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

Can you charge a 12v car battery with capacitor?

  1. May 18, 2015 #1
    can you charge a 12v car battery with capacitor
  2. jcsd
  3. May 18, 2015 #2


    User Avatar
    Gold Member

    Sure, if you have a big enough capacitor. That would have to be one BIG capacitor though and you'd need a circuit to control the charge, else you'd likely destroy the battery. Also, the inefficiency of doing so would make it a rather silly thing to do in practice, even though it is theoretically possible.
  4. May 18, 2015 #3
    Okay so theoretically it is possible - so here is a scenario - i have one 200 watt solar panel on my roof and i would like to run my house off the electrical grid - with that 200 watt solar panel can i recharge enough capacitors to run my house during sunlight hours and then using batteries at night and the capacitors then also recharging the batteries
  5. May 18, 2015 #4


    User Avatar

    Staff: Mentor

    I'm not quite following:
    1. If you are using the solar panel to charge the capacitors during the day, what is powering your house?
    2. Why not just use the solar panel to charge the batteries directly, since it would be easier/more efficient?
    3. How much energy does your house use a day? A 200W solar panel will provide about 1.5 kWh of electricity on a good day. That's almost enough to run a standard size refrigerator and nothing else.
  6. May 18, 2015 #5
    well thats my point - a capacitor as i understand it has can store vasts amount of energy/volts - if you can regulate its release - with regulator and inverter etc - to run a house off solar you need a lot of panels - would the use of capacitors not solve that problem
  7. May 18, 2015 #6

    But there are commercial products that essentially do that same thing. (They don't use capacitors for long term energy storage.) try Googling "off grid solar kits". And no, it's not a good do it yourself project unless your an electrical engineer.

    Even as an engineer I would do some other work until I had enough money to buy one. (Who am I kidding, a backup generator is cheaper and easier.)

    Do study the issue though. There are lots of fly-by-night systems that don't work.
  8. May 18, 2015 #7
    Yes but capacitors can be used for long term energy storage - and this is all just an idea in my head - i stay in South Africa and well we face up to 10 hours a week load shedding (blackouts) - to solar power a standard house off the grid is about $80 000 US (half a million rand here) -- Probably 80 panels and a couple of decent batteries --- our electrical bill tariff increases yearly have become ridiculous because 4 million people are paying for 20 million people who can afford to (not all are poor) - but dont want to pay

    so i am thinking of ways to reduce the cost
  9. May 18, 2015 #8
    and fuel prices are just as ridiculous - government adds nearly 40% on fuel for taxes etc
  10. May 18, 2015 #9
    Give up on the capacitor idea. They are way too expensive for bulk storage. Batteries are used for that for a reason.

    The thing I think you're looking for is called an uninterruptable power supply (UPS). There are lots of them on the market, including solar powered ones. They come in many sizes from units for PCs to ones for factories.

    How much power do you need? Do you need to be off grid to avoid hook-up fees? When does the power go out? Is it scheduled?

    In any case there are companies that sell UPS solutions in South Africa. It's really not a DIY project.
  11. May 18, 2015 #10
    Yes i know about UPS etc -
    A) - load shedding is not scheduled what is guaranteed in more chaos and mismanagement and a 10 nuclear power station deal (trillion rand) 100 billion dollars that actual cost will be double to triple in the end and someone has to pay for that --

    but this is going off topic - i want people off the grid at costs well lower than being quoted - going DC with panels during the day plus to charge batteries for night time usage well there is not enough roof space for the amount of panels needed and amount of batteries needed would in fact probably be a lot.

    that is why i am thinking capacitors (surely they hold/store more energy than a battery (as for the new 10KV tesla battery coming out well cost would be in the region with tax, import markup etc R50 000 -- something that i hear is going to cast about $3500 in USA - in South Africa a lot of people (average) are lucky to earn average 10 - 15 thousand rand

    1. - (another question lets say normal 12v car battery how long would that run a standard size fridge) - forget anything like stove or geezer with elements
    2. - and how many panels and wattage would one need to charge a normal 12v car battery
  12. May 18, 2015 #11


    User Avatar
    Gold Member

    Sure, if you never run more than a couple of 60Watt bulbs in your house.
  13. May 18, 2015 #12


    User Avatar
    Science Advisor
    Gold Member

    Surely they do not.
  14. May 18, 2015 #13


    User Avatar

    Staff: Mentor

    Well, no -- in order to store the electricity, something else has to make the electricity: the solar panel in this case. So you can either use the panel to power your house or to charge a battery or capacitor, but not all of the above at the same time unless you get a bigger solar panel.

    A capacitor can't do anything a battery doesn't already do better.
    A standard car battery holds about 45 Amp-hours, so 12V*45Amp-hours = 540 Wh.

    A fridge uses about 140 W and runs about half the time, so that's 540 Wh/(140W) = 7.8 hours. So you need about 1.5 car batteries to get the fridge through the night.
    If you get 6 equivalent full hours of sunlight, that's 45Amp-hours / 6 hours = 7.5 Amps. Then 7.5 Amps * 12V = 90W. So one solar panel can charge two car batteries a day (leaving nothing extra to power your house during the day).

    FYI, I'm single and live in a 1500 square foot townhouse and I use about 8 kWh per day on days that I don't use my heat, air conditioning or laundry (and my water is heated by gas). The fridge alone is 1.7 kWh.
    Last edited: May 18, 2015
  15. May 18, 2015 #14
    Would that the sun shine at noontime levels all day every day with no clouds. And dirt/dust/corroded contacts never happen. I'm not sure what the derating is for solar panels, but I'm sure it's hefty. And let us not forget the 80% efficiency of lead acid batteries (car batteries). That means to get 80 Watt hours out you need to put 100 Watt hours in. And that's only true if you have magic electronic controllers with no losses of their own.

    Things are so much nicer in sunny South Africa. A land where equipment never breaks leaving me without power until I can afford expensive repairs.

    Electric utilities really do a very good job providing cheap, consistent power. Doing it yourself is way more expensive and way less reliable. Shanevan might be in a position where he's going to miss out on that benefit of civilization, but I hope I never am.
  16. May 19, 2015 #15


    User Avatar
    Gold Member

    Lets see ---- What size capacitor would supply the same current as a 4.2V 1500ma-h battery, that is 1500ma for 1 hour
    (the battery would stay at 3.2 - 4.2 volts, while the capacitor would discharge to zero

    1500ma for 1 hour where I = C dv/dt so 1.5 = C*4.2/3600 C = 3600*1.5/4.2 = 1286 farads.

    Assume we only wanted the capacitor voltage to droop 1V, then the answer is 5400 farads

    That is for a typical cellphone battery.

    I just looked at 1 web page and it had 5,000F, 2.7V ultracapacitors - $175
  17. May 19, 2015 #16
    Ultracapacitors have a few problems. The ones I've seen aren't high voltage. But their biggest drawback is that they leak current when full. For some applications such as emergency lighting, that's fine. But it won't do for bulk energy storage.

    So try to stick to conventional batteries. How much do 5,400 1F capacitors cost? [That was rhetorical.]
  18. May 19, 2015 #17
    Thanks people and really thank you for the above responses - have only perused it and it is actually something i have learned so will copy and paste to my word and disseminate it properly and can make sense of LOL (as i said was not born wired for science) - i just thought because a Capacitor/s etc can hold unlimited volts and e.g. like a stun Tazer small rechargeable battery fills it up to give you 50 000V or more at a time constantly - i was trying to think of capacitors being used (with regulators etc) to supply the wattage needed to run whatever but using less panels - because as i understand wattage ----simple example a kettle has a 1500 watt element one would need 15 x 100 watt panels for DC to use the kettle (not counting efficiency and other factors)

    So basically my idea sucks and i need to go back to the drawing board - like inventing the Tesla Mark II battery lol
    And no people am not deluded enough to think that i can do my own DIY installations and projects - just got a thought and decided to look into it

    anyway am going into the business and decided to start small and that is doing solar power to run pool motors and then work my way up from there as i learn and become expierenced
  19. May 19, 2015 #18
    There will be lots of work for solar power installers in the coming years. It's a good business to get into. Selling and installing solar systems is a growth industry.

    But understand the design and production of the systems will take hundreds of engineers working together for years. Big companies will dominate the design side. They will want companies to market and install though.
  20. May 19, 2015 #19

    Yes i realize that and was never interested in design - not even really interested in the actual installation - but will do it with help starting small - ultimately i would want to just market and sell it and get finance for those customers - and leave supply (warranty) and installation headaches to others
  21. May 19, 2015 #20
    There are plenty of people who like installing but hate the business headaches. Find some and you should do well. Good luck.
  22. May 19, 2015 #21


    User Avatar

    Staff: Mentor

    Yes, I think the main issue is not recognizing that voltage is not energy, so a high voltage capacitor doesn't actually store much energy.
  23. May 19, 2015 #22


    User Avatar
    Staff Emeritus
    Science Advisor

    If you're getting into the solar business or any alternative energy business, you really, REALLY need, at minimum, to take a basic electronics class. Without understanding how circuits work, you will not be able to do very much and you run a very real risk of electrocuting yourself or someone else, or burning someone's house or yard down. Not only is this bad for business, you also don't want to have an avoidable accident like that on your conscience.
  24. May 19, 2015 #23


    User Avatar
    Gold Member

    Taking a basic electronics class is also important from a business perspective since it gives you the background required to evaluate vendor claims and alternative solutions. Without the basic technical background, you will need to depend on, and likely fall prey to, marketing hype.

    You are going into a new, growing and highly technical field, and scams and optimistic claims abound.
  25. May 23, 2015 #24
    Thanks people - realise that it was just a thought and not a project to get involved in
  26. May 23, 2015 #25


    Staff: Mentor

    Coincidentally, my wife and I actually live comfortably on 210 watts of solar power. But I live on a sailboat, not connected to shore. My total electric energy consumption is only 0.6 kWh/day. 80% of that goes to refrigeration, the rest to LED lighting and recharging my digital devices. We are hardly unique. There are about 10,000 cruising sailors on the US East Coast who live similarly.

    If you can reduce your housing needs to about 150 square feet of space per person, and if you move to a place with moderate climate to avoid heating and cooling, and a place where you can do your business and shopping on foot or bicycle, then energy conservation is easy.

    I know that's not for everyone, but it is not a hardship, nor is it magic, nor high tech.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook