Long ago, if you needed an IBM 5170 battery, you could drive 5 minutes to Radio Shack and they would have one. It might cost $10, but the time you saved was probably worth it. But those batteries disappeared sometime around Y2K, and of course Radio Shack has been gone nearly a decade now too. Here’s how I cobbled together a suitable IBM 5170 battery for around $6.
This battery was something of a standard, so a lot of motherboards, even into the 486 era, had a connector for this type of battery. So it’s useful for more than just an IBM 5170. If you have a 4-pin battery connector on your board, chances are this solution will work on it too.
I found a dual CR2032 battery holder at Micro Center. It cost about $3, and it holds two CR2032 batteries in a plastic enclosure with wire leads and a two pin JST connector on the end. The IBM 5170 wants 6 volts, which is slightly less than a pair of 2032s, but that’s generally accepted to be within its margin of error.
The JST connector isn’t right for the 5170 of course. But a pair of DuPont jumper wires, the same ones I use to extend the case LEDs, can be persuaded to fit in the JST connector socket. It’s a tight fit, but it works, and I don’t have any plans to use those battery holders in anything else.
The 5170 used a four position DuPont connector, with the positive lead on the side of the board closest to the ISA slots, and the negative lead on the side closest to the power connector. You could use a four position connector, but I just used two jumper wires.
Testing the voltage and polarity
Before plugging it into the board, double check your voltages and your polarity. You don’t want to damage the real-time clock or the CMOS memory on your 5170. There are no markings on the CR2032 case, but on the one I got at least, you plug one battery in facing up and the other battery facing down. I used my multimeter to check the voltage across the black and red DuPont leads. If you measure negative voltage, change the batteries around so you get positive voltage when you match them up with the colored leads on your multimeter.
I got about 6.5 volts out of my batteries. What you get will vary on the quality of the batteries and how long they’ve been sitting on the shelf. If you use high quality batteries, the solution can last 10 years. Then you just swap the batteries for a new pair and get another 10. Cheaper batteries won’t last quite as long.
Connecting the battery
I chose to just use two jumper wires. The battery connector is marked j21 and sits right behind the keyboard connector. The positive wire is on the side closest to the ISA slots. The negative wire goes to the side closest to the power connector. The two middle pins are unused, and one of them was missing to provide a key for the original battery.
If by some chance you have an old 5170 battery laying around and it hasn’t leaked and destroyed the case, you can cut off the wire connector and solder it onto the new battery holder. I saved one for exactly this purpose, and of course I couldn’t find it when I went looking for it. It is perfectly OK to reuse the connector. But any battery that old is prone to leak, so don’t keep the battery around, just the connector.
Dropping the voltage
If 6.5 volts seems like too much, you can put a diode on the positive line. The negative side of the diode goes to the positive connector on the motherboard. The positive side of the diode goes to the positive connector on the battery. The 5170 doesn’t have a charging circuit, which is the normal reason people put diodes inline with replacement batteries. In this case, we’re not trying to keep the battery from charging. We’re abusing another characteristic of a diode. A diode drops the voltage by about 0.4 volts. So putting a diode in line drops the current from the battery to more like 6.2 volts. That is well within the tolerance of the 5170s components, and as the batteries age, that voltage will drop further.
Enjoying my 5170
It took me about a week between the time I got my motherboard working and when I was able to make a trip to Micro Center for parts. I was getting tired of booting off a floppy and running a setup program to tell the system what kind of floppy and hard drives it had, and how much memory it had. Having a suitable battery in place makes the 5170 much more enjoyable to use. As you can imagine.
Some newer systems can just detect the hardware if the CMOS battery is dead, and the only side effect is having to hit F1 and not having a correct date and time set. But those ideas came a few years after the 5170 did.