36 volt solar to 12 volt battery

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?):
http://www.solarelectricityhandbook.com/solar-irradiance.html

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)

Jan	Feb	Mar	Apr	May	Jun
4.92	5.43	6.29	6.78	6.38	5.92
Jul	Aug	Sep	Oct	Nov	Dec
5.91	6.11	5.95	5.53	5.39	4.87

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. 

-Bill

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:

https://www.solar-electric.com/c40.html

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.

-Bill