Testing existing system to size for upgrade in van

marybmaryb Registered Users Posts: 2
Sorr6 if this is too much info, but I am erring on the side of including too much as opposed to leaving any out. I am full time in a van for 2 years. My system is 295watt mono panel..two 100ah AGM batteries...Mppt controller..3 individual DC outlets run directly from batteries, 10 gauge wire, fused inline with 10amp fuses. For the past 2 years I have run my 12v Cpap machine, iPad and cell phone no power issues. I introduced a 12v cooler/frig, Iceco vl35. I was stuck at mechanics waiting for parts when I first plugged it in, parking spot not receiving sun entire day, batteries depleted after 5 days, I assumed it was the half sun situation turned off frig. Got to full sun situation turn on frig, batteries depleted after approx 10 days. 
Next stop RV repair outlet, asked that they check and test all my connections. they kept van all day testing, said batteries were fine, solar panel output 10.9. 
Next day find that battery wire going into the mppt was very loose and one outlet has gone dead (fuse intact).
Tightened the wire to mppt.
Contacted the frig manufacturer to get more info on expected power usage, they became defensive when I asked about the low voltage shut off feature that had not kicked in, yelled at me to keep the frig they would refund my PayPal and hung up,  their reaction was very odd but oh well🤷‍♀️.
Next I thought to test all my devices for actual real world usage instead of relying on the labels. My thinking was, this will tell me if I need to upgrade.
I purchased a watt meter and spliced it into a DC extension cord so I could see ah usage in real world application.
The results so far are
CPAP 12 hour test overnight
17.38ah used voltage at 12.10
Frig after 4.5 test during sunlight hours
5.47ah used, voltage at 12.78 
Do I need more tests to make a conclusion of what to upgrade or???


  • PhotowhitPhotowhit Solar Expert Posts: 5,576 ✭✭✭✭✭
    maryb sai"...including too much as opposed to leaving any out."
    Right idea...

    If it's a compressor type fridge it will draw max power on and off, so need a 24-48 hour test.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • BB.BB. Super Moderators, Administrators Posts: 31,684 admin
    Welcome to the forum Maryb,

    Can you tell us a bit more? Where is the van now (nearest major city--figure hours of sun per day)?

    Panel is mounted flat to roof (no tilted/facing south, Vmp/Imp rating of panels)?

    What Voltage and AH rating of batteries (2x 12 volt @ 100 AH each?)?

    How are you measuring Amp*Hour for your loads/fridge?

    Guess that CPAP around 15-20 AH per 8 hour night with humidifier (heater) turned off... Your 17.38 AH @ 12 VDC for overnight usage seems like a relatively low/accurate number.

    Solar panel output: 10.9 Amps (?) * 30 volts Vmp(?) = 327 Watts (pretty high output for 295 Watt panel--Typical max around 227 Watts near noon, perfect/cool day, etc.). Need more information on panel and testing (10.9 amps @ ?? Voltage--Was this solar panel output, or MPPT controller to battery output)?

    Was this 5.47 AH @ 12 volts for 4.5 hours? If so:
    • Worst case usage: 5.47 Amp*Hours * 1/4.5 hours used * 24 hours per day = 29 AH per day @ 12 volts (probably worst case)
    • Typical usage from web page: 0.67 AH (@ 12 volts @ 25C/77F) * 24 hours per day  = ~16 AH per day @ 12 volts
    Typically, the CPAP and Refrigerator/Freezer are the big energy uses... If the fridge/was "warm" when you first plugged it in--Then is probably uses about 2x more Amps (Amp*Hours) for the first day until it cools down.

    Can do more when we know more (specific battery capacity, where/what what measured for solar panel 10.9 Amps/Volts/etc.)...

    But, as a guess--Say still relatively low sun (late winter/some clouds--Don't know your location) at 3 hours of "noon time equivalent" sun per day (panel mounted flat to roof). Assume MPPT controller working well. This is what I would guess your system can produce. Note: following link, you can estimate hours of sun per day by month / array tilt):
    • 295 Watt array * 0.77 panel+controller derating * 1/14.5 volts charging = 15.7 Amps best case (few times a year at solar noon/clear/cool day, battery bank less than 80% state of charge)... You will typically see less (if battery bank >~80% state of charge, etc.)... Down to 1/2 that on "average" weather day of 8+ Amps charging/DC current from MPPT controller typical max is not bad.
    • 295 Watt array * 0.61 DC system eff * 3.0 hours of sun per day = 540 WH per day average harvest (@ 3.0 hours of sun per day)
    • 540 WH / 12.7 volt battery bank = 42.5 AH per day (3.0 hours of sun) estimated average harvest
    • 18 AH CPAP + 29 AH Fridge (measured) = 47 AH @ 12 volts estimated usage
    • 200 AH battery bank * 0.50 max suggested discharge = 100 AH recommended daily max discharge (25% is better for full time off grid and solar only charging)
    • 100 AH / 17.38 AH CPAP = 5.8 days to 50% (no sun/charging)
    • 200 AH / 17.38 AH CPAP = 11.5 days to "dead" (no sun/charging)
    If you add another ~16 AH per day for fridge/freezer (to CPAP usage) and "no sun"/charging, taking the battery bank dead in 5 days makes perfect sense:
    • 200 AH * 1/(17.38+16) AH per day = 6 days to "dead" with average loads, no charging/sun
    Normally, for a full time off grid solar power system--I would be suggesting that your "base loads" should be no more than 50% of "predicted output" for optimum battery/system performance... On nice sunny days, you can add other loads (Instant Pot for cooking, vacuuming, charge laptop, etc.)...

    Your system (without the Fridge) was nicely sized. Guessing but your solar harvest was about 2x your daily average usage--And as long as you got "reasonable" sun, the system would normally keep up without using AC or genset charging.

    When you add a fridge, you (roughly) doubled the energy usage--And adding poor/no sun during that time--Simply not enough (reliable) solar charging to keep ahead of your daily loads.

    Just to look at the numbers--An "optimum" system design would be 2 days of battery capacity (2 days no sun) and 50% max planned discharge (for longer life), the size of the battery bank would look like (assuming all loads are @ 12 VDC):
    • 16 AH Fridge (average cool weather)
    • 17.38 AH CPAP
    • 8 Watt Ipad * 5 hours * 1/12 volts = 3.33 AH per day
    • 10 WH Cell Phone per day / 12 volts = 0.83 AH per day
    • Total: 37.5 AH per day
    • 37.35AH per day * 2 days storage * 1/0.50 max discharge = 150 AH @ 12 volt battery bank suggested minimum
    • Your 200 AH battery bank looks like a good fit, even with more loads
    • 37.35 AH per day * 12.8 volts = 478 WH per day
    And calculating the size of the array--One based on battery capacity (more batteries, more solar panels). And the second based on your daily loads, hours of sun per day, seasons you are "off grid, etc....

    First based on battery capacity. 5% rate of charge suggested for summer/weekend usage. 10-13% rate of charge suggested minimum for full time off grid use. Typically, deep cycle batteries suggest a minimum of 10% rate of charge:
    • 200 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 188 Watt array minimum
    • 200 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 377 Watt array nominal
    • 200 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 490 Watt "Typical" cost effective maximum array (RVs and poor sun may require larger array)
    And based on hours of sun per day and your loads... Say you have "not great" winter sun at 2 hours per day and want to run from solar only:
    • 478 WH per day average load * 1/0.61 off grid DC system eff * 1/2.0 hours of (winter) sun per day = 392 Watt array "break even"
    • 392 Watt array * 1/0.50 max base load (loads you "always" want to run) = 784 Watt array with "base load fudge factor"
    So--You think you had "too much" information? How about my overly long post with lots of guesses thrown in.  :D

    At this point, your 200 AH @ 12 volt AGM bank is not bad... And doubling your array from 295 Watts to 590 Watts would not be a bad choice. Note you need to "pick" the correct 2nd solar panel (Vmp in parallel, or Imp if in series) with the input specifications of your MPPT solar charge controller (not overly complex process--But is confusing the first time through).

    Regarding your batteries--Taking Lead Acid (AGM) and other batteries to "dead"--Can severely shorten their life (to months or even a few weeks of life)... Charging a "dead" Lead Acid battery right away can help extend its life (AC power, genset, etc.)--But best to avoid.

    Note that the Low Voltage Disconnect for Charge controllers, AC inverters, DC Loads, etc. are typically set to 10.5 volts... That is "dead" for a lead acid battery. The LVD is not really there to protect the battery bank from over discharge (picking 11.5 volts is "better" disconnect set point), but is there to protect the loads (from AC invert taking too much current, DC motors from stalling, etc. and overheating/failing from over temperature). You really do not want to ever run your system to 10.5 volts "dead" battery level... This is really hard on most battery chemistries.

    I would suggest 1) new 2nd solar panel (matching MPPT controller input specs., and rated current):
    • 784 Watt array * 0.77 panel+controller derating * 1/14.5 volts charging = ~42 amp suggested minimum rated MPPT controller output (40 amp is "close enough")--As always check the controller's specifications. Not all are the same.
    I do like to suggest that you "over panel" your system--That way you can use it without micro managing your loads, weather, parking issues... Like the system was without the refrigerator (it worked!).

    At this point, you did not talk about backup genset, AC shore power, etc... So I would guess those are not great options for you (back to "over paneling" your system for "easy" power/energy solution).

    Your thoughts/corrections/etc.?

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • marybmaryb Registered Users Posts: 2
    I could cry! I think I finally found the right place to get my answers. Ty! It has been a very frustrating road, finally I said, I will just try and learn it myself.
    First I should have added, When I purchased the system from the DIY Solar Warehouse(should be renamed to Don’t know much about solar warehouse but we accept cash and cards on overpriced merchandise)...There was a 3,000watt inverter included in purchase, this inverter never worked, it always tripped some beeping and shut off, 3 repair people later I said just bypass it for now. That was 2 years ago and I have found I never needed AC anyway, just DC works for me. So everything I do is straight DC.
    To answer your questions,
    1. I am in El Cajon, East San Diego County, lots of sun, very few  overcast days per year. I park in full sun most days for the entire day. 
    2. Panel is not tilted, it is just mounted on my van roof, kind of crappy mount, it is mounted to 2x4 so there is air circulation. I don’t know the rating of the panels.
    3. I will include a pic of the one of the battery but I don’t think it will help, I can’t remember what the salesman told me, beyond they are supposedly Deka AGM 100ah each, so I was told I had 200ah but who knows..of course at that time I didn’t know not to discharge them beyond 50%..it was never an issue until the recent introduction of the frig.
    4. To measure my amp hours (if this is totally stupid you have permission to call me a dumbass)..I bought a watt meter and a DC extension cord and spliced them together then plugged the extension into a dedicated outlet and the device into the extension, I made sure both the extension cord and the watt meter were the same gauge wire.  I know there are better ones that just hook over the wire but those were out of my budget and this cost 20 bucks to make, so MacGyver meter for the win!
    5. My Cpap is 6v and I switched to a DC cord for it, I don’t use the heater or the humidifier on it, so no adapter cord etc.
    6. I do not have access to shore power or generator etc., My van is not so great, so I can’t really go traveling, I am just stealthy here in Southern California for the forseeable future, so it is my self contained system only.
    7. I have a DC battery pack oxygen concentrator I would like to add into my power mix, I have not used it or tested it yet.
    I found a supplier where I can buy the cells and bus bars(?) and bms to build a 100ah lithium battery, I thought maybe 2 of those? Instead of keeping the AGM. So 2 295w panels, 2 100ah lithium, and a new mppt? Idk.
    My other power needs are not anything power wise. I use Usb rechargeable lights that I only have to recharge once every few months. No instapots, no blow dryers, no heaters.
  • BB.BB. Super Moderators, Administrators Posts: 31,684 admin
    Mary, You are doing fine... OJT (on the job training) is pretty much where all of us were at one point or another.

    The reason I "do the math"--Is it is cheaper than buying lots of hardware (which may or may not work), and "level set" expectations.

    Things that are hard to see at first... There are two major power (a rate, like Miles per Hour), and energy (like total miles driven) numbers to understand. The battery bank, obviously, store 'energy' (Watt*Hours or Amp*Hours*Voltage). Like a 10 gallon gas tank. But there is also how much power (Watts or Amps*Volts) the output over short periods of time.

    So, for example, say you work from the van and need a reasonably powerful laptop and your cell phone for networking. Roughly 30 Watts for laptop+cell phone (power/rate of energy usage). And you want to use if for 12 hours per day (8-10 hours working, plus some reading/movies in the evening). And a microwave which may take 1,200 Watts (lots of power)--But only 10 minutes a day (short period of time).

    For a typical battery bank--Sizing the storage (2 days storage * 1/0.50 max discharge or 4x daily usage is typical "off grid cabin" flooded cell lead acid battery design):
    • 30 Watts for Laptop * 12 hours = 360 WH per day
    • 1200 Watts for Microwave * 1/0.85 AC inverter eff * 1/6 hour per day (10 minutes) = 235 WH per day
    So the laptop (small power usage but lots of hours per day) uses more "energy" than the energy hog microwave (but 1/6th hour per day)

    Same with fridge and CPAP--Relatively small loads that run 12-24 hours per day--Driving a larger battery bank.

    But the other calculation is the short term power usage... The laptop takes 30 (to 60) Watts... And the microwave takes (1,200 Watts * 1/0.85 AC inverter eff) 1,413 Watts from the battery bank... So the battery bank needs to be "large enough" to supply that >1,412 Watts... When converted to the DC battery bus:
    • 1,412 Watts DC / 10.5 volts (battery cutoff voltage for inverter) = 134.5 amps on 12 VDC bus
    For a Flooded Cell Lead Acid deep cycle battery, the fastest discharge is in the Range of C/5 (these are rough numbers for sizing not accurate to x.xxx decimal places):
    • 200 AH bank / 5 hour discharge time = 40 amps
    • 40 Amps * 12 volts = 480 Watts max
    For an AGM or typical Li Ion LiFePO4 battery, maybe C/1 hour discharge rate:
    • 200 AH AGM bank / 1 hour = 200 Amps
    • 200 amps * 12 volts = 2,400 Watts max
    So--A 3,000 Watt inverter is a pretty big load for a 12 volt @ 200 AH AGM bank... And you also need to look at the cabling:
    • 3,000 Watts * 1/0.85 AC inverter eff * 1/10.5 volts cutoff = 336 Amps (at max 3,000 Watt continuous load)
    That is awfully heavy copper cable to carry that current at 12 volts and 0.5 volts max suggested wiring voltage drop

    Normally, for a Lead Acid @ 12 volt battery bank, I would be suggesting a maximum 1,200 or 1,800 Watt AC inverter to keep the current "reasonable" and have a relatively reliable system (high power, partially discharged battery bank, in cool weather).

    For example, a 1,000 to 1,200 Watt AC inverter with a 700 Watt microwave is probably a nice fit for your system size/RV usage...

    And if you did not need that high of power--A small 300 Watt or so PSW 120 VAC Inverter would work well too (you can run smaller AC devices without having a separate DC adapter for each one). For example, use the 300 Watt inverter to charge your batteries, O2 machine (maybe--Need more specs), and such.

    Next Page:
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Super Moderators, Administrators Posts: 31,684 admin
    With solar (and in an RV), going small/efficient with your loads will save you money and heart breaks (of expensive equipment/failing batteries from under charging, etc.).

    Lets look at "your needs" (as I understand them).

    O2 portable concentrator... Guess at 75 Watt. As always varify your power/energy numbers. For 120 VAC, a "kill-a-watt" meter or similar from Amazon/Ebay/Homedepot is a great tool:

    For O2 machine (note: I like to do all calculations in Watts and Watt*Hours--If you do in in Amps and AH, need to know if 12 VDC or 120 VAC--Can get confusing). At the moment, all 12 VDC loads (an AC inverter is roughly 85% efficient--So if AC, :
    • O2: 75 Watts * 4 hours per day (as needed) = 300 Watt*Hours per day
    • CPAP: 17.38 AH * 12 volts =  209 WH per day (DC side power)
    • Fridge (after 1-2 days of cooling): 16 AH * 12 volts = 192 WH per day
    • LED lights: 7 Watts * 2 bulbs * 5 hours per night = 70 WH per night
    • Total: 771 WH per day (with O2 machine estimate)
    • Total 471 WH per day (without O2 machine estimate)
    To size the battery bank for Lead Acid/AGM, 2 days + 50% max discharge works well--And your loads are relatively small--So your AGM battery bank is not very large or heavy:
    • 771 WH (DC) * 2 days storage * 1/0.50 max typical discharge = 257 AH @ 12 volt battery bank (O2 machine)
    • 471 WH (DC) * 2 days storage * 1/0.50 max typical discharge = 157 AH @ 12 volt battery bank (O2 machine)
    You can see that adding a (guesstimate--As always get the numbers/measure you loads yourself--I am OK at estimating, but not an expert by any means) of 300 WH per day for an O2 concentrator is a big hit to your energy budget...

    The math for a 10% rate of charge for Lead Acid/AGM battery bank(suggest 10-13% rate of charge minimum):
    • 257 AH * 14.5 volt charging * 1/0.77 panel * 0.10 rate of charge = 484 Watt array nominal (10% with O2 concentrator)
    • 157 AH * 14.5 volt charging * 1/0.77 panel * 0.10 rate of charge = 296 Watt array nominal (10% without O2 concentrator)
    And sizing for your location and estimated loads for array mounted flat to roof, San Diego California:
    http://www.solarelectricityhandbook.com/solar-irradiance.html (solar estimator)

    San Diego
    Average Solar Insolation figures

    Measured in kWh/m2/day onto a horizontal surface:


    So, 3 hours per day of sun was a close guess... Remember, these are 20+ year averages, and if you are in a valley/have some shading from trees, marine layer, etc... It all affects the harvest. Using 3.09 hours of sun for December use 50% to 65% base load derating (array sized larger for variable weather/energy needs--I.e., a larger array makes things easier (if it fits on RV Roof, and you can swing the costs):
    • 771 WH per day * 1/0.61 DC solar system eff * 1/3.09 hours per day =  409 Watt array December "break even" O2 machine
    • 409 Watt array / 0.50 derating fudge factor = 818 Watt array assuming all your loads are required "daily" + O2 machine
    • 808 Watt array * 0.77 panel+controller derating * 1/14.5 volts charging = ~43 Amp minimum rated MPPT controller (O2)
    • 471 WH per day * 1/0.61 DC solar system eff * 1/3.09 hours per day = 250 Watt array "Dec break even" no O2 machine
    • 250 Watt array / 0.50 derating fudge factor = 500 Watt array assuming all your loads are required "daily" no O2 machine
    • 500 Watt array * 0.77 panel+controller derating * 1/14.5 volt charging = 27 Amp minimum rated MPPT controller (no O2)

    Depending on your O2 needs (and if you ever want to add an induction cook top, etc.)... Lead Acid flooded cell deep cycle batteries are the cheapest and most forgiving... Just check electrolyte levels once a month, add distilled water as needed), check specific gravity once a month, etc... AGM are maintenance free and much "cleaner" than FLA (and tend to be more efficient than FLA).

    LiFePO4 batteries are just about the "perfect battery" (very efficient, very low losses, no maintenance, support high(er) discharge rates). Their downside, usually a BMS (battery management system) is used to "protect" the battery from over/under charging, over current). Some folks choose to run LiFePO4 batteries without a BMS--It can be done--You do need to check each "cell" (or parallel group of cells) for balance with the other groups (all cell voltages should match--If one cell is high or low, then need to manual balance). And their one big advantage is they charge much more quickly than Lead Acid batteries--For solar power systems with limited hours of sun in the sky (especially during winter), fast charging can be a huge advantage... The fact you can use a smaller/lighter Li Ion battery bank--Very nice for RVs with limited space/weight carrying capacity.

    More or AGM are 2x the cost of FLA batteries... And LiFePO4 batteries are probably 2x more than AGM... However, there are lots of different sources for Li Ion batteries and battery systems.

    I don't really know enough about what is out there for Li Ion battery and BMS systems... Others here can help you more with that.

    If you decide to look at Li Ion w/ BMS, there are solar charge controllers that can integrate with the BMS. This is really nice (better system control and less chance to damage the battery bank)--Again, very nice--But not everyone does that (typically more expensive solution).

    Get the "paper design" close to what you want--Then you can look for hardware and do some pricing on the different options.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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