. Thats how I am doing things right now, with rising prices dramatically climbing, the cheapest charge controller's online are 2/3 more expensive than they were in 2019. And I'm not needing a huge battery bank for everything right now, just some lightbulbs would make the difference between night and day. Thanks!

I would do it in a crisis.  It is nothing I would plan a system around.

I am about to renew my efforts on a project like this, and a quick search online revealed this MPPT controller at a modest price.  https://www.amazon.com/dp/B0894CTHCY/?tag=cleanenergysm-20&th=1
I assume it's ok to post Amazon links here, new to the forum.  
In my past, I experimented more with old laptop chargers, or wall warts to charge, with dry cell batteries and had modest results.

I have often read in the past that deep cycling auto batteries is really hard on their lifespan.  

I look forward to reading more replies on this thread, as the more I watch going on, the more likely I think an EMP is going to be a part of the next war waged on the US.  You know, like one floating over us at 60,000 ft in a Balloon, I mean UFO.  

Stephan Halasz wrote:I am about to renew my efforts on a project like this, and a quick search online revealed this MPPT controller at a modest price.  https://www.amazon.com/dp/B0894CTHCY/?tag=cleanenergysm-20&th=1
I assume it's ok to post Amazon links here, new to the forum.  
In my past, I experimented more with old laptop chargers, or wall warts to charge, with dry cell batteries and had modest results.

I have often read in the past that deep cycling auto batteries is really hard on their lifespan.  

I look forward to reading more replies on this thread, as the more I watch going on, the more likely I think an EMP is going to be a part of the next war waged on the US.  You know, like one floating over us at 60,000 ft in a Balloon, I mean UFO.  

Stephan a fair warning that is not an mppt charge controller as it's understood these days. It's made for 12 volt panels and is a pwm controller.

Stephan Halasz wrote:I am about to renew my efforts on a project like this, and a quick search online revealed this MPPT controller at a modest price.  https://www.amazon.com/dp/B0894CTHCY/?tag=cleanenergysm-20&th=1
I assume it's ok to post Amazon links here, new to the forum.  
In my past, I experimented more with old laptop chargers, or wall warts to charge, with dry cell batteries and had modest results.

I have often read in the past that deep cycling auto batteries is really hard on their lifespan.  

I look forward to reading more replies on this thread, as the more I watch going on, the more likely I think an EMP is going to be a part of the next war waged on the US.  You know, like one floating over us at 60,000 ft in a Balloon, I mean UFO.  

Thanks for the link, I was looking at one yesterday that seemed very similar, but it has more negative reviews.
https://www.amazon.com/WERCHTAY-Controller-Tracking-Regulator-Multiple/dp/B09XF1S3W1/ref=sr_1_4?crid=NJSBFYFC5ZS1&keywords=mppt%2Bcharge%2Bcontroller&qid=1676880127&sprefix=mppt%2Bcharge%2Bcontroller%2Caps%2C88&sr=8-4&th=1

amazon review wrote:
Largest issue: in my specific setup (4*100w 12V panels) this solar charge controller will not consistently provide power to the battery. While the controller does appear to be able to handle the current (most I saw as just above 10 amps) it is unable to sustain this current. It will intermittently stop providing power to the battery for no apparent reason. I even confirmed that there was power by using a Multi-Meter. (22~23Voc and 10 amps)

I did not notice this intermittent charging issue until I installed my latest 2 solar panels ( I noticed I was barely charging the battery (maybe 120wh on a sunny day out of 2 100W 12V panel), but I initially contributed this to the fact that it was winter.

Lessons Learned: do not cheap out on your solar charger.

So depending on the type of panel you have... but I always try to look at the review, your link however does not have many bad reviews, though they look so similar! Smiling battery and everything...

I certainly appreciate your comments too about the state of the world, I am very much suspecting a complete down of many systems we take for granted. A powerful enough EMP blast could also fry all the circuits inside a charge controller, so depending on such systems is still only a crutch if things get really bad out there. But I'll take all the crutches I can get! And though car batteries are not deep cycle, they can be obtained for free if you find the right places. That means that they can be replaced easily enough too. The same cannot be said for other batteries. So basically you could have a large bank of "starter batteries" that you get for FREE (that's a pretty daggone good deal) with a small controller (a cheap microprocessor) that controls charging and discharging so as to keep them at optimal levels. The simple fact is lighting options are extremely limited without electricity. thats what made electricity a thing back in the day... Almost everything else has a work around, if you read about paul wheaton's underground freezers even refrigeration is possible... But lanterns just don't cut it for doing anything in the dark...

David is very correct.  That is a scam product designed to separate you from your money.  It is a cheapo PWM controller marketed by a company called "MPPT Solar".

There are a couple of clues for you to look for in this ad.  Reading the questions and answers section, you see...

3. Q: My solar panel is 36V 200W, can I charge 12V battery?

A: To charge a 12V battery, the working voltage of the solar panel can be between 17V and 23V, and for a 24V battery, the working voltage of the solar panel can be between 36V and 46V.

The fact that the solar panel can only have a max voltage of 23V is a BIG clue that it is a fake.  Real MPPT controllers, start at 100V, and go up from there.  

Another clue is that shipping weight of 12oz.  A real MPPT controller has lots of heavy copper wire toroid-rings.   The MPPT acts as a transformer, taking high raw solar voltage, transforming it down to battery charging voltage, making extra charging amps out of the extra volts.  A real MPPT controller will have a shipping weight measured in pounds, not oz.

Please forget the 12V starter battery idea completely.  You will quickly destroy the batteries, waste the money you spent on them, and get disillusioned about solar.  Let me give you some suggestions for putting together a reliable system that won't fail you.

Start with golf-cart batteries.  They are designed to be deeply drained, have fairly high capacity, and can be found just about anywhere.  Typical GC batteries are 6V, so you need to buy two to make a 12V battery bank.  You wire the two batteries in series.  Wire the negative terminal of battery #1 to the positive terminal of battery #2 to make 12V.  An economical choice right now is the 210Ah GC from Costco, which is 110$ right now.  Get two of those.  A step up from those is Trojan's T-105 GC.  Those are 250Ah, but a bit pricier.

Next, the charge controller.  A good for the money entry-level controller is Epever's Tracer 4210AN.  It can be used for either a 12V or 24V system, has a 100V limit, and can charge the battery with up to 40A of current, if the power is available.  You can pick that up on Ebay or Amazon for ~125$.  You connect the positive terminal of battery #1 to the + battery terminal of the controller.  You connect the negative terminal of battery #2 to the - terminal of the controller.  Once you let the the controller boot up, it will display the battery voltage, which should be ~12.5V or so.

Now, the solar.  Skip the little 12V panels designed for the automotive market.  You get far higher W/$ going with high-voltage residential grid-tie panels.  Shop for those locally on Craigslist, with local cash and carry pickup.  That way you avoid the high shipping charges associated with mailing panels.  Expect to get 3-4W/$.  Two 240-250W grid-ties (~75-80$ each) wired in series would be good, or maybe a single 350-400W panel (~175-180$ each).  Connect the positive and negative leads from the panel(s), to the + and - terminals of the controller.  Do this ONLY after you have booted up the controller first on battery power.  NEVER connect the solar to the controller before connecting the battery.  Build a frame to hold the panels directly facing the sun at noon.  On an average day, the best you can expect to see in terms of real-world solar output is ~85%.

This will give you a system that can power just about any RV appliance you want to buy.  It's best to not drain flooded batteries less than 50%, so this system will give you about 1.2kWh of power per day.  12V lights, a 12V TV, a 12V laptop power supply.  

Buy a gallon of distilled water and check the electrolyte levels monthly.  Top them off as needed, maybe once every 2-3 months.

You may at some point want to buy an inverter that can convert battery DC into regular household AC.  Make sure you buy "sine-wave" if you run anything powering an electric motor, a refrigerator for example.  Keep in mind though that an inverter just being left on is load on the system, and may consume 250-750Wh per day, depending on the brand.

Good luck!

Joshua Plymouth wrote:I have been doing research on solar panels and I even bought some panels of my own. I have been considering using a large array of used car batteries and perhaps replacing some of the liquid inside of the batteries, as I have found these videos online about DIY old car battery repair

Though many say car batteries cannot work with solar, I have also read experiences of people using them and only allowing the batteries to drop 10%, and to do so using a charge controller. I'm curious if anyone has thoughts about such a system, or knowledge on what charge controller/ components could help build this system.  I have read one of the best options to go would be a MPPT charge controller using an Arduino-based system. However I do not have a lot of experience with programming an Arduino, so I am looking at other options, however charge controllers are all over the place in types (and price). Please share your thoughts and experience. Thanks!

So... Yes you can add Epson salt to distilled water and top off your batteries with it. It will break up some of the sulphates and they will fall to the bottom of the battery. It weakens the battery acid so some people add some concentrated acid back. Please don't drain the acid and clean with baking soda as suggested unless you have a safe legal way of disposing of the acid because it has lead disolved in it and is toxic and corrosive. Some garages will take it back but fewer all the time. If your battery was over discharged and froze it probably won't work as the plates will be bent and shorting. So what happens when you have a large bank of used batteries is they are all slightly different in their ability to charge and hold so they continually charge discharge each other leading to one or more failing then needing to be replaced. I can't talk you out of it it's a lesson everyone needs to learn themselves; how much is your time worth?
As far as charge controllers the epevers have proven to be a fairly reliable inexpensive true MPPT charge controller. Those other ones linked to are made for 12 volt panels.

I've had good luck using a solar panel to bring a bad battery up to 16 volts and "equalize" it.
Battery starts out with only 11.5 volts or so.
The first sunny day I can't get it to rise too much more than 12.7 while hooked up to the panel, then it drops back down over night.
Each day it will go a little higher and hold maybe 12.5 volts over night.
After a couple days it will rise past 15 volts and hold 12.7 overnight.
Keep doing it and it will get up to 16 volts during the day and hold 12.9 on it's own.
Once it's at that level it will play well with other batteries in parallel.
I don't use the battery for power storage, this is just how I bring my batteries back when they sit in the car too long and get low.
It makes batteries like the one for my riding mower last longer than if they sat all winter unused.
I also use a power pulse, electronic desulphator to help clean the plates and help bring the battery back.
The power pulse will die if you run the voltage up much over 14.5 so I don't have the battery hooked up while the power pulse is connected to it.

I don't like running good batteries up to 16 volts for very long so I have loads (fans) I put on the battery bank.
One runs a fan on a hot air box that pumps air into the house, if the clouds come in it cools off and disconnects.
Clouds help keep the voltage low on the bank. If the sun comes out the fans soon come on.
Dedicating one cheap 30 watt panel for this setup.

David, what is a  "pwm controller." please?
Joshua, I used a 400mm x 500mm self regulating panel I was lent for 5 yeras.
It worked ok for that time.
I will defer to the others who have informed me of far more than I knew at the time and even now,
so consider the suggestions.

John C Daley wrote: what is a  "pwm controller." please?

Pulse Width Modulation (PWM) is a method of quickly turning a transistor (electric switch) on and off to provide voltage that is lower than the input voltage. A battery needs higher voltage in order to put energy into it, but would be damaged if the voltage was too high. There is also a difference in voltage of a panel between no load (usually expressed as Volts open circuit, or Voc) and maximum load (Volts max power or Vmp) as well as current (capital I for current measured in amps, so short circuit current is Isc and amps max power is Imp). Even when sunlight isn't perfect, the panel puts out a voltage higher than what the battery needs and the charge controller takes care of that.

PWM controllers only turn power on and off from the panel to the battery so that the average voltage over time is the appropriate voltage for charging the battery. This is less efficient. The more efficient method is Maximum Power Point Tracking (MPPT). This uses a DC converter to put a particular load on the panel that keeps the volts + amps out of the panel at a point where it is most efficient, which is then converted to the appropriate voltage to charge the battery.

For small scale solar, often times the cost of a quality MPPT controller is around the cost of a cheap PWM controller plus a 100 watt panel. An extra 100 watt panel will give much more power than the losses incurred by using a PWM controller on this scale. It's only when you start getting to the kilowatt and above power output that the percentage of power savings from a quality MPPT controller actually pays for itself vs just adding a panel. Panels also generally last much longer than a charge controller which may be an important factor in some setups. I will give one caveat - I haven't used a name brand MPPT controller, so I don't know how clean of a signal comes out from various controllers, but the PWM controllers tend to switch on the slow side and can produce flickering in lights while charging or full.

I took my panel down last year when a hurricane was coming and wasn't physically able to put it back. I was afraid someone might walk off with it, so when a friend started talking about solar I told him to take it. My batteries lasted around a decade and I got a lot of good use and knowledge from that cheap setup. I know it's a gamble with the cheap PWM controllers as sometimes ones that look the same are different inside, but my old one still worked when I last used it. For getting your feet wet with solar I really think a cheap controller paired with one or more panels of 100 watts or more gives a great price to performance ratio. You are better off spending money on good batteries. If you don't have a ton of experience with it then I don't think it is wise to DIY multiple kilowatts and thousands of dollars of equipment, and in many places you need someone certified to legally install something that large.

Daniel Schmidt wrote:

John C Daley wrote: what is a  "pwm controller." please?

Pulse Width Modulation (PWM) is a method of quickly turning a transistor (electric switch) on and off to provide voltage that is lower than the input voltage. A battery needs higher voltage in order to put energy into it, but would be damaged if the voltage was too high. There is also a difference in voltage of a panel between no load (usually expressed as Volts open circuit, or Voc) and maximum load (Volts max power or Vmp) as well as current (capital I for current measured in amps, so short circuit current is Isc and amps max power is Imp). Even when sunlight isn't perfect, the panel puts out a voltage higher than what the battery needs and the charge controller takes care of that.

PWM controllers only turn power on and off from the panel to the battery so that the average voltage over time is the appropriate voltage for charging the battery. This is less efficient. The more efficient method is Maximum Power Point Tracking (MPPT). This uses a DC converter to put a particular load on the panel that keeps the volts + amps out of the panel at a point where it is most efficient, which is then converted to the appropriate voltage to charge the battery.

For small scale solar, often times the cost of a quality MPPT controller is around the cost of a cheap PWM controller plus a 100 watt panel. An extra 100 watt panel will give much more power than the losses incurred by using a PWM controller on this scale. It's only when you start getting to the kilowatt and above power output that the percentage of power savings from a quality MPPT controller actually pays for itself vs just adding a panel. Panels also generally last much longer than a charge controller which may be an important factor in some setups. I will give one caveat - I haven't used a name brand MPPT controller, so I don't know how clean of a signal comes out from various controllers, but the PWM controllers tend to switch on the slow side and can produce flickering in lights while charging or full.

I took my panel down last year when a hurricane was coming and wasn't physically able to put it back. I was afraid someone might walk off with it, so when a friend started talking about solar I told him to take it. My batteries lasted around a decade and I got a lot of good use and knowledge from that cheap setup. I know it's a gamble with the cheap PWM controllers as sometimes ones that look the same are different inside, but my old one still worked when I last used it. For getting your feet wet with solar I really think a cheap controller paired with one or more panels of 100 watts or more gives a great price to performance ratio. You are better off spending money on good batteries. If you don't have a ton of experience with it then I don't think it is wise to DIY multiple kilowatts and thousands of dollars of equipment, and in many places you need someone certified to legally install something that large.

I would agree with the description but not the conclusion
1) The price point where you are better off with an MPPT charge controller is now down to about two hundred watts. The Epever is available for less that $100 and is a decent mppt controller, with it you can use the utility panels which cost less than 50 cents per watt the 100 watt 12 volt panels usually go for $1 or more per watt. So a 300 watt utility panel for $150 plus $100 for the epever = $250 for 300 watts. 3x 100 watt 12 volt panels= $300 plus the $30 controller.
2)The MPPT charger will allow you to gain energy to the batteries on cloudy days, earlier and later into the days then the PWM allowing another 10-20 percent gain. A pwm no sun equals no charging. There is a very good reason that as soon as this technology became available professionals never installed another pwm charge controller again. Now that there are low cost versions available even for a starter system its the way to go.

Michael Qulek wrote:David is very correct.  That is a scam product designed to separate you from your money.  It is a cheapo PWM controller marketed by a company called "MPPT Solar".

There are a couple of clues for you to look for in this ad.  Reading the questions and answers section, you see...

3. Q: My solar panel is 36V 200W, can I charge 12V battery?

A: To charge a 12V battery, the working voltage of the solar panel can be between 17V and 23V, and for a 24V battery, the working voltage of the solar panel can be between 36V and 46V.

The fact that the solar panel can only have a max voltage of 23V is a BIG clue that it is a fake.  Real MPPT controllers, start at 100V, and go up from there.  

Another clue is that shipping weight of 12oz.  A real MPPT controller has lots of heavy copper wire toroid-rings.   The MPPT acts as a transformer, taking high raw solar voltage, transforming it down to battery charging voltage, making extra charging amps out of the extra volts.  A real MPPT controller will have a shipping weight measured in pounds, not oz.

Please forget the 12V starter battery idea completely.  You will quickly destroy the batteries, waste the money you spent on them, and get disillusioned about solar.  Let me give you some suggestions for putting together a reliable system that won't fail you.

Start with golf-cart batteries.  They are designed to be deeply drained, have fairly high capacity, and can be found just about anywhere.  Typical GC batteries are 6V, so you need to buy two to make a 12V battery bank.  You wire the two batteries in series.  Wire the negative terminal of battery #1 to the positive terminal of battery #2 to make 12V.  An economical choice right now is the 210Ah GC from Costco, which is 110$ right now.  Get two of those.  A step up from those is Trojan's T-105 GC.  Those are 250Ah, but a bit pricier.

Next, the charge controller.  A good for the money entry-level controller is Epever's Tracer 4210AN.  It can be used for either a 12V or 24V system, has a 100V limit, and can charge the battery with up to 40A of current, if the power is available.  You can pick that up on Ebay or Amazon for ~125$.  You connect the positive terminal of battery #1 to the + battery terminal of the controller.  You connect the negative terminal of battery #2 to the - terminal of the controller.  Once you let the the controller boot up, it will display the battery voltage, which should be ~12.5V or so.

Now, the solar.  Skip the little 12V panels designed for the automotive market.  You get far higher W/$ going with high-voltage residential grid-tie panels.  Shop for those locally on Craigslist, with local cash and carry pickup.  That way you avoid the high shipping charges associated with mailing panels.  Expect to get 3-4W/$.  Two 240-250W grid-ties (~75-80$ each) wired in series would be good, or maybe a single 350-400W panel (~175-180$ each).  Connect the positive and negative leads from the panel(s), to the + and - terminals of the controller.  Do this ONLY after you have booted up the controller first on battery power.  NEVER connect the solar to the controller before connecting the battery.  Build a frame to hold the panels directly facing the sun at noon.  On an average day, the best you can expect to see in terms of real-world solar output is ~85%.

This will give you a system that can power just about any RV appliance you want to buy.  It's best to not drain flooded batteries less than 50%, so this system will give you about 1.2kWh of power per day.  12V lights, a 12V TV, a 12V laptop power supply.  

Buy a gallon of distilled water and check the electrolyte levels monthly.  Top them off as needed, maybe once every 2-3 months.

You may at some point want to buy an inverter that can convert battery DC into regular household AC.  Make sure you buy "sine-wave" if you run anything powering an electric motor, a refrigerator for example.  Keep in mind though that an inverter just being left on is load on the system, and may consume 250-750Wh per day, depending on the brand.

Good luck!

Thanks so much for your suggestions Michael! Thanks for tipping us off about this product, detailed knowledge and experience is exactly what I want. The first thing I did after reading your post was look into some cheap golf cart batteries around, you are right I found some on facebook marketplace that are significantly cheaper than brand new ones. But I would not call them cheap! Next I looked at the Epever's Tracer 4210AN Great product, And after more digging I found many reviews about different MPPT controllers, and some common complaints and desired features, the price range is whacky on these products! It reminds me of something an Amish woman told me about the most expensive watermelon.... People often choose it over the cheap ones, even though she just places one of the cheap ones on the expensive table after a customer leaves...
Here is a great video I found about MPPT controllers. Though the video is a bit old, many of the products still exist, with small variations in prices.
https://www.youtube.com/watch?v=kF_cVEYxj3E

Now about the solar panels, I actually found some great solar panels that i managed to get for a whopping $35 a piece... They are 260wt panels. However they have cracked glass. I will get more info on them. Here is the specs on the back.

Made in China
Maximum Power (Pmax)  260W
Maximum Power Voltage (V mp) 30.6V
Maximum Power Current (Imp) 8.50A
Open Circuit Voltage (Voc) 38.2V
Short Circuit Current (Isc) 9.00A
Maximum System Voltage IEC1000V/UL1000V
Maximum Series Fuse 15A
Module Application Class A

To address your first comment to just forget about 12v car batteries. It is clear there will be some expenses, and make no mistake it is not simply that I am trying to find a way to cheap out (though thats the only practical way to live right now). It is also the simple fact that old car batteries are over-abundant sometimes laying in scrap, or along roadsides. Free and easily attainable. If there was a way to work them into a solar system, not only would that be very practical, but it would also be more sustainable in a bad societal situation. If your $900 deep cycle batteries go bad and are unable to be replaced, what then will you do? If I have a collection of old car batteries.. And I just happen to have that, I worry less about bad batteries at night. After your comments I dug deeper trying to find more information and I actually stumbled upon this quirky fellows youtube channel. Who did exactly what I was talking about. He has an array of old car batteries restored with distilled water and epsom salts, He has a few panels, and a charge controller. Definitely something that will interest a lot of folks in this thread. Here are three of their videos that I feel are very relevant to this topic, but there are a lot more.
https://www.youtube.com/watch?v=e9q4noZo0BY
https://www.youtube.com/watch?v=hEESf-PoQu4
https://www.youtube.com/watch?v=CJdPTLZees0

I wouldn't use car batteries, especially used batteries of different sizes or manufacturers as they will fight against each other if one is capable of 12 or 13 volts and another is only capable of 10 or 11 volts.

On my toy hauler trailer I installed 200 watts of solar and used four NEW 6-volt golf cart batteries with excellent results.  Connect 2 batteries in series to make 12 volts and then link both sets in parallel to act as 2 large 12 volt batteries to store the power.  I had a large inverter and could power the microwave, run a small electric heater all night, run the TV/DVD player all night, and I had either 12 volt lights or electric lights to use as needed.  I kept the fridge set to propane to make sure I didn't kill the batteries on cloudy days and to make sure the food kept cold.  I lived in it using the solar/batteries for2 years until I hooked up electricity to the house I was building.  The batteries lasted about 7 or 8 years.

I have been getting core batteries from a junk yard for $20. each and reviving them with a small stick welder using this technique posted on YouTube: https://youtu.be/VYtkn-N_p4s?si=7tu6E_17PfVjVOFX
So far, 95% of the batteries have come back to full voltage and amperage with some of them holding more amperage than stated on the battery. Right now, I have 10 of them ($200) wired in parallel with a single, 300 watt solar panel and an inexpensive basic charge controller I got on amazon. They have held their voltage and amperage for months and have provided me with lights, TV,  small appliances, and small tools like drills, skillsaws, etc. via a 3000 watt inverter. They are still working, holding full voltage and amperage, and providing my off grid camper with power. I do not try to run anything that draws a lot of power like air conditioning or any appliance that has a heating element like coffee pot or toaster, etc. I use a generator when i need lots of power. My plan is to monitor the batteries weekly and if any in the bank goes bad, pull it out, scrap it, get another 20 dollar core and repeat the revive process and re insert it into the bank. I've read that it takes 3 car batteries to equal 1 deep cycle battery. Hope this helps.