installing 12v marine batteries for 24 v solar panels

Using our rules of thumbs for an off grid system...

Generally, suggest 2 days of storage, and 50% discharge is a pretty nice mid price/performance point.

And you can run the math from several directions... 1) is design the system to support your load. 2) Have solar panels, what battery bank/loads can be supported. Or 3) have the batteries, what size solar array needed and loads that can be supported. Obviously, I am a big fan of #1 (you do vary the design based on how much $$$ you want to spend and adjust your loads--Conservation is generally cheaper than generating power).

I assume that batteries are the big expense--So you design and run the system to "keep the batteries happy".

You have 3x 265 Watt array = 795 Watt array.

Generally, design the solar array to provided 5%/10%/13% of charging current for your battery bank. 5% can work for a weekend/summer/backup system. 10%+ suggested as a good starting point for full time off grid (9+ months off grid use a year). At this point, solar panels are "relatively cheap" and batteries are "relatively expensive". So, "over paneling" saves batteries (and reduces generator+fuel usage--If needed).

• 795 Watt array * 0.77 panel+controller derating * 1/29.0 volt charging = 21.1 Amps typical max current
• 21.1 Amps * 1/0.13 rate of charge = 162 AH "suggested minimum AH" battery bank @ 24 volts
• 21.1 Amps * 1/0.10 rate of charge = 211 AH @ 24 volts nominal battery bank
• 21.1 Amps * 1/0.05 rate of charge = 422 AH @ 24 volt maximum battery bank (solar only/primary solar)

Then there is how much energy you can get from your system. I assume that you charge during the day and use battery bank at night. For an FLA battery bank, suggest 25% discharge per day. For Deep Cycle batteries, that would be 50% discharge for 2 days of "no sun/no genset" use. For Marine Batteries, I would suggest a daily discharge of 25% (to 75% state of charge)--Or that you need sun/genset the next day to bring the battery bank to "full" (90%+ state of charge). In either case, assume don't discharge below 50% state of charge for longer battery life.

Assuming a 10% rate of charge (nominal) as an example:

• 211 AH suggested / 90 AH batteries = 2.34 strings ~ 2x 12 volts in series times 2 parallel strings (90 AH each) for a 24 volt @ 180 AH battery bank

From the get go, that is probably a much smaller battery bank than you would wish. But, for the math, this is how it works (you can assume 5% rate of charge with a genset to help, weekend/summer only--I don't know):

• 180 AH * 24 volts * 0.25 (one day discharge) * 0.85 AC inverter eff = 918 Watt*Hours per day of 120 VAC power

And to recharge that energy usage, we need to know where the system will be installed (and used summer/winter/etc.). 
http://www.solarelectricityhandbook.com/solar-irradiance.html

Kalamazoo
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 48° angle:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
3.06	3.64	4.19	4.62	4.88	5.36
Jul	Aug	Sep	Oct	Nov	Dec
5.38	5.05	4.80	3.80	2.58	2.48

Toss the bottom three months (assume genset usage/cabin shutdown for winter). Use February @ 3.64 Hours of sun per day (long term average) as the "break even" month (you may or may not need a genset depending on how "dark" it is "this month"):

• 918 Watt*Hours * 1/0.52 off grid system eff * 1/3.64 hours (Feb) = 485 Watt array minimum suggested
• 795 Watt array * 0.52 off grid sys eff * 3.64 hours (Feb) = 1,476 WH "average available" power (Feb)

That is not a "bad fit". You have more sun than you need to fully recharge the battery bank, and some energy left over to run appliances during the day ("excess" solar power), and still run 918 WH at night (lights, tablet computer, cell phone, 24 volt RV water pump).

Always suggest that your "base loads" (loads you need to run every day--such as lighting, computer for work, etc.) is no more than 50-65% of your loads... Optional loads, such as pumping to cistern, washing clothes, vacuuming, etc., you run when you have a sunny day.

Then there is wiring your present solar array... If you use a "less expensive" PWM charge controller, you need the Vmp of the panels in the range of Vmp=35-40 volts. You would wire them in parallel and run a (795 Watts / 35 volts Vmp = 23 amps) ~25-30 Amp PWM solar charge controller.

If your panels are Vmp~30 volts (typical for 60 Cell Grid Tied solar panels), then you need the array to be Vmp-array over 40-50 volts minimum. That would be 2 panels in series (Vmp-array=60 volts), and one panel "left over", or 3x panels in series for Vmp-array=90 volts.

MPPT solar charge controllers that can run > 90 Volts Vmp-array, they are out there and plentiful, but tend to be larger/more expensive units. Great if you plan on adding to your system later (typically in the 60-90 Amp rated output range).

And for your 180 AH @ 24 volt battery bank--I would be suggesting a maximum AC inverter capacity of 230-460 Watts (assuming 500 Watts per 100 AH @ 24 volt battery capacity). The most continuous power from a FLA battery bank would be:

• 180 AH * 24 volts * 1/5 hour discharge rate * 0.85 AC inverter eff = 734 Watts continuous

And, maybe 2x 734 Watts = 1,468 Watts starting surge (induction motor based saw, water/well pump, etc.).

The "average usage" of power--Say 5 hours per night from your battery bank would be:

• 180 AH * 24 volts * 0.25 over night usage * 1/5 hours per night = 216 Watt average AC load

A 2,000 Watt inverter is relatively "oversized" for a 180 AH @ 24 volt battery bank... Larger AC inverters can take 20-40 Watts just "turned on" (Tare losses)... Smaller inverter will take in the range of 6-10 Watts--And waste less power.

I will stop here... Lots of guesses made. And relatively conservative calculations which are just a starting point for discussion.

-Bill

Wulfdan,

For me, I prefer to wire up the batteries in series, then connect in parallel (this is for Lead Acid batteries).

The reason is that if you have a bad battery, you can just do a quick check on each battery and see if the voltages all match (i.e., 6.4 volts per battery resting/charged * 2 = 12.8 volts).

There are other reasons too... If you have a a bad battery in parallel with a good battery, then you can only see the "combined" voltage (i.e., one good at 6.4 volts and the other one is at zero volts because it is "open" internally).

If the batteries are all in good shape, then functionally, there is not much difference. If there is a problem/failure, Series first, then parallel the strings--Much easier to find the "problem cells".

Balancing--With flooded cell lead acid batteries, you do a controlled over charge (equalization). The "good cells" simply gas a bit (from overcharging). And the "weak cells" are "charged to 100%" State of Charge by the leakage current.

Note that "over EQ'ing" a lead acid battery bank, while needed once a month or so--Too much EQ is hard on the batteries (causes them to run hot, use more water, damage plates, and cause corrosion of the positive grid/plates.

AGM batteries are (generally) not EQ'ed.

And Li Ion batteries, are never EQ'ed because they do not have "leakage current" to charge the weak series cells. Many banks have a BMS (battery monitor system) that checks (and some can adjust/balance voltage between cells). And it is common that many Li Ion banks are paralleled cells first, then put in series (reduces the wiring for BMS, and easier to "balance" if manual balancing).

To wire parallel strings of batteries, this is a good description:

http://www.smartgauge.co.uk/batt_con.html

In general, batteries fail from under charging/over discharging, and poor maintenance (letting batteries "go dry" and expose plates, not using distilled/other "pure" water, etc.).

-Bill