# Best way to charge 12v battery. 36v 12v MPPT

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?

## Comments

• Registered Users Posts: 3,118 ✭✭✭✭✭
The amps needed depends on the total capacity of the battery(s). For example, if you have 200ah (capacity at a 20hour discharge rate) flooded lead acid, you would want something like 40a charging capacity. The wattage is a function of amperage (amps x volts = watts).

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.
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
• Super Moderators, Administrators Posts: 28,302 admin
Batteries are designed (and by nature) to "hold" a stable voltage, regardless of the current going in or going out (within limits). A 12 volt battery will hold around 11.5 to 15.5 volts in normal operation.

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
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 22 ✭✭
edited September 2017 #4
If I have a 24v panel, and it is putting out 180w, will my watts be cut in half when I putt my MPPT on it to take it down to 12v??

• Super Moderators, Administrators Posts: 28,302 admin
No, MPPT controllers are ideal for high voltage array + low(er) voltage battery banks. For MPPT controllers (excluding losses) Power In from Array = Power Out to Battery Bank.

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
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 22 ✭✭
edited September 2017 #6
Thats cool. What I actually have is two 36v panels, and I am going to charge three 12v marine batteries. I think they have about 80AH each. I have a MPPT controller though so it should work. I will pulling more watts than the panels can provide, so will still have to charge batteries, but it should slow down the use of the generator.

• Super Moderators, Administrators Posts: 28,302 admin
500-600 watts of panel on a 240 ah @ 12 battery bank is a good match. Next time, 2x 6 volt @ ~220 ah golf cart deep cycle batteries will do you better.

Drawing more energy than the panels produce. ... That is a different issue (tilt panews if farther north/winter,  conservation,  and gasoline for genset).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 1
I have 2- 305w/36v solar panels.
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
• Solar Expert Posts: 650 ✭✭✭✭
jimy said:
I have 2- 305w/36v solar panels.
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
Hi jimy,
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
3600W PV, MNE175DR-TR epanel modified, MN Classic 150, Outback Radian GS4048A, Mate3, 54.4V 207AH LiFePO4 no BMS, 4500W genset.
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