I will update you on Ni-MH AAs in the ProPoly 4AA

[Smaug]

Since I just got this light, I was curious about this. I recall a thread a while ago about this, and two people felt comfortable doing this. I looked in the head of my ProPoly and the resistors were indeed 22 Ohms each.

So I will bring this in to work and measure the current with fresh alkalines vs. fresh Ni-MH's. In case I forget to do that, I will just use it with Ni-MH's for a bit at my own risk. I'm pretty confident that since they are of lower voltage, they will drive less current.

On the other hand, the manual expressly forbids against using anything but alkalines... Hmm... Were they thinking of lithiums when they said this? ...or did they not bother to rewrite the manual when they changed the resistor value? We will see. I bet lithiums would even be OK with the new 22 Ohm resistor. To design it any other way would be folly, IMO. It would be placing too much trust in the consumers (like us) who would just have to try the Ni-MH or lithiums despite what the manual says, heehehe.

BTW, where do you get dummy cells? I think I have a couple that came with an old handheld CB, but does Radio Shack sell them?

 

[shankus]

I just got two dummy cells from Orion, in B/S/T. He hadsold the ones he listed, but looked around and found some others he wanted to sell. Perhaps he has more?
Orion's Thread of Sales!!!


I think the idea may be that rechargeables are a problem in the 4AA due to their lower internal resistance. Even though the voltage is lower, they can supply more current to the load due to this lower resistance, especially if they are warm, like fresh off of the charger (from what I understand).

 

[asdalton]

I have been using NiMH batteries in my SL 4AA (new version) without any problems. You shouldn't use lithiums though--lower internal resistance and higher voltage. I know that some people have successfully used three lithium batteries plus a dummy cell.

 

[Smaug]

OK, here are the results:

I charged the Ni-MH batts overnight in preparation.

I hooked the cells up to the LED module and completed the circuit with the DC ammeter on a Fluke 79III.

Alkalines - Vin = 6.12 Voc, I = 600 mA (85.71 mA per LED)
Ni-MH - 5.52 Voc, I = 510 mA (72.86 mA per LED)

So it is indeed OK to run Ni-MH batts in a Streamlight 4AA.

Furthermore, the internal resistance of a AA battery is negligible compared to the 22 Ohm series resistors provided with the LEDs. Therefore, if lithiums damage your 4AA, it's because of the higher current pushed by the higher voltage. Later this afternoon, I will calculate the internal resistance of alkalines vs. Ni-MH. If you just find an ammeter which will handle the short circuit current of the cells, you can easily calculate the internal resistance of each battery with Ohm's Law. I will let you know. If anyone wants to send me a AA lithium, I will also let you know the short circuit current of that as well.

Now I'm left wondering about how Ni-Cads compare to Ni-MHs as far as internal resistance and open circuit voltage are concerned... After all, my Ni-MHs are marked 1.2V, but fully charged average out to 1.38V.

 

[Doug S]

[ QUOTE ]
Smaug said:
OK, here are the results:

I charged the Ni-MH batts overnight in preparation.

I hooked the cells up to the LED module and completed the circuit with the DC ammeter on a Fluke 79III.

Alkalines - Vin = 6.12 Voc, I = 600 mA (85.71 mA per LED)
Ni-MH - 5.52 Voc, I = 510 mA (72.86 mA per LED)

So it is indeed OK to run Ni-MH batts in a Streamlight 4AA.



[/ QUOTE ]

I believe that if you repeat your measurements after only 15 minutes of runtime on each battery set you will see why they recommend alkalines only. Even though the alkalines can briefly sustain a higher voltage and current when new, over the long haul the NiMh will supply higher voltage and current and hence the integrated time of grossly overdriving the LEDs is greater with NiMh [or NiCd].

BTW, L91 lithium AA cells contain a PTC overcurrent protection device that will trip if the cell is short circuited for very long so you would need to get your measurement quickly. The device will reset once the cell cools down.

 

[asdalton]

See this thread for my calculations for this flashlight. Your results are consistent with my equations, but with an added resistance of 0.5 ohms inside the ammeter. This means that the currents during actual use will be higher.

The easiest way to find the internal resistance of batteries, at least for the commonly available types, is to look up the data sheets on the manufacturer's website. I know that Energizer gives data for their batteries, for example. These values are small but not negligible, because the 22-ohm resistors in the SL 4AA are in parallel.