Running laptop off 12V.

[Nick Y]

Continuation of a topic that seems to have disappeared about using a laptop on 12V.

A suggestion I made, just before the topic disappeared -especially for the chap in Sumatra running on solar cells and presumably a 12V 'car' battery, was to remove the laptop battery and connect the 12V direct to the mating terminals. (As there are 4 terminals, it may be necessary to bridge them into two pairs.)

Although I think that the batteries are quoted as 10.5V, my battery was reading 12.34V on 100% charge using NHC. If this works, it will be important to remove the battery as charging directly could cause damage and coule be hazardous. A fast acting fuse would be an idea, too -e.g. 5A

If this works, and I don't see why not, there would be advantages in removing inverter RF; reduing weight, etc.

(BTW, I haven't tried the above!)

[Paul Unger]

Yeah . . . wonder where that topic went?!

So when NHC indicates that "Battery Voltage" is 12479 mV I guess I just have to move the decimal three points to the left to get a number I'd recognise, eh? I are so samrt So that'd be 12.48V; and I just pulled my battery to check and it reads "NOM 10.8V 4.4AH" (6-cell Li-ion; FRU 08K8193). It even indicates a +/- on two of the terminals on the battery; + being furthest to the 'inside' and - being furthest 'outside'. Maybe the other two are 'sensor'/charge regulator leads? Hmmmm . . . almost you tempt me to try this thing! But I'd be DEAD if I smoked my computer I've got a few friends nearby who are electrical engineers--I'll pose the hypothetical situation to them and see what they say.

[JaneL]

Yeah . . . wonder where that topic went?!

That topic went away because it degenerated into a name-calling contest and none of us have had time to clean it up yet.

[Nick Y]

Hopefully, as an electrical/electronic engineer of many years experience, it will work as long as there are no voltage spikes-but then I wouldn't take my word for it; I may have missed something and I have yet to see the schematic for my laptop!

(-reminds me of Marshall McLuhan 'You mean my whole fallacy’s wrong?' )

[leoblob]

Maybe put a Zener diode with the correct voltage across the lines, with a fuse in series with the external power source. Any spikes will go thru the Zener and pop the fuse.

[440roadrunner]

This scheme seems to me a bad idea. I have a 600X battery, which I dissected to try and figure a way of replacing the cells. I believe my old 380XD used the same scheme, which is, lith ion cells, 3 sets of two in parallel, with the 3 pairs of 2 ea wired in series---in other words, 3 LI cells in series.

According to what I've found, the nominal cell voltage is 3.6V-3.7V, which would come out to 10.8-11.1.

I would think a voltage OVER that is simply surface charge, a vast difference than the hundreds of amps that a charged lead acid battery can deliver, because that surface charge will drop quickly in the case of the LI cells.


I spose you could build a simple analogue regulator.

[Nick Y]

I would think a voltage OVER that is simply surface charge, a vast difference than the hundreds of amps that a charged lead acid battery can deliver, because that surface charge will drop quickly in the case of the LI cells.

I am unclear as to your point. I have 1W night light connected to a supply capable of producing 13A at 230V (or possibly 100A, if I connect it across the main house input terminals -and neither current ratings allowing for the rupture current.) A powerful Zener is a good idea to limit spikes, but a straight regulator will do nothing to the current, except die in smoke if the power rating is exceeded.

[440roadrunner]

My point was possibly poorly made---the voltage you measured on your fully charged LI cell pack was in essence a surface charge, and does not reflect the "true" design voltage---what the pack would output under a nominal load

A fully charged 12 lead acid battery, WITHOUT the surface charge, is quite a bit above that voltage, and because of the immense capacity (amperage) of the auto battery, it won't be dropping anytime soon.

In other words, it seems to me to be a dangerous plan to connect an auto battery directly to your Thinkpad, because the voltage is too high. Once again, LI batteries are rated at about 3.6-3.7V, or 11.1V total. In the 10-12V range, even a change of as little as 1V is an 8-10% change--no small amount at that level.


It may well be that the circuitry at the battery terminals is capable of handling that much, but without some hard technical facts, I sure won't take that gamble with mine.

[Nick Y]

OK, but my LI cells sit at over 12V for several days. A good point, though, as a car battery will go up to 13.5V at full charge. Perhaps, going back to the regulator, a simple design with a 12V zener to the base of a power transistor, with bleed resistor from collector. Emitter will then be at 11.3V roughly. with up to 2V dropped across power transistor. (Still could be 10W dissipated in the transistor.)

[jsmit86]

Why not just get a 12v dc air/car adapter for the thinkpad, and wire a 12 lighter outlet to the battery. This would seem much safer.


look here

[Nick Y]

Why not just get a 12v dc air/car adapter for the thinkpad, and wire a 12 lighter outlet to the battery. This would seem much safer.

This thread is the restart of a deleted thread, where such items were discussed. (Not your fault for not seeing the previous thread.) Two problems with solution suggested above: 1) additional weight/ bulk carried around by some people, to/from a boat and 2) RF noise produced by the inverter kills radio reception, etc. when working away from mains power -i.e. using solar power.

[GomJabbar]

A good point, though, as a car battery will go up to 13.5V at full charge.

Actually, when the engine is running the charge voltage can be as high as 14.4 volts or even more. I would think a voltage regulator would definitely be in order.

[Nick Y]

The AC adapter is supposed to provide between 15.5V and 17.0V. (Page 42, ThinkPad Computer Hardware Maintenance Manual; July 2005; for ThinkPad T43/T43p (MT 2668/2669/2678/2679/2686/2687) and ThinkPad Dock II (MT 2877))

So a minimum of 15.5V enters the laptop and this then drops to +12.6V, when fully charged. (12.6V for fully charged battery, less than 11.0V for discharged battery, again according to above manual.) The difference, i.e. the voltage dropped inside the laptop, is dropped across a regulator, which is adapting the charging voltage and current quite precisely so as to charge, when required, but not damage the battery.

One thought: The 15.5V to 17.0V are on no load and hence I would expect this to drop below the 15.5V when on load, i.e. charging a discharged battery or actually powering the laptop. Question is, to what does it drop?

[fcampbel]

I hope you guys are still monitoring this thread. I was a very poor student in physics and chemistry but I have some questions on this topic and someone up there said they were an electrical engineer.

The way I understand it, voltage can be equated with pressure and you have to run a battery in reverse to charge it. So, if the battery is putting out 11 or 12 volts, a higher voltage is required to run the current backwards to recharge the battery, hence, the 15 to 17 volts from the AC adapter.

Now, if you just plugged a wire directly from a car adapter into the DC in port on the laptop, what would be the result? What bad things could result from having sufficient amperage but lower than expected voltage? Would it still run the computer but not charge the battery? Is there any danger in plugging one in that is turned off and seeing if the power light comes on?

And -- -- I have always used a 12 volt inverter to get 120 volts AC in my car so I could use the regular power brick. But switching from 12 volt DC to 120 volt AC and then back to 12 volt DC has always seemed unnecessarily complicated, and heat is generated at every conversion point showing that energy is being wasted. How can the 12 volts from a car cig lighter be stepped up to 16 volts in one step?

While reading the schematics of power suppies for desktop computers, it seemed to me that the maximum voltage a computer deals with is 12 volts. Where do they use a higher voltage?

[jdhurst]

Amperes are more related to pressure (but the analogy is not that good).

The power adapter gets 16 Volts out as the result of a switching power supply inside the brick. It is not related to a battery running backward. Switching power supplies can be made very small (much smaller than transformer supplies, for example).

People apparently run their laptops off car batteries, but I would not, for the same I would not try to run my refridgerater off 90 volts. Too low a voltage can cause electrical stresses as well as causing an appliance just not to run.
... JD Hurst

[tom lightbody]

let us not forget some TPs run on 12v (760 series,
eg--and others): unlike the familiar 16v. yellow tip,
their native chargers use a weird trapezoidal plug w/ 4
contacts. But 12v is a huge advantage onboard, the
biggest problem being to find aforesaid trapezoidal
connector to wire direct into the cigarette lighter
socket:-)

[Terrahawk]

I'd be interested in trying out hooking 12V up to the power input of my R40. It has a 16V adapter, and I notice that the batteries charge up to 16.8V (4.20V per cell in the battery), which is slightly higher than the 16 supplied. I suspect that there is a DC-DC converter somewhere in the charging circuit.

It is true that Li-Ion cells are nominally 3.6V, but the way to tell when they are fully charged is to charge them until their terminal voltage is 4.20 +/- 0.05V. If you watch the charging cycle with the Battery Maximiser's "Battery Information" window, you will see that is indeed the case.

So, if there is a DC-DC converter in the charge circuit, there shouldn't be too much problem hooking up 12V - apart from the extra current required from the 12V source compared with the 16V one.

[rkawakami]

Hooking up 12V directly to the DC power jack of any system which is spec'ed at 16V input, may or may not work, depending upon the power conditioning being used in the laptop. First, there could be an undervoltage detection circuit which would lock out some or all of the various power supplies within the system. Secondly, if the laptop attempts to draw more current than normal, you could blow a fuse.

In short, not a recommended thing to do unless you have a spare system or can fix any damage that may occur.

Best solution, but one that can cost more than a simple DC-to-AC inverter, is to use an IBM AC/DC combo power supply. Power input into the adapter is 110/220VAC or 12VDC via a cigarette lighter plug. That plug also separates into an Empower jack for use onboard aircraft. Output is 16VDC, 4.5A via the standard "yellow" plug. IBM FRU is 22P9010. However, from reading earlier posts in this thread, if RF noise is an issue, the AC/DC combo power supply may not work either. Since it would appear to contain a DC-to-DC converter (basically it steps up the 12-13.8V power provided in the car to 16V), there might be some RF interference.