Advice about adding a protective circuit into a flashlight

[gpturner]

I would like to state something before I ask my question. I love this forum and have learned a lot from it over the years, but this is my first time starting a thread so please politely let me know if I do this wrong or if you have feedback on how I can do it better in the future.

So here is what I would like any offered advice on.
I have a ROP Mag Mod I made years ago, it is based on a 2D body and when I originally built it I used AA rechargeable batteries in cartridges, since then I have abandoned the AA’s and started using a couple of 18650 protected batteries. I have been pretty happy with this arrangement since I have a good charger and been have using 18650 cells in my LED lights for years and feel comfortable with them, I also find it a lot easier to charge a couple of 18650 batteries than fussing with multiple AA rechargeable batteries. Recently I have come into possession of several good UNPROTECTED 18650 cells. I learned the hard way about over discharging unprotected 18650 batteries in this flashlight. I would really like to use my other unprotected batteries in this light without worry, any advice or opinions about the feasibility of adding a protection circuit into the flashlight it’s self to guard against over discharging?
My other specific thoughts:

• I don’t want to build a custom single unit battery pack, I don’t have a proper hobby charger for something like that and one of the reasons I want to be able to just pop a couple of unprotected cells into the light is that I have enough I could go on a long hunting or camping trip and have extra batteries
• I know DX now sells individual PCBs for individual 18650 cells but this option does not appeal to me for many reasons.
• I have seen more than one person on the forum create nice custom 18650 adapters for D Mag bodies, I was wondering if it would be easier to add a protection circuit to an adapter like one of those adapters than the light it’s self. I thought maybe someone has already thought about and or done something like that but didn’t find any reference to anything like that in the forums.
• As you can tell by my hard lesson learned by my experience ruining those two batteries by over discharging them that I am not exactly an expert, so please feel free to be honest is this just a bad idea? Should I just stick to only using protected cells in this ROP mod, and forget this whole idea? :) (Although I might add I understand the basics of over charging and over discharging and have faith in using my charger safely and telling if a cell is good or bad)
• If you think this is a good idea, any advice on parts and how to do it? I wish there was just a unit you could remove the tail spring and just drop into the tail cap :)

Any and all advice and shared wisdom will be much appreciated, sorry this is long winded but I wanted to be thorough and not waste other people’s time trying to get the information needed to give good advice.

 

[SilverFox]

Hello Gpturner,

Welcome to CPF.

The best bet is to use protected cells.

The next best is to incorporate a protection circuit into your light.

The most practical is to keep track of your cell usage and simply use your unprotected cells. However, you have to have enough discipline to recycle any cell that becomes over discharged.

It sounds like you have a number of unprotected cells so this should not be a big concern, but some people have an aversion to throwing out a cell that looks perfectly good.

When using unprotected cells you have to first do a test to see what your run time is. This will involve running the light all the way down. Then immediately measure the cells voltage and get the cell back on a charger. Now you have an idea of how long the batteries will run, so only use a part of that. For example if you get 60 minutes of run time during the test, in actual use you only want to use it for 50 minutes. The trick to this is that many times a light is only used for short periods of time and it is difficult to track its actual use.

This is where you drag out your voltmeter. Check the open circuit resting voltage of the cells. When you see the cell voltage drop below around 3.6 volts, it is time to charge it back up.

If you can develop some good habits you can safely use unprotected cells. Protected cells are safer. With unprotected cells you become the protection circuit.

The nice thing about Li-Ion chemistry is that you can recharge often. With some planning, you simply charge the cell just before you take off on a lighting adventure. If you loose track of how much you used it, and have the cells already out for a battery check, just pop them on the charger and reset the clock.

Tom

 

[gpturner]

Thanks for the welcome and thanks for the advice, your suggestion about incorporating a protection circuit is what I want to do and what I was asking advice about doing BUT something you said is confused me.


You said
"When using unprotected cells you have to first do a test to see what your run time is. This will involve running the light all the way down. Then immediately measure the cells voltage and get the cell back on a charger."

If I run the light all the way down then haven't I just over discharged the batteries and therefore they are now ruined and will not take a charge?
I think I have figured out what the confusion is so let me know if you think I am right. It is my understand that most modern LED lights have built in protection so when you run the “light all the way down” as you put it your batteries still have a charge in them ~3.6 as you stated and they still can be recharged. Protected cells have the same built in functionality in the cells themselves. But this is not a LED light with that protection.
I took the advice as I quoted you above and I did stick a couple of cells into the light and ran the light “all the way down” until the cells completely discharged and now they will not take a charge at all, I think this is because this particular light is a direct driven incandescent light so it just flat out drained the batteries ALL THE WAY DOWN and I think my charger has a safety feature that will not charge a battery below 2.7v or something close to that. It’s my understanding that if they get that low they are bad anyway.

So what I think I want to do is to incorporate protection circuit into this ROP MAG MOD light so I can run it with 2 18650 cells without worrying about over discharging them. What is the best way to do that?

 

[Microa]

I recommend you to try Ricoh R5460N series protection IC for 2 cells in serial. I employ the R5460N-212AF (Latch type) to build my battery pack for years and it perfroms perfectly well.

 

[gpturner]

Thanks Microa, I think that is the direction I want to go in unfortunately I looked up the parts and I think there is a lot I need to still learn before I can understand how to use them to build my own PCB myself.
I see items labeled like “Protection Circuit Module (PCB) for 7.4V Li-Ion Battery Pack” on sites like battery space. Is there something out there like that I could wire in line between the positive and negative of the flashlight that would interrupt the flow from the cells if the voltage dropped too low. That way I could charge my unprotected 18650’s individually and drop a couple of them in the light in series and the circuit would protect the cells from over discharging too far. That is what I have in mind does anyone know of a Circuit protection board I could use that way?

 

[Microa]

You should use the automatic release type such as R5460N212AA. You should consider that as if the protection circuit is removed from the protected battery to your flashlight. Some li-ion battery holder for bicyclelight also employ this IC.

 

[Microa]

Sorry, I have omitted to consider the common point of the battery 1 and the battery 2 for this circuit. I think you should add an individual protection circuit module to each unprotected cell.
The schematic of R5460

The PCB layout