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Replace Ni-Cd BIOS battery with Ni-MH? Is that safe?

kyodai

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Well today i messed with one of my old Fujitsu INTERTOP Model 10. Was nearly getting crazy since it wouldn't save changes to BIOS, was back on default on every reboot.

So i opened up that little beast and turns out the CMOS battery can't be saved - already started getting these little crystals that form when battery acid leaked and dried again.

Checked the internet and seems this type ("GS SAFT 3GF20") kinda went out of fashion and disappeared around the end of the 1990s... :(

So i have been looking for a replacement, searching for 3.6V 20mAh on ebay... Seems these tiny Nickel cadmium batteries got out of fashion completely. There are however similar size Ni-MH ones.


Question - is it save to replace it by a Ni-MH battery? I don't know so much about batteries but i remember Ni-Cd have a bit different characteristics than Ni-MH. So just wanna hear that it's save and they will not catch fire or explode because they have different charging characteristics or anything.


Any comments welcome.
 

zombienerd

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If the voltage is correct, then go ahead. Know that the NiMH will not last as long as a NiCD when charged like a NiCD. They use two different specs to know when they are fully charged, so your MB will constantly 'overcharge' the NiMH which would shorten its lifespan.

The most common method for terminating charge for NiCd is typically referred to as Negative Delta V. You try to capture the point at which the voltage comes down from its peak by a prescribed amount (usually 5-10mV/cell). Because of NiCd’s tolerance for overcharge, older versions of chargers may employ a timed charge using a C/10 charge rate for 16 hours or simply a low-rate float charge indefinitely.
The recommended method for charging NiMH is the dT/dt method (change in temperature with respect to time) that is typically set to 1-2degC rise per minute, at which point the charge is terminated.

Going beyond this point is considered overcharging NiMH. While NiMH can accept some overcharge, going to a Negative Delta V termination will typically cause faster degradation and reduction in cycle life (again going in the wrong direction).
Source: http://www.edn.com/electronics-blog...-DIY-NiCd-battery-pack-replacement-strategies
 

KC9UDX

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NiMH is generally accepted to be drop-in replacement for NiCad (albeit incorrectly as zombienerd points out.)

You may be able to still get a NiCad from Asia, if you can find a cross-reference.
 

Dwight Elvey

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If you remove the charging resistor ( easy to find in most cases ),
you can run the wires to a battery holder with a 3 "D" cells.
These will last their shelf life of about 10 years.
You also have that advantage that these can be mounted way from
the mother board such that they don't do damage if ( and when ) they
leak.
Most boards use a diode and resistor to charge but some boards
have a power management chip that does these things. You'll
have to look and see.
Dwight
 

Chuck(G)

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The downside of using a primary cell instead of a rechargeable is that the 'look" is different. NiMH look very close to NiCd.

Personally, I'd just stick in a multi-cell alkaline battery holder and use a diode to block the charging current.
 

Dwight Elvey

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If you really want to use a NiCad, you can still find portable
phone battery packs. They look flat with the three button cells
but work fine.
I've even seen them at the drug store.
Dwight
 

KC9UDX

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The downside of using a primary cell instead of a rechargeable is that the 'look" is different. NiMH look very close to NiCd.

Personally, I'd just stick in a multi-cell alkaline battery holder and use a diode to block the charging current.

I'd put Eneloops in and let em charge.
 

Chuck(G)

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Well, I like Eneloops a lot, but I'm not sure how long they'd last with a steady, unsophisticated trickle charge. My impression is that you charge them, use them, then charge them... Batteries have improved a lot, but so have charging circuits.

You could also stick in a couple of lithium AA cells, including the series diode and not worry about them for a decade or more.
 
Last edited:

konc

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I recently got a couple of Ni-MH for the same purpose and on the package was written in a large font "Direct Ni-Cd replacement". So I'd say even if they're not perfect (I don't know the details of charging etc mentioned earlier), at least they'll do no harm
 

kyodai

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Thanks to everyone for your valuable feedback, ideas and thoughts. Really appreciated. Well i guess will order some Ni-MH then. :)
 

KC9UDX

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Well, I like Eneloops a lot, but I'm not sure how long they'd last with a steady, unsophisticated trickle charge.

I've been using them for years in handheld transceivers that I rarely use. They stay plugged into their chargers intended for NiCd battery packs. When I do use one, it tends to be taken off the charger and intermittently used for a month, and then rotated to a different one.

I don't have any data to say that they work as well as they did when they were new. But I've replaced standard NiMH and lithium packs that were the same age, in the same time frame.

Batteries that last a long time go unnoticed. The battery in my wife's car died recently, and I was furious. I paid a lot of money for that Optima battery, not that long ago. Then my wife pointed out the tattered receipt which shows that I bought it twelve years ago.
 

mbbrutman

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I'm a bit concerned about replacing NiCADs with NiMH batteries.

They are a 'drop in replacement' when being used to power a device. My previous reading tells me that they are not a drop in replacement when being charged. So if you put them in laptop and the laptop charges the batteries, that could be problematic.

I'm not an electrical engineer so I'm not about to start redesigning charging circuits in laptops. My approach would be to replace the cells, but do not attempt to charge them with the existing charging circuit.
 

Chuck(G)

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Exactly, Mike. Primitive charging circuits found in old PC clock+SRAM are essentially constant-voltage affairs (take a look at any reference design from that era).

Really, if it's a clock+SRAM type of application, use a couple or three AA-sized alkaline primary cells with a blocking diode and keep the bloody things off the PCB. Replace every 10 years or so. I've got several systems set up that way and have no regrets.
 

Dwight Elvey

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I tried to convince him but he then turned around and
decided to go with Ni-MH. It seems to me that for the cost and hassle
some large D alkalines would out last the fellow.
Dwight
 

kyodai

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Well I don't think I'm the right guy to remove the correct resistor from such a miniature motherboard and i also don't paln on using the laptop very often - usually i get it out like once in a year, play a bit with it and that's it. I do like the idea of having some AA batteries and a diode as a replacemt, will sure try that in an old desktop one day, but these wouldn't fit in the Intertop as it really doesn't have much space in there.
 

CincyJim

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Try GOOGLING for the specific battery type with your location included.

For just "3.6v 200mah nimh battery" GOOGLE gave me About 512 THOUSAND results (0.71 seconds)
 

Dwight Elvey

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I know, fiddling with arduino and clock modules, there are rechargeable
button cells ( I think similar to the 2032 ). I just forget the number.
Most don't realize when they buy these modules that you have to cut
the jumper trace, especially if running at 5V, that the cell will go open
in a short amount of time. These cells should normally last for 5 years
on the typical drain.
Some of the modules do come with the right cell but they don't tell
you that they don't get charged when running at 3.3v.
The switch/display modules have similar design errors.
Dwight
 
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