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

[Aro2220]

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?

 

[rcgldr]

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.

 

[Aro2220]

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.

 

[rcgldr]

I don't think it would be very efficient to do it that way.

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.

 

[mrspeedybob]

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) ;-)

 

[Aro2220]

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.

 

[rcgldr]

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)?

 

[meBigGuy]

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.

 

[Centauri1st]

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!

Cheers

 

[evanado]

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

Cheers

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!

http://dx.com/p/dc-dc-step-down-cc-...ltage-power-supply-module-152998#.UrnIEvRDtq0

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.

 

[evanado]

Here are a couple pictures of the 'art' that I've been building.

https://scontent-b-sea.xx.fbcdn.net/hphotos-ash4/1526500_10201287385611319_408907924_n.jpg

https://scontent-a-sea.xx.fbcdn.net/hphotos-prn2/q72/s720x720/1471923_10201287546695346_1176124242_n.jpg

https://scontent-a-sea.xx.fbcdn.net/hphotos-ash3/q72/s720x720/1471997_10201287546575343_728248896_n.jpg

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.

 

[evanado]

alas

I *think* I've solved mine, and the OP's problem.

http://dx.com/p/power-led-driver-constant-current-charger-power-supply-module-cc-cv-156788#.UrnVjPRDtq0

I can't really figure out why this wouldn't work to do everything..

This wouldn't be a 'trickle' charging solution. This would be a full-on DC~DC charging solution.

Voila?

 

[meBigGuy]

looks like an interesting unit.

The lm317 can do CC regulation, BTW. Just Google LM317 conctant current.

 

[CWatters]

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.

 

[CWatters]

evanado.. This might be what you need..

https://www.amazon.com/dp/B00005OWK2/?tag=pfamazon01-20

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 ??

 

[CWatters]

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.

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.

 

[OmCheeto]

Here are a couple pictures of the 'art' that I've been building.

...

So yes, these are just fun devices that nobody really needs.

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

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.

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.

 

[evanado]

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-constant-current-charger-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

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.

http://dx.com/p/diy-lm317-adjustabl...wer-supply-module-w-rectification-blue-187296Thanks again, in advance.

Thanks to everyone who responded.

I know there's 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.

 

[evanado]

evanado.. This might be what you need..

https://www.amazon.com/dp/B00005OWK2/?tag=pfamazon01-20

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 ??

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 that's it. THe box, and two cheap, compact ways to charge it.

 

[evanado]

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?

 

[OmCheeto]

...
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.

I don't need a working unit. I just need a cigar box. At one time or another, I've built everything that you are putting together.

Wait! I think there's a cigar box in my attic. Never mind.

But thanks for the idea. And I think I'll add some copper tubing to the exterior. Steam Punk is so in fashion.

Thanks to everyone again.

You're welcome.

 

[CWatters]

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 don't know the details of that product but at the very least it must boost the voltage from the source/car 12V battery (which might be lower or higher than 12V depending on it's charge state or other factors such as.. is the engine running and the alternator charging it?) to some higher voltage that depends on the charge state of the target/your 12V battery.

I can think of a lot of other features that it should have but who knows what it actually has..

Current limiting to prevent damage to the target battery. How fast can your target battery be charged?

Current limiting to prevent a melt down if the target battery is almost flat or faulty when connected? Might come for free with the above.

A feature that switches itself off if the source battery is nearly flat?

I'm not aware of a free circuit that will do all this this online but I suppose their might be one out there somewhere.

Perhaps try an electronics forum? You might find an electronics engineer willing to design you one.

 

[CWatters]

PS: Could you change your battery to a 6V SLA? If you can then that would simplify the circuit as you wouldn't need to boost the 12V first. For example there are several circuits on the web based on the LM317 that would work. You would need to ensure the LM317 was mounted on a sufficiently large heat sink.

 

[evanado]

PS: Could you change your battery to a 6V SLA? If you can then that would simplify the circuit as you wouldn't need to boost the 12V first. For example there are several circuits on the web based on the LM317 that would work. You would need to ensure the LM317 was mounted on a sufficiently large heat sink.

6V? Then you'd have to bump it back up again to have a voltage that's useful. (Lights, charging systems are all 12v)

Hard to believe this isn't a thing...

When the entire world is on 12v, it will be thing!

 

[OmCheeto]

...
Hard to believe this isn't a thing...

...

There is such a thing. Only it comes in a larger package, and is called a car.

They do make smaller "things" similar to yours, though none I've seen look like cigar boxes.

 

[CWatters]

6V? Then you'd have to bump it back up again to have a voltage that's useful. (Lights, charging systems are all 12v)

My point is that it might be easier to change the light bulb to 6V than it is to make a 12V to 12V charger.

Hard to believe this isn't a thing...

When the entire world is on 12v, it will be thing!

The problem with using 12V for the "entire world" is that if you need to draw lots of power the current and losses become too high..

A 3kW fan heater at 230V draws 13A
A 3kW fan heater at 12V draws 250A !

If the wires have 0.01 Ohm resistance then the losses in the wires are roughly...

230V system about 1.7W
12V system about 625W !

Actually it's worse than that because I ignore the voltage drop in the wires.

 

[CWatters]

This looks similar to the thing you are trying to make...

http://www.tooltime.co.uk/shoppingcart/products/12V-Car-Engine-Starter-Battery-Booster-Jump-Start-with-USB-Output-%252b-Torch.html

They claim it can even recharge the car battery from it's internal 1300mAH battery but how useful that would be is debatable.

 

[evanado]

This looks similar to the thing you are trying to make...

http://www.tooltime.co.uk/shoppingcart/products/12V-Car-Engine-Starter-Battery-Booster-Jump-Start-with-USB-Output-%252b-Torch.html

They claim it can even recharge the car battery from it's internal 1300mAH battery but how useful that would be is debatable.

Yep---Thats pretty much it, although, you still can't charge it FROM the car battery. It too, has an 15v ac adapter.

I think the bottom line here seems to be, you can't do it. Or at least, you shouldn't do it. What I am trying to build would be the exact opposite of this unit, where the car charges the unit, not the other way around.

Fact is, its likely dangerous, I suppose. But...this will not deter me.

In response to using 6v batteries---It is a really good idea, still, and I think for my up-coming electric bicycle components project, it may be ideal. My only concern is that phone/device chargers are all designed to use 12v. I COULD theoretically put a 5v USB port instead of the cigarette port, but then, you're right----I'm stuck using 6v bulbs, which... I'm sorry, are just too pathetic. 12v MR16 LED arrays are ALL the rage, with good reason. Check them out, if you haven't.

I have been building an outdoor 12v lighting system in my yard as well. I wanted to have certain lanterns that had batteries that, would charge off the system when plugged in, but you could unplug them and move them to where ever the party was. (Yes, I throw a LOT of parties) I make many of my lights/lanterns myself. I'm also somewhat of a stained glass artist. I've also built some ground canisters out of old rail-road-telephone insulators. (I can post some pictures, if anyone is interested in seeing this stuff)

It is true that similar devices already exist, but the thing is, I didn't build them, and they are ugly, plastic, and have no soul! I'm not giving up on this stuff---I am actually hopeful of using the LM317 unit as well as the charger module I had previous posted. (I did order both, and can't wait to try them out--OUTSIDE, of coarse.


I appreciate all the help, folks. I'm still thinking the ticket is the modules---My only concern, is the fact that a battery would be connected to the output side of the module---I'm thinking that would likely fry the module(s) and they need to be protected from the battery somehow.

 

[evanado]

My point is that it might be easier to change the light bulb to 6V than it is to make a 12V to 12V charger.



The problem with using 12V for the "entire world" is that if you need to draw lots of power the current and losses become too high..

A 3kW fan heater at 230V draws 13A
A 3kW fan heater at 12V draws 250A !

If the wires have 0.01 Ohm resistance then the losses in the wires are roughly...

230V system about 1.7W
12V system about 625W !

Actually it's worse than that because I ignore the voltage drop in the wires.

HAH! No, I wasn't speculating on the idea that we'd convert the world to 12v, I was speculating the idea that someday, we might not have 230v, or 120v, or high voltage, at all!

Well, maybe you will, but someday, I want to be off the grid! I would someday like to use so little electricity, that I could take care of all my basic necessities using only 12v electricity. (a ton of deep cycle batteries) Wind, Hydro, solar, bio-deisel.

In other words, heaters, furnaces, motors, computers, and appliances in general, are out of the question, basically. This might sound like living hell for some people, but for me and my wife and dog, it sounds pretty peaceful.

What can I say, I'm a bit of a post-apocalyptic fantasist, and I don't plan on living that long.