DIY external battery pack for a light?

[h2o_polo_Boi]

I was looking at pulling the trigger on the CygoLite TridenX Ni-MH. Instead of shelling out another $70+ on a battery pack, I was wondering if it was possible to make my own. The stock battery pack looks like this:

It is a 6 volt Ni-MH battery pack. I have several Ni-MH batteries @ 1.2 volts each and a 6 AA battery holder. That means I would need only 5 batteries (5 x 1.2 = 6 volts) and must short one slot on the holder. The only thing i'm missing is the DC connector which i should be able to find at a local Radio Shack.

Given all the circumstances, do you think it will work? Is there anything factors I have forgotten consider?

 

[Martin]

Hello and :welcome:h2o_polo_Boi !

It would be good to know if the original pack has any electronics to control the charge process. If it has and you want to use the original charger, then you need to add such electronics, too.

If you use a universal NiMH charger (or if the original pack just has the cells), you should add a fuse (4 .. 6.3 A time-lag) in series with the cells, that's it.
The fuse will prevent the pack (actually the holder) from melting, if you do something bad to the cable / the plug.
You can find the fuse and a holder at Radio Shack.

 

[HEY HEY ITS HENDO]

The $70+ battery pack is not AA though ......
so you will have much shorter runtime

........................................http://www.cygolite.com/2-Products/18-TridenX-NiMH.htm

 

[h2o_polo_Boi]

Thanks for the responses and welcome guys. I was thinking of charging my DIY external battery back separately. By that i mean, taking out the AA batteries when they are low and charging it in their own charger or swapping in a new pair of AA NiMHs. I have the new generation NiMH batteries called the Sanyo Eneloop 2000 mAh which are slow discharge cells. Am i making any sense or am i just embarrassing myself?

I just want to know if it was possible provided my external battery pack puts out the same amount of voltage as the stock one. I don't want to blow out anything. It would be great to just swap some batteries in and not have to wait for a recharge.

 

[HEY HEY ITS HENDO]

the SAME light is available with a 7.8v li-ion battery pack http://www.shop.cygolite.com/category.sc?categoryId=50
.... so it will most probably run on 6 AA batteries (increased runtime)

 

[Martin]

..I just want to know if it was possible provided my external battery pack puts out the same amount of voltage as the stock one. I don't want to blow out anything. It would be great to just swap some batteries in and not have to wait for a recharge.

Go ahead, it should work. Just use common sense (no short circuit, no reverse polarity).

 

[h2o_polo_Boi]

the SAME light is available with a 7.8v li-ion battery pack http://www.shop.cygolite.com/category.sc?categoryId=50
.... so it will most probably run on 6 AA batteries (increased runtime)

The li-ion one actually has a shortter runtime for some reason. The NiMH is heavier but lasts 3.5 hours on high. The li-ion while lighter, only lasts 2.5 hours on high. it's mind boggling. the FAQ states they both give off the same light. what kind of circuitry would allow that in simple terms?

in any case, are NiMH batteries still compatible with Li-ion?

 

[HEY HEY ITS HENDO]

its the same head, ...........just a different power source
although one has only slight higher voltage the other has much greater capacity and thus will run longer
not so mind boggling when you apply logic

 

[h2o_polo_Boi]

its the same head, ...........just a different power source
although one has only slight higher voltage the other has much greater capacity and thus will run longer
not so mind boggling when you apply logic

i see, so its possible that both can be of the same brightness?

 

[Martin]

..what kind of circuitry would allow that in simple terms?

in any case, are NiMH batteries still compatible with Li-ion?

Such a circuit typically drives a controlled current through the LED. The LED voltage is a direct result of the current but it's not being controlled.
A certain power is needed to operate an LED at the desired current, and this power is drawn from the battery. If the battery has a lower voltage, the converter simply draws a higher current so that the power is the same.

NiMH and LiIon are different in many ways. If I had to design a device that feeds on both, I would first design the device to work with NiMH / to charge from NiMH.
I would then design the LiIon battery pack and add electronics that adapt it to what's there. This adaption circuit is virtually free, because every LiIon battery pack has a complex safety circuit (to prevent it from burning) and this can be configured to do the adaption as well.

 

[NovaNath]

The li-ion one actually has a shortter runtime for some reason. The NiMH is heavier but lasts 3.5 hours on high. The li-ion while lighter, only lasts 2.5 hours on high. it's mind boggling. the FAQ states they both give off the same light. what kind of circuitry would allow that in simple terms?

in any case, are NiMH batteries still compatible with Li-ion?

Firstly, if you were thinking of using both types of battery together, don't. As far as i know, mixing batteries of different types is unsafe. Especially Li-ion. (Li-ion also needs a special charger)

Right, next is a bit of electronics..

P = I*V

That's Power = Current (I) multiplied by Voltage.
So the power of the lamp comes from the current it is drawing multiplied by the voltage it's taking it at. Worked example:

P=0.9A*3.75V = 3.375 Watts

It will take this power from any battery pack.
This means that for the Li-ion and NiMH packs you refer to, for an equivalent(sp) voltage, the NiMH must have a grater capacity (mAh) and it is this that determines the run time.

Does that make sense?

So it might be:
Li-ion 7.4V, 1600mAh
NiMH 7.2V, 4000mAh

Hope this helps
Nathan

 

[NovaNath]

By the way, if it's dead anyway, you can probably prise the ends off the pack you have and install new cells. Common ones are C sized, and are available with solder tags on the end! Thats what's inside my knackered Electron light.