Voltage readings differ at controller and battery... why?

RussOnTheRoad · February 2015

I'm not totally green at solar but still consider myself rather new. I thought this topic would be a little more advanced than the New to Solar section of the discussion board so I'm posting here in the advanced section.

Here is my issue: My controller, a Blue Sky Sun Charger 30 (30 amp PWM) has a digital display panel which among other things reads out what I understand to be Battery Voltage. While under charge during daylight, the voltage reading I see on the display of the controller is the same as what my multimeter displays if I measure the voltage where the leads to the battery come out of the controller. However, if I measure the voltage at my battery terminals it is quite a bit lower, sometimes as much as a volt or even as much as 1.5v, sometimes less. At night when there is no charging the voltage displayed on the LED of the controller is the same as the multimeter reading I take at the batteries. Why would the voltage reading differ at the controller and my batteries under charging conditions but be the same when not under charge?

Other information which my be relevant: I have two 60 watt panels that came as a portable kit, so my total potential output is 120 watts. Their open circuit voltage is as stated in the literature. Right now I have about 40' of wire between the controller and my batteries. I think the first 10' is 12 gauge and the 30' extension is 10 gauge. I have two Group 24 12 volt batteries in parallel which are supposed to be deep cycle marine batteries. They are made by Johnson Electric and were marketed under the WalMart name. They are rated at 101 Ah using the 1 Ah draw down method so with the 20 Ah draw down method they would probably be rated at about 75-80 Ah. They were new as of June 2013. The electrolyte level is fine. I have never seen them at less than 12v of charge and seldom less than 12.2 or 12.3 measured in the morning with maybe a .5 amp load on them overnight. Recently I left them fully charged and disconnected for a couple weeks and upon returning they tested at 12.7v when I measured them. Despite the Walmart branding I think they are probably OK.

So, any ideas about the disparities in the voltage readings? I'm sure there is an explanation. I just don't know what it is.

Thanks.

westbranch · February 2015

RussOnTheRoad wrote: »

Why would the voltage reading differ at the controller and my batteries under charging conditions but be the same when not under charge?

Welcome, to get a good picture of your system , what is the measured distance and gauge of the wire from the CC to batteries? I suspect the problem is either a loose connection, doubtful if your other readings are correct, or too small a wire gauge...

BB. · February 2015

I agree with WestBranch, it sounds like voltage drop from the battery bank to the charge controller... Too small of copper wire, too long of wire run, corroded connections, loose connections, etc.

-Bill

Photowhit · February 2015

Perhaps Russ added this after the other replies?

RussOnTheRoad wrote: »

.... Right now I have about 40' of wire between the controller and my batteries. I think the first 10' is 12 gauge and the 30' extension is 10 gauge. I have two Group 24 12 volt batteries in parallel which are supposed to be deep cycle marine batteries.

So under charge the 120 watts of panels can produce about 90watts (derated) the charge controller can put out 90 / 12.7= 7 amps.

Now plug those numbers in here! and you get 4.67 % voltage drop, call it 5% since I used 10 gauge rather than the long math.

5% of 15 volts is .75 volts, not quite as bad as you've seen, but pretty ugly! Is your inverter near the charge controller? If so you likely have much heavier cables running to it...

RussOnTheRoad · February 2015

Photowhit wrote: »

Perhaps Russ added this after the other replies?

So under charge the 120 watts of panels can produce about 90watts (derated) the charge controller can put out 90 / 12.7= 7 amps.

[URL="http://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=3.277&voltage=12&phase=dc&noofconductor=1&distance=40&distanceunit=feet&amperes=7&x=38&y=11"]Now plug those numbers in here![/URL] and you get 4.67 % voltage drop, call it 5% since I used 10 gauge rather than the long math.

5% of 15 volts is .75 volts, not quite as bad as you've seen, but pretty ugly! Is your inverter near the charge controller? If so you likely have much heavier cables running to it...

Thanks to all that have replied so far. It's very much appreciated.

To Photowit:
The information about wire gauges and length of run was included in my original post and not added in a subsequent edit.

Also to Photowit: My controller is on my portable PV panels. My inverter is inside my coach, but I'm not sure why you ask. I don't think it's a factor here since it is off 95% of the time. I'm in a motorhome and only turn it on when I need AC which might be for a little while every few days as a general rule. It's been off the whole time of this discussion. i hope that clarifies things a little.

To All:
I have taken a more careful look at the wires. The solar panel kit came with about 10' of wire attached to the battery terminals of the controller with an Anderson type connector on it. That wire is clearly marked AWG 12.

I later purchased a 30' extension from the panel vendor (GoPower). I thought this to be 10 gauge, but now I'm not so sure. The wires are inside a weather resistant sheathing which is marked CCC A0566849 Red Banner 227 IEC 53(RVV) 300/500V 2x6mm2 (where 2 = the symbol for squared) GB5023.5-1997. Man, that's a mouthful. I think--and I'm sure someone else will know--the wire thickness here is expressed in mm squared of the cross section rather than in the AWG system. I tried to find the AWG equivalent of 6mm2 (squared) but I I'm not sure I understood the charts I was looking at. It did appear to be the equivalent of 10 AWG if I'm not mistaken. The wires from the panels to the controller are marked 2x4mm(squared). I had to remove a few strands of copper from the wires going to the battery connection in order to get them to fit into the connectors on the controller so I don't see how a gauge any heavier could be used.

Perhaps I should add that the last foot or so of the wire run from the controller to the battery is also 12 AWG. That's where the connector from the extension clips to the connector mounted to my motorhome and runs to the batteries.

I've made sure the connectors on the leads are clean. In measuring the resistance of the long extension I could not get a reading except zero, but I'm not sure if I did that correctly. I think I did.

Assuming that the voltage loss between the controller and the battery is just due to the length of the wire run, would it be safe to turn up the output voltage of the controller in order to produce the desired voltage at the battery terminals and if so would it make sense to do so?

Thanks again!

waynefromnscanada · February 2015

40 feet of wire between controller and battery bank is huge, and all the more so with only 10 gauge wire. OUCH! No wonder you're seeing voltage drop!

BB. · February 2015

You can use a voltage drop calculator like this one to see what happens:

http://www.calculator.net/voltage-drop-calculator.html

40 feet (one way run) or 10 AWG wire and 10 amps of current:

Voltage drop: 0.80
Voltage drop percentage: 6.67%
Voltage at the end: 11.2

Not exactly the drop you are seeing--But given that this will cause the charge controller to turn off sooner/reduce charging current too soon to the battery bank, you can see what will happen.

In general, keep the battery bank to charge controller connection short (and heavier gauge cable)... For a 12 volt battery bank, you want around 0.05 to 0.10 volt maximum drop for optimum charge controller operation.

If you need longer wire runs, put them between the solar panel and the charge controller (but for a 12 volt PWM system, this can require fairly heavy gauge wire too). There are other solutions, but nothing really cheap.

-Bill

westbranch · February 2015

Definitely use the calculator and look at 4, 6 and 8 AWG wire losses , if even just for information. I use #4 wherever it will fit in my system wiriing... here is a link for AWG to Metric http://www.technick.net/public/code/cp_dpage.php?aiocp_dp=guide_awg_to_metric

Photowhit · February 2015

RussOnTheRoad wrote: »

My inverter is inside my coach, but I'm not sure why you ask. (?)

Inverters must be connected by larger wires due to the large amount of amps passing from the battery to the inverter. Most home/cabin systems have one area near the batteries where the inverter charge controller and batteries and fuses and/or breakers live. Setting up the system so that there is only a single heavy set of wires going from the main breaker to the battery bank. Heavier wire means less voltage drop, hence the question and right after that the answer to your question; "...If so you likely have much heavier cables running to it..."

RussOnTheRoad wrote: »

Assuming that the voltage loss between the controller and the battery is just due to the length of the wire run, would it be safe to turn up the output voltage of the controller in order to produce the desired voltage at the battery terminals and if so would it make sense to do so?

I think the best option is to move the charge controller closer to the battery bank, the voltage drop will vary depending on the amount of current passing through the wire, so the charge controller will not be easy to fix by changing it's parameters in it's current location. If they are adjustable setting a higher point will allow for correct current when max is passing through but as the current tappers off the . You may still have an issue with having enough voltage to properly charge the batteries of sunny days when the panels are hot, the voltage drops along with the line loss, you might not have enough voltage.

Do you also charge these off the alternator?

BTW- These seem to be trolling motor batteries? "...two Group 24 12 volt batteries in parallel which are supposed to be deep cycle marine batteries."

These aren't very good at heavy deep cycle use, Life span of 12-24 months, they may be showing reduced capacity. You might look at revamping your system and switching to 2 - 6 volt golf cart batteries in series, which will cost just a tad more, normally not available at WalMart, but Sam's club and Costco sell them for around $80 each. Properly care for they should last last 3-5 years.

Voltage readings differ at controller and battery... why?

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