LC III/475 battery

[arroz]

Hi!

I'm thinking about adapting my LCs to use CR2032 instead of the prone to leak 3.6V batteries. However, I looked into the schematic and realized there is no diode preventing reverse voltage, only a 1K resistor. I powered on one of the Macs without any battery in the slot, and I measured around 2.3V in the battery slot.

AFAIK providing power to a non-rechargeable battery is not great. So… is it safe to use a CR2032? What happens when the battery is drained, below the 2.3V the Mac itself provides when powered on? Is there a possibility the battery catches fire, or shorts internally?

Regards,

Miguel Arroz

 

[saybur]

Can't speak to the Mac electronics here but if you're concerned, adapting a Not-A-Varta to fit the 1/2AA slot might be a good workaround. It's basically just a MAX40200 inline to protect the battery.

Does seem weird that Apple would supply current back into a non-rechargeable battery. I didn't find a spec in a Tadiran 1/2AA datasheet but someone online noted a max 1uA reverse charge on a CR2450 for comparison.

 

[Phipli]

It isn't just exposed straight to a voltage, it is specifically connected to VBATT on U28. I can't find a datasheet for "LS1176" though. Unless it is actually a bus transceiver.

The voltage being there with no battery doesn't necessary mean a significant current would necessarily flow into the battery. You often get a small voltage with absolutely no power behind it in disconnected circuits.

 

[saybur]

That got me curious, apparently the 1176 refers to 343S0120. See here for context.

 

[arroz]

Hi!

Well that at least clarifies that mystery, i was wondering what was that chip.

Yeah this is probably not a problem. I’m just surprised there’s apparently nothing protecting voltage from going in the battery or against the battery being installed the wrong way. Maybe the chip has an internal diode but I would feel better with an external one. Which I can add easily, and take the hit of the voltage drop. I tested and a 3V battery plus diode are enough to make the 475 happy. Not sure for how long, I wonder what’s the minimal voltage that keeps the CUDA alive.

Regards

Miguel Arroz

 

[Phipli]

Hi!

Well that at least clarifies that mystery, i was wondering what was that chip.

Yeah this is probably not a problem. I’m just surprised there’s apparently nothing protecting voltage from going in the battery or against the battery being installed the wrong way. Maybe the chip has an internal diode but I would feel better with an external one. Which I can add easily, and take the hit of the voltage drop. I tested and a 3V battery plus diode are enough to make the 475 happy. Not sure for how long, I wonder what’s the minimal voltage that keeps the CUDA alive.

Regards

Miguel Arroz

Don't add a diode, the coin cells already have a lower voltage than the stock battery and a diode would drop another 0.7v. The chip  will have a diode in it.

I think you might be over thinking things a little. You're replacing a non-rechargable battery with a non-rechargable battery. You don't need to redesign the circuit.

 

[mayacintosh]

Are the demi-AA batteries really prone to leak when not 30 years old? I don't see the point to change the form factor when these batteries are still in production

 

[Phipli]

Are the demi-AA batteries really prone to leak when not 30 years old? I don't see the point to change the form factor when these batteries are still in production

Some of them leaked within 10 years back in the 90s. They might be better....

They might not. Why risk it?

 

[mayacintosh]

i had good experience with SAFT batteries, the originals in my SE (1988) and my G3 (1998) were dead but physically clean. i bought new ones of the same brand and i will check it. but i prefer 3,6V over 3,3V (last longer) and i prefer to keep the original socket on the board. but ultimately it's personal taste

 

[arroz]

Hi,

AA-style batteries leak all the time. I have a drawer with a bunch and every time I go grab one, there’s new leakage. We’re talking about less than 10 year old batteries. A friend ordered a BIOS battery for an old 486 already during the pandemic and it already leaked.

Although I have never personally seen it happening, CR style batteries also do leak. The purpose is not just replacing the battery but using something like this to keep it as far away from the board as possible (either between the HD and FDD near the speaker, or even outside the Mac, under the case, using the fan grill to route the wires outside). The wires can be soldered to the board with minimal fuss or a 3D thingy can be printed mimicking a battery where the wires go in somehow to keep the board immaculate.

And yes I admit I’m overthinking. As said I was surprised to not see an external diode so I decided to look into it. After all the old batteries tend to leak and sometimes explode but it’s all when the Macs are off for years in a row so it’s not due to any voltage coming from the U28 chip.

Regards

Miguel Arroz

 

[Phipli]

the originals in my SE (1988) and my G3 (1998) were dead but physically clean. i bought new ones of the same brand and i will check it.

SEs usually have VARTA brand cells, and those early ones are actually a different chemistry to the later ones. They're usually labelled as a nominal 3V, although they actually have a higher (~3.6v) Voltage when new. Also, the SE seems to consume very little current for the battery backed circuit and so part of the reason the batteries are still good is because many of them aren't actually flat yet. I've removed over 30 year old SE batteries that still hold 3.0V.

None of this applies to the IIcx / SE/30 and newer, and at 35+ years old, you should replace SE batteries anyway.