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Futaba-RC . A Main Hobbies . Boca Bearings

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e-Electric Batteries & Chargers > Is this right? (1.2 X 12 = 16.8)
 
 
raytor
Senior Heliman
Location: Southern US

I can't figure this one out. I have a 12 pack NiCad and when I charge it up and install it my ESC won't start because of overvoltage above 16 volts. I measured the pack with a digital voltmeter and it reads 16.8 volts hot off the charger. If I drain it down (with a light bulb) or let it sit a couple of hours to drop the voltage down to 16.5 volts, the ESC fires up.
I thought a NiCad potential was 1.2 - 1.3 volts so at most 12 X 1.3v = 15.6v !
So how did it get to 16.8v ?
10-05-2003 Over year old.
HOMEPAGE  
 
 
vitek
Key Veteran
Location: Corvallis, OR

Yes, they are rated at 1.2v/cell. When fully charged, they will store slightly more, as you have discovered.

Travis
10-05-2003 Over year old.
 
 
chriss
Senior Heliman
Location: Sunny Florida

Actually, if your peak charging at a fairly high rate, say something over 2C, you can expect the peak voltage to be well over 1.5 v/cell. So expect a 12 cell pack to peak out at over 18 volts, this will of course settle pretty quickly to something around what you have discovered. I'd recommend fewer cells or a bigger speed control.

CHris
10-11-2003 Over year old.
 
 
raytor
Senior Heliman
Location: Southern US

What I have discovered is that the voltage rating seems to be a room temperature value because if I let the pack cool after charging the voltage drops to where one would expect it. I suppose the increased temperature causes increased chemical activity in the cells and thus a higher potential when hot off of the charger.
This might pose a hazard to some of the ESCs that don't sense overvoltage and may explain some of the unusual malfunctions posted over this forum.
10-12-2003 Over year old.
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rscamp
Veteran
Location: Ontario, Canada

No. This is not a temperature effect. You could cool the batteries to room temperature during charge and they would still give you a voltage of around 1.5V/Cell.

Rob
10-12-2003 Over year old.
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raytor
Senior Heliman
Location: Southern US

Well then what explains the fact that the NiCads are rated at 1.2v/cell - is that the average potential over their lifetime? - I got a 3300ma NiMH pack and it never overvolts. Is it the intenal resistance the eventually drains the NiCd pack down to a lower voltage?
10-12-2003 Over year old.
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Eddi E. aus G.
Senior Heliman
Location: Gerlingen. Germany.

The 1.2 V/cell is the voltage the cells can keep up under load (I don't know how many times C though, depends on the type I believe). In fact under high loads it's about 1.15 V/cell for most of the NiCds available

For comparison the values I experience:

Sanyo 2000 RC (NiCd) peak at 52+ V (30 cell pack, nominal 36 V)
Sanyo 2400 RC (NiCd) peak at 50+ V (dto.)
Sanyo 3300 HV (NiMH) peak at +/-47.5 V (dto.)

Charging rate is 4.75 amps average.

The latter pack delivers about 40 Volts for the first few minutes when discharged at 5 amps ...

The effect has something to do with the chemical processes inside the cells or so I read on a german forum. The internal resistance becomes more an issue when it comes to high current discharging.

Born to fly. Forced to work.
10-15-2003 Over year old.
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e-Electric Batteries & Chargers > Is this right? (1.2 X 12 = 16.8)
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