# Newbie still trying to figure it all out!

Registered Users Posts: 4
I have a simple question that I don't see answered anywhere online. I have a 250 watt panel It puts out about 40 DC volts not hooked to anything in bright sunlight. When I hook it up to my battery bank (805 ah-12 v deep cycle batteries) via a charge controller (30 amp) the output shows only about 13.25. Is this normal? What happens here to reduce the output of the panel when I hook it up to the batteries? I have a 120 watt panel hooked in there, too. it shows around 20 DC volts and hooked to the batteries shows 12.40. ?? I would really like to find an answer. Am I doing something wrong? Or is this normal? Thanks!

• Solar Expert Posts: 5,433 ✭✭✭✭
Re: Newbie still trying to figure it all out!

The short answer is,,, it is normal. A panel will essentially "take on" the voltage of the battery. If your battery is "full" that might be in the ~14.4 vdc range,, discharge something below 12 vdc. That said,, using a 40 voc panel for twelve volts is not a good idea unless you run it through a MPPT controller. You are likely losing a substantial output as a result,, and if are running it through a PWM controller (or no controller at all) you are likely to exceed the input voltage for a 12 volt controller,, and if you are not using a controller at all, you run the real risk of killing your battery quite quickly. Solar panels are essentially constant current devices. Under full sun they will put out ~ the designed name plate current, at the voltage of the battery. So using a 250 watt panel, might put out ~ 5 amps into 40 volts,, or ~ 200 watts net. Putting that same 5 amps into ~ 12 volts will yield only ~ 60 watts. That said, if you are using a good MPPT controller, it can convert the excess voltage and turn it into lower voltage current. A PWM controller cannot.

Additionally, you 805 ah battery bank wants ~40-80 amps of charge current. If you are trying to charge them with ~300 watts of panel, you are likely SEVERLY undercharging them. I suggest you read the following links;

http://www.windsun.com/Batteries/Battery_FAQ.htm#Lifespan%20of%20Batteries

http://www.batteryfaq.org/

Welcome to the forum,, and keep in touch,

Tony
• Solar Expert Posts: 3,009 ✭✭✭✭
Re: Newbie still trying to figure it all out!

What you're seeing is completely normal. Actually it's a good fit for the panel that shows 20 volts open circuit (not connected to anything). When you load it down, the max power output is a bit over half the open circuit voltage. That said, you're wasting power using the 40 volt OC panel on a 12 volt battery unless you're using a MPPT controller between that panel and the battery. In this case, the MPPT controller can make use of that extra voltage and turn it into extra amps, harvesting the max wattage available.
The 12 volt battery is capable of absorbing all the current your panels can deliver, thus the battery, not the panels, dictates the voltage.

EDIT: OUCH! I hadn't noticed that you were trying to charge a 805 amp battery with just 250 watts of panel, not until I read the post by Icarus, who posted while I was yet composing. Icarus is most definitely right on needing more panel watts for such a large battery!
• Registered Users Posts: 4
Re: Newbie still trying to figure it all out!

Let me see if I have this correct. My battery bank requires ~ 40-80 amps of charge current. So what you're saying is my total of 370 watts will not do the job. I figure it to be around 28.7 amps (?). What size battery bank would work better with what I have now? I haven't reached the right so called "balance" yet. I will look into the MPPT control and read the links you posted. Thank you for your answers. I appreciate all the input!
• Solar Expert Posts: 5,183 ✭✭✭✭
Re: Newbie still trying to figure it all out!

the target range is between 3% and 13% of the batteries rating, better to be at or above 5%. therefor you need to half your battery amp rating.
An Mppt CC will produce more amps than the panel rating through electronic wizardry.

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
• Registered Users Posts: 4
Re: Newbie still trying to figure it all out!

One more thing! What size MPPT controller should I look into?
• Solar Expert Posts: 5,183 ✭✭✭✭
Re: Newbie still trying to figure it all out!

depends on how many panels you will connect to it. Each has different Amp limitations.
To start pick a 30 amp MPPT and compare it to the one you already have. Rogue is a good one to look at http://www.roguepowertech.com/products/mpt3024.htm

add; it is 12 and 24 volt

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