Cheapish headlight system

[barlows]

Guys, been reading here for a while, but now have a question or two.

First, I've been running a DIY headlight on my commuter with 3 SSC P4s from the bikeled.org instructions. It's works fine and I love it. Now I'm looking to add a light for my Rando bike. I'm thinking of using a P7 light from DX. I know they're not top notch, but the price is right. I'm thinking of using THIS LIGHT, and running the battery remotely. The proposed battery solution will be an 8 cell 18650 pack, run in parallel for something around 20Ah. Here's where my questions come in.

1) The light states that it is regulated for 2800 mAh on high. If it's true regulation, I would think running extra cells in parallel would have no impact on that. Am I wrong in assuming that?

2) I'd like to run some cell protection. My trouble is in deciding between individual cell protection, or just a single board for the pack. Since most protection circuits are based on voltage only (IIRC), then a single board might seem sufficient. But I'm thinking that individual cell protection might be better if the cells have differing rates of self-discharge, and may help to keep the pack balanced.

Thanks in advance for your opinions. I know this setup may not be perfect, but I think it should work okay for my needs. And then I won't have to keep stealing my commuting light for brevets.

Steve

 

[FrontRanger]

The proposed battery solution will be an 8 cell 18650 pack, run in parallel for something around 20Ah.

Presumably this is some type of Li-based chemistry. Due to slight variations in voltage it may not be perfectly optimal but it is indeed possible to wire these in parallel, as is the case in various commercially available products.

Here's where my questions come in.

1) The light states that it is regulated for 2800 mAh on high. If it's true regulation, I would think running extra cells in parallel would have no impact on that. Am I wrong in assuming that?

A battery is "trying" to be an ideal voltage source. Putting more identical cells in parallel will only lower the source impedance. This will not degrade the driver's performance (and could even enhance it slightly).

2) I'd like to run some cell protection. My trouble is in deciding between individual cell protection, or just a single board for the pack. Since most protection circuits are based on voltage only (IIRC), then a single board might seem sufficient. But I'm thinking that individual cell protection might be better if the cells have differing rates of self-discharge, and may help to keep the pack balanced.

I can't really answer this one, but I can make an observation: When you wire cells in parallel, a short in one cell is a short in all the cells! (This is not the case in a series-connected battery of cells.) So I suspect that individual protection would be a good idea, but I don't know enough about it to say it's necessary. Perhaps someone more knowledgeable can say for sure, or perhaps you can find the answer on the battery-specific sub-forum here.

I can't help but ask -- why not just replicate your other light, the one that works fine?

And lastly, :welcome:

 

[herulach]

That light you linked is definately not regulated - It's direct drive. You could regulate it quite easily using the instructions floating around for a 7135 'sandwich' as for cell protection, I dont suppose there'dbeany harm in both, however, sourcingtabbed cells with protection boards could be hard.