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Old June 12th, 2007, 10:53 AM   #1
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Using NiMh Batteries in pro gear

A number of equipment manufacturers are advising against the use of NiMH batteries in their older products simply because they can generate high current which may cause damage to transistors in the input power circuits.

I've discovered that the newer products have some sort of current limiter to allow the use of NiMH and so are safe.

Now I'm not an electrical engineer - so I am guessing that a simple current limiter circuit may be all that is required either placed in the battery chamber - if there is space or inside the device itself - these circuits only utilise around five or six tiny components - mainly transistors and resistors - or indeed there may be an integrateed component which can do the job.

The crunch is that I am looking for advice from someone who may have done this and suggestions on a good circuit/component to use.

TIA
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Old June 12th, 2007, 11:58 AM   #2
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Hi John.........

If I understand your post correctly, the answer is quite simple:

1. If a manufacturer says "don't use" NiMH power - don't! No matter how you gild the lilly, if you do and something goes wrong, the gear and your warranty are toast.

2. Modifying your "pro gear" so as to use NiMH power if it is not designed to do so will, almost invariably, void your warranty and so you are still toast.

3. There is nothing inherantly different about NiMH power except the possibility of extremely high current in the event of a short circuit (due to their extremely low internal resistance). A "short circuit" means your gear is toast.

4. The most effective remedy for high current draw in the event of a short circuit is.................a fuse!

5. You really are kidding, right?

Cheers,


Chris
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Old June 12th, 2007, 01:41 PM   #3
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Quote:
Originally Posted by Chris Soucy View Post
If I understand your post correctly, the answer is quite simple:

1. If a manufacturer says "don't use" NiMH power - don't! No matter how you gild the lilly, if you do and something goes wrong, the gear and your warranty are toast.

2. Modifying your "pro gear" so as to use NiMH power if it is not designed to do so will, almost invariably, void your warranty and so you are still toast.

3. There is nothing inherantly different about NiMH power except the possibility of extremely high current in the event of a short circuit (due to their extremely low internal resistance). A "short circuit" means your gear is toast.

4. The most effective remedy for high current draw in the event of a short circuit is.................a fuse!

5. You really are kidding, right?

Cheers,


Chris


No I am not kidding - my question still stands - even if you choose not to answer it :)
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Old June 12th, 2007, 03:25 PM   #4
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Actually, I thought I had..........

but I'll try again.

Even if (and it's a very big if, considering that you do not know what components, where, you are trying to protect from what current) you could design a "current limiter" that would do the job, which in itself did not add to the risk of failure AND could be shoe horned into the battery compartment of whatever it is you wish to power with NiMH batteries, AND it did not trigger with the initial "in - rush" current at power on (thus killing it stone dead) you are still back to square one:

If the manufacturer states "Do not use NiMH batteries in this equipment" and you do and there is a failure - the equipment and your warranty are, er, toast.

Could such a circuit be designed, knowing all the parameters of all the components it is designed to protect, their in - rush currents, steady state currents, possible failure modes, etc etc etc - yep, sure could, you'll find them in a lot of modern equipment. I could knock you up the circuit diagrams of about 4 off the top of my head right now. Are you going to find one "off the shelf" for "any bit of kit going"? Not a chance.

More to the point, do you think anyone could or would, hand on heart, say "This will let you use NiMH batteries in your gear without fear of damage either to it or the warranty, even tho' the manfacturer says don't"?

I really don't think so.

If the gear you are trying to power really is "old" (and possibly out of any warranty anyway) and you are determined to use NiMH batteries in it despite warnings not to, why not do it anyway and just bin it if it goes "phut"?

Would I advise it? Nope. In the event of a failure that "phut" could be quite serious indeed.

In my PO, this is a fools errand.

Cheers,

Chris
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Old June 12th, 2007, 03:36 PM   #5
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quote:"A number of equipment manufacturers are advising against the use of NiMH batteries in their older products simply because they can generate high current which may cause damage to transistors in the input power circuits"

power cells, whatever the technology they use, do not generate high or low current. They give current according what the device is asking for.
So NiMH are not supposed to generate more current that any other cell.
In the contrary, New power cell technology (li-ion or nimh) are pretty limited in current they can provide.
Where a lead acid battery can provide easily 10x the nominal charge (i.e. a 10 Amps battery can deliver shortly 100Amps) , nimh or li-ion cell are usually limited to 2x, or they can be damaged.
additionaly, the elements delivers less voltage than regular cell (1.2V versus 1.5V) so many devices expecting steady voltage (i.e. 6V) have trouble when they see lower voltage (4.8V) for the same number of cells.
if you increase number of cells (provided you got the space in the device) you could provide more voltage than required.
Another factor is when devices allows recharging cells inside the device.
the recharging curve for ni-cad is very different from the one for ni-mh, and you could shortly destroy the battery by swapping technology.

the biggest problem could come from Lithium power cell (non rechargeable) versus Alkalin. the lithium elements have a very high voltage (in the contrary of ni-mh) that can destroy circuit.
This can be the case too, for some rechargeable batteries when freshly recharged, providing much higher voltage than required (li-ion are a bit like that).

My final advice would be to make sure that if you provide power supply to a device, just make sure that the source is giving the required steady voltage and can provide the required current with correct polarity . If you do that , there is no reason that a device could suffer from being powered by a technology or another.

And again if the manual says NO WAY, the safe decision should be to follow the advice.
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Old June 13th, 2007, 10:48 AM   #6
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Ok, some good advice there from you both.

Being mindful of the safety issues, and the manufacturer advice not to use NiMH in the battery chamber.

I am looking toward the following solution

1 use dummy batteries cabled out through the battery door to a battery clip.

2 for 6v 1A supply use 6xNiMH in the battery clip, for 9v 1A use 9xNiMH in the battery clip

3 the battery clip output is passed through a low drop voltage regulator circuit given the data sheet http://eu.st.com/stonline/books/pdf/docs/1318.pdf


this should provide a clean regulated supply for eco - friendly longer endurance
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Old June 13th, 2007, 01:20 PM   #7
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this will not work.
if you use 6x1.2V to get 6v regulated it means your input will be 7.2V.
the regulator you choose is not a LOW-DROP model.
it means it ask for a difference of voltage between the input (7.2) and the output (6V) higher than 2.85V. (here at least 6+2.85=around 9V)
so either you add more cells (8x1.2V=9.6V) or choose another regulator.
LOW-DROP models can go down to 0.8V of difference between input voltage and output voltage.
if you go for a high-tech battery, you better go to li-ion elements.
they are better than Ni-mh.
they come in larger cylinder of 3.6V 2A.
you can purchase a kit with battery and charger, and even some box that come with selectable voltage output and charger.
check http://www.batteryspace.com
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Old June 13th, 2007, 04:23 PM   #8
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thanks Giroud
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Old June 14th, 2007, 08:25 AM   #9
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The issue probably has nothing to do with the ability of the battery to supply higher current. It's more likely related to the difference in the discharge characteristics of the differing battery chemistries.

A lot of gear, especially anything with a HDD in it needs to know when it's about to loose power so it can shut down the disk. This isn't just a matter of powering the disk down, this is updating the FAT etc. If the unit were to suddenly loose power and that update not take place at the least you can loose a lot of data, at the worst trash the directory requiring the drive to be formatted.

An example of this is my Edirol R-4. It'll run on conventional AA or NiMH cells, so long as you tell the unit which batteries you're using . Edirol advise against using the new "Oxygen" cells as this could cause problems even though the cells do provide much more power.

So in your case you're probably OK subject to two conditions.
Firstly some form of current limiting circuit to limit the amount of damage in the event of a component failure. Even without that happening when you're running leads outside a unit to a large capacity battery it's very wise to have a fuse INSIDE the battery. It's all too easy to short a lead and with enough current start a fire or get yourself burned.

Secondly you'll need some form of battery monitoring so you have enough warning of the battery voltage dropping too low for the unit to keep working so you can shut it down.
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