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Author Topic: Can a NiMH pack be used in place of NiCAD ?  (Read 4049 times)
N2NJO
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Posts: 141




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« on: July 22, 2008, 08:15:08 AM »

I have a Tempo (not the radio name) SLM (Signal Level Meter) that is around 8 years old. It has a 2300aH NiCAD 8 C cell battery pack. I want to replace it with a 5000aH NiMH pack.

Is there a problem with the different type of cell and/or the higher current capacity?

One battery outlet said a NiCAD charge circuit uses a pulsed output and a NiMH does not.
Comments here.
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WA7NCL
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Posts: 625




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« Reply #1 on: July 22, 2008, 09:33:49 AM »

They probably said the NiCAD is better at suppling loads that demand a pulsed current.

Mostly NiMH are being used to replace NiCADs because Cadmium is very toxic and presents an environmental hazard.

They both are compatible with low current trickle charging.  But if your charger is a fast charge type, there might be problems with over charging and charge termination.

Most cheapo wall wart chargers are trickle chargers so that they are cheap and consumers wont over charge the battery packs.  So I would guess you are OK using NiMH battery packs for your application.
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KB1LKR
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« Reply #2 on: July 22, 2008, 09:43:07 AM »

In general, yes. Only in very high (relative to cell size) current applications (e.g. cordless drills, saws, etc.) do you need NiCd's (which have lower internal resistance than comparable volume NiMH's.

Generally the charging protocols are the same, constant current, say 0.1C rate for slow/trickle charging, or ~1C rate for fast charging where end of charge rate of voltage change detection (dV/dt) and/or cell temperature rate of change (dT/dt) is often used. NiMH cells may use use zero dV/dt instead of negative dV/dt however.

A NiCd cell's voltage rises during constant current charge until complete, then the terminal voltage drops slightly (and cell temperature rises quickly) whereas NiMH's terminal voltage may only level off, vs. actually dropping slightly (cell temperature still rises suddenly though).

For more on dV/dt charging look up Maxim's MAX712 or MAX713 NiCd/NiMH fast charger controller IC's
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WA1RAW
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« Reply #3 on: July 22, 2008, 09:48:37 AM »

Well, it appears that you actually may have two questions.

First -- you can absolutely use an NiMh in place of a NiCad, in almost all applications.  The numbers you provided are only the long term "capacity" of the batteries.  Larger typically equals a longer run time.  Think about it:  1 and 1/2 volt batteries come in several sizes, right..?  a, aa, aaa, etc...  the only difference being that the physical larger cell just has a longer life (given the same application).

Second -- given what you said, which isn't entirely clear, it appears as if the electronic equipment that you are hoping to use the NiMh in place of the NiCad may have an automatic battery charging circuit built into it..??  If so, is there a switch that has NiMh or NiCad on the unit..?  Because the different battery types do recharge differently.  If you don't have a selectable switch, you cannot recharge the NiMh in the equipment, but you can still use the NiMh batteries in it (and recharge them with an external recharger).

Third -- lastly, there are some, though rarely encountered nowadays, pieces of electronic equipment that a VERY voltage dependent, and there is a slight voltage difference between MiMh and NiCads type cells.  If yours is one of them, then you're probably out of luck...

But the big issue here is #2, the charging circuitry.  Be careful.

 
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N2NJO
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« Reply #4 on: July 22, 2008, 09:49:30 AM »

The device draws 850mA. The internal charging circuit is speced for 8 hour recharge time with that orginal 2300aH pack.

Which of those two packs I linked would last longer as a replacement?
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N2NJO
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« Reply #5 on: July 22, 2008, 10:08:03 AM »

What part wasn't clear?
There is no switch. The device can run on between 10-15v.
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N8EKT
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« Reply #6 on: July 22, 2008, 01:10:21 PM »

Nicad chargers can't be used to charge NIMH batteries.

If you intend to use NIMH batteries you will need a NIMH charger.
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WA3SKN
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« Reply #7 on: July 23, 2008, 04:22:02 AM »

Sure you can replace the batteries.
The issue is in re-charging them!  The ni-cad charger may not be designed to re-charge your replacement batteries, take a close look at the design of the charger.
73s.

-Mike.
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N2NJO
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« Reply #8 on: July 23, 2008, 01:45:57 PM »

Kinda hard since it is a internal charger. Others have stated it is ok, except it would take longer.
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KC2MMI
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« Reply #9 on: July 24, 2008, 12:46:24 PM »

If the internal charger is the typical "overnight" charger, which charges the batteries at a 1/10th C rate?

Yes, it is perfectly safe and acceptable to use NiMh cell sinstead of NiCd. Even more so because you are going from 2500mAH to 5000mAH, so you now have a slower (1/20th C) rate charger.

There are differences between NiMh and NiCd but there is a wide area where their needs overlap and almost all "overnight" charging situations will fall neatly into it.

I've done it to a marine VHF, tripled the capacity, needs 24 hours instead of 8 to charge now--but runs three times longer, too. After five years, still no problems.

Whether a charger is pure DC or pulsed DC makes no difference. Pulsed DC is slightly more efficient for charging some battery types (particularly wet lead) but won't harm any of them.
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