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I wanted to use my laptop for way longer than the time I could do with the internal battery alone, so I went ahead and bought 16 batteries (NiMH 2600mAh), connected them in a series to reach the 19V (16 multiplied by 1.2V), which is the same as the power cable.

The computer ran for about 3 seconds, and then shut off. Why?

I've measured the voltage, and it reached 19.5V. Also some research revealed that this kind of batteries have an internal resistance of 0.05 Ohm, which means that they should be able to deliver about 24 ampere according to Ohms law (24=1.2/0.05). My power cable can only deliver 4.7 ampere, so this should not be the problem.

Any ideas?

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I can't answer your question, but why not just buy something like this instead, which is guaranteed to work? bixnet.com/unpowbat.html –  bwall Oct 6 '11 at 13:14
    
Thanks for the link, that's a really cool device to have! There's a few reasons I'm building it myself. For one, I'm curious to see if I can do it. Another reason is that NiMH batteries are really cheap. The main thing is probably curiosity and the need to build :-) –  Mika Oct 6 '11 at 13:26
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16 cells in series = 16 x 0.05 Ohm = 0.8 Ohm as every cell has this effective resistance in the whole circuit surrounding it; and then I=V/R = 1.2/0.8 = 1.5A –  Linker3000 Oct 6 '11 at 15:15
    
Most notebook batteries are not just batteries, there is internal circuitry to deal with all the issues of discharge, charge and overheating. Most notebook bios's can detect what is connected to the DC jack, maybe it is not happy with your home made battery pack. –  Moab Oct 6 '11 at 15:23
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@Linker3000 But the voltage would also increase, so shouldn't the equation become I=V/R = 19.2/0.8 ? –  Mika Oct 8 '11 at 14:18
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1 Answer

up vote 4 down vote accepted

Batteries, if only they were ideal.

When you have a power brick for charging on your computer this device has a regulator circuit. This means that it will hold its output voltage over a wide range of load conditions. Batteries are not a regulated source, they change their output voltage as a function of current out even without changing the charge of the battery.

Lets look how much a battery will have its voltage change just during one use.

image of discharge of batter from:http://shdesigns.org/batts/battcyc.html

Now, before you use this persons technique, they were measuring the batteries voltage out of circuit after a recovery time. This in my experience is much higher then the actual voltage the load will see. As a battery fails they are very aptly modeled by a nearly continuous voltage source with a series resistance and a small parallel resistance causing leakage. This still brings the point across, as you use your battery it is very very quickly going to change voltage. Measurement techniques are mostly not required for this, but if you would like to know more, please feel free to ask on Electronics.SE.

Why does it not work?

Now if your computer is expecting a regulated input voltage and it sees a good voltage, starts to pull current and then drop off, nothing in your computer is going to be happy. I know this is a pretty long answer for something that can be explained with, the batteries are not regulated, but it is important to understand why. For example, if you succeed in finding a DC/DC regulator that accepts your battery voltage range as an input and outputs what you need for your computer input, you are ready to go.

DC/DC converters

For DC/DC converters, if you pick a linear regulator, or something termed LDO, you must have higher input voltage then output voltage. There are other regulators out there, for example, something like this DC/DC converter would do the job. The issue being that you need a very small input range, not ideal for dealing with battery power. I can fine 8 million DC/DC converters, but finding one that matches exactly where you want to operate takes quite a bit of time, so I will leave you with one that works and allow you to find exactly what you are wanting for the number of batteries you want to use. For example, here is a pretty long list of options.

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Thank you for the detailed answer! As you say, it is important to understand why. It also explains my recent experiments of 2x16 batteries. After the initial use, voltage dropped, just like on the chart. Now this should do the trick (right?): elfa.se/elfa3~eu_en/elfa/init.do?item=73-067-77&toc=19829 –  Mika Oct 7 '11 at 16:14
    
@Mika, only if your input voltage stays atleast 1.5V above the regulated output. It needs to regulate to the voltage you need, you have to purchase them for that. And at 4As, it will get REALLY REALLY hot. –  Kortuk Oct 7 '11 at 16:30
    
@Mika, I added more info, let me know if that helps. –  Kortuk Oct 7 '11 at 17:02
    
Thanks for the help, think I got everything I need to move forward. I will keep this thread posted on my progress :-) And about 4Amps, I believe this is the very maximum the computer would pull. With new internal battery 4400mAh the device would run for about 4 hours, which indicates that normal usage will be around 1A. –  Mika Oct 8 '11 at 14:29
    
Oh and about the 1.5V above the regulated output: I'm considering connecting all my 32 batteries serially, resulting in 40V, and using the regulator to get 19V output. All of them are of same brand and model, so at least theoretically that should work. –  Mika Oct 8 '11 at 14:36
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