Low-Voltage detection circuit

[MrAl]

Hello again,

FrogBoy:
That's great...i hope you can build this circuit
without too much trouble.

Doug:
You bring up another very good point Doug, as usual
The rationale here is that the draining current when
the light is off is compared to the self discharge
current of a typical NiMH cell. If this draining current
is a small percentage of the normal self discharge then
the drain current is considered acceptable, perhaps even
very acceptable. Now it just so happens that using one
LM339 chip turns out to be approximately 1/8 (that's
one eighth) of the self discharge of a 2000mAh NiMH cell,
so it's deemed 'acceptable'. Using the other comparator
chip specified with the schematic brings even that down
to a very acceptable level. Put another way, the self
discharge initially looks like about a 900 ohm resistor
while the comparator chip looks like around 7500 ohms.
The question then is how much faster do the cells drain
down with 900 ohms in parallel with 7500 ohms as compared
to the 900 ohms alone. Using the CMOS comparator doesnt
even show up on the chart

Of course the reference diode networks bring this up a
little too, about 50ua each. If that's not good enough
then a constant current source would have to be included
for each ref diode. The current decreases quite a bit
when the batterys drain down too.


Take care,
Al

 

[Doug Owen]

Al,

While what you say might be true in some cases, it's not for AAs. I read the graph as 20% loss in 20 days at 25C from self discharge. A 339 is close to one mA open circuit. That makes them about equal. A second 339 will double that, of course. Half or a third the standby lifetime depending. IMO significant.

Perhaps a small signal PNP could be rigged to shut the 339s down (although I'm not sure of the effect of the inputs at higher voltages is off hand.....).

Anyway, fun topic. Thanks.

Doug Owen

 

[pbarrette]

Hi all,

I may be talking straight out of my bum-hole here since I haven't really looked into undervoltage monitoring before, but what about something like the MAX6709/6714 IC?

They operate down to 2v and have 4 inputs for voltage monitoring and the datasheet schematics show the use of an absolutely minimal number of components. I can't seem to find them on digikey or mouser, so availability may be an issue here.

The package is a 10 pin uMax, so soldering would be tricky for those without a steady hand, but this looks like it could be the backbone for getting the job done very easily. The datasheet circuit also shows low voltage indicator LEDs for each input which seems to be one of the "nice touch" features discussed here as well.

Hope this helps,
pb

 

[FrogBoy]

MrAl,

Would it be possible to actually run a subset of this circuit against the entire voltage (e.g. 2.4 for 2 AA's) rather than for each battery, or is that not reccomended?

 

[MrAl]

Hello again,

Doug:
Thanks for bringing that up. I was looking at the supply current
per section, and there are 4 sections to one IC chip so that's
800ua per chip. That's about half of the self discharge, but
even though that's still somewhat acceptable i dont like another
spec on the LM339 for this particular app... The common mode
input range isnt all that good for what we are doing here,
while the LMC7225 has input rail to rail operation. This makes
the 7225 greatly preferred over the 339. Input current is
only around 5ua per chip, which knocks the current down
to very acceptable levels and at the same time puts the
input common mode range in a better area. This means
the LMC7225 should be the only chip used and the LM339
shouldnt be used at all for this circuit. The LP339
looks promising too, but again the common mode range
is crap
With all nice changes comes some downfalls, and this
update is no exception. The downside is the max
number of cells that can be used with this circuit
and the LMC7225 will be 5 cells, not six or more.
I'll update the schematic as soon as possible to
reflect this change.

pbattette:
Hey that's a nice chip, thanks for bringing it up.
If you find any more chips that look promising, please
let us know.
The only problem here is that when monitoring a number
of cells wired in SERIES we want to measure the cells
individually, and that really requires a differential
input voltage measurement. See, our circuit is really
just a "differential input to single ended output"
circuit. The 'differential' part means we need a
measurement of both tabs on an individual battery,
not just the positive tab. This requires a voltage
sensor that has two inputs (not including ground).
The Max chip has just a single input for each
channel so we cant really use that chip here, although
for other apps im sure it could be quite handy
(such as a single cell monitor) so i think we should
keep it in mind.

FrogBoy:
Well, in the case of a single measurement, you
can simply eliminate all of the batts and their associated
subcircuits leaving just the top section (batt 2 and it's
circuit).
The only problem with using a single measurement to watch
say two cells is that the cells have to be matched, and
have to drain down at almost the same rate. This is
pretty hard to find in actual practice. Say we set
the circuit to trip off at 2.00v (two cells in series
hoping to trip at 1.00v for each cell).
If the lower cell is 1.4 volts and the top cell drops
to 0.9 volts the total is 2.3 volts, still way over
the trip point. Although we dont want any cells
operating below 1.00v the top cell ends up operating
down to 0.6 volts in this case, which is not good
at all. That's why we are using individual
measurements instead of a single measurement for
all the cells.

Remember though that the preferred comparator
now is the LMC7225 chip, not the LM339. We're going to
have to drop the LM339 completely and im going to take
it off the schematic.


Take care,
Al

 

[FrogBoy]

MrAl,

How likely is it that the batteries would run down differently like that?

If the batteries are the same brand, and rating and have been always been charged/discharged together then in reality what is the likelyhood that they will behave in that manner?

 

[MrAl]

Hello there again FrogBoy,

Well, from what i've seen in NiCd batteries even with
a very carefully controlled charge it is more likely
that they run down at different rates then together.
I guess the reason for this is that the cells have a
slightly different storage capacity so one runs down
before the other even though they are both charged
to the same exact level to begin with.

One additional point is that among the sets ive seen,
although one cell runs down before the other the second
cell is close behind, meaning one cell follows the other
by some minutes (such as 5 minutes). With a setup that
only detected a single voltage for two cells, this would
mean one cell would be slightly undervoltage for maybe
5 minutes or so. Unfortunately i dont have any data
that relates run time undervoltage to cell lifetime, so
you'd either have to contact a battery manufacturer or
take a slight risk when using the single measurement method.
It's entirely possible that very little damage is done
when running only a few minutes undervoltage and it might
be worth a try, but i cant tell you to do that because
i simply dont know exactly if it will damage the cells or
not, and if so, what exactly the extent of the damage is.
I think there are people out there doing this already
so there might be some merit to it.

An experiment would, of course, be easy to set up.
You would charge two cells (probably individually)
to the same level and then wire them in series.
Measuring the voltage of each cell individually, run
them down at the same rate as your flashlight would
typically do.
While the voltage drops, wait for one cell to reach
the cutoff point and note how long it takes the
other cell to decrease to the point where the series
voltage is under your cutoff v times two.
For example, say you want 1.1v cutoff. This means 2.2v
for two cells. When you drain them down, one cell might
reach 1.1v while the other is 1.15v, which of course still
isnt a total voltage of 2.2v yet, but one cell is now
about to run below the cutoff point so note the time.
After a little while longer, the higher cell goes down to
maybe 1.12v while the lower cell goes down to 1.08v .
Now the series combo is down to 2.2v so note the time
again. Subtracting the times will show the time that
the lower cell has been running while undervoltage.
Since 1.08v (in this case anyway) isnt very much lower
then 1.10v i wouldnt expect much damage to the cell.
On top of that, if it's only been running for 5 minutes
like that i cant see a problem with the single cell
method.
The only other thing to consider next is what happens
once the cells begin to age after many charge cycles.
If once cell gets 'worse' then it was, then it may be
running at a low voltage for an extended period of time,
but it's hard to say just how long that would be, and
since the cells aged anyway it might not matter as much.
There is always the chance that the higher cell ages
differently then the lower cell and 'catches up' with it

The choice is really yours, and what you are willing to
try out. Perhaps you can monitor the cells once every
two months or so to get an idea how well they are
tracking each other.

It should go without saying that they should both be
fully charged, and if you have good enough control
over the charge for each battery you can perhaps adjust
the charge in the higher voltage battery to make it
track the lower one a little better. You can also
put an adjustment on the voltage detect circuit
to compensate for a battery that has consistantly
higher voltage then the other(s).

Now you have an idea what this problem entails and why
we are using differential voltage measurements instead
of just a single top battery voltage measurement.

Any comments are welcome

Take care,
Al

 

[FrogBoy]

MrAl,

Just ressurecting this thread /ubbthreads/images/graemlins/smile.gif

So what do other CPFers do, or do they not use rechargeables in their flashlight mods?

 

[MrAl]

Hello again Frogboy,

Oh that's good...maybe we'll hear from some other people
thinking about low voltage battery protection circuits
and related.
I guess there might be a thousand and one designs for
this sort of thing

Take care,
Al

 

[moraino]

Just came across this circuit from Reed Electronics.

Circuit protects battery from overdischarge

http://www.reed-electronics.com/ednmag/article/CA74431?pubdate=4%2F12%2F2001&spacedesc=designideas

I have no idea where to get this IC.

It'll be great if someone can workout a circuit that most of the horwire guys here can apply to their mod lights.

Henry

 

[MrAl]

Hello there,

Hey that's pretty neat.

I noticed they didnt show a hookup for multiple
cells however. This would take some doing because
the outputs would all be referenced to higher and
higher voltages. For example, the second detector
output would be referenced to 1.1 volts, while the
third detector output referenced to 2.2 volts.
I guess they expect you to use those mosfets to
logically 'AND' the outputs, possibly with one drive
resistor per MOSFET, but probably more like two.
I guess that would simplify the circuit a little
but not as much as i hoped, except of course for
one single cell.
The maker of the ic is also shown as a link on that page
too.

Would be nice if they had one for two cells, three cells,
four, etc.

Take care,
Al