# How fast does a Generator Charge your battery bank?

Registered Users Posts: 20 ✭✭
Curious how do you determine how much run time you'll need on a Generator in order to charge your battery bank.  Any formula suggestions?

Generac 20kW Generator (90 amp, 240 vac)
two XW+ Invertors (48v,6800 watts each)
6 x Fortress Power 5.4 kWh batteries ( 105 ah, 100 amp, 51.2 vdc)
Tagged:

Says less than 55 Amps @ 54.4 volts (BMS disconnect @ 56 volts). Operating range from 48 volts to 54.4 volts...

6 batteries in parallel for 105 AH * 6 (630 AH total @ 48 volts) and charging 55 AH * 6 (330 Amps max)

You have a 20 kW genset... So working backwards from the genset... Suggest running the genset at 50% to 80% loading (most fuel efficient and longer life--assuming propane or gasoline).
• 20,000 Watts * 0.80 = 16,000 Watts maximum (longer generator life)
• 20,000 Watts * 0.50 = 10,000 Watts minimum (fuel efficiency)
Assuming that the XW is roughly 85% efficient (between electrical efficiency and power factor) and 54.4 volts max charging voltage):
• 16,000 Watts * 0.85 "XW eff" * 1/54.4 volts = 250 Amps @ 54.4 VDC charging current
• 10,000 Watts * 0.85 XW eff * 1/54.4 volts = 156 Amps @ 54.4 VDC charging current
Say each XW is programmed for 100 Amps @ 48 volts charging current. Assume that you charge to 90% battery state of charge and discharge to 20% state of charge (longer battery life--my suggestion--perhaps 95% max charge OK too).
• 90% full - 20% empty = 70% capacity (from empty to full charge)
• 650 AH * 0.70 capacity = 455 AH used
• 455 AH used / 200 Amp charge rate = 2.275 hours
Anyway--A rough framework of how to do the calculations. Li Ion batteries are >98% efficient--So ignoring those losses.

Also, some batteries say stop charging at 54.4 volt setpoint--Others suggested holding 54.4 volts for up to several hours to complete charge...

I am no expert at Li Ion battery chemistry--As always read the documentation closely.

Others here can give you much more specific answers to your questions.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 6,150 ✭✭✭✭✭
Are you open or closed loop?
"we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
http://members.sti.net/offgridsolar/
E-mail [email protected]

Just a quick look through the glossy in the above link, add 60 minutes for "absorb" (holding at 54.4 volts for 1 hour).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 20 ✭✭
edited December 2020 #5
Awesome Thanks, Bill!  I attached my system layout for everyone's review.  Pretty happy with the current layout we're just trying to iron it all out to submit plans to our city.  We're also assuming a 20W generator is ideal for our PV system (9920 watts) as well I may have some large loads coming from my shop here in the future namely a welder and other equipment.    Would you use a smaller Generator all in all?  Also, the Generator will be on (LP) Propane gas.  I was really curious how long the generator might be running for during a battery bank charge.  Thanks for the review and information.  P.s. Yes that is the Battery I will be using. Fortress Power 5.4 kWh EFlex

• Solar Expert Posts: 1,460 ✭✭✭✭
edited December 2020 #6
I would not charge the battery packs at more then about 50-55A each.  You will not have perfect matching of the six parallel packs.  I would expect about a 10-20% variance in current between the six parallel packs and will likely get worse over battery aging.

Each XW+ should be capable of about 120-130A at average voltage during charging of 3.35v x16 = 53.6vdc based on wattage limit of XW+'s  So you will be limited by two XW+'s for charge current max of 240-260A, giving an average of 40-45 amps per battery pack.

80% capacity recharge needs 84 AH x 6 = 504 AH's.  504AH / 250A = 2+ hours.  If you take to full charge, 3.65v x 16 = 58.4v, you will have about hour and half to two hours of current taper off time once you get to 58.4v absorb voltage.   XW+'s should exit absorb when current drops to 25-30 amps for each unit ( 50-60 amp net charge current).  Overall I would expect 2.5 to 3.5 hours to replace 80% DOD.

If you float long term, set XW+ float to 3.35v x 16 = 53.6v.  That still will give you over 95% SOC and reduces excessive SEI layer thickening rate on LFP cells which will increases their internal impedance and eat up available lithium that reduces capacity.  Try to set XW+'s LBCO so XW+'s shut down first on low battery, not the battery's BMS under voltage shutdown.  Sometimes that is tough due to battery cable resistance votlage drop with heavy inverter loads prematurely tripping inverter's LBCO.

Gen fuel consumption will be efficient with 13kW loading although you might consider having ability to use excess power capability for water heating or air conditioner during charging time.

• Registered Users Posts: 20 ✭✭
Thanks, @RCinFLA for the detailed description and I'll consider your sound advice. I appreciate the insight and help.
• Solar Expert Posts: 3,854 ✭✭✭✭✭✭
Something to be aware of is the charge controllers also have the potential to feed current a sudden clear break during an overcast day whilst inverter charging which could exceed the maximum charging current, the cumulative total of both. This is something I discovered personally whilst testing, fortunately I was able to shut the generator disconnect off as soon as I realized the sharp increase in current. Below is an excerpt fom the attached pdf which mentions this, they are referring to the 60 150 controllers in this case

A practical way to avoid this is to only use generator charging when there is no sun, I choose evenings because it's almost guaranteed to clear up if done in the morning and I never charge more than what is needed to get me through 24 hours. This is particularly important when AGS is incorporated.

1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS
Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.
5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
• Solar Expert Posts: 6,150 ✭✭✭✭✭
edited December 2020 #9
Overcurrent is a major killer of LFP. This is why I asked the OP if he is running open loop or closed loop. Closing the loop with the BMS and Schneider charge sources can be done with this battery. The previous post has an outdated link to what you want for a system this size. It is an example of open loop charging. This can kill batteries with these inverter/chargers. You want to close the loop and let your BMS do what is best for the battery.

I was involved in this with Fortress and I think you will be OK with the number of batteries and not run this risk. They did just finish this so it is early.  Be sure and buy the device that Fortress recommends to close the loop.

Two XW's is 280 ADC plus the solar. As a newbie you might want to only charge at night or turn-off the mppts during daylight cloudy weather. You can contact me if you want or need more on this. Good Luck!
"we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
http://members.sti.net/offgridsolar/
E-mail [email protected]

• Registered Users Posts: 20 ✭✭
Dave,  I haven't heard the term closed or open loop used with a PV System what are you referring to?
• Solar Expert Posts: 6,150 ✭✭✭✭✭
The BMS and it's charge / discharge requirements communicating that to the Schneider xanbus system. Read my post again. You should also consider emailing me. I do not bite much
"we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
http://members.sti.net/offgridsolar/
E-mail [email protected]

edited December 2020 #12
Basically, the battery BMS system "knows" how much charging (and discharging) current it is currently (spelling) receiving. Tie that back to the XW/Schneider hardware, and the units can be programmed to limit current to your battery bank.

An open loop system--The XW/Charger hardware does not know what the battery is doing... And if you have several charging sources, they can add up and damage your bank... Or you have to limit your charging from all sources, or you have to charge with the genset at night, etc...

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 6,150 ✭✭✭✭✭
The discharge part also. It knows if you are drawing to much current from the bank. The BMS knows what the bank is comprised of. This discharge can easily damage an LFP bank with (2) XWP's. Most of the people out here do not use big systems. They do not have the issues of killing the battery with a 4KW inverter/charger. They just hear how great LFP is and all of the youtube propaganda. Big systems need sanity or a big check-book. Because of LFP expense, people often do not buy enough capacity. This bites them even in small systems. I have been called in on these often for warranty and it gets denied often because of the design.

The old school batteries also were forgiving of user issues in that they gave a warning, usually gradual,  bad maintenance or poor design/installation. The failures were usually over months. An LFP battery system like Fortress will just be dead one day. Not trying to discourage the OP just a heads up.
"we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
http://members.sti.net/offgridsolar/
E-mail [email protected]

• Registered Users Posts: 20 ✭✭
We did have the 3.8 Simpliphi batteries on our radar, but we're offered the Fortress 5,4 so we're just figuring them out now. No loyalty to anyone one battery.  What are your opinions about Simpliphi batteries?
• Solar Expert Posts: 6,150 ✭✭✭✭✭
An even worse choice! There is no way to close loop with Schneider!  You should give me an e-mail !
"we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
http://members.sti.net/offgridsolar/
E-mail [email protected]

• Registered Users Posts: 179 ✭✭✭
I second that you contact Dave. I used his services when installing my system, including a Discover AES Lithium battery with XANBUS.

Great experience!
Building Off-Grid in Terlingua, TX
14 CS 370 watt modules. HZLA horizontal tracker. Schneider: XW6048, Mini PDP, MPPT 80-600, SCP. 3 - Discover AES 42-48-6650 48 volt 130ah LiFePO4 batteries