How to power up a hair trimmer: NiCd vs. NiMH battery

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GreenBear

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Hi all!

I have a hair trimmer which is powered by an AA NiCd rechargeable battery. Over time the battery's capacity went too low. The trimmer still works with it but only for a short time, and then the battery has to be recharged, which takes about 10 hours. This is very inconvenient, especially if one only managed to trim, say, one side of a head.

So I really need an alternative power source. I tired to use one AA NiMH battery with a capacity of 2700 mAh, but it does not work well: while the motor is powered up and rotates when blades are detached, with the blades attached it does so only occasionally, suggesting insufficient power.

The strangest part is that even after I connected in parallel four AA NiMH batteries each with capacity of 2700 mAh, they still were not able to power up the trimmer efficiently, performing only slightly better than one NiMH battery. And the old and worn-out NiCd battery still powers up the motor with the blades every time it is switched on.

Also I have a power supply which can produce 1.6 V voltage, but when I connect it to the trimmer and switch the motor on, the voltage drops to almost 0 V.

1) Can anybody explain why do four NiMH batteries connected in parallel under-perform one NiCd battery?
2) Is it possible to assemble a circuit to power up the trimmer with one NiMH battery?
3) Is it possible to power up the trimmer with a power supply?

Thank you.
 
Did you get a NiMH charger at the same time?
It sounds like the trimmer takes quite a bit of current. Maybe your supply was not big enough. How many amps?
 
ronv said:
Did you get a NiMH charger at the same time?
It sounds like the trimmer takes quite a bit of current. Maybe your supply was not big enough. How many amps?
Click to expand...

Hi, ronv!

Thanks for the help. No, the power supply was bought separately, it was defective from the beginning in sense that it did not work according to the specification, but it did produce the 1.6 V voltage, so I used it for charging the NiCd battery in the trimmer (it can not be taken out easily). This supply has a 500 mA marking but I doubt it will perform up to it's markings.

For charging the NiMH I use a separate charger, which has several protective functions built in lest to overcharge.

I also think that the motor takes a lot of current but it does not explain the performance of the batteries.
I measured the voltages: the single NiCd battery produces 1.57 V when in standby, when the trimmer is working the voltage drops to 1.24 V; the four NiMH batteries produce 1.36 V in standby, 1.32 V then the motor is working and 1.26 V when the motor is "on", but unable to work. The surge current is 6.5 A.
It is really a rather small trimmer, nothing fancy. It is hard to think it may require more than 6.5 A to work.

I will try to measure the current when the motor is powered from the NiMH batteries.

As to the power supply what I meant: is it possible at all to power the trimmer from any generic power supply, may be with some additional circuits?
Also this thing with the batteries just drives me crazy - I can not think of any reason to explain these facts.
 
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A single AA NiMH should perform better than a single AA NiCad. Their voltages should be similar with the NiMH having a lot more capacity. It sounds like there's a high resistance connection, using too small wire gauge and/or too long cabling when attaching the NiMH's to the trimmer. Thus, the power is being dropped across wiring and connections and not reaching the motor.

creakndale
 
I have the same problem with a cordless Remington trimmer. Uses a pair if NiCd AA in series. This unit worked well for about 4 years so I figure I will just replace the NiCd batteries. All the cutters are still fine so why not? Even though a like item has a replacement cost of under $20 USD I just can't see throwing it away. I'm just going to replace with the same batteries it uses.

Ron
 
I have a Norelco beard trimmer that used a single AA NiCd battery which went bad. I replaced it with a low-self-discharge type NiMH AA and that works fine. The main difference is that the NiMH has a higher capacity and so takes longer to charge (I measured the charge current at 100mA so it takes at least 24hrs). And with the low self-discharge it goes more than a year without needing a charge (of course I only use it about once a week).

I wouldn't think there would be enough difference in internal resistance such that a NiCd would work and a NiMH wouldn't.
 

Hi,

As Ron suggested, the best bet is to replace with the same kind of cell that was in there when it was new.

I've had this exact same problem but mine used two smaller NiCd cells in series.
So far i have used the following:
1. Two NiMH cells instead
2. Two Alkaline cells instead
3. Power supply of 3 volts to 5 volts.

The problem with the NiMH cells was that after about 6 months of every day or every other day use the cells died from being constantly charged. The NiCd's last longer in this mode. Charging the NiMH externally however works ok. I eventually used a two cell battery holder and popped them out to charge and popped two already charged back in when using.

The problem with the alkaline is they run down eventually.

The power supply works but needs a cord.

Replacing with two new NiCd cells is the only true fix that works for a long time if you use the shaver a lot and want to use the same charger that came with the shaver without mod.

You can also consider replacing it with an Li-ion cell but you'll have to charge externally with a special charger and maybe drop some voltage for your unit (not with mine).
 
A Ni-Cad battery can be continuously overcharged at 1/10th its capacity. An AA cell is 600mAh so it can be overcharged at 60mA continuously.

A Ni-MH battery can be continuously overcharged at 1/40th its capacity. An AA cell is 2400mAh so it can be overcharged at 60mA continuously.

Is it a coincidence?
 
I think you have come across an interesting fact. It would appear that the two difference battery chemistries can both absorb about the same amount of overcharge level based mostly upon their physical cell size and not the cell capacity. Thus I think it unlikely to be a random coincidence.

I wonder if the absolute overcharge rate will stay constant for future increases in cell capacity?
 
Hi, guys!

Thanks for your help. I also think that NiMHs should perform just as well as NiCd, and that is why this is so weird that they do not. If we all assume that they should, then let's call this troubleshooting, since the trimmer still turns on with NiMH only occasionally.

I would buy of course a new NiCd, if only I knew that that particular kind would work. But how can I know that if the battery inside the trimmer does not have any markings regarding its brand or type. There is a marking "SAN 06-01 S", but I was unable to find anything in the Internet using that.

The blades can be detached, and then the motor can spin without any mechanical load. The blades seem to get a little stuck in certain positions, and to move them from those positions requires extra power from the motor. The NiCd seems to be able to provide that extra power to the motor, while four NiMH seem not.

I found the following passage in Wikipedia: "With a relatively low internal resistance, a NiCd battery can supply high surge currents. This makes them a favourable choice for remote-controlled electric model airplanes, boats, and cars, as well as cordless power tools and camera flash units."

Please note the words "favourable choice". And this is despite their memory effect and the bad ecology.

I did some measurements when the trimmer is powered from four NiMH batteries:
----------------------------------------------------------------------------------------------
.........................................................................current...........voltage
----------------------------------------------------------------------------------------------
Off.........................................................................-................1.34 V
----------------------------------------------------------------------------------------------
On, with blades, but unable to move them.............1.95 A...........1.20 V
----------------------------------------------------------------------------------------------
On, without blades, motor rotates ..........................0.50 A..........1.31 V
----------------------------------------------------------------------------------------------
On, with blades, blades are moving........................1.30 A..........1.26 V
----------------------------------------------------------------------------------------------

If I short circuit the four NiMH batteries, they produce current 6.0 A (6.5 A when they are freshly charged).
How could this current not be sufficient to move the blades ?????

Today I replaced the connector, it did not help. It does not seem to be the problem with wiring since I measured also the voltages inside the trimmer, they were very similar for both NiCd and NiMH, so there was no notable drop. The wire gauge is 0.75 mm, which should be Ok.

Carl, mine is also Norelco. Do you by any chance remember the brand and specific type of the NiMH battery which worked for you?

I am just so perplexed. Today I started to think that there might be a curse. Either I am cursed or Philips is. Or China.

How can I troubleshoot this? What do I need to check? Any ideas?

Also, is it difficult to assemble a power supply that could power up this trimmer? I already have a spare transformer and some parts.
Or, say, is it possible to modify the "500 mA" power supply to this end?

Thanks for your patience and help.
 
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Something is wrong with your four AA Ni-MH cells in parallel. Maybe they are poor quality or maybe they are not fully charged.

The graph on Energizer's website shows that a single cell has a voltage that is higher than 1.3V when it begins with a 2.3A load then the voltage drops to 1.2V for most of its discharge.

I didn't find the original Ni-Cad cell on Sanyo's website.
 

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Hi again,

It is starting to look like you may not have a single NiCd or that your NiMH cells are bad, or that there may even be an Li-ion cell in there.

You took lots of measurements with the new cells, but how about some measurements with the original cell in place? We need to see how it works when the original cell is in there with the shaver in full working order. Take some new measurements and that will tell us what is wrong.
 
You are absolutely sure the initial batteries were in parallel? My trimmer uses two NiCd but in series.

Ron
 
Hi Ron,

Yeah, i was asking him for some measurements so we can tell if he identified the old original cell(s) correctly. What tipped me off was that he was unable to find the part number but seemed to know the chemistry. I guess we'll find out soon.
 
In the first post the trimmer seemed to have only one AA Ni-Cad cell.
Then he tried a single 2700mAh Ni-MH cell but it didn't have enough power.
 
Hi there Audioguru,

Yeah, and i was asking for a measurement to confirm that or show that it's actually two smaller cells in pack. All of the trimmers i have seen and shavers too have two or three cells.
Then again, did we ask him to try a brand new fresh alkaline yet? That would be a nice test to see.
 
MrAl said:
... did we ask him to try a brand new fresh alkaline yet? That would be a nice test to see.
Click to expand...
Energizer's datasheets show that their AA alkaline cell has 10 times the internal resistance of their Ni-MH cell so it is no good at high currents.
 
Hello again audioguru,

10 times sounds like a lot, im not so sure i believe that data. I've run several shavers off of AA alkalines (not AAA though) and they all ran fine. Maybe he's got a Herman Munster shaver
 
MrAl said:
Hello again audioguru,

10 times sounds like a lot, im not so sure i believe that data. I've run several shavers off of AA alkalines (not AAA though) and they all ran fine. Maybe he's got a Herman Munster shaver
Click to expand...
Many shavers use fairly low current so they can be run from ordinary alkaline cells. But Ni-MH cells have a much lower internal resistance (it is low even when half-discharged) so they can supply much more current.
 
Hi audio,

I have to agree that the NiMH should be better, but i guess we have to wait for some test results to find out why his NiMH are not working with his unit.
 
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