Universal LOW-batt switch-off circuit. - Page 1
Forum section: electronics / projects

author: jippie
author_rank: Unknown
timestamp: March 20, 2014, 06:56:38 pm
content: I regularly use rechargeable batteries for various little projects and over time I have had quite a few batteries that have died due to too deep discharge. Now I was thinking about making a small universal automatic power off switch that disconnects the batteries from the load, but can be started with a simple push of a button. The idea is not mature though and I was hoping some of you would have a smart idea how to accomplish it.The idea is to use a CMOS latch (40xx-series, maybe couple NAND-ports suffice) to keep current state of the switch. When used as a static ON or OFF the CMOS device won't draw current, which is a good property. The latch can then drive a (logic) MOSFET to allow for a larger load. I need a comparator that compares battery voltage with a given low voltage (say 4.0 - 4.4V for a quad-AA battery pack, but should ideally be easy to adjust), the comparator should be taken off power together with the MOSFET to detach as much load from the batteries as possibleandI need some sort of reference voltage that is reasonably accurate for this goal, but doesn't draw too much current.At this point in time I am looking for smart suggestions, I'm in the brainstorm phase. Does anyone know of an existing circuit (that would simplify things a lot) or have any suggestions on well available low power comparator / reference voltage parts? Suggestions for smart Google search terms are welcome too, because the results I found so far search results haven't been too helpful.

author: Andreas
author_rank: Super Contributor
timestamp: March 20, 2014, 10:30:45 pm
content: Hello,ICL7665 is your friend.Programmable comparator points + hysteresis.On the other side you could use a low power microprocessor (in SO-8 / SOT23-6 package) which sleeps most of the time and wakes up all seconds to check the supply voltage. This will give around 1uA average current consumption.With best regardsAndreas

author: Mr Smiley
author_rank: Frequent Contributor
timestamp: March 20, 2014, 10:39:47 pm
content: http://danyk.cz/p_ochr_en.html

author: jippie
author_rank: Unknown
timestamp: March 21, 2014, 06:34:30 am
content: Quote from: Mr Smiley on March 20, 2014, 10:39:47 pmThat looks like a smart solution indeed, I definitely have to check that one.Quote from: Andreas on March 20, 2014, 10:30:45 pmI feel pretty comfortable with AVRs, but the downside is that they require a voltage between 2.5(ish) and 5V. I can't use the circuit for 4 or more batteries without a regulator.Quote from: Andreas on March 20, 2014, 10:30:45 pmICL7665 seems an all in one solution (apart from the 'power'-MOSFET), good tip.

author: muvideo
author_rank: Frequent Contributor
timestamp: March 21, 2014, 07:49:50 am
content: Quote from: jippie on March 20, 2014, 06:56:38 pmHello, I'm not answering your question, others have,but I'd like to know what chemistries you use and how.Deep discharge protection depends on chemistry, andchemistry depends on use you make of the battery(on board charger, fast-slow charge, temperature,voltage etc...)AFAIK NiMh are pretty immune to deep-dischargedamage, and last "ready" ones (low self discharge)seem good canditates for electronic circuits power supply.Lithium and Pb ones are damaged by deep discharge,and must be protected.For me the downside of NiMh vs Li or Pb is charge management,single cell Li or Pb are very easy to charge, the criticalparameter for end of charge detection is voltage that is easyto monitor with accuracy.For NiMh I prefer tu use external charger for fast charge(less than 6-7hours), slow trickle charge can be used onlywhen is acceptable to wait 10hours or so for a full charge.

author: jippie
author_rank: Unknown
timestamp: March 21, 2014, 04:53:58 pm
content: Quote from: muvideo on March 21, 2014, 07:49:50 amI mainly use NiMH and have a track record for forgetting to timely remove the batteries and charge them. Batteries only seem to die after too deep discharge, while some of them can be 'rescued' by retrying couple times.

author: muvideo
author_rank: Frequent Contributor
timestamp: March 21, 2014, 05:04:51 pm
content: Quote from: jippie on March 21, 2014, 04:53:58 pmSo you have unrecoverable elements due to overdischarge?That's interesting, I suppose is due to cell reversal due tounequal capacity. Does it happen with high cell count and/orhigh discharge rate?

author: jippie
author_rank: Unknown
timestamp: March 21, 2014, 06:30:29 pm
content: Quote from: muvideo on March 21, 2014, 05:04:51 pmI usually use 4 AA cells and never mix sets.

author: npelov
author_rank: Frequent Contributor
timestamp: March 22, 2014, 10:39:14 pm
content: I bought 3 sets 4 AA batteries SANYO NiMH(labeled made in japan). I was very careful with then, didn't drop them under 1V, I rotated them to wear them evenly and never mixed sets. None of the sets was even after 3 months and few refresh cycles didn't fix it. All 3 sets had batteries that reach 1 V when others were 1.1 or even 1.2V. Before charging I always drained them to 1V. I hate NiMH.I tried the same thing with cheap chinese Li-ion batteries. They were relatively the same capacity and ware out evenly. Well, at least better than NiMH.

author: dannyf
author_rank: Super Contributor
timestamp: March 22, 2014, 10:53:01 pm
content: There are tons of such things all over the web. The tricky part of those designs is so that it is latching. Otherwise, it may oscillate.The simplest would be a relay + resistor. Pick the combination so that it would cut out after the voltage becomes insufficient to latch the relay anymore.