There are going to be many times when the sun isn’t shining, the wind isn’t blowing and your batteries are low. The only choice you might have is to charge your batteries using your fossil fueled generator.
As you may already know, there is not a more expensive method of battery charging than using a 120/240 volt 60hz (230 volt 50hz) AC fossil fueled generator.
There are 3 ways to charge your battery bank and they are listed below in order of best to worst (most efficient to least efficient) methods of charging your battery bank. Most efficient just means you get more kWhs per dollar spent (or liter of fossil fuel consumed).
Ideally you would have enough solar, wind or micro-hydro power (but that can be almost impossible in some climates).
The second best would be a DC generator made from a fossil fueled engine and 3 phase alternator that is rectified to DC.
Using an off-the-shelf or industrial 120/240 volt 60hz (or 230 volt 50hz) fossil fueled generator.
Making your own electricity with a gas, propane or diesel generator can cost anywhere from $2.50 t0 $5.00 per kilowatt hour compared to the $.05 to $.60 per kilowatt hour most utilities charge. It is horribly expensive and environmentally a disaster. Most fuel powered generators have no or very little emissions controls and are made to wear out in a matter of months when used to charge batteries in an off grid power system.
STEP 1: Bulk charge at 10% of the battery bank”s C20 rate
The first step in setting up your system to charge batteries efficiently is to program your inverter/battery charger to charge at 10% of the battery bank’s C20 AH rating.
Consult your battery manufacturer’s literature for the C20 amp hour (AH) rating.
For example:
Trojan’s L16s have a C20 amp hour rating of 370 AH at 6 volts.
If you were to take four of these L16s and place them in series you would now have a 24 volt battery bank rated at 370 amp hours.
Simply take the C20 AH rating and multiply by 10% or .10. This is your bulk charge current setting.
In the above example you should try to put 37 amps (370 multiplied by 10%) at 24 volts into your battery bank. This would require about 880 to 1110 watts depending on the actual battery voltage.
To get the most “bang for your buck” try your best to achieve the 10% of the bank’s AH rating. Obviously there will be times when this is not possible such as having too small of a generator or too small of a charger. You can charge at a lower current, it just won’t be as efficient.
If you charge with more than 10% of C20, you risk damaging the batteries due to over heating and over gassing.
Let’s try another example:
This time we have sixteen Trojan T105s (225 AH @ 6 volts) wired in series and parallel to make a 48 volt battery bank. There are two banks of eight batteries that are paralleled to make 450 amp hours (C20) at 48 volts. This is pretty common battery bank and is shown in the picture below…
In this example, it is best to set your inverter/charger to bulk charge at 45 amps (450 amp hours multiplied by 10%). Forty five amps is approximately 2160 to 2700 watts depending on the real voltage of the battery bank.
Even if you have a much larger battery charger/generator, 45 amps is the target charge rate for this particular battery bank. Any less will result in excess generator run time and fuel consumption, any more can result in damage to your batteries.
STEP 2: Program correct bulk voltage into inverter/charger
Consult your battery manufacturer for the correct bulk voltage setting. Next examine your inverter/charger’s manual to learn how to program the bulk voltage.
You need to be careful here as some manufacturers list:
the bulk voltage per cell
the bulk voltage per battery
the absorption voltage (same thing as bulk voltage)
the bulk/absorption voltage (same thing as bulk voltage)
the bulk voltage as a range (i.e. 2.35-2.45 volts per cell)
the daily charge voltage (same thing as bulk voltage)
All of the above terms like absorption, daily charge and bulk voltage are all referring to the bulk voltage setting.
If your bulk voltage is listed per cell, think of each cell as being two volts.
a 6 volt battery or battery bank has three cells
an 8 volt battery or battery bank has four cells
a 12 volt battery or battery bank has six cells
a 24 volt battery or battery bank has twelve cells
a 48 volt battery or battery bank has twenty four cells
Once you know how many cells you have simply multiply the volts per cell by the number of cells you have. If your battery bank is 48 volts you know it will made up of 24 cells.
For our first example we have 4 Trojan L16s. Trojan lists the bulk/absorption voltage as 2.45 volts per cell.
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