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Sizing a battery charger

Discussion in 'Electronic Design' started by G. Morgan, Aug 14, 2012.

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  1. G. Morgan

    G. Morgan Guest

    What is a good formula for the needed minimum current (and over voltage)
    for charging batteries? Use a 12V car battery for example, how much
    'trickle charge' is needed to keep them alive when not being used?

  2. mike

    mike Guest

    The form of your question suggests that you're interested in more
    than just car batteries.
    Each battery technology and sub-technology has specific requirements.
    Best source of info is the vendor for the EXACT battery you're charging.
    Even car batteries have sub-technology differences.

    Remember that most battery chargers are used infrequently and
    "not even close" is close enough to get the desired result...your car

    Google will give you temperature curves and voltage/current charge
    profiles. Typically, more than one current and voltage is used
    to determine the charge profile.

    And you might make for solar when you have a use it or
    loose it charge source.

    Maintenance during prolonged storage is a subject of much disagreement.
    I subscribe to the theory that some amount of higher current cycling is
    preferred over
    continuous trickle.

    There's lots of snake-oil out there for batteries.
    A solar energy group might be a place to get guidance.

    Lots depends on exactly what you're trying to accomplish and how
    much you're willing to spend to get that extra bit of performance...
    depending on your definition of "performance".
  3. Martin Brown

    Martin Brown Guest

    There is no one answer - it depends significantly on the battery
    chemistry and slightly on ambient temperature as well if you want to be
    precise. Lead acid is fairly robust. Keeping a car battery alive just
    requires replacing the losses due to self discharge on average and not
    letting it ever get completely flat. Try:

    For a quick intro at the better than an unregulated current source
    charging profile.

    Martin Brown
  4. G. Morgan

    G. Morgan Guest

    You are very perceptive. I am thinking about keeping 3.7V rechargeable
    lithium's charged. I disassembled some solar landscaping lights and
    isolated the charging circuit (a 3v rechargeable lithium was in there).

    Open circuit on the solar panel is about 4.5V, once it goes through the
    regulator I get about 4V.

    I'd like to buy some more panels and put them in series to get 12V, thus
    keeping a 12v high-current source available if the lights go out (I live
    in hurricane alley). I can simply plug the 12V into an inverter for
    110V AC, or leave posts sticking out where I can pick off 3V DC for a
    22/2 cable that I can run through the house, adding 3V LED lamps where

    I do have a gasoline generator, but solar for LED lighting is a
    no-brainier in that situation.
  5. Uwe Hercksen

    Uwe Hercksen Guest


    running a 3 V DC cable through the house is not a good idea, you will
    loose too much voltage over the long cable.

  6. G. Morgan

    G. Morgan Guest

    Thanks Martin,

    The batteries will be lithium. I'd like to put enough of the 3.7V ones
    in a series/parallel circuit to get 10-15AH or so. It will be able to
    provide 3V DC and 12V DC in case I have to use an inverter.

    Will high temps (110°+) make this much harder to do? I would like to
    keep the batteries in the garage to keep the wire run as short as
    possible to the panels.
  7. G. Morgan

    G. Morgan Guest

    I'll compensate for voltage drop in the copper.
  8. G. Morgan

    G. Morgan Guest

    I'll compensate for voltage drop in the copper.
  9. G. Morgan

    G. Morgan Guest

    Thanks Martin,

    The batteries will be lithium. I'd like to put enough of the 3.7V ones
    in a series/parallel circuit to get 10-15AH or so. It will be able to
    provide 3V DC and 12V DC in case I have to use an inverter.

    Will high temps (110°+) make this much harder to do? I would like to
    keep the batteries in the garage to keep the wire run as short as
    possible to the panels.
  10. MrTallyman

    MrTallyman Guest

    There are power fail LED lamps that have a battery storage in them and
    turn on when the power in the plug they are in halts.

    Buy some flameless candles.
  11. MrTallyman

    MrTallyman Guest

    That is what the higher voltage is for.
  12. Martin Brown

    Martin Brown Guest

    Depends whether you want to burn your garage down or not. Lithium
    battery recharging is about the most volatile battery chemistry that you
    can imagine. It is not for nothing that portable PC batteries have anti
    self imolation disable circuits in them to protect from adverse
    conditions. Even then some units have notoriously caught fire.

    Lithium cells are most unforgiving and potentially very nasty if you
    don't do *everything* *EXACTLY* right. Unless you are expert in safe
    charging techniques do not even think of reusing lithium cells.

    Lead acid is relatively easy provided that you don't accidentally short
    out the terminals and follow the charging rules more or less.

    Low voltage fuses (or cutouts) in any high capacity battery circuit are
    essential to avoid fires. A modest high capacity battery will source a
    hundred amps or more for plenty long enough to make copper wire red hot.

    Martin Brown
  13. mike

    mike Guest

    Depends on a lot of variables, but I'd be surprised if you could get
    more than 30mA out of a yard light solar cell.
    And the batteries are going to be very low capacity.
    I buy my solar yard lights from the dollar store.
    They've been thru three levels of profit and transshipped several
    times from China and they still sell for a buck.
    Can't be very good solar or lithium cells in there.

    available if the lights go out (I live
    Executive Summary: Give it up!!! That really is a no-brainer.

    Lithium batteries are a POOR choice for what you want to do.
    High cost, low capacity.
    Solar is a POOR choice for what you want to do. You don't get much
    sunlight in the period around a hurricane.

    Depends on chemistry, and you probably can't find out what the exact
    is used in the lights...but for the types of cells used in older laptop
    computers, you MUST keep the battery voltage below 4.2V and above 3ishV.
    In series, they must be balanced. It's a battery management nightmare
    for no real benefit. Battery management failure can cost you your
    house in a ball of fire.

    go buy a deep cycle lead-acid battery and stick it in the garage.
    Put it in a box with a vent to the outside so your furnace doesn't
    ignite the hydrogen it gives off under overcharge.

    Decide on a charging regime...I suggest a 1-2A trickle charger with
    good voltage control and hit it for an hour every once and a while.
    Numbers to be determined by the exact battery and charger.
    Run your inverter or leds off the 12V. Stick about 3 leds in series
    and use a resistor to get the current you want at 10-14.2V range of
    battery voltage.

    Use the generator to charge the battery for prolonged outages.

    For general lighting, use LED flashlights/lanterns.
    $20 at Costco will get you a lantern that runs off 8 D-cells
    and claims 100 hours run time...and it's almost guaranteed
    to light up when you push the switch after a few years sitting there
    in readiness.

    Keep it simple.

    I was gonna suggest a more robust permanent system if you're serious
    about solar, but the hurricane is gonna blow the panels off the roof
    anyway...or put a tree thru 'em.

    I lived on the Gulf Coast for 18 years. I know a little about trying
    to keep outside stuff from being destroyed by a don't!!
  14. G. Morgan

    G. Morgan Guest

    That's what I'm playing with. $2 ones I saw at Big Lots that are 1.5V,
    plus some bigger 3V landscaping lights I found somewhere and just held
    on to them. Hell, I didn't even know they made 3.2V lithiums in "AA"
    size until I took the two big ones apart. <g>

    The goal is to build a portable solar box with really cheap parts that
    are already in these things. I figure if I stack enough of them
    together I could maintain a pretty stout power supply.

    With the (3V LED) lamps removed I can modify them a little to easily
    clamp to a bus, alligator clips maybe. Then for the bus I grab a box of
    22/2 and run it where lights are needed for 3V.

    I'll start small, just a box that lights up the LED's - but I would like
    to eventually build a battery bank of some sort (auto batteries?) to
    handle 12V high-current to power multiple inverters along the bus. Even
    if I use 18/4 the voltage drop will be negligible, the whole cable
    length would be less than 200' in my estimate.
  15. G. Morgan

    G. Morgan Guest

    That was an idea for a quick portable system that can be deployed in 15
    minutes. I have some 3000 mA 18650's that can deliver a lot of power. I
    just don't know how to charge them using solar correctly.

    Great idea. That is a second project. May even include a transfer
    switch to a circuit or two.
  16. G. Morgan

    G. Morgan Guest

    These 3000 ma 18650's could do some damage, I agree. I just liked the
    idea because they pack so much power into a small container. I thought
    if I can keep them charged with cheap solar cells I'd always have a
    high-capacity 12v source.
  17. Martin Brown

    Martin Brown Guest

    And are inclined to protest by self immolation if provoked.
    I have yet to find an application where a solar panel can beat
    physically moving a larger lead acid battery to and from mains power for
    The one where unexpectedly a freebie that I got at a trade show came in
    useful was one of those solar powered mobile phone rechargers. Our local
    mains got taken out by a major thunderstorm as did the East Coast
    mainline train system and this little gadget allowed me to have a
    working mobile phone despite the thing deciding to play dead soon after.
    Nothing beats a generator if you are off grid for any length of time.

    Martin Brown
  18. mike

    mike Guest

    I can't argue with your voltmeter, but I do have some observations...
    Rechargeable lithiums in AA size are a product liability disaster waiting to
    happen. Joe sixpack is gonna stick 'em in every AA socket he can find.

    The two-cell yard lights from 5 years ago contained two AA NiCd batteries.
    The current batch of $1 yard lights has a single AAA NiCd with a
    QX5252 boost converter to run the LED. The spec sheet is extremely terse,
    but there seems to be no mechanism to control the charge current or
    voltage. At least, there's no spec for it.
    You get whatever the solar cell puts out.

    Lithium batteries are a poor match in cost and functionality for a yard
    Certainly possible, except for the cost/value/stout ratio.
    But my experiments suggest that it's unlikely.

    It's certainly gonna vary with the design, but remember what they
    were designed to do. In the ones I've dissected, if there's light,
    there's zero output, none, not any. It's in charge mode.
    If there's no light, the up-converter runs and stuffs pulses of CURRENT
    into the LED load. I spent some effort trying to figure out how to make a
    5V supply out of them for a solar powered microcontroller. I gave up.
    You can buy 'em at any auto parts house or any department store or most
    anywhere. 12V lead acid battery with optional lights and optional
    120VAC. Biggest bang for the buck.

    This is a bad idea. You want 12VDC>>120VAC inverters as close to the
    battery as you can get 'em.
    If you mean 18 gauge wire, 200 feet of it running 20Amps into an inverter
    is not's a FIRE.
  19. Sjouke Burry

    Sjouke Burry Guest

    Trickle charge a battery only, when you want to destroy
    the battery.
    Periodic charge with a modern charger is better, there are modern
    ones which will only restart charging when charge drops to 70 or 80
    Else use a mains timer , to power the charger ~10 minutes a day.
    That should keep your battery happy.
  20. mike

    mike Guest

    Think this through before you start spending money...I didn't.
    I found a brand new 5KW generator at a garage sale for $100.
    I couldn't pass it up.
    Then I found a transfer switch for $50. Might as well get that too.
    Then I found some big wire at a garage sale for $20. oooh, need that
    too. Then the $15 each plugs to hook it together.

    Then, I started thinking about how to do it. The electrical stuff
    was trivial. But the permit to install it cost more than the generator.
    And my service entrance doesn't meet current code, so there's risk
    that the inspector might require a major $$$$ change there if I modify it.

    My major concern was to run the furnace air handler so that the pipes
    wouldn't freeze in winter. So, I'd just put a plug on the furnace
    electrical wire.
    Well...the electrical code forbids that. General consensus, verified by
    talking to the local inspector, suggests that the "rule" is generally
    ignored. But what is the fire insurance underwriter gonna say when
    my furnace juice doesn't meet code and the place burned down? His job
    is to DENY claims.

    Bottom line...anybody wanna get this generator out of my garage???

    A sensible approach is to put a pair of isolation diodes and an
    additional deep-cycle battery in your car or truck.
    Stick an inverter adjacent to the battery and you're done.
    You have a general purpose portable power source that can
    supply a LOT of power for a short time. And it's always ready.
    And it comes with a two-ton charging machine.

    Another option is to buy a used UPS. The batteries are always
    dead, but the rest often works. Make sure it's a 12V system, not all are.
    Hook it up to a deep cycle RV battery and plug it in.
    They're not designed to run for a long time, so you'll probably
    need a fan on the heat sink.

    What follows is a rant. You know where your "next" button is...

    The hardest part of any project is deciding what you want to accomplish.
    It's easy to lump multiple disparate objectives together and poorly
    specify the desired result.

    The power going out for a few days after a hurricane is a camping trip.
    If you're on life support, that's an emergency.
    Having a solar powered led infrastructure in your house is a hobby.

    The definition of a hobby is spending WAY too much time and money on
    that gives you pleasure, but the result is inconsequential.
    Lithium batteries and solar is a fine hobby. Just expect to spend
    way too much time and $$$$ on it. ;-)

    If I were building a lithium standby system out of 18650 lithium
    cells, I'd start by figuring out how many peak amps I'd want and
    put that many cells in parallel. So, for 100W of 120VAC out of an
    inverter, I'd have at least 10 cells in parallel.
    If you want longer run time, put more cells in parallel.
    Then I'd figure out the voltage output I wanted.
    For a load that could tolerate 9V-12.6V, I'd series three.
    for 12-16.8V, series 4. Notice that these numbers exceed
    the requirements on one end or the other for most inverters.
    So, I'd go buy 40 brand new cells from the same batch from
    a reliable vendor and weld them
    up in sets of 10 with high-current plugs.

    Remember that random 5 year old Chinese cells reclaimed from
    worn out laptop battery packs and sold on EBAY are not
    what you want.
    You want recently manufactured cells from a reliable source.
    Those are EXPENSIVE!! Fry's gets about $13 each for 'em.
    There's a reason a Tesla car costs a hundred grand.

    I'd charge them to 3.8V, yes exactly the same voltage.
    The optimum voltage may be different. Point is you want
    40% of so state of charge.
    You want the
    state of charge to be EQUAL.
    Then I'd stick them in the fridge at just above freezing.
    One school of thought holds that at near freezing the actual
    state of charge has less effect on cell life...YMMV.

    When the hurricane watch goes up, take 'em out, warm 'em up
    and charge each pack to 4.2V...Not about 4.2V... LESS than or equal to
    Notice that I didn't say plug them together and charge to 16.8V.
    I said make sure the state of charge is equal and you don't
    exceed 4.2V on any pack.
    Plug them together and run your system.
    Have a mechanism that disconnects the load when any cell pack
    gets below 3V or so. Don't rely on watching a meter. Fall asleep
    and you can snuff your batteries.

    THESE NUMBERS WORK FOR SOME, BUT NOT ALL battery chemistry variants.
    ASK THE VENDOR FOR NUMBERS and use those.

    There's not much solar energy available in the wake of a hurricane,
    so you don't need the solar part. IFF the cells are balanced and you
    don't go overboard, you can get away with a few charge cycles without
    monitoring every pack individually, but I'll deny saying that.
    There's typically a pressure disconnect in each cell.
    If you over-stress the system and one cell disconnects, that
    puts even more stress on the others and the dominoes start to fall.
    You have to balance the probability of some fault against the consequences
    of that fault and how much liability insurance you have.

    Then I'd look at the plan and decide that it's clear that
    I should go buy a 12V lead-acid battery instead.

    Garden lights make damn fine garden lights...leave them where they
    18650s make horrible laptop batteries, but they're the best we can
    afford. Leave them in laptops.

    Are we having fun yet?
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