Coin battery in computer drains overnight

[Averagesupernova]

Kind of says it all I think.

Lol. These machines can likely go for ten months without being powered up. A twenty year old lithium coin battery is expected to soon be at the end of its life. No doubt. But the service tech claims a fresh one gets drained overnight. What I really suspect happened is they put in a new battery the first time I shipped it off when they likely found a dead one. I would not have been surprised either to find a dead one that is around twenty years old. The rest of the unit was checked out, powered down and returned to me. The unit probably had already drained the battery by the time I received it back. I just didn't check it out immediately after I received it. Bigger things to worry about at the time.
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I did manage to find a factory reman of this unit from the OEM. The price tag is $6500 with exchange. Dealer price. Not an e-bay seller. Without the exchange the price goes to $8500. I won't be going this route.

 

[Averagesupernova]

The latest:
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The unit arrived back to me yesterday. I put it on the bench tonight and here is what I found: The lithium battery is in fact dead. I removed it and hooked a three volt supply in place of it with a 100K resistor in series. The voltage drop across the 100K is 95 mV. The thing is drawing less than a uA. I question whether there was an attempt to even repair it. I have to say it was not the most fun thing to disassemble. For whatever reason the tech decided it would be a good idea to blob the little battery with black RTV silicone. Worried about vibration or wanted to make sure the next guy to deal with it would have fun? Here are some pix.
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[Averagesupernova]

Here are a few more:

Not sure what the little nicad bank in there does. I assume provides enough power to make sure nothing is lost in a power loss. Powers unit long enough to save everything I would guess. I never really want to find out. Lol.

 

[AlexCaledin]

The current may be 1 microampere at some logical state of the CMOS chip and more than 10 milliamperes at another state, if the CMOS is damaged.

 

[Baluncore]

For whatever reason the tech decided it would be a good idea to blob the little battery with black RTV silicone.

Was that a neutral cure silicone, specified to be an excellent insulator?
It only takes a finger print on a coin battery insulator to discharge that battery if there is moisture condensation later. Don't handle coin cells with your fingers.

Check for a ceramic capacitor across the Real Time Clock chip. If it is multilayer ceramic then it may become mechanically or thermally intermittent, but with insufficient power available to fuse that layer, which is expected to happen in most power supplies.

 

[Averagesupernova]

@Baluncore I don't know what kind of silicone it would have been. I thought it was a silly place to put it. There are a lot of other parts in this box that vibration will get long before the little 2032 battery. I am quite green about the specifics of motherboards. The only thing I see that I know would be associated with the 3 volt supply is a PhoenixPico BIOS chip. But, it's me, not someone who is familiar with this part of a computer. I'll continue to poke around.

 

[Wrichik Basu]

I removed it and hooked a three volt supply in place of it with a 100K resistor in series.

Is this an AC-DC adapter? I thought of that alternative some days back, but all I found on the internet were 3.3V supplies.

The repair guy could replace the soldered battery with a battery holder. In that way, replacing batteries would have become far easier.

 

[Averagesupernova]

Is this an AC-DC adapter?

Bench supply.

 

[AlexCaledin]

In old comps I used 4.5V batteries to replace the 3V, it worked all right.

 

[Rive]

I question whether there was an attempt to even repair it.

A basic computer tech is not expected to do anything else but replacement. Sad but true.
But this one above is just awful. By my humble opinion it is safe to say that the 'repair' attempt itself might be the reason for failure. If you clean it up and put a new CMOS battery (maybe a beefier 3V one, and not the 'naked' type but one with https://uk.farnell.com/panasonic-electronic-components/cr-2032-hfn/battery-lithium-2pin-horiz-cr2032/dp/1298246 or full insulation) on (or around) the board then it'll likely work and you can close the case.

 

[256bits]

I question whether there was an attempt to even repair it

I wonder.
Do they make these anymore? or Obsolete tech.
Parker Hannefin I suppose>

Is this the unit
https://www.bigiron.com/Lots/AgcoFalconIIController

 

[DaveE]

In old comps I used 4.5V batteries to replace the 3V, it worked all right.

Um, OK... Kind of like those guys on Youtube?

In Silicon Valley, back in the days when they didn't have to write SW to make money, we used to call some DIY types "Hackers". It had a very different connotation than the current usage. Now it's like a badge of honor or a job description; Mark Zuckerberg loves those guys. Back then hackers were why design reviews were invented. "I tried this and it seemed to work OK" wouldn't get you to the finish line.

OTOH, I'm sure @Averagesupernova will appreciate the tip if he can't find a 2032 3V coin cell, but he can get a 4.5V one; or if he's desperate enough to think that raising the battery voltage would have a significant effect on the discharge rate of a backup battery.

 

[Averagesupernova]

If you notice there is a 2 pin header next to the little coin battery. Removing the jumper that connects these two pins disconnects the coin batt. There are also pads on the board next to this header that could hold other pins. Should be able to get an arrangement of pins there that allows a battery with leads and a two pin connector to plug in. Right now nothing else is connected to the motherboard. I'll try to reassemble with my test setup left in place tomorrow and see what happens. Supposedly this board has not been available since '02. I find that hard to believe considering these were still being built less than a year before. I'm sure its no longer available now. That's to be expected.
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@Rive you are correct. Average computer techs are not expected to do much more than plug and unplug connectors when it comes to hardware. Thing is this guy asked me to send it in.

 

[Rive]

Um, OK... Kind of like those guys on Youtube?

Really old motherboards (up to early 486 as I recall) had a connector for optional 'external battery'. That connector did not provide charging current but had some zener-and-stuff circuit, so it could receive anything.

Some guys sold a ton of quad AA battery holders to companies saying that it's safer, since no CMOS loss will occur in the expected lifetime of the computer

Thing is this guy asked me to send it in.

Maybe he thought it'll be just the usual coin replacement without soldering?
No idea.
But anyway, he did no good there

 

[Averagesupernova]

@256bits yes that is the one.

 

[Tom.G]

Two main points (and a conjecture):
1) Since this is a mobile unit, probably with much vibration, do NOT use a battery holder.
2) Remove ALL of the silicone rubber. That stuff is also hygroscopic and can supply a leakage path to kill the next battery.

As for substituting a 4.5V supply, that may work. I suspect that a reason for having two series diodes between the clock chip and the 3V coin cell is to reduce the voltage enough to put the clock chip into a Sleep mode. This would keep the clock running but power-off the support circuitry such as the bus interface. IIRC, the lower voltage also reduces the power consumption of CMOS chips proportional to the square of the voltage.

Cheers,
Tom

 

[Averagesupernova]

Maybe he thought it'll be just the usual coin replacement without soldering?

He is the one who told me it was soldered onto the board when I called prior to shipping. And before I sent it I opened it up to have a look. There was no blob of silicone on it prior to shipping. The whole scenario doesn't really smell right to me. a lot of these repair depots will just not bother shipping your unrepairable product back to you unless you ask for it. So I have suspicions.
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I'll hopefully be able to spend a bit of time on it again tonight. I'll certainly not leave anyone hanging.

 

[nduka-san]

That's not very nice. You do not explain why you think the battery is dead. Did you get a message that says "your battery is dead"?

But hey, you want to act this way to people who are trying to help you, your choice. In the words of Barbara Billingsley, " Chump don' want no help, chump don't GET da help! "

thanks for the cool quote

 

[DaveE]

This kind of thing requires quite some work to clean up (and it'll never be like the new ever again )

At every company I ever worked for this PCBA would be scrapped instantly. Easier, and better to just build a new one. Except, maybe, for that summer job I had at Atari, they were really cheap. If it worked when it left the factory, it was deemed reliable enough for them, LOL.

This is a big problem with obsolete products; sometimes you just can't do what you want. This is one, of several reasons, that big companies may choose to build their own. If you expect a long lived product, you want to be able to control the supply chain, even though it's more expensive up front. My last company is still making and selling some pretty complex (at the time) PCBAs that I designed 30+ years ago. Their policy is product support for at least 10 years after obsolescence. They also sell expensive things. You don't get that for stuff you buy from Amazon.

 

[Rive]

At every company I ever worked for this PCBA would be scrapped instantly.

Some of those motherboards worth more than hundred dollars for a collector.
And they are like old wine. One piece dumped, one less. So when you finally found the one you are looking for for ages, you will try everything possible...

Once I've found one of https://www.ebay.com/itm/Alaris-Leopard-LX-386-Motherboard-Rev-D-486SLC2-Vintage-IBM-PQ-14/353290613318. With battery a bit worse shape than on that picture above. I could cry... Removed all the RAM slots and many SMD components, repaired many tracks... Took dozens of hours... Ended as a donor

Ps.: by the way, check those pictures about the battery there

 

[DaveE]

Some of those motherboards worth more than hundred dollars for a collector.
And they are like old wine. One piece dumped, one less. So when you finally found the one you are looking for for ages, you will try everything possible...

Yes, that makes sense in that world.

Many custom PCBAs cost hundreds to thousands of dollars in the other world. But then it isn't so much the cost to make or repair it, it's the cost of reliability, down-time, field service, etc. Vendors in that world are often chosen by reliability and ability to provide good support as much as performance of the product when it's new. Suppose that PCBA was in a laser that costs $100,000 dollars and if it fails, the laser quits, the semi-conductor inspection tool quits, and a semi-conductor fab line stops. Those people will spend anything to stop the bleeding ASAP. Similar issues at car factories, textile manufacturers, big clinical labs, etc. Actually, what most of those guys do is have spares on site or a central depot, if they can afford it. Sometimes the panic is "we used our spare unit and need to replace it ASAP."

Then the post-mortem analysis starts: What, why, how likely, how to avoid repeats, etc. There are cases, rarely, where you spend more money diagnosing a failure than the replacement value, just to get the data. So the decision often goes one of two directions: just throw it away and send a new replacement; or send a new replacement and spend a small fortune in engineer's salaries analyzing what happened, then throw it away. Those companies would go out of business in a month if they had to compete in the consumer product markets.

A $200 motherboard often isn't worth the cost of the technician's time if repairs aren't simple. Unless they are obsolete, then a used one could be a great deal at (almost) any cost.

 

[Averagesupernova]

Last night I spent a little time attempting to see if there were any of the other boards inside the case that would cause an increase in CMOS battery current when plugged into the motherboard. I found no changes, which is what I expected. Still was drawing just under a microamp.
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So today I removed the display and keyboard and brought to the bench to run as a system while watching the current draw from the CMOS battery. I applied 3 volts again through the 100K resistor and found the unit still was drawing just under a microamp. I powered the unit up and the drain on the 3 volt supply dropped to .35 uA. I assumed it would drop to zero. No idea if this is normal but it isn't a concern. I ran the unit through setup, set the clock/calendar and shut it down. This time the current draw went to about 1.4 uA when powered down. I have to assume this is normal. My guess is that the real time clock that is buried in the south bridge/wherever does not run until the computer boots up. That's the reason just under a microamp before. When powered up again the CMOS battery current dropped back to .35 uA again and the clock and calendar were correct. I really don't know what else to do other than put a battery in and go with it.
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I have to assume one of several things:
1) There is an intermittent leak that hasn't shown up on my bench.
2) I was simply lied to about the battery draining overnight. I know it was dead when it was sent in for repair but after 20 years in the field it's no surprise. By the looks of the old battery I have suspicions that it is the exact same one that was on the board when I shipped it in for repair. I'm wondering if the big glob of RTV silicone was simply put there at the last minute before they shipped it back so it would be difficult for me to tell what wasn't done.

 

[Tom.G]

At 1.4uA, that 235mAH battery should last just over 19 years. I think you got your money's worth out of it.

Datasheet here:
https://data.energizer.com/pdfs/cr2032.pdf

Cheers,
Tom

 

[Averagesupernova]

At 1.4uA, that 235mAH battery should last just over 19 years. I think you got your money's worth out of it.

Datasheet here:
https://data.energizer.com/pdfs/cr2032.pdf

Cheers,
Tom

Since I have not owned it for more than a small portion of those 19 years I didn't cash in on that battery life but it's likely someone else did.
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The latest is I installed a new battery Friday evening and everything seemed to work. I checked it two days later after having been powered down with no change. All works as it should. I think the conclusion will be that the story I got from the repair tech is a big fat lie.