Running one out of three 18650s protected, OK?

[Juggernaut]

Is it a good idea to run 3X 18650 batteries and have two of them not protected and have one them with protection (AW) I was thinking that if I run a LED bulb off of them then instead of having to pay extra for all the protected batteries I could just have one to shut them all down when the voltage becomes to low. I mean if the voltage is low enough the protected one will stop working and basically stop the current going though the other ones. Does it matter which position to put the protected battery "front, middle, or, back:shrug:"? Where talking the difference between $39 and $26.58 not to mention you could do this set up twice over and save more then $24:thumbsup:.

 

[DonShock]

That will sorta work, but the assumtions that have to be made can bite you in the rear in actual use. Yes, once the protected cell hits the low voltage setpoint and cuts off that should cut off current flow through all the cells. But you are making the assumption that all the cells are equally charged to start and equally discharge in use such that the individual cell voltages are all the same when the protection kicks in. In the real world, it doesn't work that way and that assumption can be a dangerous one. It is possible that one of the unprotected cells could have a much lower voltage than the protected cell and thus be damaged long before the protected cell cuts off.

Personally, I use either all protected cells or all unprotected cells in a given application. That way if I am using unprotected cells I know that I am the only source of protection. In reality, I treat all my cells as unprotected and do not rely on the protection circuits.

 

[Juggernaut]

Thanks for the advise:thumbsup:, I will not let the protection circuit kick in if I can:shakehead. I have this weird thing were I never let a light die:shrug:. I have only seen a LED or Incandescent light die twice in the last 6 years. Both cases were on purpose to see what run times I could get. I always charge Li-ions when they hit 4.0 "sometimes more" just so I can be sure my lights will never quit on me. The chances of that one cell having to kick it's protection on will be very improbable. It will be for a back up set of batteries anyway. So I would very rarely use it in this set up, I just wanted to make sure it would work.

 

[mdocod]

I would be best overall to just bite the bullet and get all protected cells, doing so will also help improve safety during charging...

What charger are you going to use?

 

[Juggernaut]

I use a Cheapo DSD charger "which is locked in a steel lock box which is in turn taped to a thick and heavy piece of stainless steel" This way if they go :shakehead worst case I should only be awoken and loss $30 worth of cells and charger but be ok, Whenever I go to put the cells in a multi cell light I chuck the cells in combs on the charger for a few seconds just to try and eliminate any voltage difference. Not sure if this is a smart thing to do but I make sure the cells are at lest 0.01 same in voltage before use. I just ordered them and I decided not to take such a big gamble and I won't run the back up set with any less then 2x Protected and 1x unprotected to minimize problems. I only own 1x unprotected and "when I get them for Christmas 4 more protected" so total 5x protected This way I can have one in my EDC-P7, one in my Taskforce, 3 in my M90X and the last one "the only unprotected" will be the, but in some sort of weird scenario I can at least run 2x full sets of batteries though the M90X:thumbsup:. I wanted some of those IMR 18650s for the M90X since there cheaper and would probably give me a tiny little brust in output using something like a EO-13 LA but there was none in stock at LH so I said what ever:sigh: and went with the normal ones "there probably wouldn't be a big difference anyways since the bulb only pulls 1C and those IMR cells seem only good for really high draw rates as for as I know:shrug:.

 

[mdocod]

As long as the protected cell has less capacity than the unprotected cells then it's theoretically going to work fine... but you would need to do some testing to determine this...

What if AWs protected 18650 performs 2.2AH and his unprotected cells only perform at 2.1AH? and you go to discharge them down, and the protected cell is still at 3.6V and falling, but the unprotected cells are already at 1.7V and falling fast........

If this is a buck-regulated LED light, there will be no way to "see" this happening as it will stay bright all the way down when you have 3 x 18650 cells in series...

 

[Juggernaut]

Yes I could see how this would be a bad idea in a LED light:shakehead, I guess if I ever do use this set up I'll only run it with an Incan and watch for the steep drop off.

 

[mdocod]

The other issue is that protected cells have a higher resistance than unprotected cells due to the protection circuit, I'm not sure how this would play out in a series circuit to be quite honest but might be worth considering..

If I were in your shoes, and wanted to see this through, I would run some simple tests....

Charge the 3 cells up, figure out what the load of the configuration is on the cells, and calculate an approximated amount of running time required to drain off say, 25-50% of the total stored capacity of the cells. Then, remove the cells, and test the voltage of them all. If the unprotected cells are equal or higher in voltage to the protected cells, then you are ready to roll and shouldn't really have any problems. If you want to make double-sure, continue the test with more draining of the cells in the configuration, checking every 10% of estimated drain time on cell voltages to make sure those unprotected cells are keeping up with the protected cell in voltage. Towards the end of the discharge, one of the cells will drop off quickly and you'll want to find out which one it is, ideally, a protected cell will be the one to drop off first for everything to work out well... Or if they all start to drop off at the same time that would be great. (what I mean by this sudden drop-off is when they start reading less than ~3.5V, it doesn't take long at all to drop a lico cell from 3.5 to 3.0 or less once it is down that low. )

Eric

 

[Juggernaut]

I'll report back when I try it. Though like you said it all has to do with internal cell resistance and if there isn't a big difference my results may vary with other people so it still wouldn't be a foolproof setup, but we'll see. Thanks for all the help:thumbsup:.