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Plus ER14505 PRAM replacement?


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Has anyone tried using a 3.6V lithium AA (ER14505) as a replacement for the Plus PRAM? The 4.5V A21PX/523's are rare and expensive in Canada ($22/ea + shipping is the cheapest I can find - unless someone else has a decent source --- they're plentiful and less than 1/2 the price in the US), but I can get 4x of the ER14505's for $20 on Amazon... thoughts?

Edited by desertrout
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3 hours ago, SuperSVGA said:

If you can't find any other solutions you could always line up 3x1.5v coin cells.

 

Came here to say this; I got some replacements a little while ago that turned out to be 3x 1.5v cells of the right size (i.e., 1/3 the height of the 4.5V battery) coated in some kind of shrinkwrap to keep them in place.  Worked fine, probably DIYable.

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4 hours ago, LaPorta said:

Not sure if available in Canada, but this is what I purchased and still use:

 

https://www.amazon.com/gp/product/B00FED2M9K/ref=ppx_od_dt_b_asin_title_s00?ie=UTF8&psc=1

 

Yup, those are the ones I was tending to find, price and availability is loopy. $40 for a battery is loopy to me, at least. I can find them pretty inexpensive at some US retailers, but apparently they can't be shipped to Canada. I leave near the US/Can border and it's common to have such items shipped to receiver on the US side (also tends to be faster and cheaper for most US purchases), but that's not an option at the moment...

 

I've seen the coin cell option tossed around - one member here 3D printed a AA-sized housing for them, seems like a neat option.

 

I've gone ahead and bought a couple of the 3.6V ones in the meantime just to see.

 

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I'd have to look into the components a bit more, but my theory is that the older computers with 4.5V battery backups had a 5V CMOS chip acting as the "power manager"(ie soft power, Real Time Clock, ADB in some of our macs). 4.5V represents the low end of a 5V +/- 10% rail. Most of the pre-mac 8-bit machines(Like the Apple II) used exclusively 5V Digital logic(as well as +/-12V analog circuitry).

 

Later machines, including many Macs, had mixed 5V and 3.3V logic as 3.3V was sort of new in the late 1980s-early 1990s. The lowering of voltages(particularly core voltages) was all part of the quest for faster speeds and lower power(power is related for frequency and voltage squared - so if you cut the voltage in half, the power is quartered!). Many of our later Macs(Like the Classic and the LC) likely have 3.3V CMOS power management circuits. The cells are 3.6V to allow power "OR-ing" through a schottky diode network, which typically have a 0.3V drop associated. This allows a circuit which will not draw the battery down when the unit is plugged into the wall. Having said that, I'm not sure why some later LCs and even PowerMacs had the cube shaped 4.5V battery. Perhaps those also had a step-down regulator which allowed the battery to last longer due to a much higher margin(the 4.5V battery would have to drain until it's voltage was below 3.0V before you lose functionality). I do recall seeing some power conditioning circuits on the battery in some of the older Mac schematics I've looked at...

 

Anyway, i'd be careful using any lithium battery where the alkaline was as there may not be the proper circuit protection to prevent current from flowing the wrong way and a lithium battery contains more "violent" chemistry(thats why you get more energy storage in a smaller package). It's possible that the alkaline based system allows some reverse trickle current to lengthen the life of the battery(alkaline batteries can be sort of recharged or at least maintained by very small current injection). I'd have to look over the 512K Mac Schematic I have to see what the battery/main power merging looks like to be sure...

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Well I bought a variable power supply and did some tests, and I wasn't able to figure out at what point it stops working. I set my power supply to 2v and unplugged the Macintosh Plus, and it drew around 2uA and retained its time and volume settings just fine.

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On 1/25/2021 at 1:14 AM, taddy said:

I don't live in Canada so I don't know the exact shipping cost,Please check the shipping fee on the following site.

$ 9.95

https://www.batterymart.com/p-exell-a21-a133-4_5v-alkaline-battery.html

$ 10.95
https://www.batteryjunction.com/exell-a21px.html

$ 12.38

https://canada.newark.com/dantona-industries/v21px/battery-alkaline-4-5v-3lr50/dp/18C0285

 

Thanks @taddy for the assistance, but I've looked. But to illustrate the issue I put two batteries in each cart and went through the checkout. Batterymart shipping is $15.65 + $19.90  = $35.55 USD * exchange rate =  $45.48 CAD. Battery Junction shipping is $19.87 + $21.90 = $41.77 USD * exchange = $53.47 CAD. Newark is Canadian with only $11.50 shipping, so with tax = $41.53 CAD. So, not quite the $40 per each I said above, but $20 per at the cheapest is still pretty silly to me if there are viable alternatives.

I've been using the ER14505's in two Pluses for the past week, and they're holding time and settings like champs so far (a week is not long, I know). $20 for four from Amazon, free shipping, so at least from a cost perspective it seems to be shaping up as a good option (as expected based on what @SuperSVGA saw).

That said, I so want to acknowledge what @macdoogie is saying... I'm very curious to know if you can determine anything of note about the battery/main merging.

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10 hours ago, LaPorta said:

FInding alternatives like this is also an important goal if the real deal is unable to be obtained in the future.

 

Yep, we are now in a constant state of trying to find modern solutions for obsolete components... batteries, flybacks, SCR's, tubes, backlights, etc. etc. etc.

 

I do think the coin cell (3 x LR44) alternative has potential as a long-term solution. Readily available and will only continue to be (knock on wood), inexpensive (I can get 30 - i.e. ten batteries' worth - for $9), the correct voltage, alkaline... the housing the only hurdle but can easily be 3D printed and constructed. Downside is they don't have the capacity of the original (600mAh vs 145mAh in series, 435mAh in parallel). ER14505's on the other hand have 2400mAh...! So depending on whether or not lithium batteries are safe to use and how long 145mAh will last... well, at least for the moment there are options.

Edited by desertrout
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Having bought one of these batteries as a replacement before, it usually is just a stack of LR-something alkaline coin cells connected together in series with nickel strips and then shrink-wrapped. Works as a dimensionally exact replacement. I'll have to take the battery out of mine to see what cells exactly they use.

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