Building custom battery pack.

[deejayspinz]

I am working on a custom P7 bike light project. I have the lights built and want to build a custom battery pack. Here is what 'think' I want to do. Looking for input on if and how this would work.

I would like to get the most runtime and was thinking of running 3 or 4 18650's 240 mAh in parallel @ 3.6V or maybe 2x2 (combined series/parallel @ 7.2 V). My current lights handle up to 8.6V.

So my question is, a) can I get some protected cells from say DX and solder them together in either of the configurations? ..and b) what charger would I have to use to charge them?

Ideally, what I am trying to get is a enclosed battery back that I have a connector coming out of. When running the lights, the connector is attached to the light. When charging is needed, I just take it off and connect to a charger..

Am I crazy? or can this be done?


Thx.

 

[RGB_LED]

deejayspinz, I had thought about picking up some tabbed 18650's to solder together for a previous project but thought, what's the point when they come soldered together with PCB and leads? Fyi, my project consisted of powering two handlebar-mounted Cree Q5 bike lights which had 2.5mm female dc connectors so I just picked up a 7.4v 2200mAh Li-Ion battery pack - similar to the following here - soldered a male dc connector, added heat-shrink to the ends, then plastidipped the whole works and added velcro to the body for easy attachment.

As for running the light / charging the battery: it is as you described... when running the light, the battery pack simply plugs into the dc connector from my lights. When charging, I simply unplug the battery and plug the charger in, light turns green on charger when complete, unplug from charger and back into light. Since it's only 2200mAh, I get just under 2 hours for both lights but almost 4 hours with one light. I use up a basic 7.4v Li-Ion charger similar to this one. If I can get my Imageshack a/c to work, I'll post pics. The cool thing about this setup is that I am planning on soldering dc connectors to my PT Apex headlamp so that I can use the same 7.4v battery pack on that head lamp.

Powering your P7 light should be easy to do but I'll let others comment about the required voltage and draw as I'm not too familiar with that emitter and what's required.

Btw, there are some threads debating the need to have a balanced charge on any multi-cell battery configuration - I don't know enough about it to comment but wanted to mention it.

One more note: coincidentally, there's another thread that talks about building 18650 battery packs here and there are some good words of advice... here is another thread.

Good luck with your project!

 

[mdocod]

It would be best to avoid soldering directly to any li-ion cell, the heat is not good for it. Li-Ion cells should have tabs welded on for safety.

Any li-ion pack with cells in series should also have balance taps wired to each section of the pack. (treat parallel wired cells as larger capacity single cells in the pack, but wire up a connection in-between sections of the pack that are in series)...

Then ideally use a charger with balancing capabilities.

or charge each section of the pack individually with a regular "1-cell" output charger.

For pack applications, you're going to want cell-to-cell consistency. I would not build cells from DX into a pack if you paid me to do it. I'd personally look into an external protection circuit designed for the pack in mind, and use unprotected name brand cells, like LG, Sony, Panasonic, etc.

AW protected cells would also work, but you don't really need a protection circuit in every cell in a circuit where you have cells in parallel, just a circuit for each set that's in parallel.