Battery charging with generator

Re: Battery charging with generator

You don't say what AH capacity your battery bank is and how much power you use in a day...

Sounds like a might be an Off-Grid system?

Anyway--generally size the battery charger current based on 5% - 13% of your banks AH capacity (we like the 20 Hour rate).

Size the generator to 50-75% load of battery charger (x1.25 or so to allow charger losses)... You can go with a lightweight (inexpensive) genset if this will only be used during wither (propane, gasoline, diesel)... Or a prime power rated genset if this is an off-grid application and you use the genset a lot (say to power your welding/metal shop).

Just to pick some numbers... Say you use 5kWhrs per day. Battery bank would be roughly:

• 5,000 WH per day * 1/48 volt bank * 1/0.85 inverter losse * 3 days of no sun * 1/0.50 maximum batt discharge = 735 Amp*Hours

Assume 5-13% rate of charge, 59 volt battery charging and 80% efficient charger, the load on the genset would range:

• 735 AH * 59 volts batt charge * 1/0.80 charger losses * 0.05 = 2,710 watts min
• 735 AH * 0.05 = 37 amps min charger
• 735 AH * 59 volts batt charge * 1/0.80 charger losses * 0.15 = 7,047 watts max
• 735 AH * 0.13 = 96 amps max charger

So--Roughly, find a battery charger (or programmable battery charger) and size a genset from ~5kW-14kW range...

Just a rough range and how I would look at the question... Plug in your own "real" numbers.

If you don't have one--I would suggest a Battery Monitor so that you can just look at the meter and read out battery capacity in XX% (not 100% accurate--but should be close enough to keep you out of problems... For sealed batteries, where you cannot measure specific gravity, almost a must have).

If you have a choice--try to pick a battery charger with a remote battery temperature sensor (battery voltage is temperature sensitive) and an AC charger with Power Factor Correction (lowers the "VA" required by the charger and can allow you to use a smaller genset).

Avoid a "too large" genset if there will be a lot of hours on the genset. Most gensets have minum fuel flow around 50% of rated electrical output. If you run the genset at 10% rated output to charger your battery bank--you will consume ~5x as much fuel as you really need.

Diesel gensets should be operated at 50-60% more so minimum load for a long/low maintenance life.

"Suggestions" for running genset:

• 75% state of charge, start the genset next morning and/or shed loads
• 50% state of charge, start the genset now
• Charge until 80-90% state of charge. And/or watch current and when it drops off by 1/2--turn off the genset--let solar panels continue the charging.

Anyway, the above is just some basic recommendations and rules of thumb. If you have different needs, please feel free to ask. Nothing is written in stone.

-Bill