Author Topic: Sense check on wiring resistance please...  (Read 3369 times)

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DamonHD

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Sense check on wiring resistance please...
« on: April 05, 2016, 01:12:10 PM »
Hi,

I have a very small off-grid system (with a decent store) used for keeping my Raspberry Pi server running the whole time.  It generally only uses a couple of watts, so I could run it over wet string.

However, I can shift about another 12W/1A of load to it from the grid when the battery is full enough.  This seems to cause ~0.25V drop, ie implying impedance from battery, via charger, to load (which is very close to the measurement point) of 1/4 ohm.

Cabling run from battery to controller to load is mainly 4mm^2, about 4m, so 8m there and back, and for that I estimate about a quarter of the voltage drop that I actually measure.  You can see the sharp movements in the purple line where the extra load is switched on and off in the graph below:



a) Given the small load, is my wiring resistance reasonable?

b) What might be the rest of the drop?  My couple of fuses either side of the controller (~20A MAXI automotive IIRC)?  The SS-MPPT-15L controller?  Bad connections (yes, I periodically check and tighten).  Pixies?

Rgds

Damon

mab

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Re: Sense check on wiring resistance please...
« Reply #1 on: April 05, 2016, 01:32:16 PM »
I assume the 0.25v drop is measured at the load? have you measured volts at the battery terminals? I suspect the voltage drop could be the battery itself. If the normal load is much less than 1A then you will see it sag with the larger load.

For info 4mm2 is tabulated as 9.22mOhms per meter (for supply and return) so 4m should be 36.88mOhms.

DamonHD

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Re: Sense check on wiring resistance please...
« Reply #2 on: April 05, 2016, 01:40:52 PM »
Thanks

1) Yes, the calculator I was using using two-way not one-way distance, so the expected wiring loss is in fact nearer an 1/8th of what I'm actually seeing.

2) Voltage is measured at the load.  0.2V+ sag at 1A from 400Ah of gel batteries seems high, but am I just wrong?  (I am possibly going to tweak the wiring at the batteries a little.)

Rgds

Damon

dnix71

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Re: Sense check on wiring resistance please...
« Reply #3 on: April 05, 2016, 03:18:11 PM »
What keeps the 400ah gel pack full? The yellow line is way too high for lead acid gels and 13 isn't high enough. How old is the gel set and have you removed the caps so you can add water?

mab

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Re: Sense check on wiring resistance please...
« Reply #4 on: April 05, 2016, 03:27:28 PM »
voltage sag on a lead acid is rather non-linear, and there are a lot of factors including the age of the battery (particularly if you haven't watered them as Dinx71 suggests), etc.  That said, .2v for 1a from 400Ah does sound a little high, if you could measure the sag at the battery terminals it would confirm how much is the battery.

I've just re-read your post and realised the 1A if passing through the 15A controller(?) - so you may well be losing some volts across the FET of the LVD (assuming that's how it works).

DamonHD

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Re: Sense check on wiring resistance please...
« Reply #5 on: April 05, 2016, 03:59:37 PM »
Measuring the sag at the (gel) battery is an excellent idea and I shall attempt to do it tomorrow.

Thanks

Damon

Bruce S

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Re: Sense check on wiring resistance please...
« Reply #6 on: April 05, 2016, 05:02:56 PM »
Damon;
I agree with mab on this, perhaps a whole system sag measurement is in order.
I have had to do this with EV scooters to get the BM on the different chems.

Bruce S
A kind word often goes unsaid BUT never goes unheard

DamonHD

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Re: Sense check on wiring resistance please...
« Reply #7 on: April 06, 2016, 04:29:10 AM »
A bit difficult at the moment as charging is going on, but voltages at the battery seem rock solid and its only at the house end of the wiring that there seems to be much of a change, and that instantaneous change on adding/removing the load seems much more like 0.13V, of which ~0.03V is probably attributable to the wiring as per mab's numbers above.

So down to maybe 0.1V still needing explanation, since I'm prepared to believe that a wobble of ~0.1V from surface charge effects, etc, at the battery is perfectly possible.

I am tightening connections as I go today, also.

Rgds

Damon

DamonHD

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Re: Sense check on wiring resistance please...
« Reply #8 on: April 06, 2016, 02:18:17 PM »
Hmm, I think that that mystery 0.1V is getting lost somewhere between the SLAs and the controller.  I think that I will revisit and refresh, and possible replace or beef up all of that wiring.

Looks like only ~60mV is getting lost between controller and load over a much longer cable run and in the outbound fuse, and within the bounds of sense.

But I may be losing ~40mV across each 20A MAXI fuse at 1A, which does seem excessive.

Needs a little more investigation.

Rgds

Damon

DamonHD

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Re: Sense check on wiring resistance please...
« Reply #9 on: April 06, 2016, 02:26:56 PM »

SparWeb

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Re: Sense check on wiring resistance please...
« Reply #10 on: April 14, 2016, 12:49:45 PM »
Hi Damon,
You're getting into the range where the type of meter you are using is critical to getting a correct measurement.
No one believes the theory except the one who developed it.  Everyone believes the experiment except the one who ran it.

System spec: 135w BP multicrystalline panels, regulated by Xantrex C40, DIY 8ft diameter wind turbine, regulated by Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024

DamonHD

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Re: Sense check on wiring resistance please...
« Reply #11 on: April 19, 2016, 06:45:12 AM »
Working on bolstering the battery bank internal connections first, to see if that makes a noticeable difference...

Rgds

Damon

DamonHD

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Re: Sense check on wiring resistance please...
« Reply #12 on: April 23, 2016, 01:15:24 PM »
I am throwing a little bit of money and copper at the problem.  Just received and fitted one pair of 35mm^2 jumpers, which I have fitted in the middle of the bank to avoid creating any imbalance, and have ordered slightly more accurately sized (shorter) ones for the rest of the bank now that I have established it should work.

In passing I may have found a small but measurable drop between the -ve power-take-off corner and the next battery from the currently undersized cable.

So, if all goes to plan, this time next week my bank will be sorted for wiring!

Thanks for all suggestions...

Rgds

Damon

DamonHD

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Re: Sense check on wiring resistance please...
« Reply #13 on: May 02, 2016, 10:59:27 AM »
I put in the new wiring, which mostly went as expected, and did significantly reduce the sag as seen at the load.

Now I added some more load and the sag is back.  B^>

But it is more load that I'm taking off-grid, even if small.  I'm trying a new scheme to reduce night-time loads with this...

Rgds

Damon

DamonHD

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Re: Sense check on wiring resistance please...
« Reply #14 on: May 07, 2016, 12:25:20 PM »
Progress: found a bad connection on one of the main panels.  Hoping that a full-sun day tomorrow will show everything working...

I have also updated my algorithms to explicitly allow for battery sag under load and rise when charging:

http://earth.org.uk/RPi201406/code/powermng.c

Only about 5 years late...  %-P

Rgds

Damon