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Replaced soldered in Ni-cad cmos battery

smeezekitty said:
I would be cautious of raising the voltage too much if it calls for 3.6V. Especially if you go over 5V
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One example of that might be the 486 based motherboard for which the user's manual is [here].
On page 6-2, in reference to the external battery connector, is "3.6V - 4.5V"
The author has excluded 6 volt batteries.
6 volts too much for the chipset?
 
modem7 said:
One example of that might be the 486 based motherboard for which the user's manual is [here].
On page 6-2, in reference to the external battery connector, is "3.6V - 4.5V"
The author has excluded 6 volt batteries.
6 volts too much for the chipset?
Click to expand...

Considering that most other circuits on the motherboard operate at 5v I would say 6 is too much unless it original called for 6 (some do)
 
smeezekitty said:
Considering that most other circuits on the motherboard operate at 5v I would say 6 is too much unless it original called for 6 (some do)
Click to expand...

3 AA batteries should meet the requirements perfectly. My thinking for my own board is that the two button cells at 6v will last another 15-20 yrs. It's good to have a PC that can keep time as well as a good watch again! :D
 
My 486 motherboard manual (such as it is) indicates the external battery connector is for a 3.6v battery. I believe the motherboard is a PC Chips Deep Green board.

What I did yesterday was order three different AA size battery holders off eBay. I got 10 each of a 4 battery, 3 battery and 2 battery holder - all for less than $20 delivered. That should cover me for all situations. What I will do is try using the 3 battery holder first (4.5v) and if that works, I'll stick with it. If it seems the clock is losing time or I lose the BIOS settings, I'll switch to the four battery (6v) holder. I got the two battery holder in the event I ever replace a 3v lithium.

I have used rechargable AA batteries in the past without issue, so I will continue to use them. They usually don't give you the full 1.5v per battery so that lessens the effect of the (seemingly) extra voltage.

The one issue with the battery holders is they do not come with a plug at the end of the wire. So I took a chance and ordered the four hole female plug (all my motherboards have four pins on the external battery connector) and the female wire connectors that go inside them. The size was 2.54mm. I am hoping that's the right size, but if not it was only $8 for 100 plugs and 500 wire connectors.

Joe
 
I use the Moto MC14818 datasheet and app note as the ultimate arbiter in this matter. It's the archetype for every PC RTC since the 5170. The Vdd input is rated to +8v; Since the backup power is fed through a diode or transistor, what's seen at the Vdd pin is going to be 0.7 volts or so lower. So with +6v applied at the battery, you'll see +5.3V tops there.

Plenty of safety--and, more importantly, plenty of battery life until the battery output drops to somewhere around 2v.

If someone has a datasheet for a different PC RTC chip that shows other information, please post it here.
 
There's a good reason to use a 6V supply if you're using alkaline. As the cells age, their voltage slowly decreases from 1.5V to somewhere around 1.0V before they fall off the "cliff" and the voltage drops to a few tens of millivolts. Since the clock chip itself doesn't draw much, you want to use as much of that slope as possible. With 4 cells, you still end up at around 4V, still enough to keep the RTC going. (see below)

Think of it as a downhill ski run. If you start midway down, the run-out on the bottom isn't going to be as long as if you started at the top.

Hope this makes sense.

AA-100mA.png
 
They last so long it really doesn't make a huge difference TBH. I stick with 4.5V in my machines that take external batteries.
The only disadvantage is 4.5V battery packs are less common.

In fact failing to work before it hits the sharp slope might actually be a good thing because if you use the machine reasonably frequently, you will likely notice the near dead batteries before they start leaking.
Once batteries are completely discharged they seem to be much more likely to leak
 
4.5V battery pack? Why use a pack? AA cells are cheap (as are battery holders) and long lasting.

One thing that old cells are liable to do is swell, which is probably a factor in leaking also. Get a few swollen ones in a Maglite and very often, you have to resort to a hammer.
 
Chuck(G) said:
4.5V battery pack? Why use a pack? AA cells are cheap (as are battery holders) and long lasting.

One thing that old cells are liable to do is swell, which is probably a factor in leaking also. Get a few swollen ones in a Maglite and very often, you have to resort to a hammer.
Click to expand...

When I said "pack" I meant holder
 
smeezekitty said:
When I said "pack" I meant holder
Click to expand...

Ah, okay--I was thinking of the assembly that was several cells bonded together, more or less permanently. I have several machines that I'm lucky to power up every 5 years or so. I just checked one that's gone 12 years without power and the clock's still ticking, though it's about two weeks behind real time.

While I'm at it, I should mention that if your mobo has no external battery connector and you want to replace the rechargeable with a lithium, forget the diode--just use a MOSFET--no significant voltage drop. Even better than a Schottky diode--and about as inexpensive.
 
Hi Guys,
For some reason, my previous comment did not get posted.

I think I just purchased enough stuff to give me any option I need.

For AA battery holders, I was able to get 10 packs each of the 4 battery, 3 battery and 2 battery holders. That should give me the ability to have either 6v, 4.5v or 2v. Cost was less than $20 for everything delivered.

The holders have wires, but no plug on the end. I was unable to find them locally, so I ordered them online hoping I got the right size. The four hole plug and the corresponding female wire connectors (which go on the four pin motherboard connector) are described as Dupont 2.54mm. I guess I'll find out when they arrive. Cost for 100 plugs and 500 wire connectors - $9.

With 30 battery packs, 100 plugs and 500 wire conenctors, I think I'm set for every old PC I have. I'll start with the 3 battery holder and if it holds up well, I'll stick with it. Otherwise, I will go with the 4 battery holder which sounds from the above comments as though it should be safe to use.

Thanks...Joe
 
Here are some pics of the battery case for my dual coin cells. Has an on/off switch. (Cost a $1.89 for two of them on Ebay). It is mounted on velcro tape to the inside case. Even after taking these pics etc. the pc fired up with the exact correct time. :p The ground wire may have become necessary because of my own error. When I put the mobo back in the case after desoldering the old ni-cad, there were only two screws (seen in the 2nd pic) that are attached to the chassis, the rest are plastic push pins ; I noticed when re-installing that one of the screws had an insulator and the other one didn't. So then I thought maybe I dropped it or lost it when removing the mobo. So I added one to the other screw. But considering the ground problem I ran into, I am now thinking that one of the screws is supposed to NOT have the insulator. Oh well. It's working fine as is, and I don't remember which one was which now. :( Maybe next time I'm inside I'll check resistance between screw mounting holes and ground.

battery 001.jpgbattery 002.jpg

Update: I found the pic below which proves that both screws should have the insulator, so I DID lose one of them in the process of removing the mobo. (it's easier to see when the photo is larger and enhanced) So the ground wire is just necessary with the external battery I guess.

misc 386 001.jpg
 
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I bought everything off eBay. Here are the links for all the pieces I just bought. I have not received any of them yet. Also be aware they all ship from China. I have had one other dealing with a Chinese vendor off eBay (molex power splitters) and there were no issues.

Wire Connectors:
http://www.ebay.com/itm/251554916504?_trksid=p2059210.m2749.l2649&ssPageName=STRK:MEBIDX:IT

Plugs:
http://www.ebay.com/itm/261267213927?_trksid=p2059210.m2749.l2649&ssPageName=STRK:MEBIDX:IT

Holders:
http://www.ebay.com/itm/110892076126?_trksid=p2059210.m2749.l2649&ssPageName=STRK:MEBIDX:IT

http://www.ebay.com/itm/110946605538?_trksid=p2059210.m2749.l2649&ssPageName=STRK:MEBIDX:IT

http://www.ebay.com/itm/121152971152?_trksid=p2059210.m2749.l2649&ssPageName=STRK:MEBIDX:IT

Thanks...Joe
 
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