How Many 12v Solar Panels Needed To Charge 24v Battery?

rss2qrss2q Keep It SimplePosts: 75Solar Expert ✭✭✭
Part of me doesn't even know why I'm asking this question, but just need to confirm this with a few people.

I have 2x 12v batteries hooked up in series to produce 24v, and I need to confirm that I only need 2x 12v solar panels in order to properly charge them? Of course I'll have a charge controller and etc., but I'm only focused on the question above.

I remember when I asked someone how many panels I needed to charge a 36v battery, I originally thought I needed only 3x 12v batteries, but according to them that would not be enough, so that's why I'm double checking about this.

Don't know why this wasn't a popular Google question :p

Comments

  • CariboocootCariboocoot Gone Fishing... Posts: 17,615Banned
    Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

    Nominal Voltage-wise, 2 times 12 = 24.
    Vmp-wise, 2 times 17.5 = 35.
    Providing they are standard "12 Volt" panels two in series will produce sufficient Voltage to charge 24 Volts of battery.

    The only missing bit: how big are the batteries and how big are the panels?
  • rss2qrss2q Keep It Simple Posts: 75Solar Expert ✭✭✭
    Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

    The only missing bit: how big are the batteries and how big are the panels?

    I'm thinking of getting 2 of these 30W solar panels since they are on sale: http://www.ul-solar.com/30_Watt_12_Volt_multicrystalline_solar_panel_p/stp030p.htm

    As for the batteries, they are 12v, 35Ahm batteries.

    Does the battery size change something, as if so, maybe that's what they were referring to?
  • CariboocootCariboocoot Gone Fishing... Posts: 17,615Banned
    Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

    The Amp hour capacity of the battery determines how much you need in Watts for proper recharging:
    V * A = W
    So a 12 Volt, 35 Amp hour battery would be recharged at (minimally) 14.2 Volts * 1.75 Amps = 24.85 Watts. Now, panels don't actually put out their "nameplate" rating all the time; they put out an average that's usually 80% or so of the rated Wattage. A 30 Watt panel would average 24 Watts - so it is just about the minimum for recharging that battery.

    Put two of those batteries in series and you get 35 Amp hours @ 24 Volts. Put two of those panels in series and you have the minimum for recharging the 24 Volt battery bank.
  • rss2qrss2q Keep It Simple Posts: 75Solar Expert ✭✭✭
    Re: How Many 12v Solar Panels Needed To Charge 24v Battery?
    The Amp hour capacity of the battery determines how much you need in Watts for proper recharging:
    V * A = W
    So a 12 Volt, 35 Amp hour battery would be recharged at (minimally) 14.2 Volts * 1.75 Amps = 24.85 Watts. Now, panels don't actually put out their "nameplate" rating all the time; they put out an average that's usually 80% or so of the rated Wattage. A 30 Watt panel would average 24 Watts - so it is just about the minimum for recharging that battery.

    Put two of those batteries in series and you get 35 Amp hours @ 24 Volts. Put two of those panels in series and you have the minimum for recharging the 24 Volt battery bank.

    Okay, so where did you get the 1.75Amps from? Like is there some type of table you go by that tells you the minimum value?

    Also, would it be better to just get in two of these 20W 24v panels so that I'm not so close to the minimum value: http://www.ul-solar.com/product_p/stp020p-wp.htm

    Or should I go the 12v 30W route. This is something I'm a little fuzzy on as well. As the 20W 24v panels can only deliver .61A which is going to increase the charge time, where as the 30W 12v panels can deliver 1.77A.
  • CariboocootCariboocoot Gone Fishing... Posts: 17,615Banned
    Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

    Ah, the magic formula! The recommended charge rate of 5%-13% of the battery's Amp hour rate. You can read about this and many other fun facts in the Battery FAQ's here: http://www.windsun.com/Batteries/Battery_FAQ.htm

    What you'd really like to have is twice that 30 Watts for the 12 Volt, or four times for the 24 Volt:

    3.5 Amps (10%) * 14.2 Volts = 49.7 Watts, less derating = 62.125 Watts.
  • rss2qrss2q Keep It Simple Posts: 75Solar Expert ✭✭✭
    Re: How Many 12v Solar Panels Needed To Charge 24v Battery?
    Ah, the magic formula! The recommended charge rate of 5%-13% of the battery's Amp hour rate. You can read about this and many other fun facts in the Battery FAQ's here: http://www.windsun.com/Batteries/Battery_FAQ.htm

    What you'd really like to have is twice that 30 Watts for the 12 Volt, or four times for the 24 Volt:

    3.5 Amps (10%) * 14.2 Volts = 49.7 Watts, less derating = 62.125 Watts.

    Well if space is limited, and I mean, very limited, which route would you go with if you had to pick one, as both will work, but as you've just calculated, it will be charging at the very minimum...but it will charge the batteries, which is the important thing for me. I just want to check to see which one is faster, since we have an Amp difference.
  • CariboocootCariboocoot Gone Fishing... Posts: 17,615Banned
    Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

    More Amps = faster charging.
    The other thing to remember is net charge rate is the gross charge rate less any current going out to supply loads. So the 5% rate works for a system that doesn't have large or continuous loads during recharge time.
    My rule-of-thumb is to "shoot for" 10% and you'll get a workable rate between 5 and 10 percent.
  • System2System2 Posts: 6,290 admin
    Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

    I have a 48v solar system, can I mix a 36v 280 watt panel with a 12v 15 watt panel to get 48v ? I am new at this trying to learn, Thanks
  • BB.BB. Just some guy Posts: 24,225Super Moderators admin
    Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

    No, not really--It is like connecting a 12 volt car battery with a set of "D" cell flashlight batteries in series to get 24 volts.

    Yes, you will get 24 volt's -- but only at the current level of the battery cells--not the big car battery.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • waynefromnscanadawaynefromnscanada New User Posts: 3,009Solar Expert ✭✭✭
    Re: How Many 12v Solar Panels Needed To Charge 24v Battery?
    I have a 48v solar system, can I mix a 36v 280 watt panel with a 12v 15 watt panel to get 48v ? I am new at this trying to learn, Thanks

    A 93 watt 12 volt panel would be a good match for the mentioned 280 watt 36 volt panel.
  • rss2qrss2q Keep It Simple Posts: 75Solar Expert ✭✭✭
    Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

    I just noticed something after deciding to go with 20W panels instead of 30W panels due to the overall size of the panels.

    The 24v 20W panel produces 0.61 Amps as can be seen here.

    Hooking two of those 24v 20W panels in parallel is going to double the Amps to give us: 1.22A total.


    The 12v 20W panel produces 1.17 Amps as can be seen here.

    Hooking two of those 12v 20W panels in series is going to keep our Amps the same and still give us 1.17A total.

    So in this case, it's better to go with the 24v setup above correct? Again, I had to change from 30W to 20W panels because again I'm limited on space I found after measuring, the 30W panels would be a little too long. :cry:
  • CariboocootCariboocoot Gone Fishing... Posts: 17,615Banned
    Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

    Vmp on the "24 Volt" panel: 33
    Vmp on two "12 Volt" panels in series: 34.4

    What this means in terms of charging a 24 Volt battery with a PWM controller: not much difference.

    The current the panel produces at its Imp is not necessarily the same as the current the battery charges at once the controller and all the wire is in place.

    Typically a "24 Volt" panel will have a Vmp of 35. The two "12 Volt" units in series come closer to this by 1.4 Volts. But that could make up for the Voltage drop through the wires.
  • rss2qrss2q Keep It Simple Posts: 75Solar Expert ✭✭✭
    Re: How Many 12v Solar Panels Needed To Charge 24v Battery?
    Vmp on the "24 Volt" panel: 33
    Vmp on two "12 Volt" panels in series: 34.4

    What this means in terms of charging a 24 Volt battery with a PWM controller: not much difference.

    The current the panel produces at its Imp is not necessarily the same as the current the battery charges at once the controller and all the wire is in place.

    Typically a "24 Volt" panel will have a Vmp of 35. The two "12 Volt" units in series come closer to this by 1.4 Volts. But that could make up for the Voltage drop through the wires.

    So basically, you are saying it's better to go with 2x 12v 20W panels hooked up in series instead of 2x 24v 20W panels hooked up in parallel?
  • CariboocootCariboocoot Gone Fishing... Posts: 17,615Banned
    Re: How Many 12v Solar Panels Needed To Charge 24v Battery?
    rss2q wrote: »
    So basically, you are saying it's better to go with 2x 12v 20W panels hooked up in series instead of 2x 24v 20W panels hooked up in parallel?

    It looks that way to me. There's about .9 Watts difference in the choice.
  • System2System2 Posts: 6,290 admin
    Re: How Many 12v Solar Panels Needed To Charge 24v Battery?
    A 93 watt 12 volt panel would be a good match for the mentioned 280 watt 36 volt panel.
    Thanks for the info, I was unsure whether to purchase a MX60 or solar panels to balance the voltage.
  • nielniel Solar Shotgun Posts: 10,309Solar Expert ✭✭
    Re: How Many 12v Solar Panels Needed To Charge 24v Battery?
    rss2q wrote: »
    Part of me doesn't even know why I'm asking this question, but just need to confirm this with a few people.

    I have 2x 12v batteries hooked up in series to produce 24v, and I need to confirm that I only need 2x 12v solar panels in order to properly charge them? Of course I'll have a charge controller and etc., but I'm only focused on the question above.

    I remember when I asked someone how many panels I needed to charge a 36v battery, I originally thought I needed only 3x 12v batteries, but according to them that would not be enough, so that's why I'm double checking about this.

    Don't know why this wasn't a popular Google question :p

    normally 2 12v pvs of the proper current rating through a controller will charge a 24v battery bank. it really is governed by the vmp the pvs output and if not enough voltage is there to overcome resistive losses, operating losses of the cc, absorb voltage point of the batteries involved (and in the case of fla types the eq voltage), and temperature effects then putting another like pv in series can be done with at least trying to match up the imp ratings in the case of differing pvs. there are pvs out there that can put out enough to operate at a 24v potential without the need to series other pvs, but these are in the higher wattage range as well as expense range.

    i don't recall the question in the other thread going into the number of 12v pvs it would take to charge a 36v battery bank specifically and i believe if i'm remembering correctly that you wanted to split the batteries into differing voltage loads which you should not do on a battery bank. simply, if a pv can properly charge 1 12v battery then suffice it to say that 2 of the same pvs will charge 2 of the same 12v batteries. do realize too that when we refer to a pv as being a 12v pv that the output vmp of that pv is far above 12v and that 12v rating is for the typical charge requirements presented at that nominal 12v rating that is used by batteries.

    somehow, i feel you are leaving pertinent questions out of this like is a 20w pv enough to charge your particular battery and what these batteries are to power.
  • rss2qrss2q Keep It Simple Posts: 75Solar Expert ✭✭✭
    Re: How Many 12v Solar Panels Needed To Charge 24v Battery?
    niel wrote: »
    i don't recall the question in the other thread going into the number of 12v pvs it would take to charge a 36v battery bank specifically and i believe if i'm remembering correctly that you wanted to split the batteries into differing voltage loads which you should not do on a battery bank. simply, if a pv can properly charge 1 12v battery then suffice it to say that 2 of the same pvs will charge 2 of the same 12v batteries. do realize too that when we refer to a pv as being a 12v pv that the output vmp of that pv is far above 12v and that 12v rating is for the typical charge requirements presented at that nominal 12v rating that is used by batteries.

    Well the method for charging that 36v system has come a long way since that thread and a solution has already been found as far as how to successfully charge that portion of the system.
    niel wrote: »
    somehow, i feel you are leaving pertinent questions out of this like is a 20w pv enough to charge your particular battery and what these batteries are to power.

    I can't remember who it was here, but these have been covered as well.
  • mbmoshermbmosher New User Posts: 2Registered Users
    Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

    Hello,

    I've got a system similar to the original poster's that I am trying to set up. I've got two 30W 12v panels wired in series that I'd like to charge two 12v 55Ah gel acid batteries, so it's really a 24v system on both ends.

    The problem I'm having is finding a solar charge controller that could handle this set up, does anyone have any recommendations? I have a cheap instapark 24v 10A one in place, and while it passes the battery voltage to my load, it doesn't seem to charge the batteries at all. The solar array is sending out ~38v.

    Thanks for any help you can give me.
  • westbranchwestbranch Not So New User Posts: 4,032Solar Expert ✭✭✭
    Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

    without more specs for the panels, one cannot give a definitive answer.... but... you are shy by at least 2 more panels probably as much as 4 short or more... that is why they are not charging, not enough amps total
     
    CL 647 asleep  24V 900Ah C&D AGM
    CL 4592  175A E-Panel WBjr, 2 x 4s 140W to 24V 900Ah C&D AGM + 4s 140W 
    Linksys Wet54g WiFi Bridge ,  ASUS RTN10 router, Hughes Sat Modem
    Omnicharge 3024, Cotek ST1500W 24V Inverter,
    1680W & E-Panel/WBjr to come, Eu3000i & 1000i Gens,
    West Chilcotin, BC, Canada




  • BB.BB. Just some guy Posts: 24,225Super Moderators admin
    Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

    Another issue is the charge controller itself.

    It will stop current flow if it thinks the battery is full... Some controllers are set with a switch/jumper between 12 and 24 volts, some will even automatically configure based on the battery voltage--So if you had "dead" 24 volt batteries, it may think they are supposed to be 12 volts.

    In any case, what is the battery voltage at the terminals of the charge controller? Either it is less than ~25.4 volts (for charger to turn on--fixed, had 24.4 volt typo before. -BB) or over ~29 volts (charger reduces/stops current flow). And, in between the controller may choose to let current through or not.

    You can also just put a jumper from PV + to Batt + and see what current flow you have--It should be the maximum supported by the panels/sunlight conditions.

    And, how are you measuring the current? Ideally, I would suggest a DC current clamp meter if you can justify it (here is a ~$60 one). A DMM set to 10 amps can work--but it may also reduce current flow (especially if set to less than 10 amps full scale or there is a blown fuse).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mbmoshermbmosher New User Posts: 2Registered Users
    Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

    Panels are Vpm 17.5, 1.68A each.

    Hadn't thought of that, I'll bring a multimeter with me tomorrow try draining the batteries a bit to see if the controller kicks in.
  • westbranchwestbranch Not So New User Posts: 4,032Solar Expert ✭✭✭
    Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

    before you start, you need to do a Voltage check on the battery. If it has sat overnight you will get a 'good' reading as to its SoC. if below 12.6/ 25.2 it did not get a good/full charge or it is failing. if below full charge and you do a load test watch it closely. take readings every 15 minutes, write it down and report back. The truth is in the numbers.. Been there done that...

    HTH
     
    CL 647 asleep  24V 900Ah C&D AGM
    CL 4592  175A E-Panel WBjr, 2 x 4s 140W to 24V 900Ah C&D AGM + 4s 140W 
    Linksys Wet54g WiFi Bridge ,  ASUS RTN10 router, Hughes Sat Modem
    Omnicharge 3024, Cotek ST1500W 24V Inverter,
    1680W & E-Panel/WBjr to come, Eu3000i & 1000i Gens,
    West Chilcotin, BC, Canada




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