Batteries show 13.0 v on multimeter but charge controller says 13.6 v

taylorpalmer
taylorpalmer Registered Users Posts: 6 ✭✭
in Caravan, Recreational Vehicle, and Marine Power Systems #1
My batteries aren’t charging and the only thing I can think of is because the charge controller is limiting the input going to my batteries since it thinks it’s at a different level of charge. All the wires are connected properly and my solar panels are in full sun most of the day. I have four 100w panels on the roof and two 200 amp hour batteries. The panels are producing power but the power is not going to the batteries. My batteries are at 30% 😭 please help lol
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #2
    Welcome to the forum TaylorPalmer,

    Can you tell us what brand/model of solar charge controller you have? PWM or MPPT type?

    Brand/model of batteries (i.e., 12 volt AGM @ 200 AH connected in parallel, etc.).

    What is the brand/model/ Vmp & Imp of your solar panels--Something like Vmp~17.5 volts and Imp~5.71 amps... How do you have them wired (1 series x 4 parallel, 2 series x 2 parallel, etc.)? Panels are facing (south or so, tilted 45 degrees, no shade on panels--including shadow from vent pipes, chimney, overhead power lines, trees, etc.--Solar panels do not produce much energy if here is any shade on one or more panels).

    With your volt meter around middle of the day:

    What is the battery voltage (measured at battery)
    What is the Vbatt voltage on the controller (measured at battery)
    What is the Vpanel input voltage to the controller?
    Does the controller show Vpanel, Vbatt, Ipanels, Ibatt...

    Yes--You want to get the system working well and the batteries charged. For Lead Acid batteries to sit for days/weeks below ~75% SoC--They will sulfate and "die" in weeks/months.

    More or less, at the battery:

    Fully charged, rested for 3+ hours = 12.8 volts to 13.2 volts or so (FLA closer to 12.8, AGM closer to 13.2 @ 100% charge)
    50% Discharged battery (no loads) ~ 12.0 volts
    50% Discharged battery (under light loads) ~ 11.5 volts
    0% Discharged battery resting ~10.5 volts (don't go here)

    Charging:
    Absorb stage (battery >~80% SoC and battery limiting current) 14.4 to 14.8 volts (~14.4 volts for AGM, closer to 14.8 volts for flooded cell lead acid)
    Bulk stage (battery typically under 80% SoC, possibly heavy loading (from 10.5 to 14.1 volts)
    Float stage (battery "full" and controller supplying current for loads and self discharge (around 13.6 to 13.8 volts)

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • taylorpalmer
    taylorpalmer Registered Users Posts: 6 ✭✭
    #3
    It’s a renogy mttp controller and the batters are 12 v life p04 200 amp hour batteries connected in parallel. I believe my panels are renogy 100 w panels but I’m not positive. The specs on that are Vmp 20.4 volts Imp 4.91amps. 2 series 2 parallel. The panels are on the roof of my van with no obstruction. The battery at mid day today was at 13.0 and the controller said 13.6. The voltage from the panels on the controller said 70 something. 
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  • Photowhit
    Photowhit Solar Expert Posts: 6,006 ✭✭✭✭✭
    #4
    taylorpalmer said:
    renogy mttp controller and the batters are 12 v life p04 200 amp hour batteries connected in parallel. 
    I believe Renogy makes a few MPPT controllers, did you see how many amps they had in output?
    What type of loads are you running? 400 watts of array isn't much, and flat on your van won't be optimal. figure you might get around 2000 watthours on a very good day. overcast days could be 20-30% of that.
    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.
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  • taylorpalmer
    taylorpalmer Registered Users Posts: 6 ✭✭
    #5
    They had between 1-4 amps of output. I’m barely drawing any power from my batteries too so 400 watts should be plenty for my setup. It has been plenty up until now. For some reason my batteries aren’t charging even though there is plenty of sun 
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  • taylorpalmer
    taylorpalmer Registered Users Posts: 6 ✭✭
    #6

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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #7
    The Panel specs. are good for your needs. Ideally, you should have 2 to 3 panels in series. With 4 panels, that should be 2x in series with 2 parallel strings and Vmp-array around 40 volts.

    If you are seeing around 70 volts, that would probably be 4 panels in series. If this is your controller (or close to it), then Vpanel max input voltage is 100 VDC.

    In summer, 4 panels in series is probably just below 100 VDC for Voc-cold. However, if you will be in a cold (sub freezing) environment, the Voc-array will exceed the 100 VDC controller input voltage.

    For your setup (with 12 VDC battery bus), 2x panels in series x 2x panel strings in parallel is the optimum configuration for your 4 panels.

    https://renogy.com/100-watt-12-volt-monocrystalline-solar-panel-compact-design/
    Optimum Operating Voltage (Vmp): 20.4VOpen-Circuit Voltage (Voc): 24.3V
    Optimum Operating Current (Imp): 4.91AShort-Circuit Current (Isc): 5.21A
    • 24.3 volts Voc (@77F STD test conditions) * 4 panels in series = 97.2 Volts Voc-array-std
    1. First step, turn off/disconnect the solar array to controller. Then disconnect/turn off the battery connection (always battery connected before solar array is connected to avoid controller damage
    2. Next step, check/reconfigure the array to 2s x 2p connections.
    3. Connect battery bank to controller, then connect solar array to controller (this resets the controller too--If needed)
    Next--Under good sun, can you tell me how much current is coming from the solar array (at what voltage)? And what voltage/current to battery bank?

    For LiFePO4 batteries, they have very little voltage drop (during discharge) and voltage rise (during charging). Your batteries should have a Voltage vs State of Charge like this:

    https://megabatteries.com/12-volt-lifepo4-battery-pack-12-ah-144wh/



    Say your array, charge controller, and battery bank is working well... Then as an estimate it would take (roughly)
    • 400 AH * 70% state of dis charge * 13.0 volts nominal = 3,640 WH to recharge
    • 3,640 WH to charge * 1/400 Watts * 1/0.77 off grid Li Ion charge eff = 11.8 hours of "full" sun to recharge
    Guessing you are around Portland OR, the amount of sun (hours per day) for July (mounted flat to roof):
    solarelectricityhandbook.com/solar-irradiance.html

    Portland
    Average Solar Insolation figures

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

    JanFebMarAprMayJun
    1.25
         		2.08
         		3.08
         		4.14
         		5.03
         		5.78
     
    JulAugSepOctNovDec
    6.24
         		5.55
         		4.22
         		2.52
         		1.36
         		1.02
     

    Or days of charging:
    • 3,640 WH to charge * 1/400 Watts * 1/0.77 off grid Li Ion charge eff * 1/6.24 hours of sun per day = 1.9 days to recharge
    • 400 Watt array * 0.77 panel+controller deratings * 1/13.0 volts nominal = 23.7 Amps about the "best harvest" amps on a cool summer day around solar noon.
    After you have configured the array, and measured the Array Vmp-array and Imp-array, and Vbatt and Ibatt--What are those values now?

    From my above guesses--You are looking at ~2 full days to recharge your battery bank in Mid-July... So, you will not see the battery reach full in a couple hours of full sun. Getting actual measurements of current and voltage will be a big help in debugging.

    If you are going to do your own work (12 VDC and 120 VAC circuits, RV circuits, DC power systems, etc.)--You might want to look into purchasing an AC+DC Current Clamp DMM (digital multi-meter). The current clamp works great. Just clip over an insulated wire, set for AC or DC current, and read the value (DC measurements, clamp meter needs to be "ZEROED" as they tend to drift a bit over time).

    https://www.amazon.com/gp/product/B00O1Q2HOQ ($50--Inexpensive and "good enough" for our needs)
    https://www.amazon.com/Auto-Ranging-Resistance-Klein-Tools-CL800/dp/B019CY4FB4 (example of "mid-priced" Clamp meter

    NOTE: There are AC only clamp meters too... They are very good and work well... Just do not work to measure DC current.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • taylorpalmer
    taylorpalmer Registered Users Posts: 6 ✭✭
    #8
    Thank you for all the info! I’ll check the array configuration tomorrow and retest and get back to you. I don’t know much about solar if you couldn’t tell lol 
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  • mike_s
    mike_s Registered Users Posts: 156 ✭✭
    #9
    You say you have a 0.6 V (13.6 - 13.0) drop between the controller and battery, at 1 to 4 A. Let's call it 2 A. Ohm's law tells us that the resistance of the wiring is 0.6 V / 2 A = 0.3 ohm. That's the resistance of about 450 feet of 8 ga copper wire. Even if that's 4 A with only12 ga wire, it would be almost 100 feet of wire.

    Check your wiring and connections.
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  • Photowhit
    Photowhit Solar Expert Posts: 6,006 ✭✭✭✭✭
    #10
    Just to question how things are being measured. What you are seeing on the charge controller is the 'system voltage'. If you are charging the voltage will be high than the battery voltage at rest. If you disconnect the charge controller before testing the voltage at the battery bank the battery voltage will be less. With little internal resistance, I would expect the voltage drop at the battery would fall pretty much instantly.
    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.
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  • NANOcontrol
    NANOcontrol Registered Users Posts: 273 ✭✭✭
    #11
    I had that situation, a substantial looking "gold plated" three fuse unit had 0.3V between the cartridge fuse clip and the mounting bus bar. It wasn't probably gold from China.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #12
    The planned voltage drop for a 12 volt bank from battery to charge controller under full charging current should ideally be 0.05 to 0.10 volt (i.e., short heavy cables, good connections).

    And why I was asking you to use your volt meter to measure the voltage (under full charge, and tell us what amperage) both at the battery and at the charger's battery connection terminals.

    Sometimes, it has been reported that Charge Controller's have inaccurate voltage/power readings. Not common. A more common issue (it seems) is a DMM meter with near dead batteries and inaccurate readings--So that is something else to check.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • taylorpalmer
    taylorpalmer Registered Users Posts: 6 ✭✭
    #13
    I took it to a friend that builds vans and we concluded that the batteries are most likely dead ): 
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  • NANOcontrol
    NANOcontrol Registered Users Posts: 273 ✭✭✭
    #14
    I take that as you don't really have any idea of the actual issue. The battery condition has no relation to the voltage drop between controller and battery. This should be a learning moment. Either it wasn't measured correctly, or you will drop in a new battery and it will happen again.  You are only seeing the result. That much of a voltage drop will create a situation where a new battery will be in a constant state of low charge and it will fail again.
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  • littleharbor2
    littleharbor2 Solar Expert Posts: 2,101 ✭✭✭✭✭
    #15
    Does your controller have the ability to adjust the voltage reading? I had a different reading on my Midnite Classic than on my Bogart Tri Metric and actual voltage reading taken with my Fluke meter. When I switched to LiFePo4 batteries I needed the controller to be accurately reading the battery voltage due to the sensitivity of the lithium batteries to high voltage. I asked on the Midnite Solar forum and found out how to adjust the voltage reading. Maybe you can do this with yours.

    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,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.

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