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Barrel Battery Replacement Options

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RadRacer203

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I got a new 486 motherboard I'm planning on building into a full machine a few weeks ago. It had a barrel battery on it that I clipped because it was just starting to leak. Unfortunately this board doesn't have an external battery connector so I need to replace the barrel battery. I know I can get a stock replacement but I was wondering if there were any alternatives, such as a cr2032 or something else that's more easily replaceable. The original battery was 3.something volts. I understand the motherboard would have a charging circuit for the stock battery so I would have to install a diode if I go with something not rechargeable, correct?
 
Yes, you'll need a diode. The voltage of the battery should be above 4 volts to compensate for the diode drop.

It was not uncommon to use 4 x AA alkaline cells in this application. They'll last their shelf life (about 7-10 years).
 
Chuck(G) said:
Yes, you'll need a diode. The voltage of the battery should be above 4 volts to compensate for the diode drop.

It was not uncommon to use 4 x AA alkaline cells in this application. They'll last their shelf life (about 7-10 years).
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Ok, I'll get a 4x AA battery holder and adapt it. What diode should I be using?
 
Any standard diode (non-zener) should suffice. A good (cheap) example is a 1N4148
 
Really, any common diode will work, even 1N400x rectifiers. You only need one. The current is in the sub-milliamp range. When I order 4148s (or 914s), I order them by the 100s; they're cheap enough that smaller quantities cost too much to ship.
 
I have 2 '386 computers, and the barrel battery died on both of them. So I clipped them out, leaving the leads sticking out of the board. Then I went to Jaycar and bought (for each) a 3.6volt battery, a holder, and some double-sided tape. I have a stock of diodes already.

Then I stuck the holder onto the inside back of the computer with the double-sided tape. The wires were soldered to the old battery's leads, one via a diode. Then inserted the battery into the holder, and it worked. Next time it goes flat I can just replace the battery in its holder.
 
It seems like I've seen both a diode and a resistor. If it is like that, just remove the resistor. If it is just the resistor without the diode you'd put a diode there to keep it from charging cells that won't be too happy. 4 AAA cells may be too much. That is 6 volts. Even with a single diode, it will still be more that 5V. That means the cells will be running the RAM, istead of the 5V line.
3 cells are enough. Although, diodes are rated at 0.6v-0.65v, that is with a significant current flow. With a couple micro amps, you'll only lose about 0.4V.
Dwight
 
There is almost a couple of added diode drops between the battery and the chip--usually a pass transistor at least.

4xAA was standard kit for many years by Taiwanese case makers. Consumption is usually around 0.1-1.0 mA.
 
I used to use the 3x or 4x AA or AAA packs, but I typically use a 1/2AA 3.6V cell in a holder nowadays. I've found the hermetically sealed Tadiran lithium thionyl chloride cells you can get in 1/2AA basically don't leak unless mechanically damaged or placed in really damp conditions where they can rust from the outside. The really long shelf life is nice, since the current draw is so low anyway.
 
glitch said:
I've found the hermetically sealed Tadiran lithium thionyl chloride cells you can get in 1/2AA basically don't leak unless mechanically damaged or placed in really damp conditions where they can rust from the outside. The really long shelf life is nice, since the current draw is so low anyway.
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I've still got an original Tadiran dated Oct., 1990 on my 486 test bed.
 
glitch said:
I used to use the 3x or 4x AA or AAA packs, but I typically use a 1/2AA 3.6V cell in a holder nowadays. I've found the hermetically sealed Tadiran lithium thionyl chloride cells you can get in 1/2AA basically don't leak unless mechanically damaged or placed in really damp conditions where they can rust from the outside. The really long shelf life is nice, since the current draw is so low anyway.
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My understanding is that the lifetime clock starts ticking the moment you put on a load on one, even if you return it to the shelf. I don't know if that's really true, however. I do know that the 3.6V 1/2 AA cells used in Power Macs will put in better than a decade, however. But I've had plain old alkalines in the same application run for 15 years.
 
Chuck(G) said:
My understanding is that the lifetime clock starts ticking the moment you put on a load on one, even if you return it to the shelf. I don't know if that's really true, however. I do know that the 3.6V 1/2 AA cells used in Power Macs will put in better than a decade, however. But I've had plain old alkalines in the same application run for 15 years.
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The one for the arduino requires something to be to be read or set to start it. I don't recall what it was, I'd need to look at the spec again.
Dwight
 
While scanning ebay, I came across someone selling NiMH batteries to use in place of NiCad batteries. These are in the typical barrel package.
Dwight
 
Dwight Elvey said:
While scanning ebay, I came across someone selling NiMH batteries to use in place of NiCad batteries. These are in the typical barrel package.
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NiMH batteries shouldn't be charged with a NiCad charger.

A NiMH charger can charge NiCd; a NiCd charger would overcharge NiMH.

NiMH cells cannot be charged with a NiCd charger as end of charge detection will not work.
 
NiMH barrel batteries have been around for a long time, so this is nothing new.

At the very low charging/discharging rates used in motherboard RTC applications, it scarcely makes a difference between NiMH and NiCd. However, I suspect that the self-discharge rate of either is higher than the consumption by the RTC oscillator.

So use a primary battery--it'll last just as long and may not even leak at the end of its life.
 
According to what I read, a 80ma NiMH can be trickled at .05C. The cells the fellow is selling, are 80maHr. Anything below 4 ma would be a trickle. Assuming the rail voltage is 5V, two 1n4148s in series with a 60 ohm resistor should be safe. The 60 ohm to protect the diodes and the diodes to keep a safe trickle at max charge. They probably won't get to maximum charge but that isn't important. 80% charge is fine, even 50%. If one cell is way out it is still basically a trickle.
Dwight
 
Reading through the whole thread, nothing much new to say so just offer my thoughts. I probably wouldn't use the AA batteries since not only do you have to do the diode fix, the AA batteries (alkaline or not) can still leak. I'd probably opt for the coincell replacements instead. The one mentioned earlier with the built in diode sound like a great option, but I also know they make rechargeable coincells (no idea if the charging circuitry is compatible but since i've heard of folks doing this, i'd think it would be fine). The good thing about the coincells vs big ole batteries is that they shouldn't leak (even if they could, i'd imagine it wouldn't be enough to do any damage).

The other things i've done in this situation is to either not bother replacing... I have a few machines i only use very rarely, so it's no small issue to just put the date in when you boot up. I have to with my 8 bit machines anyway. The other thing I've done but i'm not necessarily advocating, is in my Amiga 3000 I replaced the battery with a cordless phone battery. It can still eventually leak, so the only advantage to this is that you can run it somewhere away from the board where it won't damage anything if that happens, and of course that the charging circuitry can stay in place.
 
ngtwolf said:
Reading through the whole thread, nothing much new to say so just offer my thoughts. I probably wouldn't use the AA batteries since not only do you have to do the diode fix, the AA batteries (alkaline or not) can still leak. I'd probably opt for the coincell replacements instead. The one mentioned earlier with the built in diode sound like a great option, but I also know they make rechargeable coincells (no idea if the charging circuitry is compatible but since i've heard of folks doing this, i'd think it would be fine). The good thing about the coincells vs big ole batteries is that they shouldn't leak (even if they could, i'd imagine it wouldn't be enough to do any damage).

The other things i've done in this situation is to either not bother replacing... I have a few machines i only use very rarely, so it's no small issue to just put the date in when you boot up. I have to with my 8 bit machines anyway. The other thing I've done but i'm not necessarily advocating, is in my Amiga 3000 I replaced the battery with a cordless phone battery. It can still eventually leak, so the only advantage to this is that you can run it somewhere away from the board where it won't damage anything if that happens, and of course that the charging circuitry can stay in place.
Click to expand...

Regular lithium coin cell do eventually leak but don't cause as much damage as the alkaline cells do. I know this because one leaked after 17 years in my Suburban's key fob. I thought is was ruined. Water wouldn't touch the electrolyte. It was an oil based stuff. I eventually found that BrakeKlean worked and cleaned it off the circuit board. I had to flush the plastic fob body quickly as it was dissolving the plastic as well. Other solvents may have worked and been less trouble for the fob body but that was what I used.
Dwight
 
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