solar panel li-ion battery charger

Discussion in 'The Projects Forum' started by camerart, Feb 25, 2013.

  1. camerart

    Thread Starter Active Member

    Feb 25, 2013
    517
    30
    Hi,

    I want to charge li-ion batteries with solar cells (12V), for back pack or cycle.

    I have to make up the solar panels from cells first.

    I have a couple of little circuits for protecting each of the cells from over/under charging.

    Now I need to know how to get the varying power from the solar panels into a safe charging system.

    Cheers, Camerart.
     
  2. tindel

    Active Member

    Sep 16, 2012
    568
    193
    linear technology has several chips that can do this.

    Your question is a little bit broad. What do you need to know?
     
  3. camerart

    Thread Starter Active Member

    Feb 25, 2013
    517
    30
    Yes, it is a broad subject, so I will break it down into sections:

    I am trying to power a 12V amateur radio, with solar power, for portable use. I want to make a small pack with everything inside. Radio, antenna, charger, solar etc,as small as possible. I imagine app 1 foot square x 5 inches deep.

    1/
    I have 150 x 150 solar cells (0.5v 8w?) I gather these can be cut down into 2 or 3 or 6 (as there are 6 tabs on the positive side?) I would do this to raise the voltage (lowering the watts of course) to get to a more useful total voltage. Then encapsulate them into suitable size folding panels to give the input power.

    2/
    I have bought Buck converters, up and down with varying voltages for input and output, depending on what the solar panels give out. I'm not sure if they can cope with changes in the sun's ever changing output.

    3/
    I have Li-ion cells 18650, which appear to have over/under charge protection built in, also some similar but without protection. I have a 4X series cell protection circuit, so I can use the unprotected cells. Also some individual cell protection circuits.

    4/
    Now to join it all together safely. I have read that li-ion cells need care when charging, and this is especially what I need to know.

    If it is possible to keep any answers in there sections, I'm sure it will be more understandable for anyone interested.

    Cheers, Camerart.
     
  4. tindel

    Active Member

    Sep 16, 2012
    568
    193
    Just about every battery needs care when charging. A good charger will have two (sometimes three) ways to turn off charging once the battery is full, the second is a backup. App Note 68 will help you understand different battery charging architectures including Li-Ion. It is one of my personal favorite app notes.
    http://cds.linear.com/docs/en/application-note/an68f.pdf

    Some things you need to know is: How much power does your radio consume (if it's sold in the US, it should have a power rating printed on it somewhere). From this, you can calculate how much power you need from your battery to power it for your desired length of time. Then you can calculate your desired solar array size to charge your battery in your desired length of time. Once you know these requirements then you can size your battery and solar array (SA) and determine what sort of boost/buck converter architecture you need. Maybe you've done this already but have not told us these requirements yet.

    About your batteries - make damn sure they have protection built in, and that the protection is working before going that route. Frankly, I have a hard time trusting batteries with 'protections', and consider them a third line of protection if my primary and secondary protection fails. Read your battery datasheet and understand it. Li-Ion is not something to play with! Ask any Boing (aka Boeing) (insert awesome sound effect here) engineer.
     
    Metalmann likes this.
  5. Metalmann

    Active Member

    Dec 8, 2012
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    Thanks, very valuable info!
     
  6. camerart

    Thread Starter Active Member

    Feb 25, 2013
    517
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    The radio consumes between 12-24W depending on use, I will have to compromise radio use with battery levels.

    3/Li-ion cell charging: I read as much of the LT1510 as I could take in, it looks to be good chip, but can it cope with the varying voltages of a solar panel?
    I hope for existing tested circuits, so that I don't have to design my own with perhaps disastrous results.

    Cheers, Camerart.
     
  7. tindel

    Active Member

    Sep 16, 2012
    568
    193
    As long as your solar array doesn't exceed 30V then this chip should be fine to use.

    Maybe an amateur radio website has what you're looking for - have you looked online for what you're looking for? Maybe it's a wall charger device that you have to convert to a SA powered device, something like that. That's a bit of a hack though, and you still have to know what you're doing.

    A 12-24W radio will work for 1-2Hrs with a 2Ahr battery. Longer if your you have higher capacity in your battery.

    How long are you planning on using your radio while on battery power? How much battery capacity do you have? How quickly do you plan to charge your battery? What is the open-circuit voltage voltage of your array? What is the short circuit current of your solar array? Peak-power voltage and current of your solar array is also a useful data point.
     
  8. camerart

    Thread Starter Active Member

    Feb 25, 2013
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    I am looking online.

    I've stripped a wall charger, but I can see a protection circuit, that is impossible to reverse engineer, so I'm starting fresh.

    The whole system is being designed, so as yet no panels, battery etc. I am starting fresh. I'm hoping for advice on all of it.

    Cheers, Camerart.
     
  9. tindel

    Active Member

    Sep 16, 2012
    568
    193
    Okay, so first you have to establish your requirements:

    So again, I ask: How long are you planning on using your radio while on battery power? How quickly do you plan to charge your battery?

    Additional questions:
    What size (volume) would you like to maintain? What size (weight) would you like to maintain?

    Answering these questions will allow you (or us) to determine what battery chemistry and solar array type to use, it may design architecture to use.
     
  10. camerart

    Thread Starter Active Member

    Feb 25, 2013
    517
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    All as small and light as possible with as much power as possible :), but if I start 8X 18650, cells 4X running the radio and 4X being charged, then see how it works.

    I have the solar cells already, these are 0.5V 8.2W (Full sun), which can be cut or joined as required.

    I think the main difficulty I have is that I'm not sure how the solar variation acting on the charger with Li-ion cells.
     
  11. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    Are you attempting to build Something like this?

    If so, you may get some hints from either that design, or similar ones. They all seem to use larger panels than you plan to, however.

    A lot of your needs depend on the amount of anticipated "radio receive time" vs. "Radio Transmit time" vs. "Battery Charge only" time. Assume average of 9 hours of good sunlight (max solar output) per day since it's hard to keep an optimum angle.

    I use the Brunton "SolarPad" for topping off my GPS while hiking, it works great, but being around trees and moving diminishes output a great deal.
     
  12. camerart

    Thread Starter Active Member

    Feb 25, 2013
    517
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    I looked at your suggestion, and following the clues led me to an array of stuff to read all very interesting. Yes, something like this. Although following links to the actual controllers, I've yet to find a small enough one, and they must charge Li-ion batteries safely.

    I checked the Brunton out, and I will most likely follow something similar to that design. Very neat.

    Cheers, Camerart.
     
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