NiMH PRAM battery conversion

[YMK]

I've removed the lithium PRAM battery from my SE/30 and replaced it with a 3-cell, 3.6V, 80mAh rechargeable NiMH battery and a simple charging circuit:

Amazon.com: New Ni-MH 80mAh 3.6V Rechargable Battery Cells with Tab : Cell Phones & Accessories

Amazon.com: New Ni-MH 80mAh 3.6V Rechargable Battery Cells with Tab : Cell Phones & Accessories

www.amazon.com

The NiMH battery is about the same diameter as the original, but shorter. If you want to keep the socket, the tabs on the battery could be bent outward to make contact, but I chose to solder directly to the logic board.

With even occasional use of the machine, the battery should stay charged and eliminate the explosion/leakage risk of lithium cells.

Two components are added between pin 8 of the RTC and the battery to allow charging: a 1N4007 diode and a 56 Ohm resistor:

5V is dropped to ~4.4V through D2, then to ~3.8V through the new diode. If you want to tap a 5V source directly, you'll need an extra diode.

Battery voltage with power off: 3.740V
Battery voltage with power on: 3.747V

The SE/30's standby circuit pulls 5.8uA. With 80mAh of capacity, the battery should last for 19 months, ignoring self-discharge.

On the Color Classic, @JDW measured 63uA, which would drain the battery in only 1.7 months.

I have used Eneloops in a vintage PC with good results. That may be a better way to go with higher drain standby circuits.

Since I've just done this mod, I don't have any long term results. My machine is used fairly often. Otherwise, I wouldn't have soldered the battery.

 

[JDW]

Wow! Fantastic! Tiny but so useful. This is great.

I should mention one usage caveat that will affect some SE/30 motherboard owners out there.

Phil Greenland created an RTC replacement with memory that stores PRAM and basically everything but the Date and Time when the machine is power off, even with no PRAM battery at all. CayMac Vintage currently sells it here:

https://ko-fi.com/s/3dbe7b3977

I have that RTC replcement inside an SE/30 Reloaded board I paid CayMac to build for me, and it works great. But there’s is one design flaw (lack of optimization flaw, I mean) when it comes to current consumption. To read about that, keyword search the following web page for “1mA” and you’ll see me talking about it with Phil in reply:

New Timepiece for a Classic Mac – Part 2

Continuation of the project to replace the aging RTC module in a Classic Macintosh SE/30—now using the ATTiny85 USI interface, PCB design and pin‑compatible module details.

www.quantulum.co.uk

So anyone who has that special RTC replacement chip should probably avoid using a PRAM battery altogether because a 1mA current draw from Phil’s RTC will kill a PRAM battery super fast.

Of course, using your really neat NiMH rechargeable PRAM battery with the stock RTC avoids the 1mA battery drain problem altogether.

Not sure how many people out there have Phil’s replacement RTC, which is why I just wanted to mention it for people who are oblivious to the current consumption caveat.