Advice needed for small off grid system

jimbuckjimbuck Posts: 42Registered Users ✭✭
edited January 20 in Solar Beginners Corner #1
Hello, I am designing a small system for a friends van. It’s a 1995 Mercedes 308D T1
She is of limited budget like many folk I know.
I have a few questions.

115Ah “leisure” batteries are very easy to come by here and I am finding that to most people with vans/trailers this is a big battery and considered to be an endless source of power. I know this is not the case.

My friend already has the 115Ah battery.

I have found a great supply of used (3years old) 260w panels for £0.50 a watt!

I would like to use two of them but that would then require a 30+amp charge controller which is twice as much £ as a 15A one.

If I use a 15A MPPT controller can I over panel for winter? The Voc would be way lower than the controllers limit. Would the controller just clip any extra amps made in summer time? The panel specs are:

Pmax: 260W
VOC: 37.5
VMP: 30.4
ISC: 9.12A
IMP: 8.56A

I have read many posts on here and have found BB’s calculations very helpful!

Here are some figures based on them:

Required daily energy Wh: 435

So if I use 1, 260w panel.
260w x1 / 12v x0.77 cc derating x1 / 0.13% max cost effective charge rate = 128A bank so the 115Ah fits.

Battery bank autonomy.
115Ah x 12v x 0.85 inv eff x1 / 2 days storage x 0.50% DOD = 293 Wh. She would need to be careful on these days. (Most loads are dc not inverter?)

Winter time.
260w panels x 0.52 system efficiency x 1 hours of sun (December) = 135.2 Wh

If I used 2x 260w panels then I would get double 270.4 Wh which would still need some power conservation.

Ultimately she may end up using less than the 435Wh I calculated due to not having lights on for 10 hours a day and a laptop for 3 hours.

If money was no object I know exactly what I would propose but unfortunately it is.

Does anyone have any thoughts?

The 15A charge controller can be adjusted for max charging amps to the battery so even if over panelled it would not over charge the battery.

Thank you, Jim

Comments

  • BB.BB. Posts: 27,887Super Moderators, Administrators admin
    edited January 20 #2
    Jim,

    Very nice job... I like you took my equations and modified them so they "look like" the way you needed your questions answered--That is the whole reason I do this vs some magic spreadsheet formula and a single "fudge factor".

    If there is a chance later--I would suggests looking at 2x 6 volt @ ~200 AH "golf cart" batteries. A larger battery bank using pretty cheap and rugged batteries (gives you 12 volts @ ~200 AH battery bank).

    If the owner will (mostly) be DC, you can "drop" the 0.85 inverter losses and change the 0.52 overall system efficiency to 0.61 (flooded cell DC system eff--0.81 solar panel derating * 0.95 charge controller derating * 0.80 flooded cell battery derating = 0.61 "end to end" DC solar system eff).

    Guessing the van will be used around UK/Europe? Will the array be adjustable or fixed (flat to roof)? You can use something like the following link and see if adjusting the tilt is worth it for the areas friend will be visiting. For example, London flat vs adjustable tilt :

    http://www.solarelectricityhandbook.com/solar-irradiance.html

    London
    Average Solar Insolation figures

    Measured in kWh/m2/day onto a horizontal surface:
    Jan Feb Mar Apr May Jun
    0.75
     
    1.37
     
    2.31
     
    3.57
     
    4.59
     
    4.86
     
    Jul Aug Sep Oct Nov Dec
    4.82
     
    4.20
     
    2.81
     
    1.69
     
    0.92
     
    0.60
     

    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel where the angle is adjusted each month to get optimum sunlight.
    Jan Feb Mar Apr May Jun
    1.30
     
    2.05
     
    2.76
     
    3.86
     
    4.52
     
    4.91
     
    Jul Aug Sep Oct Nov Dec
    4.66
     
    4.46
     
    3.26
     
    2.41
     
    1.56
     
    1.08
     
    Solar power systems need sun... They do not work if there is no sun (far north, bad weather), or even just partial shading from vent stacks, car top carrier, luggage, etc..

    Regarding her solar charge controller--15 amps would work for a single panel:
    • 15 amps * 14.5 volts charging * 1/0.77 panel & charge controller derating = 282 Watt panel "cost effective" over paneling
    But for two panels--If she is in an area with cold/clear skies and a tilting array, may be giving up too much charging opportunities. In winter/cold clear/mountain skies--The array can actually output near or over Pmp ratings during mid-day. The extra size controller (or two 15 amp controllers, one for each panel, connected to the battery bus) is probably worth the money.

    Around 500 WH per day--No fans, no other loads but laptop, LED lighting, cell phone charging--Should be a decent amount of energy usage. During summer months, lots of "extra energy" (enough to run a small 12 volt efficient fan, etc.).

    Also, you may want to look at alternative charging (a DC to DC battery charger from the van alternator system, or small genset+AC battery charger--And AC charger for when in camp ground and/or friendly house to recharge from). For "permanent" mounted AC to DC battery charger, use bolted or plug in connections--"Alligator" clamps simply are not reliable enough to long term usage.

    continued next post
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Posts: 27,887Super Moderators, Administrators admin
    Lastly, mentioned an AC inverter... I really like small AC inverters (of good quality) vs running 12 VDC around a cabin to different appliances (remember that a true off grid 12 VDC lead acid battery system can run from 10.5 to 16+ volts--Under heavy loads to charging/equalization--Many "12 volt" devices do not run well under large voltage swings. A "good" AC inverter can make things easier--Just AC everywhere and DC only to the inverter).

    For a van, full time AC inverter may not be optimum. A small/good quality MorningStar AC 300 Watt 230 vac 50 Hz (or 120 vac 60 hz) uses about 6 watts "just running". 4-5 hours a day of this inverter (6 watts * 5 hours = 30 WH)--Is almost 10%-30% of  her daily harvest in dead of winter. The MorningStar is nice because it has "sleep mode" and a remote on/off input--Nice ways to reduce inverter waste when it is not needed. Some of the other smaller AC inverters (better brands) do have remote on/off inputs--Nicer than having to install a heavy DC power switch--or use the circuit breaker for on/off.

    https://www.morningstarcorp.com/products/suresine/
    http://www.samlexamerica.com/
    http://cotek.ca/ (cotek and samlex are probably related companies)

    Otherwise--Some other things to think about:
    • Hydrometer to measure specific gravity of battery cells (and log). Glass ones are cheap but easy to break. Others are nice but more expensive (this brand is from Europe).
    • DC Current Clamp DMM (note that AC only clamp meters are common, but not useful for DC power systems). A relatively inexpensive DC current clamp digital muti-meter can help with on-scene debugging of system (and van electrical). USD$60-$100 is usually a good starting point.
    • Fuses and Circuit breakers. Very good safety measure when a cable gets shorted to ground in the van, failed AC inverter, etc. Lead Acid batteries can output 100's of Amperes into a dead short. There are some very nice small fuse holders for relatively high current (input to inverter). Try to avoid "automotive" fuse holders--They are not very reliable and do fail at less than rated capacity. DC Rated circuit breakers are a bit more than fuses (and larger)--But they can save money on spare fuses (good fuses are not cheap) and also double as on/off switches.
    • Some sort of battery monitor/meter system. Most new users simply use battery power until the battery goes dead. If you cycle a lead acid battery very often below 50% of capacity, it may only have a few hundred cycles of life. And if you take it below ~20% state of charge, it can kill it. There are voltage monitors and true battery current/capacity monitors. Each has there own advantages and disadvantages.If you do not pay attention to them and how they work (and can drift over time), they will "lie" to you.
    http://www.sears.com/craftsman-digital-clamp-on-ammeter/p-03482369000P
    https://www.bluesea.com/products/category/0/54//ATO-ATC Fuses (I do not recommend this type fuse+holder--any brand)
    https://www.bluesea.com/products/category/16/72/Fuse_Blocks/Terminal_Fuse_Blocks (30-300 amp fuse holder+fuses)
    https://www.solar-electric.com/bogart-engineering-tm-2030-rv-battery-monitor.html (true current monitor)
    https://www.solar-electric.com/midnite-solar-mnbcms.html (voltage only)
    https://www.victronenergy.com/battery-monitors (true monitor)
    http://www.smartgauge.co.uk/ (voltage monitor)

    Links are just starting points for research--I am not off grid--So I have not used any of the above (other than the DC clamp meter). Some of the others have been used by members here and they can give you some first hand observations.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jimbuckjimbuck Posts: 42Registered Users ✭✭
    Wow, Thanks Bill, comprehensive as ever!

    Reading between the lines I think that I need to specify the 30A charge controller and set its charge rate at 15A into the battery (115Ah) and then if she decide she needs more capacity in the future then I can raise the current for a pair of 6v deep cycles.

    The reason I was looking at the 2 panels and the 15A controller is that in winter here with flat panels (200w) I rarely saw over 5 amps into the battery. Even though i’m South there really isn’t much sun and it’s very low in the sky.

    I looked up the insolation figures and December was 0.68 hours of sun with the panel flat on the roof!

    Again if I can persuade her to install tilts then I will but climbing 10” up s ladder to lift s pair of 260w panels won’t be an easy job. Still I think she will be stationary most of the time so it will be worth it. The summer figures were actually better in june/July with flat panels.

    I think that most loads will be DC so I will run the math again with your new deratings (thank you). I have used victron inverters and my current setup is a 300w (350kva) TSW with eco mode and remote on and off, it’s a lovely bit of kit! I am hesitant to install more than one AC wall outlet due to not being able to install an RCD box. I’ve never found any clear information on neutral earth bonding within these units.

    I’ve also insisted that there is a battery meter installed Victron BMV 700. We cant get trimetrics over here easily.

    I find myself getting more passionate about solar power but also more realistic about costs and power. I’ve spent the last year self teaching myself and I can honestly say you have helped tremendously with your math!

    I will price up the system with the 2 260w panels and a 100/30A MPPT controller with the 115ah battery to start with.

    I would like to install the panels in parallel due to potential shading issues, the MVP is high enough to reduce cable losses and make use of the MPPT controller. I need to look at whether the panels need their own breakers, I suspect not as there is only two in parallel. I will install a breaker before the controller to act as a disconnect too.

    Sorry for the long post, lots of the info is just me clarifying things in my mind
  • EstragonEstragon Posts: 2,839Registered Users ✭✭✭✭✭
    With 30Vmp and a 12v nominal system, you pretty much need mppt. With two panels in parallel, individual breakers shouldn't be needed.

    Flat panels can get more production in summer because they still produce in the morning/evening of long summer days. Obviously there needs to be clear sky for that to happen, so she'll need to park carefully ;-)
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • BB.BB. Posts: 27,887Super Moderators, Administrators admin
    If you get a solar controller that can support a remote battery temperature sensor, you can let the battery charge with >13% rate of charge (going towards 20-25% maximum rate of charge is certainly justifiable). And for something like a van in a mixed climate situation, the remote battery temperature sensor is a good idea anyway (as battery temperatures get higher, the charging voltage needs to be reduced).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jimbuckjimbuck Posts: 42Registered Users ✭✭
    Hi Bill and Estragon, sorry for the delay in replying.

    I will advise her to park in the open away from trees and to regularly clean the panels too. If she can stretch to it I will fit tilts for when she is stationary for long periods, in this country winter is pretty dismal for solar!

    Bill, at the moment the only MPPT controller that is decent and available is the victron units, they don’t have a remote temp sensor, just internal which is the only thing that lets them down. For a 100/30A it’s £180 compared to £400 for the equivalent Tristar MPPT unit. The victron is fully programmable and will be mounted at battery height and close by. Even in summer here I have never had any issues with my own bank in similar conditions but it’s worth keeping an eye on I suppose.
    I’m feeling more confident with my proposal to her now and feel that I have good math to back things up.

    Thanks, for all your help. It’s possible I may have a few more questions but right now I’m content.

    Thanks guys. Jim
  • westbranchwestbranch Posts: 5,087Solar Expert ✭✭✭✭
    edited January 22 #8
    Jim said: the victron units, they don’t have a remote temp sensor, just internal....
    that means the CC should be as close to the battery as possible, ie  at  the same temperature...but then the operator can not easily check the numbers..?? unless there is a remote panel??
     
    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
  • jimbuckjimbuck Posts: 42Registered Users ✭✭
    Hi westbranch, there will be a victron BMV 700 installed for monitoring battery SOC. also a Bluetooth dongle can be added to most victron products to look at data and adjust settings. I do wonder why a simple external temp sensor is not included / optional though.

    Jim
  • littleharbor2littleharbor2 Posts: 1,027Solar Expert ✭✭✭✭
    Jim said: the victron units, they don’t have a remote temp sensor, just internal....
    that means the CC should be as close to the battery as possible, ie  at  the same temperature...but then the operator can not easily check the numbers..?? unless there is a remote panel??

    The Victron charge controllers don't have any readouts on the controller body.


    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

  • EstragonEstragon Posts: 2,839Registered Users ✭✭✭✭✭
    A temp sensor in the controller is probably close enough for regular charging if it's close to the bank in similar ambient temps. A couple of situations to be aware of which could require manual intervention though; if ambient changes a lot fairly quickly, as might happen in an RV compartment getting hit by morning sun, will see controller temps charge much faster than battery temp because of the thermal mass of the battery. Another is during late absorb and especially during equalization, charging results in internal heating of the battery. Without a remote sensor, it would be prudent to manually monitor battery temp during EQ to be sure temp doesn't rise too much, and to spot check late absorb once in a while IMHO.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • jimbuckjimbuck Posts: 42Registered Users ✭✭
    Thanks Estragon, I will monitor the charge voltage with the data collected by the controller and I also have a thermometer that records max and min temperature which I could place in the battery compartment. In this country it would be very rare to have batteries overheat due to the sun
  • jimbuckjimbuck Posts: 42Registered Users ✭✭
    I keep posting my message to find half of it missing?
    I was also saying that maybe I could adjust the temp compensation at the beginning of summer? Don’t know if that would be any good. Also I’m surprised that the victron units don’t offer remote temp sensors, it would cost pence to incorporate one.

    Jim
  • EstragonEstragon Posts: 2,839Registered Users ✭✭✭✭✭
    I wouldn't adjust compensation seasonally. The vector (mv per degree per cell) doesn't change with temperature, and it's normally a vector that gets set (as opposed to an absolute value).

    Agreed about a RTS being a pretty low cost addition. Maybe because, as you note, daily temperature ranges are relatively small there.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • jimbuckjimbuck Posts: 42Registered Users ✭✭
    Thanks Estragon. The mv per cell can be adjusted but I just left if at factory settings. I will keep an eye on battery / compartment temperature during the summer months. I think the next controller for my own system will be a MorningStar Tristar.

    Jim
  • westbranchwestbranch Posts: 5,087Solar Expert ✭✭✭✭
    Jim, AFAIK Victron's major market is the boating set, sailboats and that sort,  and I suspect that on  a boat the temp  does not vary  too much below deck so why add a BTS for small quarters?

     
    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
  • EstragonEstragon Posts: 2,839Registered Users ✭✭✭✭✭
    The temp can vary by a surprising amount in a marine installation. On my boat, the batteries are in a box on a companionway landing over the engine. Heat from the engine migrates up to warm the batteries, but the electronics would be at cabin temp.

    In some installations, batteries are installed in the bilge, which can be much cooler than the cabin depending on the waters and season sailed. When I launch in spring, water temp is barely above freezing, and cabin is heated. The bilge wouldn't get much warmer, but the cabin can get to a cozy 70°F in a few minutes.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • jimbuckjimbuck Posts: 42Registered Users ✭✭
    edited January 28 #18
    Maybe most of their market aren’t concerned with external temp sensors
    It’s a surprise considering the marine sector covers the entire globe. I think it will be fine for my installation though. Thanks for your help
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