Off grid question

Re: Off grid question

Using this site, set for Athens Greece, and setting the derating for 52% (because off-grid system), and I had to set 1kW worth of solar panels (lowest it would allow), then multiply by .510 kW of your system (510 watts of panels), I get:

Month    Hr/day    kW/month
1     3.22    24
2     3.77    26
3     4.68    35.5
4     5.35    38
5     5.84    42
6     6.74    46
7     6.97    48.5
8     6.82    47
9     6.43    44
10    4.65    33.5
11    3.04    21
12    2.83    20.5
Year 5.04    426.5

So, you average around 40 kWhr per month 1/2 of the year, and 20 kWhrs per month the other half of the year...

For the 40kWhr / month, that is about 1.3 kWhrs per day.

Assuming you have a small but efficient fridge of 0.85 kWhrs per day (a very efficient US fridge is about 1kWhr per day--say your 85 watt fridge runs 10 hours per day in hot weather).

1.3kWhrs - 0.85kWhrs = 0.45 kWhrs per day left over (or 450 watt hours).

You add up the 11 watt light bulbs... say 10 of those 4 hours per night, 440 Watt hours... And that is about it...

At this point, just guessing, much of the year your batteries are never getting fully charged. And during the less sunny months, your loads may be exceeding the panel's capacity by quite a bit.

So, get a hydrometer and measure the specific gravity of your batteries.

Get a better idea of how much load you are really using. And, you probably need to add more panels.

Also, it would be nice to get a small generator and good charger to recharge the batteries when the solar is low. And equalize the batteries once or twice a month.

If you have been running your batteries below 75% capacity for a long time, or they go below 50% capacity often--they are probably slowly dieing.

Sorry, got to go now... Will be back later.

-Bill

Re: Off grid question

Sorry, missed the AGM part...

If you can justify the cash, the way to go is using a battery monitor. It is just about the equivalent to having a gas gauge in your car's fuel tank.

Unlike a voltmeter which is only accurate after the battery has sat for several hours with no charging or load currents, the Battery Monitor can tell you the current state of charge of your battery bank at any time.

Look at the XBM model as one that as been recommended by several people here as a model with a good range of features.

As Mike said, you cannot over (or Boost) charge the AGM's... It will just destroy them if you over heat them (or over generate gas) from over charging.

I am not sure what you mean by a "pair of shunts"... There are several electrical definitions of a shunt. 1) to short out or bypass a circuit path with another wire. 2) would be a large calibrated resistor that is put in the main current path of the battery bank (in this case) and connecting a small amp meter or battery monitor to measure the current flowing into or out of the battery bank.

In any case, this will not give you any more power or efficiency--it will just tell you more about the state of your batteries.

And, typically, AGM batteries are around 90% energy efficient when charging/discharging. Flooded (or Wet) cell lead acid batteries are about 80% efficient--so you are about as efficient as you can get there.

Another place you may be able to increase efficiency a bit--is that a 1,500 watt inverter? That sounds a bit large for your installation--it is possible that if you used a smaller inverter, you may have less losses.

Also, I did not find any specifications for that inverter... Is that a Pure Sine Wave Inverter or a Modified Square Wave Inverter (sometimes called by the marketing department a "modified sine wave inverter"). The MSW inverters cost much less, but when running with motors, the square wave will cause the motors to use, roughly, 20% more power than they would on a pure sine wave inverter... Also some electronic equipment (TV's, radios, etc.) will have more noise in the video/audio on a MSW inverter.

Lastly, MSW inverters can cause some things to fail (motors, electronics, appliances, tools) before their time. Problem is that some 90% of devices will work fine and 10% may fail--but it is difficult to know which will work and which will fail.

-Bill

Re: Off grid question

"Equalization" is where each cell in series is slightly overcharged--because one (or more) of the series cells only reaches 98% charge, instead of 100%... If this goes on long enough, the one cell that requires more charging will eventually fail from being undercharged. So, charging the other cells to a bit over 100% once in a while will also bring up the 98% cell to 100%.

Another reason for equalization is to mix the acid--it can stratify--where the heavy acid (produced during charging) sinks to the bottom of the cell and the light battery fluid (mostly water) floats to the top. Equalization, properly done, will produce enough bubbles to mix the acid throughout the battery. This is especially important for "tall" batteries.

For AGM and Gel cell batteries, they don't have liquid electrolite--so there is no reason to mix. Also, AGM (at least) only requires light equalization once a year or so...

If you equalize an AGM battery just like a flooded cell, it will generate Hydrogen and Oxygen gases--however, the battery will not be able to recombine them back to water fast enough--plus the excessive charging current and recombination will generate enough heat that the pressure seals will let out the steam--quickly drying out the battery and causing failure.

-Bill