1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How do I charge a small battery with a large battery?

  1. Feb 7, 2012 #1
    If I had a big car battery and wanted to charge a smaller battery with it (say a laptop), how would I do that without breaking the laptop?

    First of all, a battery has a certain voltage and DC current which is the same thing a laptop battery uses, but you'd have to ditch the AC adapter because that has a transformer on it for converting AC to DC.

    I know that I=V/R but how can I charge one battery with the other battery (assuming they are rechargeable in the first place). If I connect the leads directly and one is a much bigger battery won't that damage the smaller battery? Is there a way to reduce BOTH the incomming voltage and current so that it isn't harmful to the battery?

    How would you do this? A capacitor?
  2. jcsd
  3. Feb 7, 2012 #2


    User Avatar
    Homework Helper

    You could use an inverter to convert the car battery DC to AC. Assuming your laptop battery is a LiPo battery, those can catch fire or burst open if improperly charged. Radio control models use special chargers for LiPo and other types of rechargable batteries to avoid any issues.
  4. Feb 7, 2012 #3
    I don't think it would be very efficient to do it that way. First of all, if we assume the power input is going to be renewable, likely solar, then efficient is key. Plus, the computers are going to be very low voltage, probably not even a laptop but an ultra power pc for some home functions running linux or something.

    If you convert from DC to AC then from AC back to DC I think that's inefficient, no? I suppose it depends on the quality of those parts.

    You would need an inverter then a transformer. It would be better if I could use a capacitor or something to control the voltage/current. I'm just not sure how.
  5. Feb 7, 2012 #4


    User Avatar
    Homework Helper

    My concern was safety not efficiency. Many of those radio control model chargers use a 12 volt DC source, so that would work if you know the parameters of the laptop battery. Those chargers are fairly complex, in order to safely charge LiPo batteries.
  6. Feb 7, 2012 #5
    You may be able to get an adapter designed to charge your laptop from a 12 volt power outlet in a car. If you can then that would likely be your best option. If not then the inverter will be your best option. What you want to do is not the job of a simple capacitor or resistor, you are talking about a fairly complex piece of equipment. It will be cheaper to buy an extra solar panel to compensate for the inefficiency then it will be to buy all the hardware needed to build something yourself (and a half a dozen laptop batteries that you will screw up trying) ;-)
  7. Feb 8, 2012 #6
    I don't understand what good it would do buying another solar panel. Without some kind of battery to level out the variable power input (solar energy is not a constant) you're either not going to have enough voltage/current or you'll have too much. Besides, most laptop chargers use weird voltage/currents that are not easy to get to with typical cells (for example, how many 2v cells would it take to get exactly 8.3V?)

    I'm trying to figure out what is required to transform DC energy from one voltage/current to another. I'm not going to learn anything if I go to bestbuy and buy a car adapter for my laptop and plug that in.

    If the voltage was alright would the current be fine? Would the laptop just pull as much current as it needs as long as the voltage isn't so high it blows everything up? Or do you need to really control the current too?

    I'm confused.

    Anyway, the reason I was wondering about this is because I stumbled across ultra-low energy pc designs using really tiny ITX motherboards and atom-like processors and stuff. One desktop used 4 watts of energy and I saw another one using 2. I was figuring that with an ultra low energy PC, Linux and some programming ability you should be able to build a green smarthome. Or really you could do a whole bunch of neat things where having a portable computer that has infinite energy would be useful.

    I just want to understand how things work. I really think it would be handy to be able to wire up solar panels to a battery system and have nice clean power that works with sensitive electronics.
  8. Feb 8, 2012 #7


    User Avatar
    Homework Helper

    Current is an issue. The charger needs to be "smart" enough to limit the amount of current in a discharged battery, which will have lower voltage than a fully charged battery, so that would be an issue with a near constant voltage source.

    Is there an option with the lap top to use a dc voltage source for power (this would mean that there is some type of logic inside the lap top to contol battery charging)?
  9. Feb 8, 2012 #8


    User Avatar
    Gold Member

    Generally you build/buy a power supply that converts the car battery to the proper voltage for the device you want to operate. If you create the right voltage, and are capable of supplying the required current, then the system will work. This assumes you are powering a device which contains a battery charger to manage the charge currents and profiles. (check out wikipedia's article on charging lithium ion batteries)

    Power supplies take many forms, and battery chargers also take many forms. Depending on what you want to power, and how efficient you need to be there will be different answers. But one thing is for certain: Actually charging batteries without damage is never simple (unless you are an engineer with experience). Building power converters (DC to DC or whatever) is generally complex also.
  10. Mar 3, 2012 #9
    I'm just a student, but a really simple method would be build a circuit where

    1) Limit current to about 10% of the smaller batteries amp-hour current with a current limiting circuit. You can google current limiting circuits and figure out which is best for you as they can go from lossy to fancy and efficient. you could just slap a resistor in there and rate it with ohms law, or you could use an LM317. Or fancier switching circuits like the lm2937.

    2) Connect that to a voltage limiting circuit to limit voltage to around 1.5X battery voltage. so a 6 volt battery would be voltage limited to 9 volts if your charging off a 12 volt battery. The voltage needs to be less than the "gasification voltage" of your smaller battery. Again, if you just use a voltage regulator like an lm317 its burns off the extra voltage as heat so your conversion efficiency will suck.

    Now that your current and voltage are limited the smaller battery cannot be overcharged.

    NOTE: this is a 10% trickle charge, so it will take 10 hours to recharge the small battery. any higher and you risk damaging the smaller battery. If you want to charge faster or a 90% efficient circuit, you will have to invest in a battery management IC like a MAX 700 series or LM2576.

    I hope that helps get you on the right track. Study hard!

  11. Dec 24, 2013 #10
    I've come across this thread because I have a very similar request. Id like to be able to charge very small 12v SLA batteries using the 12v lighter port in my car.

    The only issue I see with the LM317 as you say, is that it doesn't seem to regulate amperage, and the control pot would be pretty much unnecessary in my case as the unit would be internal in the devices I am building.

    SO, I'm thinking of trying one of these- I looked at those other circuits and they usually don't have an adjustable current pot. (which, is why i think you said to adjust it with a resistor or something)

    For the life of me, I don't understand why this circuit wouldn't work!


    To be perfectly honest, I'm a beginner and I'm not even exactly sure how this unit works, but it seems to be just the ticket.

    I'll monitor and set it up for 'trickle charging' conditions as you say,
    but assuming the unit works for this function, I'm wonding if I can then include a shut-off circuit ----this making it possible to charge the battery at a better rate? Also, it would be really nice to have an indicator LED to tell me when the battery is full.

    Again, I'm not a physics major, and my electronics abilities are at ground-level, so take it easy on me! ;)

    Thanks in advance.

    edit: btw, these batteries recommend a 500ma charging current.
    Last edited: Dec 24, 2013
  12. Dec 24, 2013 #11
    Here are a couple pictures of the 'art' that I've been building.




    So yes, these are just fun devices that nobody really needs. They include an ultra-bright mr16 LED array, and someday, I'd like to be able to include a usb and storage HDD storage unit.

    Again, I'm just a beginner with electrics, so any and all comments/help is much appreciated. *see above comment^*

    thank you again, in advance.
  13. Dec 24, 2013 #12
  14. Dec 24, 2013 #13


    User Avatar
    Gold Member

    looks like an interesting unit.

    The lm317 can do CC regulation, BTW. Just Google LM317 conctant current.
  15. Dec 25, 2013 #14


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    I only had a quick look but that unit appear to be a "step down" converter. So no use if you want to charge a 12V battery from a 12V source. It says that the "minimum voltage difference: 2V". Google suggests you need at least 12.9V to charge a 12V SLA so it looks like you will need at least 12.9V + 2V = 15V to use that module.
  16. Dec 25, 2013 #15


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    evanado.. This might be what you need..


    It's designed to charge the battery in one car from the battery in another car via the lighter socket in each.

    The only reservation I have is that it appears intended for short term use (eg 10 mins). I don't know what it does if you leave it connected for long periods. eg does it detect if the target battery becomes fully charged and switch to a float charge or ??
    Last edited by a moderator: May 6, 2017
  17. Dec 25, 2013 #16


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    If you want to charge one battery from another then there are a bunch of issues. First question is what sort of battery? Lead Acid? NiMH? Lithium Ion? These all have different charging requirements. Lead Acid batteries need a constant voltage source, where as NiMH cells prefer constant current. Some batteries such a Lithium Ion will readily burst into flames if you abuse them.

    Numerous chargers exist for charging model car and airplane batteries from a 12V source. Some are designed for charging low voltage battery packs (eg <12V) and others are designed for charging higher voltage batteries (eg >12V). Some are simple trickle chargers, others are microprocessor controlled fast chargers that will charge multiple batteries at once....safely.

    Best bet would be to seek advice and buy a suitable charger matched to the battery technology that you want to charge.
  18. Dec 25, 2013 #17


    User Avatar
    Gold Member

    Sometimes "needs" are irrelevant. I want one.
    Please PM me if you have any for sale.

    No, thank you.

    Ps. I have a deep cycle marine battery hooked to a 400 watt inverter sitting in my living room. I power 12 VDC LED xmas lights directly from the battery, and a 120 VAC set from the inverter. I can also power my laptop with the system. I'm not sure why Aro2220 wanted to reinvent the wheel, when everything is sitting there on the shelf. I suppose I'm just old and rich, with zero spare time to build things that I used to.

    pps. One day, I'll post a picture of the 12vdc power supply I built from scratch, back in 1979. It still works! All surplus store parts. I added an LM317 circuit years later, though that section eventually died.
  19. Dec 29, 2013 #18
    Thank you all!

    Wow, thank you all so much for the help!

    Unfortunately, I think I'm even more confounded than I was before.
    So, what you're saying is that this unit --- http://dx.com/p/power-led-driver-co...power-supply-module-cc-cv-156788#.UrnVjPRDtq0

    ---Will not work according to CWATERS because it only does 'step down'. Actually, according to this page, http://www.jaycar.com.au/images_uploaded/slabatts.pdf [Broken]

    I need 13.5v to charge a 12v SLA. so according to your math, id need 15.5V to use this module.

    Okay, so what I THINK I am hearing is I would use an LM317 in conjunction to the charging module previously stated?

    So, first the LM317 unit boosts the voltage up to 16 or 17v, THEN the Power LED Driver Constant Current Charger Power Supply Module steps that down to 13.5V at 500ma.

    So guys, would THIS LM317 unit here work this way? Some of the other units I've seen have a maximum amp rating, which would be bad in a car situation because I know the outlet itself is fused for 10a.


    Thanks again, in advance.

    Thanks to everyone who responded.

    I know theres devices that do what I am trying to do here, But the idea is that it's all in one cigar-box, with as few accessories as possible.

    OMCheeto, I would LOVE to commision one of these for you---You would be my first customer!
    BUT--until I get this issue solved the way I want it, I just can't build one! I would hate if I sold someone something that could potentially start on fire or something.

    Thanks to everyone again.
    Last edited by a moderator: May 6, 2017
  20. Dec 29, 2013 #19
    Something that does this is exactly what I'm trying to build. I can't find enough info on this thing, like, how many ma does it kick out? Does it float when it's charged? Seems like a cool toy, but I wouldn't trust it, and certainly don't want to put another 20$ part into the overhead of my product.

    I want basically this exact circuit, IN my box, with the LEd indicators and everything.

    Lets say, someone buys one of my boxes. I want to ship them exactly:
    1. The box.
    2. a 17v adapter with a K-size jack.
    3. cigarette lighter male-plug with a k-size power jack on the other end...

    And thats it. THe box, and two cheap, compact ways to charge it.
    Last edited by a moderator: May 6, 2017
  21. Dec 29, 2013 #20
    Stupid question.

    I have a seemingly stupid question to ask----Do I need diodes between the battery and these modules?

    In other words, wouldn't the current from the battery fry the modules ?

    Stupid question, I know. I think I'm more looking for 'What kind of diode" do I need to prevent that from happening?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook