{req} Cable Length for a amp meter shunt Resistor Please

[PaulC]

i will need 2 x shunts

1st shunt needs to be 0.33R
2nd shunt needs to be 2R2

Cable is 4mm sq 6 AWG (this is the thickest single core copper i could find, as need High Current)

Cable resistance is 0.3951 per 1000 feet

so please could someone calculate the required length of 4mm (6AWG) Copper @ 20DegC if you need an average temperature.

Problem is i've looked on net and seen that many calculations that give different lengths it's unreal.

Added new drawing..
Hope it will be of use

UPDATED 11-04-2012

[Magnox]

You're going to need some long shunts...

Simply:

for 0.33Ω, length = 1000 x 0.33/0.3951 = 835.23 feet
for 2.2Ω, length = 1000x 2.2/0.3951 = 5568.2 feet


Out of interest, if you were using pure copper:

From R = ρl/A,

Where:
R = Resistance, Ω
ρ = resistivity (copper=1.68x10^-8), Ωm
l = length, m
A = cross-sectional area, m²

l = RA/ρ

so for 0.33Ω, length = 0.33 * 0.004² / 1.68x10^-8 = 314.3m

to check... 31430/2.54/12 = 1031 feet, so at least you are better off with your cable than with pure copper.

Edit: Oh, by the way, 6AWG according to my tables is 4.12mm diameter, =13.302mm², but I think the difference isn't going to be an issue. The 3/4 miles of cable might be though.

I couldn't resist (pun) this...

you need about 280kg of copper cable (not including the insulation) worth about $2350 at today's market prices. Wow - that's some shunt!

Edit again:

Perhaps of more help is this: the ACS750ECA-100 from Allegro. It's a hall-effect 100A current sensor with a ratiometric (vs. supply voltage, 5V) output in a sort-of TO-220 package. A bit smaller than the cable will be. I have a handful in my component stock - they are nice devices.

Just how much current are you thinking about? Allegro do devices for up to 1000A 

[PaulC]

maybe i could link in to telegraph poles..
on a series note though in the attached diagram
res Risns (current sense) for bq24450, because charger could go upto > 30 Amps i was going for 4mm cable shunt.
so maybe i will make a variable shunt as current on rsense legs is small i could adapt variable res in series.
thanks for this reply.
it seems my maths wasn't that far out then..


Updated Drawing as mosfet-p needed flipping over..
just incase someone was using drawing

[Magnox]

You could check evilbay for shunts - for instance item 200720481765 gives 75mV at 100A. Just a thought.

alternatively, see if yo ucan get Allegro to send you some free samples. That's how I got mine... (don't spread this around though, everybody might try it.  ... oops... too late!)

[pickit2]

why not use a power fet as shunt?
its been done many times.

[PaulC]

Excellent suggestion..
Looks like a plan.
Thank you.

p.s. i also asked allegro for samples too
also any ideas on this project then feel free to say so.
also feel free to use any part or all of it.

[zab]

why not use simple CT on primary of the transformer for all this? It is simple ,normally available and cost less. I have use it On battery chargers with success.

[zab]

on DC side use resistors for sensing like http://www.ttiinc.com/attach/19100 and http://www.digikey.com/Web%20Export/Supplier%20Content/tt-electronics-welwyn-985/pdf/welwyn-an-current-sense-resistors.pdf?redirected=1

[dotm]

another sensor you could use is this one:
http://www.lem.com/docs/products/fhs%2040-p%20sp600.pdf
its 4 dollars on digikey. Evaluation kits are 15 dollars.

[Wizpic]

Another simple idea, use a DC fuse say 50amp and measure the voltage drop across the fuse and then use an op-amp to boost the voltage 0-3volts for 0-30amps. I've used this many of times and I've know they also have used this idea in some industrial chargers. It's a cheap and simple method, if you think about it you should already have a DC fuse in line has protection any way so you only need to use 1 extra op-amp.

cheap has chips  
Edit just looked at your scheamtic and see that you've got the shunt in the + line normally they go in the - side so just place the fue in the - line that will do the trick

second edit. just seen what IC your using so the try the dc fuse in the +side and op-amp the same

[PaulC]

Wizpic nice idea
Another simple idea, use a DC fuse say 50amp and measure the voltage drop across the fuse and then use an op-amp to boost the voltage 0-3volts for 0-30amps.
cheap as chips is very much likeable.

[sharning]

You can use "Sensefets" which are high currnet mosfets with current sense.
here is an example part,( they probably come in many other ratings and packages too)

www.onsemi.com/pub/Collateral/NTMFS4833NS-D.PDF

[dotm]

Another simple idea, use a DC fuse say 50amp and measure the voltage drop across the fuse and then use an op-amp to boost the voltage 0-3volts for 0-30amps.

a fuse gets warmer the more current flows trough it. the warmer the fuse gets the more resistance it has. fuses with enough linearity for current measuring purposes are very expensive (see bussman fuses..).

[Wizpic]

Paul
I've not seen a desulphator before that use's a load (Rsense) resistor before I may be wrong in thinking that my thoughts on your desulphater part will discharge the battery rather than desulphate it. I thougt the idea is you pulse the batery with hihger voltage to break down the desulphation.
Here is an high power 12V version they also do the basic version if high power is not needed
 I do like the idea that you only use it when the battery is in float mode otherwise the battery would seroiusly overheat if it was used during bulk charge

[zab]

my dear if you want to make it simple Use CT otherwise Low value resistor or fuse with opamp will fulfill your needs.For For better use hall effect ICs. Secondly the desulphator section of your circuit need revision. High voltage pulse have to directed towards battery.as did in Wizpic example.There are lot of example of desulphator circuit. just google it.

[zab]

PaulC please attach the soft copy of the drawing  intead of pic. Little more modification is perhaps required.
I have indicated on the attached pic.where you want to attach transistor q14 and q15 bases and what is the voltage level of the drive circuit .q14 and q15 can not run in this configuration.you need to revise them as well.  if you supply power to circuit then it will be on in all mode.  Hope it will help.

[zab]

It will not still work because I think the transistors Q14 Q15 will be on alternatively not simultaneously.so these transistor have be connected in parallel not in series.both collector will be connected with relay coil and emitters will be ground.In present configuration transistor works as and gate and it require both transistors on simultaneously to switch on relay. In parallel if any of the transistor will be on relay would be on. I think You want switch on the circuit in both stages. Please keep on sharing.Please tell what is status of the project now.
I hope it will be helpful.

[PaulC]

hi zab
thank you for this info.
the 2 x 2n222 are to enable the desulfer circuit only in float mode without any out side switching so as not allow any mistakes from the user i.e me from using it in charge mode, the 24450 enables stat1 & 2 when in float mode.
if this will not work then any suggestions from anyone would be gratefully received.

[zab]

Hi PaulC If both transistor are on 1 State then the circuit will work.
If on transistor is on in state 1 the other is on on state 2 then you have change it.

[pickit2]

hi zab
thank you for this info.
the 2 x 2n222 are to enable the desulfer circuit only in float mode without any out side switching so as not allow any mistakes from the user i.e me from using it in charge mode, the 24450 enables stat1 & 2 when in float mode.
if this will not work then any suggestions from anyone would be gratefully received.

why not have a window compator set to the voltage of fully chaged battery and lower voltage, so desulfer only kicks in, when battery is fully charged and will not start full power charging again till charger is reset.

[zab]

Electronic gives you the choice you are looking for. Yes why not it is possible to do with comparator as well as with micro controllers.

[PaulC]

Added new drawing.. to 1st Post.
thankyou zab/pickit2
modified drawing
Hope it will be of use.
Any comments Ideas most welcome

[zab]

Modified circuit will definitely work but you have precisely adjust the variables to synchronize it with the charger control IC. in my opinion If you make little more changes there would be no need of further setting, charger ic setting will work for this as well.

One more question why are you adding 2 Ampere Meters? The reading of the both will be same.
only the difference would be of BQ24450 current(only friction of mA) . First will read bat current+BQ24450 current while the other only bat. Is there any other reason I could not think of?

Please share the result of the project after practical trial.Whether the circuit succeed in bringing about the desired results ie.Restoring the Battery,enhancing its life.

[PaulC]

Hi zab & thank you for replying.
will the 1n4148's work as a (and) and not (either)..
i.e. float mode switches both stat1 & 2 (connect to float1 & 2)..
if so this is excellent..
and i will remove one of the amp meters (1 near battery) makes sense..

[zab]

Hi PaulC the diode will switch on the output relay in both float stat 1  as well as 2.
these will work like simple OR gate. if any one of these or both present ,output will be present.
and OFF in absence of these  stat 1& 2.