Battery level tester.

[Colin Dawson]

Hi all

I've got a circuit that I use to measure the voltage of a battery. The idea
is that it will give me plently of warning of when the battery level is
getting low. It works great for the actual circuit design, but there's a
problem that I've got when it's in use. First I'll show you the Circuit
design.

<http://www.cjdawson76.btinternet.co.uk/images/astronomy_batterymonitorfulls
ize.jpg>

To explain the application a little futher. The 12V Battery is an 85Ah Lead
Acid Leasure battery (For use in boats and Caravans). I use it for my
Astronomy Hobby to power, my Telescope, Laptop, Dew heater (like a cars rear
window heater) and all the other toys that I need during an observing
session.

Although I've not measured anything specific I think that when everything is
turned on as the same time, I'm pulling about 10Amps. Which means there's
enough juice for a whole nights observing.

I've got a problem with the battery meter, in that as I turn stuff on, and
the amount of current drawn increases, the meter shows a voltage drop, and
since the difference between a full and empty battery reading is about 2V
(12V= full 10V = empty) this shows a significant drop on the readout.

Is there something that I can do to counter balance this high current
voltage drop?

My thought on this is to place a kind of Ammeter into the circuit, that will
adjust the value of VR1, in accordance to the amount of current drawn
through the circuit. Changes in current would would effect this part of
the circuit, and continuously trim the battery meter, so that the readout
remains stable. (and hopefully correct)

Regards

Colin Dawson
www.cjdawson.com

 

[martin griffith]

On Sat, 25 Sep 2004 15:17:57 +0000 (UTC), in sci.electronics.design
you wrote:

Hi all

I've got a circuit that I use to measure the voltage of a battery. The idea
is that it will give me plently of warning of when the battery level is
getting low. It works great for the actual circuit design, but there's a
problem that I've got when it's in use. First I'll show you the Circuit
design.

http://www.cjdawson76.btinternet.co.uk/images/astronomy_batterymonitorfulls
ize.jpg

To explain the application a little futher. The 12V Battery is an 85Ah Lead
Acid Leasure battery (For use in boats and Caravans). I use it for my
Astronomy Hobby to power, my Telescope, Laptop, Dew heater (like a cars rear
window heater) and all the other toys that I need during an observing
session.

Although I've not measured anything specific I think that when everything is
turned on as the same time, I'm pulling about 10Amps. Which means there's
enough juice for a whole nights observing.

I've got a problem with the battery meter, in that as I turn stuff on, and
the amount of current drawn increases, the meter shows a voltage drop, and
since the difference between a full and empty battery reading is about 2V
(12V= full 10V = empty) this shows a significant drop on the readout.

Is there something that I can do to counter balance this high current
voltage drop?

My thought on this is to place a kind of Ammeter into the circuit, that will
adjust the value of VR1, in accordance to the amount of current drawn
through the circuit. Changes in current would would effect this part of
the circuit, and continuously trim the battery meter, so that the readout
remains stable. (and hopefully correct)

Regards

Colin Dawson
www.cjdawson.com

Hmmmmm.... 85Ah at 10A for a night, not nice. Look up depth of

dishcharge (DOD). You are going to ruin the battery very quickly at
this rate of discharge for a 85A battery.

A few links
http://www.varta-automotive.com/eng/index2.php?p=2&s=4&content=produkte/antrieb_beleuchtung/antrieb_beleuchtung.html
http://www.batteryuniversity.com/parttwo.htm
http://www.uuhome.de/william.darden/
http://www.yuasabatteries.com/pdfs/TechMan.pdf
from the last link
"Deep discharge or prolonged discharge leads to harmful sulfation."




martin

Serious error.
All shortcuts have disappeared.
Screen. Mind. Both are blank.

 

[John Woodgate]

I read in sci.electronics.design that Colin Dawson <news@cjdawson.com>
wrote (in <cj4275$8ht$1@titan.btinternet.com&gt about 'Battery level
tester.', on Sat, 25 Sep 2004:

I've got a circuit that I use to measure the voltage of a battery. The
idea is that it will give me plently of warning of when the battery
level is getting low. It works great for the actual circuit design,
but there's a problem that I've got when it's in use. First I'll show
you the Circuit design.

http://www.cjdawson76.btinternet.co.uk/images/astronomy_batterymonitorf
ulls
ize.jpg

That shows your 'voltmeter' but it doesn't show the loads you connect to
the battery, and that matters; see below.

To explain the application a little futher. The 12V Battery is an 85Ah
Lead Acid Leasure battery (For use in boats and Caravans). I use it for
my Astronomy Hobby to power, my Telescope, Laptop, Dew heater (like a
cars rear window heater) and all the other toys that I need during an
observing session.

Although I've not measured anything specific I think that when
everything is turned on as the same time, I'm pulling about 10Amps.
Which means there's enough juice for a whole nights observing.

I've got a problem with the battery meter, in that as I turn stuff on,
and the amount of current drawn increases, the meter shows a voltage
drop, and since the difference between a full and empty battery reading
is about 2V (12V= full 10V = empty) this shows a significant drop on
the readout.

Presumably, though you don't tell us, the meter is not connected
directly to the battery terminals but downstream towards the loads. What
you are observing is the voltage drop in the wiring due to the 10 A or
so that is flowing.

Is there something that I can do to counter balance this high current
voltage drop?

Yes; you need THICK wires. Since I don't know how long your cable runs
are, I'll give you a possibly overkill solution. Think 6 mm^2 or 10 mm^2
twin and earth from your local electrician. This is probably the
cheapest; if you tried to buy it from a supplier you would probably have
to buy 50 m or 100 m or pay through the nose at Homebase or B&Q.

You will get *some* voltage drop even at the battery terminals, but it
should be very small at 10 A if the battery is in good condition and
properly charged.

My thought on this is to place a kind of Ammeter into the circuit, that
will adjust the value of VR1, in accordance to the amount of current
drawn through the circuit. Changes in current would would effect this
part of the circuit, and continuously trim the battery meter, so that
the readout remains stable. (and hopefully correct)

No, all that would do is to alter the calibration of your 'voltmeter' so
that 10 V appeared to be 12 V. I don't think you would be satisfied with
that!
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[Rich Grise]

On Monday 27 September 2004 03:27 pm, Clarence did deign to grace us with
the following:

"John Woodgate" <jmw@jmwa.demon.contraspam.yuk> wrote in message
news:F5sE4MN+QIWBFwMc@jmwa.demon.co.uk...
I read in sci.electronics.design that Clarence <no@No.com> wrote (in
cI_5d.20471$QJ3.2996@newssvr21.news.prodigy.com&gt about 'Battery level
tester.', on Mon, 27 Sep 2004:

The Volt meter must be connected directly to the battery terminals to be
a reasonable indication of charge.

Yes, IF that was what was *needed*, as opposed to *wanted*. The OP
*needs* to check that the battery isn't soon going to not be able to
work his telescope and stuff, and the way to do that is to monitor the
voltage at each load. At each load, because I strongly suspect he has
significant voltage drops in his cables.

If, for example, the telescope doesn't work below 10 V, a red light that
comes on when the voltage at the telescope is 10.3 V would do what the
OP needs, whether that 10.3 V is coming from a new 200 Ah battery or a
small VLA that's five years old.
--

So he really needs a new battery, (or perhaps run 24 volts,) and a
regulated
dc to dc remote sense, regulated converter?

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=46412&item=4327490986&rd=1



Cheers!
Rich

 

[R Adsett]

In article <OIadnSHptqpl9cXcRVn-gA@comcast.com>, dvanhorn@cedar.net
says...

Battery capacity metering is not anywhere near as simple as it appears at
first glance.
TI has some offerings that will help.

You can't simply relate voltage to usable capacity remaining, even when the
load is a simple resistor. When the load is dynamic, it's worse.

Which is why I suggested looking at a Curtis gauge. Maybe not as

satisfying as building one yourself in some ways, but they do work. They
work well under dynamic loads (they are designed for electric vehicles
and for vehicles like forklifts the current ranges from tens of amps
running to hundres under peaks loads (and of course 0 when off)). They
will not work under fast discharges, but in that use a clock might be
more effective.

They do have 12V versions and you may be able to pick up a used one out
of a scrap personal mobility vehicle.

They are voltage devices and only run two leads to the battery.

Also contrary to the expectations of some of the posters I suspect it
would solve both the stated and unstated issues. It would give a steady
'fuel level' reading unlike the voltmeter. Also since the voltage
available under load will be a function of the available capacity of the
battery it will also serve as an advance warning of the point where it
will be unuseable to drive a heavy load (telecope motor, laptop
invertor). This point may just be above the 'empty' point on the gauge
that's all. A voltage comparator might work, but you would have to set
it so that it would flash a light on when the voltage under load was
getting low which leaves the real possibility that it would only indicate
a problem at the same time that the device you were running failed
because the voltage dropped too low.

A reference to Curtis Instruments

http://www.curtisinst.com/index.cfm?fuseaction=cProducts.dspProductCatego
ry&catID=15

These are probably the gold standard gauges used for EVs. There other
capacity gauges I know of that match them for performance but they are
buried in drive controllers. Amp hour meters (at least theoretically)
offer greater accuracy but are considerably more expensive, and since the
capacity varies depending on load and enviroment (and the integral of
current accumulates errors) they still need a voltage algoritm to provide
a backup.

There are also gauges available that are intermediate between simple
voltmeters and the Curtis gauges.

Robert

 

[ChrisGibboGibson]

soar2morrow@yahoo.com (Tom Seim) wrote:

chrisgibbogibson@aol.com (ChrisGibboGibson) wrote in message
news:<20040927184451.04585.00001280@mb-m02.aol.com>...
soar2morrow@yahoo.com (Tom Seim) wrote:

Won't anyone give me a straight answer on how the hell to build an
Ammeter

Here:

http://www.linear.com/pdf/LTC4150_0504_Mag.pdf

You're welcome.


The 4510 is only good to 8.5 volts and as the sense resistor is in the
positive
battery lead it's not such a simple job to place it elsewhere. Not
impossible,
but probably so for OP.

Anyway I think JW is right.

Gibbo

The sense resistor can be put on the low side.

Tom

You'd better tell Linear Technology to correct their data sheet then.

Gibbo

 

[John Fields]

On Tue, 28 Sep 2004 11:17:13 -0400, R Adsett
<radsett@junk.aeolusdevelopment.cm> wrote:

A voltage comparator might work, but you would have to set
it so that it would flash a light on when the voltage under load was
getting low which leaves the real possibility that it would only indicate
a problem at the same time that the device you were running failed
because the voltage dropped too low.

---
That's why I suggested three LEDs; green = OK, yellow = You're getting
close, and red = You're there!

--
John Fields

 

[John Crighton]

On Sun, 26 Sep 2004 09:22:59 +0000 (UTC), "Colin Dawson"
<nospam@cjdawson.com> wrote:

I think everyone here has managed to completly miss the point of what I'm
asking.


There is enough voltage in my setup to power everything that I want to
power. The cables are thick enough to power everything that I want to
power, and then some. (I can easily double the number of devices without
ricking overrating the stuff that I've used. I've done it)

At the moment my "Battery Monitor" is actually a "VoltMeter". I don't want
a VoltMeter connected to the battery, as is doesn't tell me when it's time
to start thinking about recharging the battery.

What I want is a "Battery Level Meter". Just because I start pulling 10A
from my battery doesn't mean that it's capacity suddenly drops, as a
VoltMeter shows. I don't care what the Voltage of the battery is. I WANT
TO KNOW WHEN I NEET TO START TURNING OFF DEVICES BECAUSE THE DAMMED BATTERY
IS ALMOST FLAT AND MY TELESCOPE IS ABOUT TO LOOSE IT'S ALIGNMENT. (at
this point Colin has thrown his teddy out of the cot)

Get the point now?

Won't anyone give me a straight answer on how the hell to build an Ammeter
circuit, so that I can get the "BATTERY LEVEL MONITOR" to give a correct
reading?

Regards

Colin Dawson
www.cjdawson.com


Hello Colin,

I think I know what you are on about. You want some warning
that your battery is about to fail/run out of capacity. Is that it?

I saw a little circuit that was used to test lead acid back up
batteries, that were on constant trickle charge. Every hour
a heavy load, lasting about a millisecond was connected
across the battery bank. An op amp was used as a comparator.
One input was connected to the battery using a suitable divider
and the other input was connected to a referece voltage.

If the battery voltage drooped below a preset level when
the heavy current flowed in the load, the comparator's output
triggered a buzzer. The workers then had a warning that
one or more of the batteries in the bank was not performing
well and would be no good for back up power when required.

How does that help you Colin?
Well lets say that you made up a similar rapid discharge
tester with a load resistor, in series with a power mosfet
across the battery. Some 555 timer ICs could apply a similar
1 millisecond pulse to the mosfet every 10 or 15 minutes
(or whatever time period you like.)
The 50 amp or so load resistor can be made up of cheap
wire wound types of several watts, it doesn't have to be
super big, power wise, because it only acts as a load for a
millisecond. Trial and error will show you that the load resistor
can be surprisingly small. 1 millisecond every 10 minutes is a
low duty cycle.


You mentioned a 10 amp load for your telescope setup,
so connect some old car headlights up to your battery
and time how long it takes your battery to discharge to
the point where the buzzer on the rapid discharge tester
starts to sound, meaning that the internal resistance of
the battery is rising, meaning the battery is not so good
anymore, nearing exhaustion.

The problem with this idea is that towards the end
of the batterys life, the one millisecond rapid discharge
tester would cause voltage glitches that could upset
your computer. You would have to run a number of trial
and error tests and adjust the thresh hold of the warning
signal to suit yourself.

That is all a bit crude, I know, but it is cheap and
will give you a bit of an idea how much time you
can go on using your battery. You know roughly
how much time is left from your trial and error tests
with the car headlamp as a load.

Regards,
John Crighton
Sydney

 

[Colin Dawson]

"Charles W. Johson Jr." <qrus19@mindsprUng.com past to present> wrote in
message news:hXl6d.1514$Yr.132@newsread3.news.pas.earthlink.net...

"John Fields" <jfields@austininstruments.com> wrote in message
news:ap9jl057cftp6tgrhaqi6v4q6pgphj1hji@4ax.com...
On Tue, 28 Sep 2004 11:17:13 -0400, R Adsett
radsett@junk.aeolusdevelopment.cm> wrote:

A voltage comparator might work, but you would have to set
it so that it would flash a light on when the voltage under load was
getting low which leaves the real possibility that it would only indicate
a problem at the same time that the device you were running failed
because the voltage dropped too low.

---
That's why I suggested three LEDs; green = OK, yellow = You're getting
close, and red = You're there!

--
John Fields

My take on the problem is he's using an unregulated power supply and
trying to read a somewhat constant output. Anytime he changes the load the
meter bounces wildly. Just a little basic regfulation at the meter would
give him what he seems to want.

Charles

This sounds alot simpler than the idea that I had. It'll more than good
enough if it stops the laptop's harddrive causing the bounce. Does anyone
have any suggestions on how to do this? Ideally, I don't want to use a
seperate powersupply from my big lead acid battery.

Regards

Colin Dawson.