NiMh change in internal resistance when getting old?
09-27-2006, 07:55 AM
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NiMh change in internal resistance when getting old?
How do you measure the internal resistance of a power pack?
I have 2 years old 5 cells, 2700mAh NiMh freshly charged pack which, when loaded at 3 amps, gets down to 4.8V
Is this normal or is it getting old and unable to deliver the Amps when required?
Cycling the pack at C/5 still shows 2700mAh.
Ben
I have 2 years old 5 cells, 2700mAh NiMh freshly charged pack which, when loaded at 3 amps, gets down to 4.8V
Is this normal or is it getting old and unable to deliver the Amps when required?
Cycling the pack at C/5 still shows 2700mAh.
Ben
09-27-2006, 10:47 AM
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RE: NiMh change in internal resistance when getting old?
Resistance internal formula is (no-load V - full load V ) / Current Amps at load
Yes a 6V down to 4.8 V at 1 C does not sound to good. But some older generation 2700mA AAA did this when new. Really would need manufacture spec for that battery to really say it's gone bad. Always test battery after 5 cycles of good form charge and write in book save for later then you know exactly what's up.
Rich
Yes a 6V down to 4.8 V at 1 C does not sound to good. But some older generation 2700mA AAA did this when new. Really would need manufacture spec for that battery to really say it's gone bad. Always test battery after 5 cycles of good form charge and write in book save for later then you know exactly what's up.
Rich
09-29-2006, 01:21 PM
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RE: NiMh change in internal resistance when getting old?
Capacity seems OK. If it doesn't self discharge fast, why dump it?. As a rule of thumb, dump when capacity drops to 80% of original capacity. If 90% the capicity is still available after a week of inactivity, I'd say the pack was still OK.
Re reading your thread, do you mean the voltage immediately drops to 4.8V under load or after a full discharge?
Re reading your thread, do you mean the voltage immediately drops to 4.8V under load or after a full discharge?
10-01-2006, 10:25 AM
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RE: NiMh change in internal resistance when getting old?
The 5 cells 2700mAh is 6.72V without any load. This is after 12 hours coming off a fresh charge (resting time).
I apply a load of 2 Amps and the voltage drops to 5.90, or a difference of 0.82 volts.
This means that the internal resistance is 0.4 Ohms (0.8V / 2 A).
After a bit of playing around with this test, the unloaded voltage is now 6.00 volts, slowly going back up.
If I then apply a 3 Amps load, the loaded voltage drops to 4.82V, or a delta of 1.2 V, equivalent to the same 0.4 Ohm internal resistance (1.2V / 3 Amps).
Looking into the datasheet for this 2700Mah NiMh, they state that the internal resistance is 0.2 Ohms.
My batteries are 2 years old and the internal resistance has doubled.
In other words, the lung volume is still there but it developped asthma!
I use this battery with digital servos on a 35% Extra and my concern is that it doesn't support the load anymore. My snaps have been getting sluggish recently and this, together with a bad switch connector might be the culprit. I am changing everything and will see if it improves.
Thanks for your help.
I apply a load of 2 Amps and the voltage drops to 5.90, or a difference of 0.82 volts.
This means that the internal resistance is 0.4 Ohms (0.8V / 2 A).
After a bit of playing around with this test, the unloaded voltage is now 6.00 volts, slowly going back up.
If I then apply a 3 Amps load, the loaded voltage drops to 4.82V, or a delta of 1.2 V, equivalent to the same 0.4 Ohm internal resistance (1.2V / 3 Amps).
Looking into the datasheet for this 2700Mah NiMh, they state that the internal resistance is 0.2 Ohms.
My batteries are 2 years old and the internal resistance has doubled.
In other words, the lung volume is still there but it developped asthma!
I use this battery with digital servos on a 35% Extra and my concern is that it doesn't support the load anymore. My snaps have been getting sluggish recently and this, together with a bad switch connector might be the culprit. I am changing everything and will see if it improves.
Thanks for your help.
