How big a powerbank to run a led light.

[mzjo]

I now have two potential cycle front lights that will run off dc. One is a torch that I recovered from death by soldering wires onto the led contacts. The other is a cheap led headlight from the supermarket that is marked as a dynamo light in one language and a battery light in another, with terminals marked + and - ve. Obviously no battery provision!

The first of these lights has been used with an outside battery holder with 4 AAs. This is not ideal as 1 there is no switch; 2 the batteries are pigs to get in and out and 3 the wires to the battery connector are so fine and rigid that they break at inconvenient moments (like when you really need the front light to work!).
Having got a "nomad battery" for my telephone the idea came into my head that a full size USB connector would resist being plugged and unplugged much better than my battery pack. Tried out last night with positive test results.

So what are the risks, if any, of doing this for an extended right (to light or to battery)?

What size battery? Mine is sold as 4000maH but judging on its performance charging my telephone 2500 would be closer to the truth. (It wasn't expensive and does what it was bought for). Would one 2500maH battery pack be equivalent to 4 AA rechargeables at 2500 maH at 1.2v each? How does this maths work out?
Any other comments? I don't own a dynamo wheel at present and the possibility of swapping lamps between bikes with differing size wheels is desirable. The advantage of a separate battery to me is that the lamp is lighter and therefore can be mounted with a bit more liberty than a conventional battery light.

[quixoticgeek]

mah is a stupid unit when it comes to working out batteries of different types. Unfortunately marketing departments have made it a right pain.

In short, to compare these things, use watt hours. 

Watts are power = voltage x current. (P=VI)

a 4000mah lithium ion battery is roughly 3.6v, so  4*3.6 = 14.4Wh.

4x 2500mah 1.2v AA is roughly 2.5*(1.2*4) = 12wh

a 2500mah 3.6v lithium battery at 3.6v is 3.6*2.5 = 9wh

How to make this useful?

Well, if your light draws 3w, then a 9wh battery is going to give you just under 3 hours of use (cos nothing is 100%).

Does that make sense?

J

[philip]

If the 4000mAh battery is a usb power pack then it is 5V rather than 3.6V. In which case 2500mAh at 5V is very close to 4 x 2500mAh at 1.2V.

The 4 AA pack is probably above 5V when fully charged but quickly drops to about 4.8V and then gradually drops to 4.4V as it discharges. The usb power pack probably maintains a more even 5V.

[Kim]

In the interests of bigger, more impressive looking numbers, marketers of USB battery packs cite the amp-hour capacity at the battery, not the USB output.  On a really bad day, batteries in series get counted twice.

This is unhelpful when you're doing real-world capacity calculations.

[quixoticgeek]

If the 4000mAh battery is a usb power pack then it is 5V rather than 3.6V. In which case 2500mAh at 5V is very close to 4 x 2500mAh at 1.2V.

The 4 AA pack is probably above 5V when fully charged but quickly drops to about 4.8V and then gradually drops to 4.4V as it discharges. The usb power pack probably maintains a more even 5V.

You'd think that, but it's the mah value of the 3.6v (nominal) lithium cell inside, not the 5v output. It makes for bigger numbers, and we all know more megapixels is good...

The usb pack does have a dc-dc converter in there to increase the output to 5v (or in some cases to drop it from 7.2v...). Which adds some inefficiency, which is why you'll never actually get 3 hours of light for your 3w lamp from a 9wh battery pack, tho you should get >2.5 hours.

How accurate that 5v is depends a lot on the quality of the pack. As well as any intelligence (in the loosest sense of the word) is built in to it. You may find it i anywhere from 4.9-5.1v. You may also find that it will only output 2.5w (500ma), at 5v, unless your device has the right chip to talk to the pack and ask for extra power. Depending on the pack...

In the interests of bigger, more impressive looking numbers, marketers of USB battery packs cite the amp-hour capacity at the battery, not the USB output.  On a really bad day, batteries in series get counted twice.

This is unhelpful when you're doing real-world capacity calculations.

Yep. Hence my 3.6v (nominal) rather than 5v.

Most lithium battery packs will quote the wh as well as the mah, of you look closely enough on the label. Largely because planes don't allow above a 100wh of lithium battery in your hand luggage, so printing the wh saves some maths for the security goon.

J

[mzjo]

mah is a stupid unit when it comes to working out batteries of different types. Unfortunately marketing departments have made it a right pain.

In short, to compare these things, use watt hours. 

Watts are power = voltage x current. (P=VI)

a 4000mah lithium ion battery is roughly 3.6v, so  4*3.6 = 14.4Wh.

4x 2500mah 1.2v AA is roughly 2.5*(1.2*4) = 12wh

a 2500mah 3.6v lithium battery at 3.6v is 3.6*2.5 = 9wh

How to make this useful?

Well, if your light draws 3w, then a 9wh battery is going to give you just under 3 hours of use (cos nothing is 100%).

Does that make sense?

J

Yes that makes sense. On the whole it means that i am counting on much the same run time as my Lezyne rear light on constant mode. better going straight for a dynamo. Now does anyone know if running led lights will be beyond a 1950's New Watson bottle dynamo - I have a couple that work well!

 

In the interests of bigger, more impressive looking numbers, marketers of USB battery packs cite the amp-hour capacity at the battery, not the USB output.  On a really bad day, batteries in series get counted twice.

This is unhelpful when you're doing real-world capacity calculations.

I think i have seen you say something like this before and it is the reason why i think my power pack gives so much less than the figure on the packaging which I have long since thrown away (other than the fact that it pains to charge a telephone, even switched off)

If the 4000mAh battery is a usb power pack then it is 5V rather than 3.6V. In which case 2500mAh at 5V is very close to 4 x 2500mAh at 1.2V.

The 4 AA pack is probably above 5V when fully charged but quickly drops to about 4.8V and then gradually drops to 4.4V as it discharges. The usb power pack probably maintains a more even 5V.

You'd think that, but it's the mah value of the 3.6v (nominal) lithium cell inside, not the 5v output. It makes for bigger numbers, and we all know more megapixels is good...

The usb pack does have a dc-dc converter in there to increase the output to 5v (or in some cases to drop it from 7.2v...). Which adds some inefficiency, which is why you'll never actually get 3 hours of light for your 3w lamp from a 9wh battery pack, tho you should get >2.5 hours.

How accurate that 5v is depends a lot on the quality of the pack. As well as any intelligence (in the loosest sense of the word) is built in to it. You may find it i anywhere from 4.9-5.1v. You may also find that it will only output 2.5w (500ma), at 5v, unless your device has the right chip to talk to the pack and ask for extra power. Depending on the pack...

In the interests of bigger, more impressive looking numbers, marketers of USB battery packs cite the amp-hour capacity at the battery, not the USB output.  On a really bad day, batteries in series get counted twice.

This is unhelpful when you're doing real-world capacity calculations.

Yep. Hence my 3.6v (nominal) rather than 5v.

Most lithium battery packs will quote the wh as well as the mah, of you look closely enough on the label. Largely because planes don't allow above a 100wh of lithium battery in your hand luggage, so printing the wh saves some maths for the security goon.

J

Inspired by this I have just looked all over my power pack. The most obvious information on it is the maker (or probably exporter) name and address. But on the side I have 3.7v 4000mAh and PRI 5V dc 1A SEC 5V dc 2A (obviouslt dc in symbol form that I don't know how to do on a keyboard). No mention of watts, presumably anyone needing to know that would be expected to calculate it from these figures.

[Frank9755]

Referring back to the original question, you can run an led rear light off a power bank for a long time, several nights ,as they don't draw much power.
My experience is that a good front light will draw 1-1.5 amps, ie I get 7-10 hours from a 10 Ah power bank for the lights that I power in this way. I have one that gives more light but will only give me about 5 hours run time so I have ruled it out as not practical for my purposes.
NB these are summer times, I get less in UK winters