36 volt solar to 12 volt battery

flyfighing64flyfighing64 Registered Users Posts: 2
I am new and I was going to experiment a little. I received a free solar panel and a free battery. 

The solar panel has no spec sheet on it so I don't know anything about it. It has 60 cells and I my meter says it is producing 34 volts in the sun.  below are the spec's on the battery I have. My question is what controller would I use to charge this battery with this solar panel. I just want to hook up some 12 volt LED lighting over my work bench. Like I said I am new and just experimenting. 

Thanks in advance for your help.

Battery Spec Below:

Superior Plates with an expected life span of 10 to 12 years in float service applications.

 Nominal Voltage20Hr CapacityCCA
RC (min)
 Terminal Posts DimensionsWeight
Charging Current*
Charging Voltage
Float Voltage
 2.1SAE+3/8" STUD12.9"w x 6.9"d x 10"h/10.8th**91lb
25A @ 1.75/Cell  350Min

75A @ 1.75/Cell    90Min

Short Circuit Current: 3850A
Max Discharge Current: 1500A (5s)
Internal Resistance: 3.1 mOhms

Recommended Charging Options:
  • AC Chargers: Any Quality Brand AGM- Smart & Microprocessor controlled- charger with the recommended specs above can be used. Battery Charger Guide.
  • Vehicle Alternator: 14V-15V.
  • RV Converter: 14V-15V.
  • Solar Panel: 225W-500W.
  • Wind Turbine: 225W-500W.
  • Charge Controller: 20A-UP

* Warning: Do not use a charger with smaller amperage than the recommended Charging Current shown above.
                  Maintainers should ONLY be used to maintain a fully charged battery but NEVER as a charger.

**Total Height including terminal post.

Operating Temperature Range:

  • Discharge: -4~140F
  • Charge:     14~140F
  • Storage:     -4~140F

Temperature Compensation:

  • Cycle use:     -30mV/C
  • Standby use: -20mV/C


  • OldPhartOldPhart Registered Users Posts: 5 ✭✭
    No responses?
  • BB.BB. Super Moderators, Administrators Posts: 32,204 admin
    Sorry, I missed this post--And it is a bit difficult to give "an answer" without some clarifications and such.

    I am guessing that the original poster FF64 and OP are different posters?

    Anyway, some starting observations.

    First, the flood cell lead acid battery. Lead acid batteries do not like to be stored without recharging (roughly once a month, minimum for FLA batteries. AGM and other types can go upwards of 6 months between charging--Lower self discharge). So the first thing to check is the resting voltage of the battery. If it is ~12.7 volts or higher (room temperature, no loads/charging), it is pretty much full charge. If it is ~12.4 volts or lower, and has not been recharged or on a float charger for a period of months or longer, it is possible that the battery is getting/or has been sulfated. A permanent loss of battery capacity and one of the major failure modes for FLA batteries.

    Also, this is a "float service" battery. Generally used for backup of telecom and UPS (uninterruptible power supply) systems. This batteries are not generally a good fit for "deep cycle" applications--And are intended for shallow cycling (like from 100% to 85% state of charge). If you deep cycle them (75% to 50% to 20% state of charge), you may only get a few hundred cycles (or fewer).

    But "free" is hard to beat when getting batteries. And they can get you a hand full of $ for core charges when recycled.

    If the loading is light (LED lighting). An example of a long(er) life would be a ~15% or so discharge per day:
    • 155 AH * 12 volts * 0.15 light cycle = 279 Watt*Hours per day
    • example: 279 WH per day / 40 Watts of LED lighting = ~7 hours per day of daily lighting
    • Or: 155 AH * 0.15 cycle = 23.25 AH per day @ 12 volts (if you like AH better)
    As always, be safe. Wire has to be heavy enough to handle current, and you need to use a fuse or circuit breaker (for the max rated current of the wiring) to make sure that a short circuit does not start a fire (lead acid batteries can output 100's of Amperes, or more, into a dead short).

    Next, the solar panel. A 60 Cell solar panel has a Voc voltage or ~34+ volts and a Vmp~30 volts (voltage open circuit/voltage maximum power). These solar panels are not a "good match" for just connecting directly to a 12 volt or 24 volt battery (or through a PWM type solar charge controller). It will charge a 12 volt battery (bank), but the panel will lose about 1/2 of its "wattage" because of the miss-match between Vmp~30 volts and Vbatt~14.8 volts.

    Do need to know more about the solar panel... Ideally its Power rating in Watts, or Imp (current maximum power point). If you don't know, there are several ways of estimating... One is to look for a solar panel that is the same physical size and cell type (poly or mono crystalline), or simply figure out its surface area and multiply by ~15-17% (typical modern panel efficiency). For example:
    • 1,000 W per sq mtr * 0.17 eff * 1.5 meter * 0.675 meter = 172 Watts Pmp (17% is typical "monocrystalline panel cells")
    • To figure out Imp: 172 Watts Pmp / 30 Volts Vmp = 5.733 Amps Imp
    That will get you started... Very likely the "unknown" panel is somewhere around 175 to 250 Watts (depending on age of panel, and who the source of the panel was, and choices made by the original buyer).

    It is always best to follow the battery mfg. instructions for charging... But a good place to start is 5% rate of charge minimum for a emergency backup/sunny weekend use system. 10% to 13%+ rate of charge is better for a full time off grid system:
    • 115 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 108 Watt panel minimum
    • 115 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 217 Watt panel nominal
    • 115 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 282 Watt panel/array "typical cost effective" maximum
    And then there is how much solar panel/array you need to support your daily loads and based on your location. Say fixed array near Long Beach California (original poster?):

    Long Beach
    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 56° angle:
    (For best year-round performance)

    Southern California is a nice sunny region all year round. Lets day that need the lighting mostly in the winter, and don't want to use a generator / utility power very often... So pick 4.0 hours of sun for "break even":
    • 279 Watt*Hours per day (using light cycle number) * 1/0.61 off grid DC system efficiency * 1/4.0 hours of sun per day = 114 Watt solar panel "break even"...
    Choosing the charge controller... Something 'inexpensive" would be a 12 volt PWM controller. But with a Vmp~30 VDC solar panel, you would need a 2x larger (Vmp=30) volt panel because of the losses of the "higher voltage" panel and the PWM controller on a 12 volt bank.

    An MPPT charge controller will work too... We need to know the Wattage of the solar panel that the original poster has. There are USA units, that are not cheap. And there are import MPPT solar charge controllers that are cheap/reasonable price to those that are really cheap (and may simply be a PWM controller marked with "MPPT" on the case)....

    I will stop here, please feel free to comment/ask questions. 

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • flyfighing64flyfighing64 Registered Users Posts: 2
    Thank you for the information. WOW you are knowledgeable! I will measure the panel and use your info to calculate. Sorry for the late response to you. I had watched my question for quite a while with no response and I have given up, but I came back to check it today and saw what you had to say. 

    I was looking online and I can buy a 36 volt to 12 volt regulator. Would that allow me to use a standard controller rather than and MPPT? This is just an experiment since I got the panel and battery for free. I am going to only run some LED lights at my work bench. Nothing more. I just wanted to play around with it. I will try to calculate my panel if you will educate me on the possibility of using the regulator.

    Again thank you for the very detailed response.

  • BB.BB. Super Moderators, Administrators Posts: 32,204 admin
    You are very welcome Dan. You can setup your profile to get emails when people reply to your posts. And you can "bump" a post too.

    Posts with very few details and lots of questions are difficult to answer. You did have lots of information on your batteries... But the lack of details about loads and solar panels makes things a bit more difficult... And why I just made some guesses to move the conversation forward.

    Note that PWM controllers are just simple On/Off switching transistors. They do not down convert voltage--They just control "average current" (pulse width modulation) to the battery bank based on voltage/estimated state of charge.

    MPPT controllers are (typically) down converting/Buck Mode switching power supplies. Which do efficiently (>95% efficiency at rated loads) take high voltage/low current from the solar array and "down convert" to low voltage/high current to charge the battery bank.

    If you are not using much energy, and you have a PWM controller rated for higher input voltage, it will work and charge, just not as efficiently. The Xantrex (Schneider) C-40 controller would work. As would probably any 12/24 volt PWM controller:


    More or less, current in = current out for a PWM controller. So guessing about your panel:

    172 Watts Pmp / 30 Volts Vmp = 5.733 Amps Imp
    • 5.733 amps (full sun) * 14.5 volts charging = 83 Watts
    Vs, the expected solar power with an MPPT type charge controller:
    • 172 Watts * 0.77 panel and controller derating = 132 Watts "useful" into battery bank with MPPT controller
    Note that there probably are some BP Solar 175 Watt panels out there... And many had failed. Check the shorted output current of the panel and see if it is >50% of Isc (in decent mid-day sun). Isc is proportional to the amount of sun hitting the panel. If you do not get more than 2-3 amps in "typical" mid-day sun, the panel may be going bad.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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