How to power up older laptops when you don’t have an AC adapter

Thinkpad 755CSE

I am in the process of restoring my childhood laptop, a Thinkpad 755CSE. I suspected the power supply was going bad because the laptop had a random rebooting problem. Used adapters for these machines were going for $50+ on Ebay. How could I find an alternative way to power this Thinkpad on the cheap? Easy. Make a “fake battery”.

The Fake Battery

A fake battery is simply a DC power supply (of a correct voltage) connected directly to the battery terminals. This can work on just about any electronic device that uses batteries. I prefer to go through the battery rather than the standard power input because battery power inputs will have a wider range of acceptable voltages and currents, since batteries themselves have their voltage changing constantly depending on load and existing charge.

It’s easy to find a DC adapter that will fit in the battery’s varying range. You can also buy generic ones on Ebay for $5. Before you go hunting through your hideous box of power cables, you need to determine what voltage and amps you need. There are multiple ways to find this information.

  • Find a technical manual. Older computers tended to have very detailed manuals on servicing their products. Sometimes you can find information on the battery requirements, like I did here for my thinkpad.

    In the manual I found this which shows the voltage is expected to be between 8V and 16V, which is a huge range and makes the common 12V DC plug a good choice. You also see which pins go to (+) and Ground(-). There are always other pins for battery logic and temperature but in most cases you can ignore them.

  • If you can’t find a manual, look at what’s written on the battery. If you don’t have a battery, try to find a picture of one on Google or Ebay.

    You can make out on this replacement Thinkpad battery that it is 10.8 Volts. We don’t know the acceptable voltage range from this but you can usually go at least a couple volts in either direction, so a 12V adapter is ideal again here. Remember to note the (+) and (-) terminals, you’ll need to make sure they all match up.

  • Lastly, we’ll want to know how much current(Amperes) our adapter should provide. The best way is to look at what’s written on a standard power adapter. You know the drill, if you don’t have one find a picture of it on the internet. This is what I see written on my Thinkpad power plug.

    This adapter is 30 Watts. You might need to do a little math to figure out a comparable value for our custom DC adapter. We only need one formulate for calculating all our power(watts) needs:

Current(ampers) X Voltage(volts) = Power(watts)

If it doesn’t say clearly that your adapter is X watts, you can calculate it. In this example there is a range of volts and amperes in the OUTPUT but they always equal the same power:

3.0Amperes X 10Volts = 30Watts
1.5Amperes X 20Volts = 30Watts

We already know a 12Volt DC adapter will work in this system, but how many amperes of current does it need to output to provide 30 Watts? Rearrange the power equation to calculate:

30Watts/12Volts = 2.5Amperes

We need to find a 12V DC power adapter that can output 2.5Amperes. Most professionals will tell you that yes, you will need 2.5 Amperes for this job. I like to be a little loose with the rules and save a few more bucks. You can use lower power adapters at your own risk. I’m using a 1.5Amp one right now and it works fine. Most Laptop AC adapters overestimate the power the unit will consume and since we definitely won’t be charging a battery at the same time you can typically get away with a slightly less powerful adapter. That said, if you start seeing random shut downs or reboots, that’s a sign you need to up the Amperes.

Connecting the fake battery

Thinkpad 755 fake battery DC plug connector

I had an easy time on this Thinkpad 755 since I was able to take out the battery terminals. Then I soldered a DC receiving plug to the (+) and (-) terminals. You should have figured out which pins are (+) and (-) from your earlier research. The polarity must be correct! Most but not all DC power plugs have the inner pin as (+) and the outer ring as (-). Tape over all the battery terminal pins you are not using, you don’t want them to accidentally get charged. This should work on any old laptop(pre-2010). Newer computers might not work with this mod because they think you’re trying to use off-brand batteries and they just can’t let you do that. (I’m looking at you New Thinkpads!)

I first tried this mod with only alligator clips holding the wires to make sure it worked, it did, then proceeded to solder them permanently. There are other ways you can be even lazier with this, such as cutting off the end of a DC plug and attaching the wires directly to the battery terminals. But I like having a DC outlet in place so the laptop isn’t permanently attached to the power cable and I have the freedom to swap in a more powerful adapter.

Final Working Result

Thinkpad 755 Fake Battery box

Here it is completed, with my DC power cord plugged in. This isn’t fully ideal since now I have a power cable coming out from under the keyboard. But this 25 year old laptop works again, and instead of paying an unreasonable amount of money for a replacement AC adapter, I made my own. This cost me nothing since I already had all the parts, but even if you needed to buy the wires, adapters, DC plug, and soldering station, it would still cost less than what replacement AC adapters are going for on Ebay. As an extremely frugal person, this is my favorite way to get into vintage computing.