How do volts and mAh affect the torch?

[immahooo]

Hi,

I am trying to buy a good 18650 battery, and I realised that some batteries have more mAhs but cheaper... I'm guessing it's the volts that matters and differs the price.
How do volts and mAh affect the torch brightness/runtime?

 

[RoGuE_StreaK]

(inexpert opinion)
- Firstly, many of the cheaper batteries overstate their capacity, sometimes drastically so; see if you can find an "actual" mAh figure (eg. a 3000mAh may have an "actual" capacity of 2400mAh)

- All things being equal, higher mAh should equal longer runtimes. Unfortunately all things aren't always equal.

- Not so much voltage but discharge capacity, though they are kinda interrelated.
Some batteries can't deliver more than x amount of current at a time (eg. 2Amps), so they may not be able to supply the driver/emitter with enough current to reach it's highest output. This effects brightness; you may not ever see your flashlight go as bright as it could go if you use an underperforming battery.
The voltages of some batteries sags more than others under a certain load; this in turn may mean the voltage falls below the amount required to keep the driver in regulation, or less than the forward voltage of the LED (assuming LED?) for a desired current. Starts dimming.
To top that off, protected batteries can have different circuits, which may impose artificial restrictions on what the cell is capable of delivering.

Have a look at some discharge graphs made by HKJ (he posted a new one just recently), he's got a comparison webpage where you can compare various batteries he has tested and you can see how some drop off well before others, different curves at different current rates, etc., etc, quite informative/instructive. Also shows that some of the cheapies can be as good as the expensive ones.

Though one other factor could be life; the cheaper ones may well compare in output in tests, but no-one's particularly certain how they will fair in the long-run, whereas the expensive ones may(?) have a more rigorous long-term testing procedure?

 

[immahooo]

Thanks Rogue, I think I'm gonna stick with the Panasonic NCRB.

 

[RI Chevy]

In layman's terms:

mAh can best be referred to as a gas tank in a car or truck. More mAh's in a battery is similar to having a larger gas tank in that vehicle. A larger gas tank does not make the car go any faster, just allows you to go longer between fillups. The same as more mAh in a battery means longer run times before recharging.
mAh has nothing to do with brightness or power. Voltage is better referred to the power.

 

[Jakeyb]

Can't go wrong with Panasonic. This is one place you simply can't risk lower quality

 

[stevenkelby]

In layman's terms:

mAh can best be referred to as a gas tank in a car or truck. More mAh's in a battery is similar to having a larger gas tank in that vehicle. A larger gas tank does not make the car go any faster, just allows you to go longer between fillups. The same as more mAh in a battery means longer run times before recharging.
mAh has nothing to do with brightness or power. Voltage is better referred to the power.

Well, kind of, but that ignores discharge capacity, which is like having a better fuel pump to empty the tank quicker, make more power and go faster :) A bigger tank (higher mAh) generally has a bigger fuel pump (higher discharge capacity) which is capable of higher output, in some lights anyway (ie, direct drive lights).

 

[HKJ]

Well, kind of, but that ignores discharge capacity, which is like having a better fuel pump to empty the tank quicker, make more power and go faster :) A bigger tank (higher mAh) generally has a bigger fuel pump (higher discharge capacity) which is capable of higher output, in some lights anyway (ie, direct drive lights).

Only to a limit, you do find any high capacity LiIon battery that can deliver 20A, but some with less capacity can.


I do often use the following chart to explain my battery charts:

The "brightness limit" depends on the actual light and anything above that line has reached maximum brightness. In the above chart it means that until about 0.25Ah/0.3Ah the brightness will be constant, then it will start dropping. The battery with the red curve will be brighter, but the battery with the blue curve will last longer.

 

[RI Chevy]

Well, kind of, but that ignores discharge capacity, which is like having a better fuel pump to empty the tank quicker, make more power and go faster :) A bigger tank (higher mAh) generally has a bigger fuel pump (higher discharge capacity) which is capable of higher output, in some lights anyway (ie, direct drive lights).

That is why I prefaced my response with "In layman's terms." A simple response. :)

 

[stevenkelby]

Yes it's all a rich tapestry :D

 

[stripe14631]

what causes a light to pulse near the end of the battery charge???

 

[DellSuperman]

what causes a light to pulse near the end of the battery charge???

That is caused by the driver, a low voltage warning.
Some drivers do that to warn the user that the battery is running low on juice.

- JonK

 

[5pyral]

That is caused by the driver, a low voltage warning.
Some drivers do that to warn the user that the battery is running low on juice.

- JonK

That's a neat feature. I think I am going to get into making my own so I can include things like that into my wanted modes.

 

[RoGuE_StreaK]

Just adding one more bit using HKJ's graph above. Assuming an LED, and assuming a constant-current non-boost driver set @ 2Amps. Let's say the forward voltage for said LED for 2Amps is 3.5volts (made-up figure just to make the point).
If you check the graph, the Eagletac 3100 drops below 3.5volts at about 1.2Ah, whereas the AW 2600 drops below at about 1.8Ah. So even though the Eagletac has higher capacity, when discharging at 2Amps and driving an LED that requires 3.5volts at this current the AW actually performs better, you'll get a constant brightness for longer before the driver drops out of regulation and it essentially goes into direct drive and starts to dim.

After that point the Eagletac will actually run for longer but get dimmer and dimmer, whereas the AW will be brighter for a little while then drop off to nothing quite quickly.
The Eagletac will give you say 36mins (1.2Ah = 0.6h @ 2Amps) at full brightness followed by another 48mins of steadily decreasing brightness, while the AW will give 54mins at full brightness followed by another 15mins of rapidly decreasing brightness.

So if you have details of the driver, the LED datasheet for reference, and cross-reference it all with HKJ's tests, you can make a pretty good guestimate of what kind of runtimes and brightness you could get from any particular combo.