# [Howto] Powering my laptop with a car battery

## Dominique_71

Hi, I am not sure it is the good forum, but as it is for hardware issues, I put this subject here.

OK, I was having troubles with my laptop battery. The capacity of original battery was very low after 3 years of use, almost unusable. So, I was buying a new one with more capacity than the original one. Severe mistake, the new battery was powering the laptop but the laptop was unable to charge it. I brooked apart  the 2 batteries and made a new old one with the electronic of the old one and the cells of the new one. The laptop was able to charge the battery, but was just unable to work with that battery.

I begun to try to power the laptop with a motorcycle battery. I must be possible because the laptop's battery have 6 cells in 3 groups in series of 2 cells in parallel coupling, which made 3 * 4.2V = 12.6V, which is about the same voltage than a car or motorcycle battery (I will write car in the rest of this subject). That modification can also be useful for someone that want extended battery use time.

Warning: For that modification to work, the original battery must have 3 group of cells in series coupling, which give a nominal 12.6 voltage. If it is not the case, you must use a battery arrangement that give you the same voltage than the original battery. Also, it is much I don't know about those f. smart batteries, which imply I made no guaranty of any kind it will work for you. Last, the board with the connector of these smart batteries are modern electronics, which imply to use adequate tools to modify it and to sold the wires, and to know how to use these tools.

See BU-911: How to Repair a Laptop Battery for a description of a typical pinout of a smart battery connector. The pinout of the connector of the new battery is a little bit different. On the board, they are marked like P+, D, C, T and P-. I first connected the car battery on P+ and P-. The laptop was not working. The power led was ON during one second and that's all. When continuously pressing the power button, that led was On and blue and the charge led was blinking yellow, but nothing more was happening.

I measured the resistance between P- and T on the board of the battery connector. It was 3.3k ohms. At the BU-911 page, they said it must be 10 k ohms. I cut the board near to the T pin in order to sold a 10k ohms resistor, but before soldering it, i made a new try with the car battery and it just work. As it work, I will leave it so.

The OS don't recognize the battery:

```
ls /sys/class/power_supply/BAT0/

ls: impossible d'accéder à '/sys/class/power_supply/BAT0/': Aucun fichier ou dossier de ce type
```

/sys/class/power_supply/BAT0/ don't exist, but the laptop is working fine.

I will update that post later with the pinout of the connector, but be aware it can vary from one laptop to another.

EDIT 1: Another warning. You cannot use the charger of the laptop when directly coupling a battery to a laptop as explained here. That imply when the battery is plugged in the laptop, you must NOT plug in the power adapter of the laptop at the same time. If you do so, I don't know what will append, but I know for sure than, in the worst case, this will damage your laptop.

That imply you must use another power source to charge the battery. The usual car battery chargers are not stabilized, which imply it is best to NOT charge the battery when it is connected to the laptop. If you can afford to buy or make a stabilized charger, it is no problem to use it at the same time than the laptop, but event in that case, do NOT plug in the power adapter of the laptop. I can provide a schematic for a possible stabilized car battery charger of my own. It work well for me, I am writing this with the laptop coupled to the car battery with my custom charger ON.Last edited by Dominique_71 on Sat Sep 24, 2016 5:05 pm; edited 2 times in total

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## Roman_Gruber

YOU HAVE BEEN WARNED.

We are talking about chemistry here and I highly warn anyone tinkering with batteries when he does not have the background.

Li-ion batteris have circuitry which prevent most disasters but not all. Therefore those batteries should not be open in the first place.

Using car batteries with another chemistry with circuitry for li-ion batteries is a way to harm you and others.

YOU HAVE BEEN WARNED.

edit: The post was much larger, it breaks down to teh above statement. I do not want to tell details or anything else in this regard.

--

The only safe workaround in my eyes is to use a power converter for car batteries which converts the car battery voltate to 230V AC 50 hz for example, than you can use the notebook power supply.

e.g. just an example. If links are not allowed to amazon please remove it. thanks

https://www.amazon.co.uk/BESTEK-Inverter-Converter-Electrical-Outlet/dp/B00AX0OAZI

 *Quote:*   

> BESTEK 600W Car Power Inverter DC 12V to 230V AC Converter with Electrical Outlet 

 

I want to point out for those safety features which all your hack does not provide at all

 *Quote:*   

>  Rock Solid: Durable aluminium enclosure provides advanced protection from drops and bumps, allows power inverter stand up to harsh environments.
> 
> Advanced Protection: Built-in cooling fan, full protection and auto-shutdown keep your appliance and car from overheating, overloading, low load, short circuit, low voltage and over voltage. (

 

Not recommended for the average guy: Or you can create your own power circuitry for external power input, my laptop needs for example 19Volts dc with 9.23 Amps max.

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## Dominique_71

I don't use the laptop to charge the car battery, but a dedicated charger. As this charger is stabilized, I can even have it plugged at the same time than the laptop. And yes, I don't plug-in the power adapter of the laptop when using this battery. I will add this in the first post.

For my point if view, that solution to power the laptop directly with the battery is better, because I don't get the losses in the 12VDC to 115/230 VAC converter, which losses are high in most cases.

EDIT: corrected typo (replaced plus by plug-in).

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## Roman_Gruber

 *Quote:*   

> For my point if view, that solution to power the laptop directly with the battery is better, because I don't get the losses in the 12VDC to 115/230 VAC converter, which losses are high in most cases. 

 

That depends on the design of these converters and what you pay for. 

I want to ask you how you want to ensure that the notebook does not accidentically charges the battery because they are using the same teriminals. Any hack which does not really check for the safety of the user is a dirty hack and should not be shared to the public, which may not be that knowledgeable.

--

 *Quote:*   

> I don't use the laptop to charge the car battery, but a dedicated charger. 

 

You may also tell the guys to dismantle / remove the charging port of the laptop at all, so a charging of the wrong chemistry battery will not happen at all.

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## Dominique_71

 *Roman_Gruber wrote:*   

> I want to ask you how you want to ensure that the notebook does not accidentically charges the battery because they are using the same teriminals. Any hack which does not really check for the safety of the user is a dirty hack and should not be shared to the public, which may not be that knowledgeable.
> 
> --
> 
>  *Quote:*   I don't use the laptop to charge the car battery, but a dedicated charger.  
> ...

 

Yes, but it will imply to modify the laptop. Another solution would be to put a series diode in one of the wire. It will let the current to flow from the battery to the laptop and block the charging current from the laptop to the battery. But it will be a 0.6 V drop on that diode, which imply this is not a good solution. Which imply to modify the laptop would be the best solution.

On my laptop, the maximal charge current is 2.5 A. From figure 3 at BU-409: Charging Lithium-ion, it appear the charge current will not exceed the maximal charge current. A 2.5 A charging current would be fine for a >=25 Ah lead battery. Also, the voltage is limited to the end of the charge, which is good even when charging a lead battery. A 12 V lead battery can handle something like 14 to 14.5 V at the end of the charge, when with lithium ion, that voltage will be limited to 4.2 V per cell, 12.6 V in that case, which is a little bit too low. That imply it must be possible to use the laptop to charge such a lead battery, but to get it fully charged, which is good to do, you will have to disconnect the battery from the laptop and use a dedicated lead battery charger.

I will make some measurements and investigate it.

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## szatox

Car batteries are pretty safe to play with. Just make sure not to put your finger inside (you WILL get burned) and not to drop it on your foot (considerable weight guarantees lossy compression on your part)

You are not likely to damage such a battery even with rough usage or heavy duty unless you try to weld with it - yes, it is possible, no, you don't want to do that, even by accident. Don't short circuit.

On the other hand, don't try charging Li-ion cells unless you really know what you're doing. they're fireballs about to happen.

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## axl

car battery: dc 12v.

laptop input: dc 13.7v. 

what could go wrong?

nothing. so far nothing went wrong. it's funny to ssh user@car  :Smile: 

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## Irre

fully charged battery: 12.6 V

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## Roman_Gruber

that depends on the chemistry in question

e.g. my vehicle uses this, and also the previous vehicle uses this

https://en.wikipedia.org/wiki/Silver_calcium_battery

around ~2000 a college from me, ~17-18 years old, managed to empty the acid from an ordinary car battery for smaller vehicles. We used that battery for a project, and that battery was freshly purchased by myself in a shop. so the battery was new and "safe"

only because the ordinary user may not hurt himself, does not imply that you will find a technical student who is kinda nuts to leak the acid of a car battery ...

Just something from my past ...

--

I would not rely on the nominal voltage of a battery when designing a circuit...

As I had a broken alternator recently, i measured full / charging battery / compleltyl drained battery. ... The voltage range is kinda big. Also the current is kinda big which is possible (needs special equipment and knowledge to measure that ...)

--

I can accept the following statement

Nominal voltage 12 Volts, 44wh

anything else depends on the age / chemistry / charging state of the battery in question / or if any cells are dead ...

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## Roman_Gruber

 *axl wrote:*   

> car battery: dc 12v.
> 
> laptop input: dc 13.7v. 
> 
> what could go wrong?
> ...

 

Depends

Charging Voltage is much bigger as those 13.7 volts and there you may have issues

but as these days voltage regulators have a wide imput range, and lets assume those notebook mainboard designers use standard components it does not really matter. I assume the need for higher voltage is only for the cfql tubes (or what they are called) for the lcd unit. When you check lcd specs you will often see a requirement for ~13-15 Volts (dc / ac .. not sure now)

Without knowing the cirucitry of the notebook in question I would not sign that statement at all.

--

Just proofs that you have no general idea about batteries or electronics. Nominal voltage is not equal with the voltage the battery has. Or how much abuse the mainboard / lcd device can take. I would not be surprised when the voltage is directly feeded to the lcd and you ruin your display ... Just possible cause.

edit: everyone knows mainboard needs to be cheap, safety for not designed uses, will not be covered. they even use low temperature cap and than you have dead mainboards for example, cheap connectors ... they count every penny and will reduce components to the absolute minimum.

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## Zucca

I'll post here just to make another warning. This is about the types of lead acid batteries.

I myself, have played around with lead acid batteries and I've witnessed the dangers of lead acid batteries quite close by on a previous workplace I was working (first as an intern).

The normal lead acid battery most people recognize is the VRLA battery. VRLA stands for Valve Regulated Lead Acid. Those batteries have the acid in a form that can produce gas. Acid is in those usually in simple liquid form. When charging those types of batteries (especially when overcharging happens) they can produce gas that mostly a mixture of Oxygen and Hydrogen. And in short mixture of Oxygen and Hydrogen is a gas mix waiting for a slightest ignition. As you may know hydrogen is the gas in hydrogen cars. It has that much potent.

The place where I was working had one room for the lead acid batteries for their UPS system. All those batteries were open breathing types. There was a seperate air ventilation in that room, totally isolated from the building's other air systems. It was propely built room for those batteries.

However, when two tehnicians went to change few bad ones of them to new ones. Upon disconnecting one of them (UPS was of course set in bypass mode) propably a small spark ignited the little amout of hydrogen and BOOM! The other guy survived ok. But the another had some short term hearing problems and his arm got burned. Only first degree burns as the flames didn't last that long. But remember: that happened in well ventilated room.

This is why UPS manufacturers tend to use AGM type of lead acid batteries that are fully sealed. Acid in them is in fiberglass mat. That type of lead acid batteries are much safer. There are also gel lead acid batteries that (you guessed it) have the acid in gel form. Some of them might be safe also. Please, do your own research.

P.S. I'm not saying using lead acid battery for powering laptop is crazy. ;) I think this is quite interesting topic.

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## axl

Cars are not really that stupid these days. Most cars cut the power to the lighter when you are not in the car. 

What I suggested is use a simple connection between car cigarette lighter and the battery DC input from the battery. Rely on the laptop internal step-up regulator connected to the battery. For the laptop usually running with a battery of 13.7V, a battery that has 12V (which is the car lighter voltage when motor is running) is just a battery with less power. As long as you don't try to charge that battery u're ok. 

It's slightly more complicated to make a desktop to work in a car, but  a laptop is pretty easy.

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## 1clue

This is not nearly as simple as it might seem from the outside.  As many have said before me, DO NOT mix a Li-Ion charger with a lead-acid battery. When charging a battery you need a charger designed for that chemistry. When connected to expensive or sensitive equipment, you need to have some sort of smart monitoring of both the charging circuit and the laptop power supply, independent of each other. It's possible that what's in the laptop will work for the load side.

An in-line diode will prevent the laptop from charging the battery, but it will also reduce the voltage delivered to the laptop. Which may have repercussions with respect to running your laptop.

The least dangerous thing I can think of is to get/make a Li-Ion battery for your laptop, either the same or larger capacity. 

Next would be to use a UPS (which may be relatively easily converted to car batteries, given a few precautions as to picking the correct UPS) and then use the normal laptop power supply. As the OP mentioned that's not the most efficient way to go, but if your efficiency equation takes into account fire damage or acid damage then it probably is the most efficient.

In order to charge a 12v battery while using the laptop means modification of the laptop so that the lithium ion charger is completely removed from the circuit. If it's not removed then anything you do will have that circuit trying to mess with things, which can/probably will lead to unwanted results.

If you're going to pursue this at all, including the option of using any sort of modified UPS with a nonstandard battery, you need to understand some basics about battery chemistry and charging cycles. You need to take into account the laptop's Lithium Ion charger, and the lead-acid battery, and the charger you use for the lead-acid battery.  You also need to understand how drawing power from a battery while charging affects the entire charging process.  I'd look into outgassing for lead acid batteries too, and how to deal with it and minimize it.

Keep in mind that almost all lithium-based batteries have built-in circuitry to protect each cell. Some of the better ones have monitoring features. Look at RC car websites for adults if you want to get an idea. There are lots of YouTube videos of DIY batteries making white hot fires when something didn't go right. http://www.rcdriver.com/pardus-lipo-charging-bag/ is an example of a charging bag that charges the battery while still in the car, avoiding fires caused by disconnecting the battery incorrectly, as well as protecting your house from the fires caused by charging incorrectly and by charging a damaged battery. It's the first link I found when I searched on rc car charging bag. You can see hundreds or thousands of links, depending on your patience.

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## axl

i didn't suggest any kind of thing that involved a charger. of any kind. 

what i said was to connect the car lighter dc output to the laptop battery input. no charger. it's just like your car lighter IS the batter for your laptop. NO CHARGER. the laptop will think it's just a somewhat low batter, and will compensate with powerup regulator. for the car, it will be just another lighter. 

it doesn't get any simpler than this. in my example there are no batteries. no transformers. just a plain +/- connection from the car lighter to the laptop battery connection. but just as input. NO BATTERY.

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## axl

let's consider the system a bit. we have a car. standard. and a laptop. standard. 

car should output 12V on it's lighter output. 

laptop should be looking for 8/9-14 V on it's battery. NO CHARGER. NO BATTERY. just the DC inputs FROM the car lighter as if it were a battery. 

a normal battery doesn't have a constant volt output. it varies based on how charged it is. for a battery just as the laptop that works between 9 and 14 volts, the circuitry is already in the laptop to handle a steady 12v input. 

so instead of using batteries and complicated transformers, just connect the car lighter to the laptop REMOVED battery. as in input. NO charger. it will work great. 

the laptop is comfortable running at 12 V on that input, the car is happy to output that voltage on the lighter output. everything is fine. 

why not?Last edited by axl on Tue Sep 27, 2016 2:02 am; edited 1 time in total

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## 1clue

@axl,

I didn't respond directly to your post. I responded to the thread, in which a separate charger was mentioned.

Speaking from personal experience, the voltage from the cigarette lighter can go from about 10v to 18v under relatively normal circumstances. A fully charged car battery is around 11 to 11.5v at rest, meaning not being charged. When you hit the starter it can very easily dip below 10v in a fraction of a second. Normal charging goes up to 15v, but a bad regulator in the car can drive it over 18v.  As well, the "regulator" in many cars is an on-off relay switch that puts a big resistor into the circuit to avoid frying gauges. It's on or off, and the battery connection drops completely during the switch. It's not a long enough delay for you to notice the lights flickering, but it's significant.

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## 1clue

Speaking to automotive use, every laptop I've owned requires more than 12v. So you're into a power inverter no matter what. In my case they use significant current, which means you're into a really good power inverter.

Edit: In addition to rms voltage issues with automotive use, you also need to consider high frequency noise from things like spark plugs and other components being powered by the system. Some cars have electronically actuated fuel injectors that can add significant noise, for example. Now there's variable valve timing, some of which alters camshaft timing and others directly operate the valves. I would think the latter would add significant noise.

Another thing I've noticed is that many pulse-width modulation wall warts have a transformer which multiplies the wall outlet voltage to something like 170 to 300 and then uses pulse-width modulation to convert it down to 12v or whatever. I wouldn't think that's how it works, but at one point I was scavenging transformers and found this out. I don't know if it's still common practice.

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## axl

if most cars can accommodate an iphone charger, or a fridge or a tv, am sure it can just as well do a laptop. 

i said a standard car, with motor running.

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## 1clue

An iPhone charger almost certainly has conditioning circuitry in it. There's no other excuse for the price.

A refrigerator is an electric motor. They are a rotating inductive/capacitive load, effectively conditioning their own power.

A TV with a cigarette lighter cord is probably going to have fairly beefy power conditioning too.

Personally I'm going to let somebody else try it with their laptop first.  My laptop's too expensive to experiment with.

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## 1clue

automotive usb charger guts:

http://www.ti.com/lit/an/slva464e/slva464e.pdf

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## John R. Graham

 *1clue wrote:*   

> Another thing I've noticed is that many pulse-width modulation wall warts have a transformer which multiplies the wall outlet voltage to something like 170 to 300 and then uses pulse-width modulation to convert it down to 12v or whatever. I wouldn't think that's how it works, but at one point I was scavenging transformers and found this out. I don't know if it's still common practice.

  I don't think this is a very common practice as the primary purpose for the transformer in switching power supplies is isolation. Also, the rectified, filtered output voltage of a 1:1 isolation tranformer is sqrt(2)*supply voltage, which, for a 120vac input is right at 170vdc, so that may be what you're measuring.

- John

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## axl

 *1clue wrote:*   

> An iPhone charger almost certainly has conditioning circuitry in it. There's no other excuse for the price.
> 
> A refrigerator is an electric motor. They are a rotating inductive/capacitive load, effectively conditioning their own power.
> 
> A TV with a cigarette lighter cord is probably going to have fairly beefy power conditioning too.
> ...

 

i suppose if you are the type of driving person you could just use a power monitor thingie to look at the lighter output before trusting it. or use an external power regulator between the lighter output and the battery input to keep it below 12v. 

i'm not... i like small robots. and most of my robots CAN and WILL charge from the cigarette lighter. it was a necessity to fly twice. or swim twice... or whatever that dirter was doing. 

the car lighter output should be just as safe as the wall outlet. 220 in my country. sure a storm could come and explode your tv... but often it doesn't happen. 

WV might cheat about emissions but i'm pretty sure the lighter outlet is at a safe 12 v. and again, a stepup/down power regulator for 12 V would be a great intermediary between lighter/battery input of a laptop. and it would just work. 

wasn't that what we tried to accomplish from the start?

me for one, am happy cars HAVE a power output, regulated by the car own electric/electronic components. and own generator. that's why i said "while motor is running". not starting, stopping or simply not running. 

car manufacturers and goods manufacturers rely on that power output just as we rely of electricity coming out of the wall. 

and it does. you should know, most car manufacturers put a lot of thought in your lighter voltage output. clients are not happy when their iphone chargers blow up. and they dont.

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## 1clue

I really don't want this to turn into a pissing match, but it evidently has done so.

There certainly is a difference between the quality of power at an AC house receptacle and a cigarette lighter in a car. Keep in mind that the original purpose of the cigarette lighter port was to make a piece of metal glow hot enough to light a cigarette.

The consideration taken with regards to that port is in the power supply of the device being plugged into the lighter.

Many modern computer power supplies can take something like 85-270v AC. A power inverter with pwm behind it can handle a stunning range of input voltages and, given adequate current supply, convert that to a safe steady voltage.

Automotive power inverters for the cigarette port can be had for something like USD $30. To me that's a pretty decent insurance policy, because it's unlikely your computer will be damaged with it in place. It may not have enough current to do any good, but you won't get trashed.  Higher current power inverters can be had for $200 USD.

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## axl

 *1clue wrote:*   

> There certainly is a difference between the quality of power at an AC house receptacle and a cigarette lighter in a car. Keep in mind that the original purpose of the cigarette lighter port was to make a piece of metal glow hot enough to light a cigarette.

 

depends on the country you live in and the car you drive. also the inverter thingie has no place here.

ac/dc. it's just 12v dc we talking about.

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## 1clue

I'm on my phone now so pardon spelling and brevity.

If you look at the link I posted you'll see that to convert from 12v to 5v for a usb charger, they use an inverter and pwm. The same thing can be used for any output. Including 12v dirty to 12v clean.

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## 1clue

According to that pdf for the usb charger, the lighter plug supplies 6v to 14.5v with multiple 16ms surges to 40v.

Again, it's your laptop.switching supplies are reliable and cheap.

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## Dominique_71

 *1clue wrote:*   

> According to that pdf for the usb charger, the lighter plug supplies 6v to 14.5v with multiple 16ms surges to 40v.

 

I would never connect the battery plug of a laptop directly to a lighter plug. But this is not an issue for me, the only vehicle I own is a titan bike.  :Very Happy: 

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