[Q] How to measure the current consumption of a bicycle torch...

• 09-15-2013
rbid
[Q] How to measure the current consumption of a bicycle torch...
I have several flashlights that I use for my mountain bike hobby, some of good quality as the Serfas True 1500, and with a questionable quality I got in from DX (Deal Extreme), They have different LEDs, (P7 LED, T6, U2) , each giving a different amount of light. (two are mine and two of my wife)

Some of the batteries I have are good, but getting old, and the ones that come from DX, are not trust-able...

Now, I am planning to buy several LiPo batteries, and my question is capacity.
• Battery with low capacity will leave me without light halfway ..
• Battery with large capacity weigh much more and maybe are "over kill"

To do this, I want to measure that consumption of each lantern and then choose the correct battery capacity ..

Therefore, how can I measure the consumption of each torch, using the original batteries 7.4V.

Dump question that expects an correct clever answer

• 09-15-2013
canuck_tacoma
You have to put an Ampere Meter in series between the battery and bulb. Pretty easy. You may need to make an adapter to tap in.
• 09-16-2013
rbid
Current measured results...
Thanks, that is what I just made, and got interesting results.

For the Serfas True 1500+ Light at max power, I get 2.6A. That is, I need a battery with above of 9A for running for 3h 30min.

For the SSC P7-C light, the current is 1.5A (5.A for the complete 3:30)

For a new CREE LED XML-0L T6, the current is 1.25A 9around 4.34A for the 3:30)

Now, how do I measure the capacity of my current batteries?
• 09-16-2013
-Archie-
Simplest way is, to connect the voltmeter to the same adapter as well, and measure the time from switching the light on to the voltage drop to the discharge limit. To prevent overheating, household fan can be used during test.
• 09-16-2013
canuck_tacoma
Quote:

Originally Posted by -Archie-
Simplest way is, to connect the voltmeter to the same adapter as well, and measure the time from switching the light on to the voltage drop to the discharge limit. To prevent overheating, household fan can be used during test.

Yes....or you can set up a dumby load, basically a resistor. Don't short the batteries out without a load. Volt meter will be in parallel.

Another way would be to just run them until the light dies, dims, or the lights circuitry goes into low battery alarm.
• 09-16-2013
canuck_tacoma
Quote:

Originally Posted by rbid
Thanks, that is what I just made, and got interesting results.

For the Serfas True 1500+ Light at max power, I get 2.6A. That is, I need a battery with above of 9A for running for 3h 30min.

For the SSC P7-C light, the current is 1.5A (5.A for the complete 3:30)

For a new CREE LED XML-0L T6, the current is 1.25A 9around 4.34A for the 3:30)

Now, how do I measure the capacity of my current batteries?

You can look up the specs for these LED's to see if they are within range of your measurements. Keep in mind that there will be some resistance due to the circuitry too.
• 09-20-2013
androgen
Quote:

Originally Posted by rbid
Now, how do I measure the capacity of my current batteries?

this reminds me of a joke which explains that if you put two electrodes into soup and connect a light bulb you can determine whether you have already put salt in the soup ...

because apparently there is something wrong with tasting the soup to tell if you have enough salt in it ...

why don't you plug in the lights and wait for the batteries to run down ? why do you care what capacity they have - it's the run time that counts anyway.

besides, the run time cannot simply be calculated from capacity because different lights will cut out at different voltage.
• 09-21-2013
Cat-man-do
Quote:

Originally Posted by rbid
...Now, how do I measure the capacity of my current batteries?

There are several ways to do that but truthfully even if you know the exact ( mAh ) rating of the battery that information isn't going to help you unless you know the exact amount of power your lamp is using on high. Unless you know exactly how much current ( amps ) is going through each of the LED's on high only then ( along with the amp hour or Watt-hour rating of the battery ) can you determine how long the lamp will run by just running numbers.

Now forget everything I said in the above paragraph. Keep things simple. Set the lamp up in your house and do a run time test with the battery fully charged and with lamp you are going to use. Have a fan blowing on the lamp to keep it cool ( even with a fan it will still get hot ) If the lamp has voltage indicators keep an eye on those and make note at what time they change. When you're done you will know how useful the battery is to you.

If you want a better battery you need to know the mAh rating of the original battery. With Serfas 1500 you should be able to get that information from the company that makes the lamp. Send them an email and ask them what capacity the battery is ( in Milli-amp-hours ) and what brand cells were used. Ask them the voltage of the battery as well, just to make sure you don't order the wrong voltage if you replace the battery.

For something like the Serfas 1500; If you want 3hrs on high that is going to be hard to do but not impossible. A high capacity 6-cell built with quality cells should do the trick. Something in the 6600mAh - 7800mAh range should work well. As long as the voltage and connectors are compatible you should be good to go.

Now for the single emitter P-7 and XM-L lamps, once again look for the better branded cells. A good 5200-5600mAh 4-cell battery built with Samsung,Sanyo,Bak or Panasonic cells should get you your three hours.

( * Note; look over "The Battery Thread". Although some of the information is already out of date you may find something that I failed to mention. (...such as cell holders for 4-cell set-ups...:ihih: )