External battery

Motherboards used to have 3.6V barrel batteries. One good thing about them is that they are rechargeable. They charge themselves every time computer is powered on so in theory they should have never need to be replaced. Unfortunately they are like Duracell batteries, they leak and might work while leaking.

Leaking can damage motherboard so remove leaking batteries immediately. I recommend to remove them before they start to leak. Removing is easy, just cut them with pliers and there will be no need to desolder anything.

Locate the 4-pin external battery header from the motherboard. It should be close to the battery and there should be a jumper to select between internal and external battery. Pin 1 of the header is +, pin 2 is not connected (and usually missing completely so there are only 3 visible pins). Pins 3 and 4 are -. You can use multimeter to locate pins 3 and 4 since they are connected together. This is useful if your motherboard has all 4 pins available and there are no good markings.

Here are what you need to build an external battery: CD-Audio cable, battery holder, 3 AA batteries, soldering iron and some electric tape. It is possible to do this without soldering iron if you use some connectors or just tie the wires together.

You are going to need the type of CD-Audio cable that has one large connector that fits to the external battery header. That is the most common type of CD-Audio cable so it will be easy to find. Battery holders are easy to get as well. You can get those from Chinese ebay sellers and the price is very cheap even with postage. Just make sure you get holder for three AA batteries that are connected in series to get 4.5V.

You might wonder if 4.5V is too high since the barrel battery is 3.6V. I once had a “real” external battery and once it had depleted I opened it. It had 3.7V lithium battery in it. That made me try to build my first own external battery using 3.7V lithium-Ion rechargeable battery. It worked but it self discharged way too soon to be of any use. Using non-rechargeable lithium battery would have been far better but those are expensive.

Modern systems use 3V lithium batteries that are shaped like a coin. I tried those and 3V was enough to hold BIOS settings but it was not enough for RTC to tick properly. I'm told that it will work better on some motherboards.

Then I read from Vintage Computer Forum that 4.5V is not too high. Finally I found one interesting picture from a 386 motherboard. Next to the external battery header there was printed the voltage range and the range was 3.6 to 4.5V. So I finally decided to try 4.5V. So far I've tried it on two motherboards and both without problems. I don't know how long the batteries will last but I'll let you all know when I need to replace them.

Back to the assembly instructions. Here I have CD-Audio cable that has large head on the other end and large + small head on the other. This is good since this cable can be used for CD-Audio even after building the external battery. Alternatively I could have build two external batteries using one CD-Audio cable.

I should have taken first photo before cutting anything but I didn't think of it until it was too late. The pictures should be self explaining. Just be careful when connecting the small CD-Audio head back as the orange wire and thin black wire are connected together. It might be wise to use multimeter to see how the wires are connected before cutting anything. Some CD-Audio cables have different colors and possibly only 3 wires.

If you don't need to keep the CD-Audio cable usable it might be wise to use longer cable for the external battery. This helps to place it in a location where leaking does not do any harm. Finally remember NOT to use Duracell batteries since they will certainly leak eventually.

CD-Audio Cable. The short part with black connector will be used for the external battery.

Wires are now soldered.

 

First tape all individual wires, then tape them all together.

This step is optional. I inserted toothpick to the not connected hole and then broke the toothpick. This makes harder to connect the external battery wrong way. Also note that red wire is used as + wire.

External battery is now completed. The tree AA batteries are connected in series.