Charge Controller for two 120v battery banks ?

Re: Charge Controller for two 120v battery banks ?

The amount of solar power you receive will very much depend on your available sun, weather, and shade (if any). Shade will kill a solar electric panel output (pipes, chimney, overhead power lines, nearby trees, etc.). you need a shade free roof/panel mount area from 9am to 30pm or so. Shading on even a few cells of a panel can kill the entire panel (or series string) output until the shade goes away.

And what is a typical day for your location. If you live closer to Lisbon, using the Solar Electric Handbook:

[h=3]Lisbon
Average Solar Insolation figures[/h] Measured in kWh/m²/day onto a solar panel set at a 51° angle (from vertical):
(For best year-round performance)

Jan
Feb
Mar
Apr
May
Jun


4.00

4.61

5.76

6.20

6.26

6.31



Jul
Aug
Sep
Oct
Nov
Dec


6.45

6.61

6.18

5.24

4.08

3.44



Toss the bottom three months (assuming you use a generator), that gives you 4.61 hours of sun per day:

1,300 Watts * 0.52 off grid system eff * 4.61 hours per typical February day = 3,116 Watt*Hours of AC power per day

Now, if you are looking at a full time off grid system, a 10% or more rate of charge (my suggestion) on a 24 volt battery bank would be:

1,300 WH per day * 0.77 typical panel+controller derating * 1/0.10 rate of charge * 1/29 volts charging voltage = 345 AH @ 24 volt battery bank

Assuming 2 days of storage and 50% maximum discharge on your battery bank (good optimum battery bank design for lead acid batteries):

345 AH * 24 volts * 0.85 inverter eff * 1/2 days of storage * 0.50 max discharge = 2,070 Watt*Hours per day (from battery bank, no sun)

So, the above system would seem to meet your 2,000 WH per day for at around 9 months of the year... For the other 3 months of the year, you would need a combination of conservation (for bad weather) and/or backup generator/grid...

To see how much you could power you could get from the battery bank, using flooded cell lead acid battery bank design rules:

345 AH * 24 volt * 0.85 inverter eff * 1/20 hour discharge rate = 352 WH per 5 hour night (just an idea of how much average power you can pull
345 AH * 24 volt * 0.85 inverter eff * 1/8 hour discharge rate = 880 Watt maximum continuous load (would give you less than 4 hours of power to 50% discharge)
345 AH * 24 volt * 0.85 inverter eff * 1/5 hour discharge rate = 1,408 Watts maximum short term power (i.e., small microwave+lights+water pump)
345 AH * 24 volt * 0.85 inverter eff * 1/2.5 hour discharge rate = 2,815 Wattts maximum surge (few seconds to start a well pump, etc.)

So, that would make a ~900 to 1,500 Watt inverter (maximum useful inverter) for your 345 AH @ 24 volt battery bank.

Note that the solar panels you are looking at appear to have a Vmp (voltage maximum power) rating of Vmp~54.7 volts:

http://www.sol-distribution.com.au/BenQ/BenQ-SunForte-Data-Sheet.pdf

When designing for an off grid solar power system, you are limited by the standard MPPT (maximum power point tracking) charge controllers typical input/output ratings. For most MPPT charge controllers with ~140-150 VDC maximum input rating from the solar array (Voc--Voltage Open Circuit rating when cold), for various reasons, most controllers would only work with one panel in series (Vmp-array=54.7 Volts standard test conditions). Voc-cold-array with two or more panels in series could be >140 VDC in cold weather (we would have to look at this issue in more detail--if needed).

For your system, that means that you are limited to one panel voltage, and a 24 volt battery bank--You could not have a larger system with a 48 volt battery bank with most charge controllers (you need Vmp-array~70 VDC minimum to properly charge a 48 volt battery bank with ~60 VDC minimum charging voltage for the battery bank).

Otherwise, those panels should work fine for your needs.

A big issue with off grid solar power systems--It is very difficult to start small and grow them. Yes, you could start with a 1,500 WH per day system and grow it to ~3,000 WH per day (add solar panels, more batteries, etc.)... But any larger system (or even smaller), probably not easily. Inverters are 12, 24, or 48 volts--Larger system need larger inverters. Adding batteries later, batteries age (years and cycle life). It is not recommended to mix old + new batteries--If gives you a mixed age battery bank where you will have some batteries failing earlier than the others--And having to change some/all batteries later. Also, adding parallels strings of batteries has issues too (more work for maintenance, more costs for wiring, etc.).

Sort of like buying a small car, then trying to put in a larger engine/transmission, bigger brakes, etc. to turn it into a pickup truck--Can be done, but usually not a good plan.

-Bill