New Streamlight 4AA - safe for NiMH?

I got a Streamlight 4AA this week, and I like it a lot. However, I know that alkaline batteries have a lousy discharge curve, and that the flashlight will get noticeably dimmer as the batteries are consumed. Has anyone here used this light successfully with rechargeables? I've seen conflicting stories in the older posts.

The old version of the SL 4AA relied heavily on the internal resistance of the alkaline batteries to restrict the current, and NiMH batteries would fry the LEDs despite their nominally lower voltage. The new version has 22 ohm resistors instead of 12 ohm, and so the internal resistance of the batteries will matter less. But how much less?

I did some rough calculations based on the following formula for the current though one LED:

I [mA] = (1/7)*1000*(4*Vb - Vf)/(Rl/7 + 4*Ri)

Vb = voltage of one battery
Vf = forward voltage across LEDs
Rl = resistance of each LED resistor [ohms]
Ri = internal resistance of each AA battery [ohms]

I used data from the Eveready website to obtain the following parameters:

Energizer AA alkaline:
Vf = 1.5 V, Ri = 0.156 ohm (initial)

Energizer AA NiMH:
Vf = 1.4 V, Ri = 0.030 ohm (initial)

If Rl = 12 ohm and Vf = 3.4 V, I obtain the following:

With alkalines: I = 159 mA (ouch!)
With NiMH: I = 171 mA (ouch!!!)

If Rl = 22 ohms and Vf is the same,

With alkalines: I = 98 mA
With NiMH: I = 96 mA

All of these values seem too high, since I recall seeing an older post saying that the old design (with alkalines) drew 120 mA per LED, while the new design draws 90 mA per LED. Maybe I need to use a higher Vf or higher Ri. However, the qualitative trend is that the new design for the Streamlight 4AA should be much more tolerant of NiMH batteries. The NiMH batteries might even drive *less* current.

What does everyone else think?

I can't make any sense of that formula, but I wouldn't expect Streamlight to depend on the internal resistance of alkaline batteries.

If we're talking about seven parallel LED's with a resistor in series for each one, the current should be (Vb-Vf)/R where R
is the series resistor. For alkalines, Vb=6V, Vf=3.4V,
Vb-Vf=2.6V, R=22 ohms, so I=2.6/22 = .118 amps or 118 mA.
For NiMH at 1.2 volts, I=(4.8-3.4)/22 = about 63 mA.
These are really both awfully high for a Nichia-type led, I would have thought. 118 mA at 3.4 volts is around 400 mW
which is getting near Luxeon territory.

How about measuring the current consumption with fresh alkalines, and again with NiMH's? That would be the best way to see what's actually going on under power.

John

Unfortunately, my analog multimeter (old!) can't take any current larger than 150 mA, so I can't put it in parallel with the batteries. However, I was able to stick the probes through the opening in the side of the head and measure the Vf across one LED. It measured near 3.6 V, a value that may or may not represent all of the LEDs.

After recalculating based on this new Vf, I predict the following:

Alkaline AA: 91 mA
NiMH AA: 87 mA

That's for the *new* Streamlight 4AA flashlight. For the old light with the 12 ohm resistors,

Alkaline AA: 147 mA
NiMH AA: 158 mA
Three Alk. AA: 59 mA

I suspect that the internal resistance of the alkaline batteries increases at very high current loads, which might account for the value of 120 mA quoted in another thread. An internal resistance of 0.3 ohms per battery would be sufficient to achieve this.

By the way, this is my derivation for the current:

Voltage delivered by the batteries under load:

Vd = 4*Vb - (7*I)(4*Ri)

Voltage drop across resistor:

Vf = Vd - I*Rl

After solving for I:

I [A] = (4*Vb - Vf)/(Rl + 7*4*Ri)

This is equivalent to the formula that I gave originally.

Put a 0.1 ohm 1% resistor in series with the batteries, measure the voltage across it, multiply by 10 and you've got the actual current draw of the LED array! Digi-Key sells precision current measuring resistors.
Rob

I've been using mine with 3 Lithium batteries and a dummy cell. Very light and stays bright longer than alkaline.

so, is using Ni-MH safe?

my maths and electronics are no good.

I agree with previous posts regarding the internal resistance of the alkalines. You must have voltage to force current to flow, so you would have less current flow using a nicad or a metal hydride cell given the 1.2v per cell nominal voltage vs. 1.5v of an alkaline. The drawback to metal hydrides is their self discharge rate. They will go dead with no use fairly quickly (month?)so they make a terrible emergency power source. They are great fresh out of the charger though and back into the charger when done.
Kevin

[ QUOTE ]
Kevin_Falkner said:
You must have voltage to force current to flow, so you would have less current flow using a nicad or a metal hydride cell given the 1.2v per cell nominal voltage vs. 1.5v of an alkaline.

[/ QUOTE ]

That's the nominal voltage, however, which does not necessarily correlate to the actual current under a high load. The values of internal battery resistance are available from the Eveready web site, and they have a real effect. (That's why NiMH is superior to alkaline for camera flashes, despite the higher voltage and energy capacity of the latter.)

If you search through the old posts, you can find reports that people *have* burned up their old (12 ohm) SL4AA LEDs using NiMH batteries in place of alkalines.

In the end...all those posts about the SL 4AA 7LED being fried on NiMHs caused me to get a PT Attitude and run it routinely on 4 AAA NiMHs...I've been extremely happy with it!

Not TOO bright for a task light.
Bright enough for 95% of what I use a flashlight for.
(the other 5% I use my UK 4AA AS2...love it!)
Relatively cheap!
Extremely pocket-able.
Comfortable to clamp between the teeth when necessary.

I even notice that over a month's time the NiMHs don't seem to lose too much of their energy. After that...I just recharge 'em! /ubbthreads/images/graemlins/smile.gif

I always keep a 4 pack of Walgreens alkalines on hand ($1 on sale) for those emergencies when my NiMHs might have gotten low and I really need that backup power. Haven't had to use them yet!

So far...this has been a GREAT combination!
Cheap to buy...cheap to run...great to use!

Although I also see a lot of happy SL 4AA 7LED owners posting too!