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Solar charger for car battery?

Discussion in 'Electronics & Electrics' started by Sledge, May 22, 2021.

  1. Sledge

    Sledge Member

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    I'm getting one of these to keep a low use car battery in working condition:
    https://www.bcf.com.au/p/wanderer-4.8w-solar-maintenance-charger/574604.html?cgid=BCF027015#start=13
    https://www.supercheapauto.com.au/p...ntenance-charger/564510.html?cgid=SCA01060203
    But one of the reviews on the SCA website says:
    Should i be worried? Is he accurate?
    If so, is there an easy fix so i don't kill any batteries?
     
  2. paulvk

    paulvk Member

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    You do not have to pay that much here is the same thing on ebay
    https://www.ebay.com.au/itm/202632076126?_trkparms=aid=1110006&algo=HOMESPLICE.SIM&ao=1&asc=232358&meid=b429b1ad0ceb43ba86149db619c9347c&pid=101195&rk=4&rkt=12&mehot=ag&sd=231717124515&itm=202632076126&pmt=1&noa=0&pg=2047675&algv=SimplAMLv9PairwiseUnbiasedWeb&brand=Eco-worthy&_trksid=p2047675.c101195.m1851

    A 5 watt or 10 watt panel on a car battery will not hurt it at all I had a 10 watt one on my battery for 5 months with no regulator while waiting for a new gear box and that battery lasted 7 years. Note it is a good idea to have a diode in series with the panel to stop it drawing any current from the battery at night the guys at Jaycar would be able to help with that but their solar panels are too expensive.
     
  3. OP
    OP
    Sledge

    Sledge Member

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    So i ended up getting one, and now that it's here and i've tested it...
    It outputs 20v.. but pretty low amperage...

    Would that be an issue for 12v car batteries?

    (I have no idea about electricity)
     
  4. paulvk

    paulvk Member

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    20v is the open circuit voltage when loaded it drops.
    Which one did you get?
     
  5. OP
    OP
    Sledge

    Sledge Member

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    I'm guessing open circuit? Nothing was connected to it at the time...
     
  6. spludgey

    spludgey Member

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    I'd be very hesitant to use anything without some sort of voltage regulator like an PWM or better MPPT. They're not expensive either.
     
  7. OP
    OP
    Sledge

    Sledge Member

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  8. spludgey

    spludgey Member

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    That's a PWM not an MPPT, but yes, that'll do the job.
    Not sure what the minimum input voltage is though, as it doesn't have a spec sheet.
     
    Last edited: Jun 4, 2021
  9. OP
    OP
    Sledge

    Sledge Member

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    So why does it say MPPT?
    Do they not know what they are selling?
     
  10. rthy

    rthy Member

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    spludgey likes this.
  11. paulvk

    paulvk Member

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    What panel have you got?
     
  12. spludgey

    spludgey Member

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    False advertising.

    They probably know, but they're hoping you won't notice, as this PWM does the same thing, just in a less efficient know, which the vast majority of people wouldn't notice.
     
  13. OP
    OP
    Sledge

    Sledge Member

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    Would you happen to have a link to what you would recommend at all?
     
  14. spludgey

    spludgey Member

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    No, sorry. A PWM is fine if you're not trying to maximise the output. Bear in mind that your input voltage needs to be higher than your output voltage though.
     
  15. C4Z4M

    C4Z4M Member

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    What is the self discharge rate of the battery? Unless the panel manages to completely charge the battery it's never going to be an issue and I severly doubt 5w will manage that
     
  16. spludgey

    spludgey Member

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    So let's say the panel is capable of making a maximum 6Wh/W/day in summer in Adelaide, so 30Wh/day for a 5W panel. The capacity of the car battery might be 50Ah or 600Wh. Now all you have to do is ask yourself whether a brand new fully charged car battery would go completely flat in 20 days? I'd be pretty certain the answer would be no, so the self discharge rate is likely to be a fair bit lower than the maximum energy being supplied by a panel and I personally would try and avoid it.
    With lead acid batteries, the impact of overcharging isn't that bad and you'd likely just lose a bit of electrolyte, but it's obviously not ideal.
     
  17. OP
    OP
    Sledge

    Sledge Member

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    No idea, something is making it go flat if we don't use the car for a couple of weeks...
     
  18. C4Z4M

    C4Z4M Member

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    Yep Agreed that a new clean battery in good health might not go flat in that 20 days. One that has been in a car for a while and might be a little dirty across the top I'd say would likely be dropping a bit.
    I'm not saying that there is no risk of over charging but in this case it would seem that there is greater risk of the battery going flat and dying that way.

    Sledge I'd test the working current of the panel when connected to the battery on a sunny day to see just how much it is pushing into the battery. Also check how much current the regulator you choose uses as some of these will continue to draw from the battery even after the sun goes down which could negate the panels input altogether.

    Have you tested teh parasitic draw of the car when off? most cars will be drawing a small amount all the time.
     
  19. OP
    OP
    Sledge

    Sledge Member

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    how do I do that?
    No.. again.. how?
     
  20. C4Z4M

    C4Z4M Member

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    you mentioned you had tested the panel
    So I am guessing you have a multimeter that can do amps.

    To test how much it can manage to put into the car battery put your meter in current measuring mode (change the position of the positive lead to the position for amps and set the dial to say 2A if it's not auto ranging) then connect the negative lead of the panel to the batter and then the positive to one of the lead of your meter then hold the other lead onto the battery.

    For testing the draw from the car when off do the same only by removing one of the battery leads and connecting the meter between the battery and car. this will tell you how mutch it is drawing from the battery.

    Be careful not to touch your meter leads to the positve and negative of the battery while connected up for current measuring as you will likely blow the fuse in the meter or worse.
     
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