Best way to charge 12v battery. 36v 12v MPPT
bloodofheroes
Registered Users Posts: 22 ✭✭
I want to run my RV for a week using two 36v panels. I am charging 12v batteries, and unfortunately I cannot increase the voltage due to the RV system. These are deep cycle RV Marine batteries. The questions I have are:
How many Amps is too much?
How many watts are too much?
Is there even such a thing?
Why does a charge controller have to take it to 12v? Why cant they charge at 36v?
How many Amps is too much?
How many watts are too much?
Is there even such a thing?
Why does a charge controller have to take it to 12v? Why cant they charge at 36v?
Comments
Charging with too high voltage is really hard on the battery, and would likely damage a controller not intended for it. You need an mppt type controller with those panels. A 12v nominal panel would put out ~18v, which a pulse type (pwm) controller could handle.
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
A solar panel is often called a "solar battery". That is really not correct. They are "current sources" and not voltage sources. More or less, the solar panel will output current (into the appropriate load) proportional to the amount of sun hitting the panel.
Yes, it is possible to connect a 36 volt panel to charge a 12 volt panel--But this is not an optimum setup. For example, say you have a panel that is 36 volts and 5 amps (36v*5a=180watt). If connected directly to a 12 volt battery and charging the battery, the battery will hold (for example) 12 volts. However, the power (Watts) going into the battery will be less (12 volts * 5 amps = 60 Watts). In this example, you will be wasting 2/3rds of the panel's Wattage. A PWM (pulse width modulation) type charge controller (less expensive solar type charge controller) is just a computer "on/off" switch
If you used a MPPT (maximum power point tracking--more expensive) type charge controller, it sort of acts like a voltage transformer. Taking high voltage/low current and down converting to low voltage/high current to support your battery/DC loads. In this (made up) example, it would look like (ignoring losses and other details):
- 36 volts * 5 amps = 180 Watts from panels
- 180 Watts from panel / 12 volts at battery = 15 amps into charging the battery bank
You ask how much power is needed to charge the batteries. Ideally, we ask what your loads/energy needs. Then design the battery bank. And lastly, design the solar array+charge controller to charge the battery bank and support your loads (and the amount of sun you get in the locations you will be camping... Summer in New Mexico is much different sun than winter in North Dakota).But--We can suggest a range of solar panels that work with your existing battery bank. Nominally we would suggest a 5% to 13% rate of charge. 5% is good for weekend/summer usage. 10%+ is better for full time off grid. However, RVs usually have weight and space/size limitations, and that we just have to work withing those specifications.
But, for a general idea, say you have a 200 AH @ 12 volt battery bank:
- 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 * 0.10 rate of charge = 377 Watt array nominal
- 200 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 490 Watt array "cost effective" maximum
If your panels are Vmp~18 volts (voltage maximum power) rated--Then you can use a PWM type charge controller.If your panels are >>~18 Volts Vmp, then you would need a MPPT type charge controller.
Those are the basics, but your needs and details are what matter. Tell us a bit more about your power needs (like how many Watt*Hours/Amp*Hours per day you need, size of your battery bank, where/when you will be camping most, will you need/have a backup AC genset, etc.).
-Bill
It is PWM controllers that "lose" the voltage advantage when high(er) voltage arrays are connected to low(er) voltage battery bank. For a PWM controller Current In from Array = Current Out to Battery bank.
Power = Volts * Current (Amps)
Power is a "complete unit". Amps and/or Volts by themselves is "incomplete" and does not tell you everything you need to know about the power/energy in your system.
I.e., 1 amp at various voltages -- 1 amp * 12 volts = 12 Watts (battery). 1 amp * 120 volts = 120 Watts (AC mains for your house).
-Bill
Drawing more energy than the panels produce. ... That is a different issue (tilt panews if farther north/winter, conservation, and gasoline for genset).
-Bill
I need some power at my cottage....a couple lights, tv, satellite receiver, etc.
What size and how many batteries do i need.
I'm assuming a mppt charger but what kind as I see there are a few dif kinds.
I was thinking about 6 6v golf cart batteries.
Any ideas or a wiring diagram would be real helpful.
Thanks in advance.
Jim
Normally, a solar power system is designed based on the loads, (a couple lights, tv, satellite receiver, etc.), it will need to supply. In this case, you already have the panels, so we will have to work in a somewhat reverse manner. Select a battery bank that can be properly charged by your panels. If using lead acid (golf cart type) batteries you would need 2 at around 300 amp hours. Larger than a golf cart battery, but there are batteries available in that range for $200 to $250.
However, with such small loads I think this is a good situation for using a 100AH LiFePO4 battery. It will cost more, about $950 verses $400 to $500 for the lead acid batteries, but it is far easier to manage, maintain and should have quite a long life. The full 100 amp hours are usable, whereas only 50% of the lead acid battery can be routinely used. It will easily handle the 30 amp charging current coming charge controller. The only caveat is, it needs to be in an environment that never gets below 25 degrees F while in use.
Since this will be a 12 volt system with a 12 volt inverter and MPPT charge controller. I would recommend using a Morningstar 300 watt inverter and a Tristar MPPT 30 amp controller.
Link to battery:
https://www.solar-electric.com/battle-born-bb10012-100ah-12v-lithium-battery.html
Link to Morningstar controller:
https://www.solar-electric.com/morningstar-tristar-ts-mppt-30.html
Link to Morningstar 300 watt inverter:
https://www.solar-electric.com/morningstar-si-300-115v-ul-inverter.html
Rick
Yes, it can be done. MPPT controllers will take the higher array voltage and lower current, and down convert to lower voltage & higher current to charge the battery bank.
That is what MPPT controllers are designed to do. At upwards of 95% efficiency.
A PWM controller is not recommended. At best, it would only be ~25% efficient.
Of course, there are a lot of details to work out (your loads, size of solar array, brand/model of MPPT controller, wiring, etc.).
If you wish to continue, I suggest you create a new thread (discussion) and we talk about your needs there in detail.
Take care,
-Bill
You can use NOCT deratings (average solar panel harvest under actual conditions/temperatures), or we just use a derating factor of ~75-77% to get roughly the same value as NOCT power ratings.
With solar, the details matter (min/max voltages from solar array--Hot/Cold, the charge controller input max input voltage, battery bank voltage, bank Amp*Hour capacity, your loads, etc.).
This why I (we) like to go through this step by step... It is easy to get overwhelmed and/or miss stuff when you jump "in the middle" of a design.
-Bill
I will answer your questions (to the best of my ability) here... But for deeper discussions about your system, I recommend that you start your own thread. That way we can focus on your specific system and needs.
Using Trojan's user manual for AGM/GEL batteries:
https://www.trojanbattery.com/pdf/TrojanBattery_UsersGuide.pdf
The standard equation for Lead Acid Battery charging voltage compensation is:
- -0.005 volts per degree C per Cell * (Tbatt - 25C) * number of cells
- -0.005 volts/C/c * (0C - 25 C) * 6 cells {12 volt battery} = +0.75 Volts (at 32F)
- 2.40 volts per cell (AGM/GEL typical) * 6 Cells = 14.4 volts "absorb" Charging
- 14.4 volt charging (at 25C/77F) + 0.75 volts (0C) = 15.15 volts (using typical Sealed Lead Acid battery params).
So, yes, that is the normal recommendation--Increase charging voltage when cold decrease when hot. Sealed batteries (VRLA, AGM, GEL, etc.) are "not refillable" so overcharging causes heating and gassing--Which reduces life and can vent Hydrogen+Oxygen gases--And shorten life even more.One issue that people in sub freezing climates run into is that many AC inverters will shut down at ~15.0 volts (high voltage alarm). So, you are left with dialing back charging voltages to below 15.0 volts, getting a new AC inverter with 16+ VDC alam/shutdown, or shutting down inverter (and possibly other loads) when charging >15.0 volts.
And a bit of other information... When you parallel batteries, you should be careful about how you wire them so that they share load and charging currents more closely:
http://www.smartgauge.co.uk/batt_con.html (the website has lots of White Papers if you want more details on batteries)
Personally, I am not a big fan of lots of batteries in parallel strings for various reasons... In your case, I would be suggesting 6 volt batteries, 2x series and between 1-3 parallel strings. More parallel strings can be a pain to maintain (more ways to fail).
And there is the issue of recommended charging current... More or less, 5% rate of charge for weekend/emergency backup systems, 10%-13%+ for full time off grid systems (for Lead Acid Batteries @ 20 Hour capacity rating)--Details matter, always refer to mfg. instructions). For solar charging, the solar array sizing is (all solar power numbers are "fuzzy", just lines in the sand as suggested staring points:
- 300 AH * 14.4 volts charging * 1/0.77 solar panel+Controller deratings * 0.05 rate of charge = 281 Watt array minimum
- 300 AH * 14.4 volts charging * 1/0.77 solar panel+Controller deratings * 0.10 rate of charge = 561 Watt array nominal
- 300 AH * 14.4 volts charging * 1/0.77 solar panel+Controller deratings * 0.13 rate of charge = 729 Watt array typical cost effective maximum
And there is sizing the array for your loads, by season, location (hours of sun per day).To start at the beginning, I suggest your daily loads size your battery bank (a nice starting point is your loads * 2 days of storage * 1/0.50 maximum discharge for longer life)... Then go through the rest of the picking/configuring the bank, sizing the solar array, picking hardware (paper designs are cheap--Buying hardware first and trying to fit together, can be expensive and a bit frustrating).
-Bill
|| 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 ,
- 100 Watt panel * 0.77 typical panel+controller deratings = 77 Watts typical max
On subzero, reflections from ice (or sandy beach) onto the panels, the derating factor can reach near 1.0 (or even a bit higher) in peak winter weather (very cold temperatures, panel pointing at sun, clear air, perhaps higher elevations).The current into the battery bank depends on the battery bus voltage--So using the 0.77 derating, the typical best you may see:
- 100 Watt panel * 0.77 derating * 1/12.0 volts = 6.4 amps into well discharged battery in bulk stage
- 100 Watt panel * 0.77 derating * 1/13.6 volts = 5.7 amps into "mid charge/float current" to loads)
- 100 Watt panel * 0.77 derating * 1/14.75 volts = 5.2 amps into battery in "absorb" stage
In absorb and float--The battery (and loads) are controlling how much current they will draw from the MPPT charger (down to below 1% rate of charge for "full FLA" battery) as current tapers down.Note that Vpanel needs to be a couple of volts above Vbatt for the MPPT controller to operate... So if you want full charging current at 14.75 volts, the Vpanel input needs to be around 16.75 volts minimum for "optimal" MPPT charge controller operation.
This thread has some details about solar cells/panels and what "current sources" look like. Lead Acid Batteries are "voltage sources"--More or less the 12 volt battery holds "12 volts" across its terminals.
Solar panels are Current Sources... Their output current is, more or less, proportional to the amount of sun hitting the panels. Solar noon, panels pointing at sun, cool/clear day, around 1,000 Watts per sq meter of solar energy. As long as the solar panel is at Vmp or less (the MPPT controller tries to measure Vmp-actual -- Which varies mostly based on temperature--Hotter panels, lower Vmp working voltage)... As the voltage falls (MPPT controller, or PWM controller/battery bank) drop the voltage, Imp stays pretty steady from Vmp to Zero volts on the panel (current source).
Of course, the actual physics is a bit more "muddy"--As Vpanel falls, the current does rise a bit... Just a like a battery--Charged is higher voltage, as discharges the voltage falls, and very heavy loads and battery wiring/chemical processes drop battery terminal voltage a bit too).
-Bill
For Lead Acid (including AGM, GEL, etc.), the offset is around -3 to -5 mVolts per degree C per cell... Assuming 25C (77F) and -5 mV offset, at 0C/32F with an AGM typical 14.2 to 14.4 volt charging @ 25C:
- (0C actual batt temp - 25C standard temp) * (-.005 volts per degree C) * (6 cells) = + 0.75 volt offset for 0C
- 14.4 volts + 0.75 volt offset (for freezing) = 15.15 Volts charging @ 0C Lead Acid Battery Temperature
As always, check the manual or ask the supplier/battery mfg what their specifications are. Note that temperature offsets lower the charging voltage for "hot" batteries.And look at your charge controller specifications. Virtually all Solar Charge controllers have an internal temperature sensor--So you want the controller in the same space as the battery bank. You don't want the battery bank in a cold crawl space and have the controller inside a 70F room.
And many controllers have a remote battery temperature sensor--You can install the sensor on the battery (terminal, side of battery, etc.) and measure the actual bank temperature.
You can also run into issues with the "wide" voltage range on a Lead Acid battery bank. Many (good quality, older designs?) AC inverters may "alarm"/shutdown at 15 Volts... And some others may alarm/shutdown at 16-17 volts. Our friends in the great white north can have inverters shutdown (or manual shut down the inverters) when full charge/EQ charging in sub zero weather with the 15 volt max inverters.
https://www.solar-electric.com/lib/wind-sun/SureSine.pdf
High Voltage Disconnect 15.5 V
High Voltage Reconnect 14.5 V
https://www.solar-electric.com/samlex-pure-sine-wave-inverter-pst-1000-12.html
-Bill