Tweaking protection pcb for 3.0V low voltage cut-off

I’ve been googling for this for a while. I have a - poorly designed - circuit (actually it’s a usb powerbank, no leds involved this time…) which has no low voltage cutoff so I want to add one. Actually i’m running it with a protected 18650, but i still want an external cutoff because:

1) 18650 protection board cutoff kicks in at 2.5-2.4V which i think is definitely too low for daily use
2) a protection circuit should be the last resort in case something goes wrong, i don’t think it should be used on a regular basis (and i don’t know how reliable they are)
3) battery integrated overdischarge protection must not prevent the battery from operating correctly in any operating scenario, either under low or high loads… that means the cutoff is set to a very low value to avoid that with high discharge currents (high voltage sag) it kicks in before the battery has discharged completely

It looks like a simple and common problem, but apparently the solution is not as simple. I’ve found many threads on forums and many pages… a lot of talks and very few tested circuits, but here’s a couple of interesting links:
From The Readers: Low Battery Cutoff Solutions | Hackaday (read the comments as well)
http://shaddack.twibright.com/projects/method_SimpleLiPolyOverdischargeCutoff/

many of those circuits require microcontrollers, which i don’t want to use, and the simple ones have all have some drawbacks that made me discard them. So what is left are dedicated protection boards.

But again, a couple suggested in the blog post linked above Accessories - PCB - 1S (3.7) Li-Ion/LiPO - Tenergy Power looks great having the 2.9-3.0v cutoff, but that shop is only a solution for people living in the us. As i live in europe i’ve had a look to protection pcbs on ebay and aliexpress, cheap and easy, but i can only find circuits with cutoff at 2.5v or 2.4v while something around 3.0v is needed… if anyone finds something it would be much appreciated

SO:
here’s my idea, both to share and to ask for opinions (i have just a very basic electronic knowledge)… since finding a board programmed for 3.0v cutoff seems too complicated, i thought to tweak the common 2.5 cutoff board that i can easily steal from a any 18650 i have laying around or just buy a bunch of them on aliexpress for as low as 1$ shipped or less:

http://www.aliexpress.com/item/Single-18650-lithium-battery-protection-board-protection-plate-DW01/32309641658.html

Then I can use a voltage divider with just 2 resistors to set the kick in voltage wherever i want, instead of connecting the board reference directly to the positive electrode… of course resistors in the Kohm-Mohm range would be needed to avoid battery drain, and it’s probably a good idea to add a capacitor when using so high resistor values for the reason explained here http://jeelabs.org/2013/05/16/measuring-the-battery-without-draining-it/ (i don’t know much about microcontrollers, but i guess this high impedance thing might be a common issue).

Any suggestion/better idea?

My suggestion is ditch the lousy powerbank and get one that doesn’t suck. If it doesn’t have low-voltage cut-out then I’d be very surprised if the charge termination voltage was very accurate, or the boost-converter was very efficient.

The Miller ML-102 isn’t very expensive. But, if this is the electronics project you want to spend your time on, please disregard my suggestion.

actually the low voltage cutoff is the only thing I have to worry about: i’m not going to use the circuit for charging the battery (it doesn’t even have that option, i just wanted a switching regulator with decent output and efficiency)

here is my circuit by the way:

http://www.ebay.it/itm/391019124618?ssPageName=STRK:MEWNX:IT&\_trksid=p3984.m1439.l2649

Also the efficiency seems ok to me, i’ve measured 81% at 1 A output (5,0W/6,1W)

I didn’t know about that miller charger, thanks for the suggestion :wink:
Here is a good review by HKJ Review of Charger ML-102
500 mA of output is too low, but the new revision seem to have over 1A output (and hopefully they have mainted all the other good characteristics).

But, yes, as you said i’d rather play with what i have and learn something new… but i’ll suggest that Miller for friends :wink:

The newer version (7.1) of the miller claims 2A output, but the ones I have freak out at 2A and start cycling on and off when the battery voltage drops to ~3.75v. Another forum member reported that they saw similar behavior in some but not all of the ones they had. On the other hand, it does great at 1-1.5A, and the charging seems good. I haven’t traced a charging curve, but it seems to terminate accurately and charge quickly, so it doesn’t seem like it switches to CV prematurely.

One potential downside is that I don’t think they ship with resistors in place to indicate USB charging rate.

Good to know, but that’s nothing… also mine didn’t have it (as you can see from the picture, Data+ and Data- are isolated) and was limited to less than 500 mA, then I added a 200 ohm resistor.