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Sense Check On Wiring Resistance Please...


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#1 DamonHD

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Posted 05 April 2016 - 07:18 PM

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:

Posted Image

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

Edited by DamonHD, 05 April 2016 - 07:19 PM.


#2 Nickfromwales

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Posted 05 April 2016 - 08:46 PM

Can you solder all the connections?
High resistance joints would be my guess, but Do these possibly occur with changes in ambient temp?
Regards, Nick.

#3 SteamyTea

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Posted 05 April 2016 - 08:48 PM

Temperature? Plot it and see.

#4 SteamyTea

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Posted 05 April 2016 - 08:48 PM

SNAP

#5 DamonHD

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Posted 05 April 2016 - 08:51 PM

I do regularly check for bad (loose/hot) connections, and have occasionally had them elsewhere, but this seems pretty consistent and steady.

Anyhow, I did some connection tightening yesterday and will do more checking and adjustment tomorrow.

Thanks

Damon

#6 DamonHD

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Posted 23 April 2016 - 07:16 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

#7 SteamyTea

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Posted 24 April 2016 - 05:39 AM

Put up for plots in a weeks time and we can see if it makes a difference.

#8 DamonHD

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Posted 24 April 2016 - 06:32 AM

Indeed.

#9 SteamyTea

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Posted 24 April 2016 - 07:17 AM

And log the local temperatures.

#10 DamonHD

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Posted 24 April 2016 - 07:20 AM

Every 4 minutes, plus METAR data for EGLL if necessary!

Rgds

Damon

#11 temp

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Posted 24 April 2016 - 02:45 PM

Have you monitored the voltage at the battery end of the wire? eg to check if the 1/4 Ohm is mostly in the battery/charger or in the wire/joints?

It could be that your charger is limited to a few amps so that when the load is light the charger voltage is high then when the load exceeds the charger current the voltage is dragged down to nearer the 12V that the battery can sustain. Perhaps check the voltage drop with and without the charger connected.

The resistance of 8m of 4mm^2 should be around 0.034 Ohms according to ..

http://www.epanorama...ndex=calc_cable



#12 DamonHD

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Posted 24 April 2016 - 03:00 PM

Hi,

The solar controller/charger has a capacity of 15A/200W.

I did monitor voltage at the battery and it seemed relatively stable while I added/removed the extra 1A load. I am measuring voltage at the load end for the graph above.

I retightened connections and found a potentially poor one in the battery bank, so I will pay special attention to it when adding the new fat jumper cables.

I'm not expecting miracles, but getting nearer 100mV sag would be reassuring.

The other factor to consider is that the gel battery bank is now ~5Y old, and I have it paralleled with a ~10Y old much smaller gel battery, which is possible not helping things.

Rgds

Damon

#13 DamonHD

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Posted 02 May 2016 - 04:58 PM

Hi

I put in the new wiring, which mostly went as expected, and did reduce the sag as seen at the load significantly.

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