Battery charging

In general, for flooded cell lead acid batteries, if you charge at more than ~13% rate of charge--It is recommended that you use a remote battery temperature sensor. As FLA batteries get hot, the charging voltage falls... So you can get thermal runaway (higher temperature, higher current flow, higher temperature, etc.). Around C/8 rate of charge (0.125) is usually the point where (for example) fork lift batteries get active cooling and thermal management.

Otherwise, the highest rate of charge is probably 20-25% (again on the 20 Hour capacity rating of the battery bank) that I would suggest to not exceed.

Temperature rise of a FLA battery is also based on state of charge--At less than 80% SOC, FLA batteries are very efficient at charging. At over 90% state of charge, their efficiency falls to where even an extended charging time (many hours or longer) even a 5% or 2.5% rate of charge will eventually overheat the battery (basically battery gassing--Hydrogen+Oxygen+Heat).

Of course, battery voltage is important here--You only get "high current flow" with high voltage (i.e., equalization at >~15.0 volts and >~2.5% rate of charge).

And that seems to be the weakness of "car chargers"... They are designed for quick charging and many chargers do not seem to regulate the charging voltage (will go over 17 volts if you let it).

If you monitor the battery bank temperature and gassing (light fizzing OK, rolling "boil" of hydrogen+oxygen gassing not)--You can use unregulated battery chargers. Using a timer to cut the AC power after X Hours may not be a bad idea if you are trying to quick charge your bank and accidentally get distracted.

"Typical" battery chargers have inexpensive components which tend to have very poor power factor. Also, many "home" type gensets are not very happy running at 100% of rated output for many hours (aka--Typical charging of a battery from 50% to 90%+ state of charge). The rough formula I suggest is something like:

• 2,000 Watt genset (rated power, not surge) * 0.80 charger efficiency * 0.70 Power Factor of charger * 1/14.8 volts charging * 0.80 Genset derating for continuous load = 60.5 Amps charging current for 12 volt battery bank maximum continuous

If you have a "modern" power factor corrected battery charger like the Xantrex... The efficiency may be ~90% and the Power Factor ~95%:

• 2,000 Watt genset (rated power, not surge) * 0.90 charger efficiency * 0.95 Power Factor of charger * 1/14.8 volts charging * 0.80 Genset derating for continuous load = 92.4 Amps charging current for 12 volt battery charger with PF Corrected power supply
  

The above sort of estimates the "worst case" and "best case" battery charger numbers and suggested maximum output for a 12 volt battery charger on a residential genset. Note that technically we are saying a 2,000 Watt genset also has a maximum rating of 2,000 VA (volts*amps) where:

• VA = Power * Power Factor

Commercial and other larger gensets are frequently rated at PF=0.80 or so (80% power factor)... Residential gensets are typically rated at PF = 1.00 (100%).

AC math and terms (like power factor) is actually a very complex subject.

I like to be conservative in my electrical/engineering suggestions. I am not there--And things happen (forget to disconnect charger when a family emergency occurs)--I want a more "bulletproof" setup. It is not impossible for over charged/over loaded circuits/genset to fail (expensive) or cause a fire (expensive or worse). Using a genset with a limited amount of fuel in the tank (4-6 hours) does limit the charign time too.

-Bill

Just to be clear, the Xantrex charge controller is available in 20/40/60 amp @12 volt models... There is no 50amp @ 12 volt model.

More or less, a 10% minimum rate of charge is recommended for Lead Acid batteries, or 10%*208AH= ~21 amp load.

A 50 Amp (or 60 Amp) charger is pretty large and may be "overkill" for a 208 AH battery bank (once the battery is >~80% state of charge, the battery will automatically cut back on charging current--Also known as the "absorb" charging stage. "Bulk charging" state is when the charge controller outputs its maximum current into a less than ~80% state of charge lead acid battery).

For a genset--More or less gasoline engines are most efficient if they are loaded to 50% or more of rated output. If you fall back below 50% of rated output, the fuel flow (more or less) remains the same from 50% to 0% output power. The inverter generators (Honda, Yamaha, and others) are a bit more fuel efficient at lower output because the can slow down the motor RPM (inverter genset will keep 60Hz output frequency fixed and still remain fairly fuel efficient down to 25% of rated output). Most of your charging time will be in the "absorb" phase and a larger charger will only reduce the genset run time a smaller amount (for a deeply discharged Lead Acid battery bank, your system will spend ~4-6 hours in the 80-95%+ state of charger range regardless of the charger current output).

Note that Inverter Gensets seem to be more "sensitive" to high surge/high output current and will quickly shut down if you exceed their rated output (surge or continuous). Plain old 120 VAC alternator gensets are much more forgiving with temporary overloads/surge current. I am not sure which genset you have.

If genset (and AC mains) charging is "rare"--And Iota charger (IQ-4 Optional charger controller module is still nice) is less expensive and typically "good enough".

If genset charging is going to be an ongoing thing (not enough solar panels/long streaks of bad weather, AC Mains charging most of the time except during power failures, etc.)--The Xantrex is a very nice (but expensive) feature rich unit. And for your present battery bank, I would probably go with the 20 or 40 amp unit. I think the 60 amp unit is too large (and too expensive) for your present needs.

If you double the size of your battery bank to ~400 AH later--The larger 40 amp battery charger will still be a very nice fit (~10% rate of charge) with your smaller genset.

If you ever go with a larger battery bank (over ~600-800 AH @ 12 volt battery bank), I would be suggesting you look at 24 or 48 VDC instead--So the battery charger and inverter(s) have to be replaced anyway--Which may limit the amount of money you want to put into a 12 VDC (only parts) system.

-Bill