do I need a charge controller?

bobdog
bobdog Solar Expert Posts: 192 ✭✭
I'm charging a 12 volt "deep cycle" battery for an electric fence. The panel I have is a little, old, flexible amorphous type and in the winter due to trees and location, it doesn't get enough sun to keep the battery charged. So, I'm changing to a 20 watt (new) Sunwize panel I have laying around.

My question is, do I need a charge controller or can I just wire directly to the battery/fence charger? What is the highest watts I can go in order to not over charge the battery?

I am in southwest NM. Along the Gila river. Middle of no-where.

Tim

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: do I need a charge controller?

    The rough size of the panel, using a 1-2% of battery AH capacity (without charge controller), and assuming Vmp (the voltage at which the panel will produce its maximum power) is ~15v<Vmp<20v would be:

    Battery AH * 2% * Vmp = Panel Wattage

    Example, 100 AH 12 volt battery, Vmp=16 volts (Vmp can be higher, but is just a waste of money), and installing a blocking diode between the panel and the battery (to prevent the panel from slowly discharging the battery at night):

    100 AH * 2 % * 16 volts = 32 watts

    So, the panel should be around 16-32 watts in size maximum.

    Several questions--how much power does the fence use (Amp*Hours or Watt*Hours per day), and how big is the battery, and what kind of battery is it.

    Even an inexpensive charge controller ($30-$70 or so) would be helpful for your battery... You can use a larger solar panel (vs battery size) so that it will stay charged in winter and not overcharge in the summer.

    The other issue is that Lead Acid Batteries do not like to be discharged below 50% capacity (and especially, will quickly die if discharged to near 0% capacity).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • bobdog
    bobdog Solar Expert Posts: 192 ✭✭
    Re: do I need a charge controller?

    Thanks Bill. I forgot all the math. I'm not sure the battery size as it is for a fence maintenance contractor at work. He is in the Gila and i am in Las Cruces.

    But, all-in-all I will likely use a controller as I have a MS SS-10 and that will do the trick. I was just hoping a direct connect might work, but to save on batteries I should do it right.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: do I need a charge controller?

    that 1-2% i stated bill is roughly arbitrary as less than 1% could do it, but i figure losses and stc ratings and whatever else throws a wrench into it. really all that is needed on a charged battery is to overcome monthly self discharge losses and that can vary too so i figure it's better to err on the higher side for such purposes.

    now as far as your position on this bobdog, you need to get the old (or new) panel up and in the clear so as to receive proper sunlight because a new panel even though higher power will still have the same problem if mounted in the same place. the sun powers these panels no matter what size they are.
    use the controller or some other regulation circuit even though some may tell you you don't have to.
  • Todd22123
    Todd22123 Registered Users Posts: 2
    Re: do I need a charge controller?
    BB. wrote: »
    The rough size of the panel, using a 1-2% of battery AH capacity (without charge controller), and assuming Vmp (the voltage at which the panel will produce its maximum power) is ~15v<Vmp<20v would be:

    Battery AH * 2% * Vmp = Panel Wattage

    Example, 100 AH 12 volt battery, Vmp=16 volts (Vmp can be higher, but is just a waste of money), and installing a blocking diode between the panel and the battery (to prevent the panel from slowly discharging the battery at night):

    100 AH * 2 % * 16 volts = 32 watts

    So, the panel should be around 16-32 watts in size maximum.

    Several questions--how much power does the fence use (Amp*Hours or Watt*Hours per day), and how big is the battery, and what kind of battery is it.

    Even an inexpensive charge controller ($30-$70 or so) would be helpful for your battery... You can use a larger solar panel (vs battery size) so that it will stay charged in winter and not overcharge in the summer.

    The other issue is that Lead Acid Batteries do not like to be discharged below 50% capacity (and especially, will quickly die if discharged to near 0% capacity).

    -Bill

    2 questions:
    I have a 35 Ah Battery and a 5 Watt Panel on a Sailboat. The panel is to keep the battery charged in between infrequent use. Based on the above:
    35 Ah * 2% * 17.1V = 12 Watts. So I understand I should be able to use up to a 12 Watt panel without a charge controller. My panel is 5 Watts, so I should not need a charge controller. Is this a correct interpretation?

    I thought I do need a charge controller, so I purchased http://www.solar-electric.com/sg-4.html Sunguard 4.5 Amp 12 Volt Solar Charge Controller by Morningstar. I installed it, and before hooking up the battery, I just measure the voltage in the battery leads. I was expecting 14.1 V, but instead saw Zero Volts. Does this type of charge controller put Zero volts when not connected to a Battery?

    Thanks,
    Todd
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: do I need a charge controller?

    Todd,

    Pretty much correct. For a 35 AH 12 volt battery:
    • 35 AH * *2% = 0.7 amps
    • 0.7 amps * 17.5 Vmp = 12.25 watts
    Above 1%, you probably do want to use a charge controller to limit maximum voltage. The battery will be much happier.

    Monitor the battery voltage in the middle of the day while there is no power draw--If it is below 14.2 volts, then a charge controller is not really needed (but still a good idea).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: do I need a charge controller?

    if you already have the controller then use the controller. it is true that most controllers will need the battery to be present before working.
  • Todd22123
    Todd22123 Registered Users Posts: 2
    Re: do I need a charge controller?

    Thank you to all who responded. I did read that the PWM controller uses battery current to run its circuit, so there will be no potential at the battery leads on the controller if just the solar panel is connected. Here is the website and quote for anyone interested.

    http://www.cirkits.com/scc3/FAQ-SCC3.html

    "Q: If I connect my PV panel to the SCC3 input, why is there no voltage on the battery terminals?
    The SCC3 will not function if there is no battery connected to the battery terminals, that is where it gets the power to operate. Further, the battery should always be kept above 8V for the circuit to operate properly. Typically, lead acid batteries should always be kept above 10V or they will suffer internal damage and have a very short life. If you start with a new battery that is completely discharged, you may need to pre-charge the battery by connecting the PV panel directly to the battery until it reaches 12V."

    Thanks again,
    Todd