Voltage drop questions

I'm getting ready to install my panels this spring at my small off-grid cabin, and need to plan for the wiring. Some basic details of my system are: two 64 watt Uni-Solar panels (7.8 amps total), 50 feet between the panels and SunSaver 10 amp charge controller, 5 feet between controller and AGM battery. I have used the voltage drop calculator spreadsheet to work through a few scenarios, but I still have some questions.

1. The only way for me to stay within the recommended 3% drop would be to use 6 gauge wire - expensive and hard to find around here. If I use 8 gauge wire, the voltage drop would be 4.6%. Considering the small output of my panels, is it worth the extra cost/hassle to use 6 gauge wire?

2. Let's take this scenario one step further. What if I want to add another 64 watt panel to the array someday with the 8 gauge wiring in place, which would make the voltage drop about 7%. BTW, I know I would need a different controller in that case. Would a voltage drop of 7% cause any damage to my controller or battery, or am I just losing a little bit of power?

Please keep in mind that this is just a small weekend cabin that will be visited maybe 15 times per year, so much of the potential power of the system will not be used anyways.




  • mike95490mike95490 Solar Expert Posts: 9,358 ✭✭✭✭✭
    Re: Voltage drop questions

    You will find that the extra wire cost, over the long run (10+ years) will pay for itself with the extra power you would be throwing away heating wire.

    120W - 4.6%= 5.16w 1460hours year = 7533.6 watt hours lost in 1 year

    180W - 7%= 12.6 18,396watt hr lost 1 year

    Another idea to dispose of, is using the batteries heavily on a weekend, and expecting the PV to recharge over the week. After 24 hours of being below 75% full, the batteries will begin to sulfate and die.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • Steve961Steve961 Solar Expert Posts: 93 ✭✭✭✭
    Re: Voltage drop questions
    mike90045 wrote: »
    Another idea to dispose of, is using the batteries heavily on a weekend, and expecting the PV to recharge over the week. After 24 hours of being below 75% full, the batteries will begin to sulfate and die.


    I thought AGM batteries were somewhat resistant to this. According to the battery FAQ, "The Concorde batteries can be almost fully recharged (95% or better) even after 30 days of being totally discharged." My AGM battery will be nowhere near totally discharged when I leave, and should only take a couple of days to be fully charged. I also usually have a generator with that I can bulk charge it if necessary before I go.

    It seems that with 8 gauge wire I will only be losing 23 watt hours per day with 2 panels, and 53 watt hours per day with 3 panels. For the 30 days per year I will be up there, I'm not sure it matters. What I'm more concerned about is will lower voltage from the array cause any problems with my system hardware.

    Thanks for your thoughts.

  • BB.BB. Super Moderators, Administrators Posts: 32,005 admin
    Re: Voltage drop questions

    I think I read somewhere that Concorde says their batteries are "special" and do not sulphate like other batteries below 75% state of charge...

    But I have seen other places that recommend that they be recharged if below <75% state of charge when in storage. And they all have a 15.5 volt @ 8 hour "recovery" algorithm (hmm, why is that needed).

    Sulfated Lifeline Concorde AGM Batteries

    One thing to watch out for is the "totally" discharged... Many rechargable chemistries are OK to take to 0 volts on a cell. What kills a cell is to reverse bias a cell (i.e., start charging it at -2.x volts).

    That will usually destroy the cell cause it to never hold a charge properly again.

    Does loosing 5% AH capacity a month constitute a failure due to sulphating (Concorde, as I remember 20% loss of AH capacity is a "failed" battery) after 4 months at zero volts and a 20% loss of capacity?

    There is no doubt that AGMs (in general) and Concorde (specifically) are nearly ideal lead acid storage batteries. At 2x the price of normal flooded cell batteries--It would be best not to test the limits (at least, not on my dime).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Steve961Steve961 Solar Expert Posts: 93 ✭✭✭✭
    Re: Voltage drop questions


    We've already discussed the whole DOD issue with my system specifically in this thread: http://forum.solar-electric.com/showthread.php?t=6794. The consensus was that in my circumstances an 80% DOD would not be a problem.

    My original questions still stand. Is it worth the extra expense/hassle for 6 gauge wire with my small system, and is there any potential to damage a charge controller and battery with reduced voltage from an array?


  • BB.BB. Super Moderators, Administrators Posts: 32,005 admin
    Re: Voltage drop questions

    You can make it an economic decision:

    (capital cost of system) / (kWH per year * 10 year life)

    Or whatever you choose to use for 10 year system life...

    Roughly, call the cost of power at $2 per kWH for a small system--or generator fuel+cost of genset...
    • 53 Watt*Hours * 30 Days a year usage = 1,590 WH per year lost
    • $2 per kWH lost economic value (or cost to recharge with genest) * 1.6kWH per year = $3.20 per year.
    So, you at least have a rough estimate of the cost for using a genset (costs for a rarely used genset are probably many times $2 per hour).

    However, the economics go pear shaped if you are using the PV system 15 days / 365 days... Makes the apparent kWH costs 10x larger.

    You will not damage the charge controller... You may, on hot days, not get as much current (Vmp drops on hot days, add the voltage drop on wiring, further reduce the ability of the system to fully/quickly recharge the battery).

    The other option is to get a MPPT type charge controller and run the panels in series. Both reduces current, voltage drop, and makes Vmp>>Vbatt so there are no temperature/battery voltage/Vmp issues.

    But, that is a $300-$500+ controller... Replacing wire would probably be much less expensive.

    The only way you can hurt the battery is if you draw too many AH and take a long time to recharge--But, in theory, the Concorde AGMs are much less susceptible.

    It probably comes down to what is 53 Watt*Hours of quiet solar PV energy is worth to you in the grand scheme of loads and possible genset use.
    • 2 * 64 watt panels * 5 hours (summer sun) * 0.59 system eff = 378 WH per day
    In practice, you would be hard pressed to "find" that missing 53 WH of load out of ~400 WH of useful energy per sunny day.

    In any case, with 15 day a year usage, solar PV is a luxury for you (vs having a Honda eu1000i humming away in the background and a couple gallons of fuel usage). 8 AWG should work fine if 6 AWG is not locally available.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Steve961Steve961 Solar Expert Posts: 93 ✭✭✭✭
    Re: Voltage drop questions


    That is a perfect, and very detailed, answer. You're right that solar power will be a luxury for me, but I'm looking forward to the peace and quiet it provides. I already have a Honda eu2000i, and while it's fairly quiet, I'm looking forward to relegating it to power tools when I'm building something.

    Thank you.

  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Voltage drop questions

    Steve, if there is an electric motor repair shop nearby, ask them about what ga. wires they use. I got some very high grade 6 ga. wire from a local shop.

    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • Steve961Steve961 Solar Expert Posts: 93 ✭✭✭✭
    Re: Voltage drop questions


    I can find all kinds of 6 gauge wire locally, it's just that I need wire that's sunlight and moisture resistant, and suitable for direct burial. The one place I did find it, where I could buy it by the foot, wanted $3.75 per foot for it. I'm not sure an electric motor repair shop would carry UF-B wire, but in any case I'm sure the 8 gauge would be much cheaper.

    I appreciate the ideas.

  • dwhdwh Solar Expert Posts: 1,341 ✭✭✭
    Re: Voltage drop questions

    Something to keep in mind when doing voltage drop calcs - the drop is dependent on several factors:

    Wire Size.
    Wire Length.

    The fact that load varies is often overlooked in regards to calculating wire size for battery charging. If you calc say a 3% voltage drop at the MAX CURRENT OUTPUT (amperage) of your charger, say 10a, then at any load below 10a, the voltage drop will be less.

    So one question that has to be asked is; How much time will that charging system be running at maximum current output?

    If it bulks for a couple of hours, then spends a lot more hours in absorb - then the *average* load will be less. A system might spend say 4 hours with a 4% voltage drop, and then 8 hours with a 2% drop.

    Factoring in "time spent at different loads" changes the results when trying to figure out how many watts will be lost over time as a result of voltage drop.

    Naturally, someone will be tempted to shoot off a quick reply of; "So just spec the system at full load, and then at lower loads it's all good."

    Yes, I know. For the most part, that's a sound strategy. But apparently, not always.

    Recently, I've been planning to upgrade the battery charger in my camper. I was looking at a Samlex charger, which bulks to 14.4v, and comparing to that an Iota w/IQ4, which bulks up to 14.8v.

    Assume I have a battery that wants 14.4v and the charger will be 15' of wire from the battery. If I use #6, then I get a .2v drop during the 15a constant current bulk stage. that's too much voltage drop to use the Samlex - the battery will only bulk up to 14.2v. It's too little to use the Iota, the battery will bulk up to 14.6v.

    I can't increase the output of the Samlex, but I -can- increase the voltage drop, in order to get the 14.8v of the Iota down to the 14.4v that the battery needs. Of course, on the face of it, that won't work since I'd be creating a .4v drop, which would leave the absorb and float stages at far too low a voltage.

    But actually, it won't since in those stages, the current (load) will be less, and so will the voltage drop. So I can deliberately use a smaller wire to get the voltage down during constant current mode, while not significantly affecting the constant voltage mode.

    (Allow me to also mention - in re-researching battery chargers, I noticed something that I had overlooked before - the Iota IQ/4 "equalize" stage is not actually what one would expect an "equalize" to be. All it does, is after 7 days at float, it automatically kicks the charger back into the bulk stage for however long it takes to bump the battery back up to 14.8v, or 240 minutes, whichever comes first.)
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Voltage drop questions

    i'll just add a few observations here.

    i have 2 us64s and what i typically saw with mppt was around 7a give or take. you aren't using mppt and will likely see less current. going by bill's typical derating for efficiency of 77% the 3.88a stc will appear as 2.9876a and being there's 2 of them it's 5.9752a or roughly 6a. for the calculations i made it as 110ft of wire total at 12v. for 7a this is 5.14% vdrop. at 6a this is 4.41%. this was of course through your proposed #8 wire. could you live with this? probably, but if there's any chance of a better controller and/or another pv then i'd look further into getting thicker wire for the runs.

    dwh brings up good points too, but the bulk stage is usually the longest of your charging stages unless you don't drain much from the batteries for your loads and as such will carry high current for quite some time. this isn't just a loss of power as it is a gain of heat dissipated. the nec frowns upon anything over 5% in vdrop % because of the heat dissipated. that is an arbitrary figure as you aren't all of a sudden in danger at 5% as opposed to 4.9%. this is something you need to be aware of though and an insurance company could lay blame on that and deny claims even though it's not the cause of any damages. you won't go over 5% with your pwm controller in my opinion, but any improvements to the system with a better cc or another pv will put it over the edge as both will allow for more current to flow and thus more losses due to vdrop.
  • BB.BB. Super Moderators, Administrators Posts: 32,005 admin
    Re: Voltage drop questions

    I will have to disagree a bit with DWH saying that wire resistance in the panel path is not that bad...

    The problem is you really want maximum current and maximum voltage available with hot days and cool batteries... Vmp of the array is depressed with heat of sun; and Vbatt-charging is higher with a cool bank...

    Depending on your conditions, you will probably get reduced available voltage from the Vmp of the array through the wire and controller voltage drops, to the battery.

    You may not be loosing directly only because of the loss due to voltage drop, you can also loose charging current due to low available charging voltage at the battery bank on sunny hot afternoons...

    5% losses here, another 5-10% losses there, it all adds up pretty quickly.

    Ideally, as an engineer, I want the charge controller to have maximum voltage/current available and accurate voltage measurements at the battery bank so that it can make the programmed decisions.

    In your case, it may take a bit longer the second day to recharge the battery bank--or even the day after you leave.

    It is not the end, it is just this stuff adds up... We are already looking at 1/2 the available rating because of panel derating, wiring drop, controller drop, battery inefficiency, inverter losses, etc.

    But you use what you have. Get some time on the system and measure its performance to find out for sure. You may be perfectly OK for your needs.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Voltage drop questions

    good point bill as the us64 is not very high in its vmp at 16.5v. losses could kill it as a viable charge source when voltages are pulled too low. i kept losses on mine low.
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