Question: maxflex low voltage warning and 18650 unprotected cell

[bluecrow76]

After reading the discussion here about unprotected cells, I'm wondering, is the low voltage warning on the maxflex driver a sufficient warning system for voltage issues?

I'm using 3 x 18650 cells in my quad M@G build.
3 x 4.2 = 12.6 maximum Vin
3 x 2.6 = 7.8 "minimum" desired Vin

I set my low voltage warning for 8.4V. I know it won't stop a runaway reaction, but should I feel any safer knowing that I have that turned on? I'm aware that this setup won't actually limit the battery usage, only notify the user that things are getting out of the desired limits.

As I said elsewhere, I am slowly migrating to all protected 18650 cells, it's just a slow process. The unprotected cells I am using have been pulled from laptop battery packs and have been tested to be good. I check the voltage on them every few days.

 

[VidPro]

Re: Question: maxfles low voltage warning and 18650 unprotected cell

3 x li-ion
charged ----4.2--4.2--4.2
discharging -3.8--3.9--3.7
discharged -3.1--3.2--2.9
Dead -----2.4--2.8--1.0 --- and ruined

this is how i was planning on figuring things for the cutoff.
and its VERY dependant on the draw , the balance , and the cell quality, and Time.

worst case scenario
3.8--3.7--2.5
might be something like this, depending on draw balance and time
IF you do a cutoff HERE, what would be the ramifications?

i donno, but this is some of what i would be calculating, which is a LOT easier with just 2. with 2
4.0 -- 2.7 way out of wack, because balanced that would be
3.5 -- 3.5 still an early cutoff

so with 2 , you could do a 6.2V say
3.1 -- 3.1
with an offset handling of say
3.8 -- 2.4
ample offset handling available

so that is how i would figure it, your only capable of getting it close, and hoping any offset balances dont occur, with 3 you can get some math going for an offset balance, and over time totally dependant on the cell quality, and life and capacity and damage , only so close, but mabey close enough.

Which is why with "unprotected" i prefer ONE single item (wont work with maxflex and 3 leds) matched with parellel leds, and not using drivers that will suck a battery dry.

soo you gotta do some math for your max low like:
3.0--3.0--3.0 = 9V total
judge what your offset can or will ever be before death
3.4--2.4--3.2 = 9v total
insure that you balance the cells by singular charging
hope the capacity remains consistant, check what is occuring in reality, especially after time and age.
Top-Off often, add in some Hope, and do what you can do.
rested voltages will differ from loaded voltages, again dependant on the cell quality.

these are just Ideas, my math is rotten, the possibilities are endless, the idea remains. i dont recommed anything, just discussing it.

 

[bluecrow76]

Re: Question: maxfles low voltage warning and 18650 unprotected cell

3 x li-ion
charged ----4.2--4.2--4.2
discharging -3.8--3.9--3.7
discharged -3.1--3.2--2.9
Dead -----2.4--2.8--1.0 --- and ruined

.
.
.

soo you gotta do some math for your max low like:
3.0--3.0--3.0 = 9V total
judge what your offset can or will ever be before death
3.4--2.4--3.2 = 9v total

Very good points! That definitely encourages me to set the low voltage warning higher and continue checking the voltages more often.

One thing I forgot to mention is I NEVER mix different brand/model of cells. That would be more than tempting fate!

 

[VidPro]

i am still interested in how the maxflex works, in totality, from the users point of view.
is there a low Cutoff? and a warning? or just a warning?
how variable is the voltage indicator things? can you have 8.4 and 8.5 and 8.6 ? or is it whole numbers?
are you finding it easy to set , and know how it IS set in use?

i dont have one applied yet, because it wants more input voltage than my application.

 

[bluecrow76]

The low voltage warning of the maxflex is just a warning. The manual is here. In section 10.6 it explains the voltage warning setting and 10.7 explains the warning status.

The voltage warning level is set by telling the maxflex the voltage in the format XY.Z, so you can dial in the precise voltage you want to be notified of.

The voltage warning status determines "how maxflex reports the low voltage condition." This is 1 blink every 1, 5, 10, 30 or 60 seconds, or no flash. You can also connect a 20mA LED to the stat connector on the driver for a low voltage indicator, instead of or in conjunction with blinking the main LED(s).

The maxflex is a truly remarkable driver. It's the only high end driver I've used so far, so I can't comment on any of the others. The number of modes and the ability to be reconfigured on the fly add so much versatility to any light you drop one in. If only they weren't $40 shipped!!! It was pretty nerve wracking touching a soldering iron to it!