Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Solar car battery recharger

  1. Jan 26, 2006 #1
    I'm new to this forum, so I'm not sure if this is the appropriate section to post my question, but anyway, here it is:

    I'm looking into buying a solar panel to be permanently connected to my 12V car battery and recharge it constantly (or at least when the sun is up). I want to do this to experiment with the idea of shutting my car off at stop lights to conserve fuel, then restart the engine when the light turns green, similar to how at Toyota Prius works, and see how effective it is at saving fuel. Since starting the car requires a lot of energy from the battery, I'm afraid that doing this 5-10 times a day almost everyday will significantly reduce the life of the battery. I've been looking into using solar panels that I can rig on the hood or roof of the car so the battery is contantly charged; however, I can't find any clear answers as to how effective they are at recharging a 12V car battery. How many watts would I need for this kind of setup to get any significant recharging? Is this even a good idea to begin with or is it just a pipe dream?
  2. jcsd
  3. Jan 26, 2006 #2


    User Avatar
    Science Advisor
    Homework Helper

    Welcome chevymauler to PF Forums!!

    I commend your experimental spirit. You probably want to do some research on how much power (Watts) or cranking Amps are consumed per start of the vehicle. To get you going, here is one piece of information I found. A (car) starter may draw http://www.carcarecanada.ca/articles.cfm?itemid=18993&smocid=1508 [Broken]. How long does it take to start (5sec, 10sec?). If you know the Amp-Hr rating of your battery, you may be able to approximate the percent charge left. Ambient temperature is also an important consideration in calculating state of charge. In this regard, the above reference also discusses CCA (cold cranking amps) and RC (reserve capacity).

    To give you a feel for some solar battery chargers, here is a page with several commercial models. Be sure to read their solar tutorial.
    Last edited by a moderator: May 2, 2017
  4. Jan 26, 2006 #3
    I thinks your idea is so good, and many companys are studying this, but they can use it there day cuz is so expansive. but you can do it. I sure you can. about you question. you know this formula to find watt.
    W= I*V, SO battery 12 V D.C and the current of there is 50A\h so the watts will be 600 watt \ houre.
  5. Jan 26, 2006 #4
    Thanks for the response and welcoming into the forum.

    I've found a couple rollable solar panels that I can use on my vehicle. Here are the specifications:

    POWERFILM R15-300
    Operating voltage : 15.4V
    Operating amperage: .3 amps
    Wattage generated : 5 watts

    Operating voltage : 16.5V
    Operating amperage: .62 amps
    Wattage generated : 11 watts

    Both are listed for roughly the same price, which I find funny because the UniSolar generates more power, but again, I'm not sure how much I need. I'm currently trying to find the specifications on my battery. I have a 2003 Toyota Echo in case its coincidental that anyone who reads this may have the same.
  6. Jan 26, 2006 #5
    CCA seems to be anywhere from 500 to 550 amps.
  7. Jan 27, 2006 #6


    User Avatar
    Science Advisor

    chevymauler - this is basically the idea being used by GM in its partial hybrids like its pickup truck hybrid, where the engine is shut off when not needed. They take it another step, and use the oversized starter/generator to move the vehicle at very low speeds for stop and go traffic. The new Saturn Vue hybrid is similar except the starter/generator in that vehicle attaches in a manner similar to the alternator on most cars. Its not as advanced as a Prius, but its a lot cheaper and easier to implement.

    CCA is a standard for the current available at near freezing while staying above 7 volts for a short period of time like one minute. It has no long term time specified, and therefore is not the same as ampere-hour or Ah. For a larger case high performance car battery, 50-60Ah is about as good as common sizes will allow. With a smaller car and a smaller and easier to turn over engine, a smaller battery is typically used.

    Your alternator will likley charge the battery back up long before you park the car, so the solar cell will not really provide that much help. The alternator will make between 700W to 1200W or more, and you can see that this is much larger than 5W to 11W like the solar panels you listed.

    If you switch to energy (joules) instead of power (watts) to factor in how time plays a part in this, as in how much energy is output or input from the storage of the batteries, starting the motor 10 times at 5 seconds with 400A at 12V is going to be the product of all of those - 240kJ. The 11W solar cell running for the 3 hours of good sunlight at 11W (I'm being very generous with these numbers) will make 118,800J - just less than half as much. If you factor in Peukret on the discharge of the battery, the solar cell seems even less attractive.

    Now, I'm all for saving fuel and the planet, I really am, but how long does it take to pay for the solar cell with gas savings? And how much coal was burned to make that solar cell (they're electricity intense to manufacture) and its not so perfectly green anymore.
  8. Jan 27, 2006 #7
    Thanks for the information. I don't expect the solar panel to act as another alternator, but at least help it keep up with the power demand from other factors, such as the radio, accessories, lights, etc. And the beauty of the solar panel is that when the car is parked in the sunlight, it charges the battery without the engine being on. Not by much, but a little bit. I'm not really expecting to save any money from this, but more of a neat experiment with solar panels, as I can always use it for other applications. By buying it, I'm also supporting the technology.
  9. Feb 6, 2006 #8
    Good idea in theory but..
    The hybrid engines are smaller because they aren't needed as much and the starters are comparitively larger. On a standard engine, the starter isnt designed to handle more then a small percentage of use. Starting at every light would wear out a starter in short order. Even if you change em out yourself you have to ask how many gallons = a starter
  10. Feb 6, 2006 #9


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    a) You're going to destroy your starter, starter solenoid, and perhaps other components of the system.

    b) Your engine already includes a battery charger -- the alternator -- which pretty much keeps your battery topped off all the time. When you stop your car at your destination, your battery will be essentially already charged, and your solar panel will contribute virtually nothing.

    c) Compared to fuel burned while travelling at highway velocities, the fuel burned while idling at a 1-2 minute traffic light is very nearly irrelevant.

    - Warren
  11. Oct 29, 2007 #10
    Charging car battery using Solar Power

    => Thanks Chevymauler. I have liked the idea very much. I am looking for a similar solution. In our case, we are planning to setup a wireless network using few laptops thourgh a wireless router. For laptops we can arrange additional batteries to run for 8 to 10 hours.

    We wanted to power the wireless router using Solar power via car battery. As we would conduct experiment in different countries, so it would ideal to buy car battery in the local market and use them.

    Kindly let me know if there are already any solution for charging car battery using solar power.
  12. Oct 29, 2007 #11
    Several companies make them for cars but they are more long term trickle chargers. For what you want look at the marine industry. Sailboats usually have solar chargers of enough wattage for what you need. Your also going to need some heavy duty diodes because without them your alternator will backfeed and fry the cells.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook