NiMH Batteries failing under load?

[Razloz]

Well, I wouldn't say failing exactly, but here's the data;

4xAAA NiMH batteries freshly charged (well after a little use)
- 5 volts without load
- 4 volts under load
- 3 ohm coil on phoenix rba

3xAAA NiMH batteries freshly charged
- 3.98 volts without load
- 2.9 volts under load
- 3 ohm coil on phoenix rba

Ego Twist freshly charged maxed out on 4.8 volt setting
- 4.9 volts without load
- 4.47 volts under load
- 3 ohm coil on phoenix rba

Was wondering why my 3 battery setup has never performed quite right so did a little testing with multimeter on the posts of my phoenix. Is this just a characteristic of NiMH batteries? Or maybe I'm hitting the amp limit of one of my switches? The twist has always "hit harder" than my home-brew APVs using NiMH batteries, even freshly charged when it comes in at around 5.2 volts (that drops off after a few hits though). Which from this data would explain it, if it's a constant 4 volt instead of the 4.8. I'm thinking it might be my switches even though they're rated at 3A. My thought behind this is that a 4 ohm coil (1.2A) performs almost the same as the 3 ohm coil (1.6A). Maybe a 1.5A limit on the switches...Hmm...

 

[BJ43]

Well, I wouldn't say failing exactly, but here's the data;

4xAAA NiMH Batteries freshly charged (well after a little use)
- 5 volts without load
- 4 volts under load
- 3 ohm coil on phoenix rba

3xAAA NiMH Batteries freshly charged
- 3.98 volts without load
- 2.9 volts under load
- 3 ohm coil on phoenix rba

Ego Twist freshly charged maxed out on 4.8 volt setting
- 4.9 volts without load
- 4.47 volts under load
- 3 ohm coil on phoenix rba

Was wondering why my 3 battery setup has never performed quite right so did a little testing with multimeter on the posts of my phoenix. Is this just a characteristic of NiMH batteries? Or maybe I'm hitting the amp limit of one of my switches? The twist has always "hit harder" than my home-brew APVs using NiMH batteries, even freshly charged when it comes in at around 5.2 volts (that drops off after a few hits though). Which from this data would explain it, if it's a constant 4 volt instead of the 4.8. I'm thinking it might be my switches even though they're rated at 3A. My thought behind this is that a 4 ohm coil (1.2A) performs almost the same as the 3 ohm coil (1.6A). Maybe a 1.5A limit on the switches...Hmm...

Has nothing to do with the switch, it has to do with the C rating of the batteries, the NiMH just can't be discharged at a high rate. The amp rating on the switch does not restrict the current it just lasts longer than a lower amp switch.

 

[Razloz]

Ah thank you, wasn't sure. Had a feeling it was the batteries. Is there a formula or test to get the C rating, since most don't list it?

 

[Razloz]

These are 700 mAh AAA Energizer's I picked up when I got my rapid charger. From what I've read I thought all NiMH cells had the same discharge rate, going back and doing some more reading now that I've had time to use them. They work decent with 4 so never bothered to check on them but now I'm curious. Was planning on making some APVs using AA's instead of AAA's for a higher capacity but if they all have such a high drop off under load I might just switch over to lithium cells. Kinda fallen in love with this 15 minute charger though. >_<

 

[Razloz]

Well after some research I couldn't find any way of checking the C rating for a battery aside from contacting the manufacturer. From what I can tell the "C" Rating is more of a safety precaution than anything (for getting the max amps it can pull safely). I did however find some information about the voltage drop of batteries related to the internal resistance (IR) of the cell. The thing is from further testing it's a consistent 900-1100mV drop rate regardless of the current. Same drop rate with a 2 ohm coil, 3 ohm coil, and 4 ohm coil. I'm assuming it's just how NiMH operates under these conditions.

 

[TomCatt]

When I was using my Puck mods regularly (NiMH mods, 4 AAA), they would start out at ~5.6V w/o a load. Under load, 3.2 ohm clearos, they would deliver ~4.7V. I was using 950/1000mAh batteries.
As my sig says, Puck has my back

The C rating gives you an idea on how many amps a battery can safely deliver. A 2C 1000mAh battery should safely deliver 2*1000mA=2000mA (2A) of current.


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[Razloz]

So you were having the same drop rate, 900mV under load. Still love these batteries and my APV that uses 4, but in my next one I think I'm gonna have to use a chip to regulate the voltage.

When I was using my Puck mods regularly (NiMH mods, 4 AAA), they would start out at ~5.6V w/o a load. Under load, 3.2 ohm clearos, they would deliver ~4.7V. I was using 950/1000mAh batteries.
As my sig says, Puck has my back

The C rating gives you an idea on how many amps a battery can safely deliver. A 2C 1000mAh battery should safely deliver 2*1000mA=2000mA (2A) of current.


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[TomCatt]

Just a caution, with most step-down regulators you will lose a minimum of 0.5V from the start; step-up regulators require excess current (amps) to regulate voltage.


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