Monitoring (+) & (-) Current

Archiver

I would like to monitor/log the charge and discharge current
from a 36-volt battery bank using the Stamp. I am planning
on sensing the current using a 150 amp/50 mv shunt [noparse][[/noparse]which
I have] as the expected range is roughly from (+) 150 amps
[noparse][[/noparse]discharge] to (-) 25 amps [noparse][[/noparse]charge]. I also thought of using
a AD623 instrumentation amplifier [noparse][[/noparse]which I also have]
because it has provision for a reference voltage input. I
assume that this could be used to offset the +/- input and
hence convert the (-)8 mv to (+)50 mv signal to a 0 to 5
volt input to an a/d converter and the Stamp. My limited
electronic engineering skills however draws a blank as to
the details of the circuitry involved. The shunt can be
placed anywhere between the 6-6 volt batteries in the bank
and it would be convenient if the battery bank could also
supply the power for the amplifier/a-d converter/Stamp. A
separate supply is not out of the question however. Any
help would be appreciated.

Cheers, David R. Erskine

Ps. Note that this is the second query from me pertaining to
monitoring voltage and current from a bank of batteries. I
felt that keeping the queries separate would help with any
subsequent discussion and future reference.

Archiver

Take a look at Dallas DS2760 1-wire device. It can measure your current.
Chuck

Archiver

I'm using an AD620 for measuring shunt current (100 amp/50 mv shunt) from a
12 battery. I have the shunt in the -ve lead, with the sensing wires going
to the inputs of the AD620. Works like a charm. Although I am now using a
MAX 186 A/D converter, which handles bipolar signals, I did play with the
reference input to the AD620 to convert the signal to unipolar - it worked.
You have to calibrate things, of course, to determine what A/D reading the
Stamp has to use as zero.

Powering the A/D and Stamp from the battery bank via a 5 V regulator should
be no problem - just make sure you get one that can handle the battery
voltage when under charge - probably 14.4 volts *3.

Now as to your previous question - monitoring the voltage of each of the 6v
batteries independently.

You will want to use another instrument amp (or a good op amp in a
differential configuration with carefully matched resistors - this is what
I am using). It will have a gain of less than one (to convert the 6 v max
battery voltage down to the range the A/D can handle), ot you can use a
gain of one and follow the amp with a voltage divider to achieve the same
effect.

Suppose you are only monitoring the 1st 6v battery - you will want to
connect the differential amp across the battery terminals (NOT one to the
+ve terminal and one to ground) to eliminate the voltage drops associated
with both the shunt and the rest of the wiring in the -ve lead of the
battery bank.

Now to monitor the 2nd battery, you would connect (via some switching
arrangement - we'll talk about this later) the differential amp inputs to
that battery. And so on, up to the 6th battery.

Looks easy - you could use relays or a rotary switch. However, there is one
problem - the "common mode" voltage.

Take the case of measuring the last battery. The differential amp will see
+36 volts on one input, and 30 volts on the other. The differential voltage
is 6 volts, but the common mode voltage is 30 volts.

Not only do you need to use an op amp that can handle those voltages on the
input (the AD623 won't), but you need to have an op amp configuration that
can handle a large common mode signal. If you used an instrumentation amp
such as the AD623, it will have a common-mode rejection ratio specified -
for example 80db. Thus a common mode signal will be reduced by a factor of
10,000 - your 30 volt common mode signal will be reduced to 3 mv, which
will appear as if your real input was 3 mv too high (or two low). Thus your
6 volt measurement could be out by 3 mv.

Now if you don't use an instrumentation amp, then you need to use an op amp
in a differential configuration with four carefully matched resistors -
either by using high precision resistors, or matching (for example) 1%
metal film resistors. I doubt if it is possible to match them to the degree
required.

Now as to the switching - there are analog multiplexors available that can
do this - for example, select from 4 different pairs of inputs. The problem
again is going to be the high voltages - I don't know if there are any
around the will work at 36 volts or so. You can also build analog switches
using FETs, but you'll need quite a few.

You have chosen a non-trivial project, with a few hurdle to overcome.

Spend some time browsing the web sites of the IC companies such as Analog
Devices, National, Maxim, Linear Technology, etc to see what they have and
what the limitations of the devices are.

I suggest you check your public library for a copy of "The Art of
Electronics" by Horowitz and Hill. It is an excellent book that covers all
of the topics involved here, plus a pile more.

Have fun

Larry


At 02:10 PM 2/21/2003 -0500, you wrote:

>I would like to monitor/log the charge and discharge current
>from a 36-volt battery bank using the Stamp. I am planning
>on sensing the current using a 150 amp/50 mv shunt [noparse][[/noparse]which
>I have] as the expected range is roughly from (+) 150 amps
>[noparse][[/noparse]discharge] to (-) 25 amps [noparse][[/noparse]charge]. I also thought of using
>a AD623 instrumentation amplifier [noparse][[/noparse]which I also have]
>because it has provision for a reference voltage input. I
>assume that this could be used to offset the +/- input and
>hence convert the (-)8 mv to (+)50 mv signal to a 0 to 5
>volt input to an a/d converter and the Stamp. My limited
>electronic engineering skills however draws a blank as to
>the details of the circuitry involved. The shunt can be
>placed anywhere between the 6-6 volt batteries in the bank
>and it would be convenient if the battery bank could also
>supply the power for the amplifier/a-d converter/Stamp. A
>separate supply is not out of the question however. Any
>help would be appreciated.
>
>Cheers, David R. Erskine
>
>Ps. Note that this is the second query from me pertaining to
>monitoring voltage and current from a bank of batteries. I
>felt that keeping the queries separate would help with any
>subsequent discussion and future reference.
>
>
>
>
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Larry Bradley
Orleans (Ottawa), Ontario, CANADA

Archiver

>I would like to monitor/log the charge and discharge current
>from a 36-volt battery bank using the Stamp. I am planning
>on sensing the current using a 150 amp/50 mv shunt [noparse][[/noparse]which
>I have] as the expected range is roughly from (+) 150 amps
>[noparse][[/noparse]discharge] to (-) 25 amps [noparse][[/noparse]charge]. I also thought of using
>a AD623 instrumentation amplifier [noparse][[/noparse]which I also have]
>because it has provision for a reference voltage input. I
>assume that this could be used to offset the +/- input and
>hence convert the (-)8 mv to (+)50 mv signal to a 0 to 5
>volt input to an a/d converter and the Stamp. My limited
>electronic engineering skills however draws a blank as to
>the details of the circuitry involved. The shunt can be
>placed anywhere between the 6-6 volt batteries in the bank
>and it would be convenient if the battery bank could also
>supply the power for the amplifier/a-d converter/Stamp. A
>separate supply is not out of the question however. Any
>help would be appreciated.
>
>Cheers, David R. Erskine

Hi David,

Part 2. One way to do this would be to put the shunt in the ground
side of the battery stack, and then monitor its voltage directly with
your ADC. This would give only about 2% full scale resolution.

ground ---/\/\---o---6V+--o---6V+-- ....
shunt | |
|
`--- to ADC input

Better resolution would require an amplifier between the shunt and
the ADC. It would get complicated, because you want to measure both
charge and discharge current.

There are chips meant for charge/discharge monitoring, that have the
amplifier built in, for example, the MAX471/MAX472 (the '471 has the
shunt built in, while the '472 uses an external shunt) To use those
with the Stamp, you would have to place the shunt between the first
two batteries, so it would operate at ~6 volts. One nice thing
"sign" output that indicates which direction the current is flowing,
which you would connect to a Stamp input so you know if the battery
is charging or discharging. I have a short writeup on using the
MAXxxx at this URL. http://www.emesystems.com/BS2rct.htm#B_current.
It talks about using RCtime as the analog to digital converter with
the chip, but it works fine with a resistor into a standard ADC.


Another chip to look at is the new ZXCT1009 from Zetex. (Digikey).
It is a three-terminal current monitor for resistive shunts. You
would need two of them, one for charging, one for discharging.

-- regards,
Tracy Allen
electronically monitored ecosystems
mailto:tracy@e...
http://www.emesystems.com

Archiver

Could the Amploc Hall Effect Current sensor work for this? It can be fed into a
ADC and it has an offset where 0 amps ouputs 1/2 the supply voltage.

http://www.ampsense.com/AMP200.html

Hank Hagquist