4.5V Battery Replacement for Early Compact Macs?

[68kPlus]

Hi everyone,

Looking to find a way to replace the expensive, hard to find 4.5v battery for the Macintosh Plus in particular. The closest replacement is the Exell PX21, which costs around $50 AUD here in Australia, and the cheapest is the Unicell A133 4.5V which is $45.
There are some German adapters that are also expensive that convert 3x LR44 to the PX21, but I can't seem to find them cheap.

Any ideas?

 

[YMK]

If it will tolerate 4.8V, I'd use a quad AAA holder and stuff it with Eneloops.

They're rechargeable, won't leak or explode and have a low self discharge rate.

There's also the 6V 476A, which is cheap and can be dropped to 4.5V with diodes if needed.

 

[VicNor]

A buddy of mine has 3D-printed a holder for three LR44 coin cells. He uses a metal strip to connect the bottom battery to the connector in the computer. Good enough, I guess? It's not like keeping the time (over time) is super important on a Plus anyway. I'll imagine that one would need to replace the LR44/SR44 at least once a year

 

[rikerjoe]

I went with the triple LR44 route for my Mystic Color Classic. I found a battery holder here.

 

[MacOfAllTrades]

I used a cr2032 cell at 3.5V in my se. I understand the old macs tolerate a wide range of voltages for this purpose and many use 2032.
I got this one

Horizontal Holder for 2025 & 2032 Coin Cell Batteries

<p>This horizontal orientation battery holder is commonly used to replace lithium ion coin cells with spot-welded solder tabs. Replacing a custom battery with this battery holder ensures simpler future battery replacement and saves money over harder

console5.com

But many exist. Some with wire leads built in (which I think makes more sense for your PLUS).
these cells don’t blow up like the originals and they still last a long time. Any leakage they have in odd occasions is much less as they simply don’t have much

 

[pfuentes69]

Another possibility, which I used in my Plus, is to use three LR50, which just make the size of the 4.5V battery.

https://www.aliexpress.com/item/1005002834311917.html

 

[misterg33]

I've used AAAs in stuff like this:

https://smile.amazon.com/gp/product/B00P2FJA2M/

If you have one of the old dead batteries, you can snip the connector end off of it and use some heatshrink to connect it. You don't have to order from Amazon either. Stuff like this is available on most electronic and hobby sites. And I think there are some pre-made ones on ebay.

 

[vtgearhead]

Another possibility, which I used in my Plus, is to use three LR50, which just make the size of the 4.5V battery.

https://www.aliexpress.com/item/1005002834311917.html

The (expensive) Excell replacement batteries are actually (3) LR50 cells spot-welded in series. I haven't tried it yet, but I bet three individual LR50s with heat shrink around them will work just as well. The combination is a wee bit longer than the OEM battery so tension should hold everything in contact.

 

[rikerjoe]

I went with the triple LR44 route for my Mystic Color Classic. I found a battery holder here.

Update. The triple LR44 setup didn’t last very long for me, with one of the cells going flat in less than a year and thus not retaining enough voltage to preserve the settings. A little over a year ago I opted to try a 3.6-V AA-sized ER14505 I purchased from Mouser for a little over $8 - I know, a bit pricy for a battery. I opened my Color Classic Mystic this morning and measured the battery voltage, and it’s doing fine, measuring 3.66 V after a year.

 

[vtgearhead]

That makes great sense for almost everything. But, the Mac 128, 512ke and Plus have a battery compartment on the back intended for a 4.5V battery that's long unobtanium. Stacking 3 LR50 cells gives you the right voltage and fits perfectly in the existing holder. But for anything with an internal battery I'll go with the AA sized 3.6V cells. Some systems (Amiga) expect a rechargeable battery. In that case I use a schottky diode to prevent reverse current flow. Most have a forward voltage drop around 0.2V.

 

[Elemenoh]

I went the AAA holder route for mine. I popped off the top bit to get it to fit. A little snug but it works.

 

[JDW]

A little over a year ago I opted to try a 3.6-V AA-sized ER14505 I purchased from Mouser for a little over $8 - I know, a bit pricy for a battery. I opened my Color Classic Mystic this morning and measured the battery voltage, and it’s doing fine, measuring 3.66 V after a year.

After reading your post, I searched Amazon Japan for one of those ER14505 batteries and found a cheaper price than you paid:

https://www.amazon.co.jp/-/en/ER14505-Non-Rechargeable-Electrical-Equipment-Universal/dp/B08F1NBZ2M/

I see those are NOT rechargeable, making the cost of replacement something to ponder. I therefore look forward to hearing updates from you about when it dies. If it lasts two years, it's probably worth the price.

Not sure of the stock CC motherboard draws the same amount of current as the LC575, but I measured the current draw from my board (from the PRAM battery):

LC575 Current Consumption: 63µA @4.5V

With a triple LR44 kit attached, that often gives me about 70 days of use. Pretty pathetic. LR44's are roughly 115mAh rated batteries.

But your battery seems to be 2600mAh rated, albeit with a fairly large drop from the nominal 4.5v. Even so, you said your 3.6v-rated ER14505 measured 3.66v after 1 year, which suggests it is higher than 3.66v when fresh. 3.8v? Because that is unknown, let's assume 3.7v for now. Using a battery life calculator ("Disarchge Safety" set to 6%), we have this:

4.4 years

Now if that is true... Wow! That would definitely be the way to go for the stock CC motherboard and LC575 too.

 

[YMK]

Self-discharge rates should be considered for these very low drain applications.

CR123A batteries are another option. They're larger than the stock lithium batteries, but around half the size of a AA.

They're available in:

• Non-rechargeable lithium
• Rechargeable lithium
• 3-cell rechargeable NiMH

The NiMH option is a more compact alternative to the triple Eneloop setup and should be the lowest explosion/rupture risk.