Generator to ATS and battery?

James_B

I read a post by BB here, but am still entertaining of whether or not I can use my 16kw generac to charge a battery bank (looking at 15kw big battery with growatt 12k inv/charge controller). I have a well pump that consumes 2kwh per day roughly that I consider a critical load. 

Currently I have the whole home generator on an ATS besides the normal utility input. My question is, can I use the generator output to go ATS but split it to the charger prior to getting there to feed my charge controller?

Also, what's the math/formula to be able to figure out how long it would take for a 16kw whole home generator to recharge 300 ah 15kwh? Obviously it's a depends on how depleted it is and if you are balancing the batteries. Just looking for rough estimate to start with.

Thank you for your time in advance.

Sincerely,
James

BB.

Welcome to the forum James,

I cannot answer your question on how to bypass the ATS/Generator interface... That can be pretty complex. I guess you want to run the genset to charge your battery bank, but keep the rest of the home on utility power? If so, why?

The only way I can think of would be to install two manual transfer switches. One "to the home" that would take the ATS output OR Utility power and manual transfer from one to the other (either Gen ATS or Utility Power). You probably want to check--Many "backup power systems" can have a "bypass" for servicing (i.e., while working on Gen+ATS, the service person can bypass the genset+ATS manually and power the home directly from utility power).

Then you could connect a second Manual Transfer Switch between ATS output and Utility power for your battery charger/well pump/etc... Or don't bother with second MTS and just connect the Charger/well pump/etc. "upstream" on the ATS output directly and just switch the home between ATS or Utility.

Regarding charging time, the nice Rolls/Surrette battery manual has a formula (I don't understand the equation):

https://www.rollsbattery.com/wp-content/uploads/2018/01/Rolls_Battery_Manual.pdf (see page 11)

ABSORPTION CHARGE TIME - FLOODED

Where : T = 0.42 x C /I
T = ABSORPTION CHARGE TIME
C = 20 hr RATED CAPACITY (total battery bank)
I = Charging Current (Amps) (recommended 10-20% of C/20 discharge rate for Flooded models) 0.42 = ( 20%/50%) + 5% (5% is added due to losses)

EXAMPLE:
2 strings of 6 Volt 6 CS 25P models
20 hr AH rate = 853 AH x (2 strings) = 1706 AH
I = 10% of 1706 AH = 170 Amps (10-20%, 170-340 Amps)
T = 0.42 x 1706/170 = 4.2 hrs However, if maximum charger output is 120 Amps, 120 is used.
T = 0.42 x 1706/120 = 5.97 hrs

NOTE: Actual Amp output from the charge source(s) should be used. Maximum charger output applies when the generated output meets or exceeds this threshold.

I have a slightly different method... First some guesses for flooded cell lead acid battery bank:

• 16,000 Watt genset * 0.80 genset derating (don't overheat genset and alternator) = 12,800 Watts max continuous load on genset
• 300 AH @ 48 VDC (?) battery bank
• 300 AH * 10% rate of charge = 30 Amps charging minimum suggested
• 300 AH * 20% rate of charge = 60 Amps charging max suggested rate of charge
• 12 kW solar charger with 120 Amp charging output(?) from solar. What from genset/AC input?
• For 12 kWatt AC inverter, suggest minimum of 1,200 AH @ 48 volt battery bank. 300 AH @ 48 volt battery bank => 3,000 Watt suggested max inverter size.
• Suggest max of 60 Amps charging for your battery bank.

When running your genset to keep your battery bank "healthy" during a power outage... You could discharge to 50% and recharge to 80% State of Charge. That is a very efficient range for Lead Acid batteries. And as long as you are actively cycling--They should have good life. (don't store Lead Acid batteries below 75% SoC--They will sulfate pretty quickly and die). Recharge to 100% once a week or so...

• 300 AH * (0.8-0.5) = 90 AH of battery bank capacity used
• 90 AH / 60 AH  charging = 1.5 Hours

Or if charging from 50% to 100%:

• 300 AH * 0.50 capacity = 150 AH
• 150 AH / 60 Amps charging = 2.5 Hours
• Absorb charging: Light discharge ~ 2 hours of Absorb cycle. Heavy discharge = 6 hours of absorb
• 50% to 100%: 2.5 hours + 6 Hours = ~8.5 Hours

For your genset... The power draw would be roughly:

• 60 amps charging current * 58 volts charging * 1/0.85 charger efficiency = 4,095 Watts
• Suggested genset load (for max fuel efficiency) of 50% to 80%: 16,000 Watts * 0.50 loading = 8,000 Watts minimum "fuel efficient" loading

Most gasoline/propane/natural gas gensets with standard alternators are fuel efficient from ~50% to 100% loading... Below ~50% loading, the fuel usage (gallons per hour, etc.) flattens out pretty much... I.e., Genset with 0% loading uses about as much fuel  (GPH/etc.) as genset with 50% loading.

If you have an inverter genset (or Diesel), they generally are pretty fuel efficient down to ~25% loading...

Lots of guesses. Your thoughts?

-Bill

James_B

Hi Bill. Thank you very much for the thorough response. I'm just getting into the solar/battery/home backup arena so my knowledge is a bit limited. My goal on running the genset to both the house and the battery back would be to have the critical load be something that if it appears to be a long-term outage, I could shut the non-critical portion down entirely. That being said, while utility IS available, I'd rather not cycle my genset to charge my batteries at all. The battery bank is 48 VDC. I'm thinking I could either split the output from the genset to the ATS and run a leg to the charge controller. Am I incorrect in assuming that if a charge controller has utility power, that the batteries aren't used at all and the power directly provider to the secondary panel? The final goal would be to start buying solar panels to max out the charge controller. Genset backup to the charge controller would purely be short term.

I don't plan on running more than the water supply, a single head mini split (heat/cool), and a freezer on the critical load side initially. The rest in my mind would be on the non-critical side.

The generator is hooked up to a 1000 gallon propane tank. 

BB.

James,

You need to look at specific brands/models of AC inverters + solar charge controller + solar array + etc. and see how they work.

In general, the AC inverter will "pass through" the AC power coming in (utility, genset) to the AC output.

If you have a solar array (and internal or external solar charge controller), the solar will keep the batteries charged... And again, depending on brand/model of AC inverter, you can have a GT Inverter style (aka a "Hybrid AC inverter") that can even feed power back to the utility when it is sunny and utility power is good. Of course, at this point, you have to contact your utility and have them put you on a "GT/Utility Interactive/net metering" plan to do this legally. Doing a hybrid/GT inverter type system to use your solar power every day, vs just when the power is out, is not that much more costly/difficult--However, generally that will involve the utility (they have to agree both engineering and marketing departments), getting a permit and inspection from your building department, etc... 

Note that batteries themselves have a life even "floating"... Depending on type/cost/etc. of battery bank, they can need replacement every 5-7+ years just on float (this is highly variable--Just trying to set expectations).

I guess this is what you are looking at?

https://signaturesolar.com/growatt-48v-12kw-250vdc-split-phase-off-grid-inverter/

*Please Note: NOT STACKABLE. This unit maxes out at 12kW and is not a true ‘Hybrid’ when using grid power (just grid assist for low voltage switchover)

This unit does not support feeding power back to the utility... So if you install solar, the solar will only support your AC/Battery loads when AC mains have failed.

Also--What kind of batteries are you looking at? For lead acid batteries, I suggest that the minimum battery bank is 100 AH @ 48 VDC per 1 kWatt of AC inverter. Or 1,200 AH @ 48 volts for a 12 kW AC inverter:

• 1,200 AH * 48 VDC battery bus = 57,600 WH = 57.6 kWatt of battery storage suggested minimum

At this point, your 300 AH @ 48 volt battery bank will only support a ~3,000 Watt / 3 kWatt AC inverter.

Ideally, you should measure/estimate your loads (Watts for how many hours per day), 2x that for 2 days of storage, and 1/0.50 max planned discharge (another 2x factor) to size the battery bank.

Note that well pumps are typically designed for utility/genset operation. They are not very "solar friendly"--In that you need a large AC inverter + Battery bank to support the starting surge and running power. A 1 HP well pump can required >3,00 Watt AC inverter to start and a  300 AH @ 48 volt battery bank to support the surge current.

If you have a cistern or other tank(s) you can pump from, a small RV type water pump (12/24 VDC or 120 VAC) will be a good alternative (very low power requirements).

There are also "solar friendly" well pumps (surface and in well), that use around 1,000 Watts starting/running (no surge current). They are not cheap but can save money on the solar/backup power side, and even on your utility bill (usually much more efficient that a Jet Pump, as an example).

Otherwise, it may be best to run your major devices from your genset, and just a small/battery/solar backup for "other loads" (LED lighting, radio, laptop computer, LED TV, etc.) overnight/quiet time.

At this point, a 12 kWatt inverter with a 15 kWatt battery bank is not a "balanced" system design.

-Bill

James_B

Hi Bill,

This is what I was looking at actually. One battery and then add another one next year. 

https://bigbattery.com/products/48v-kong-elite-lifepo4-300ah-15-0kwh/

Optimally, I want to go the hybrid route with solar. That being said, it's not absolutely crucial. The battery specs at Growatt state that it has 12WK as the rated output with a surge output of 36KW. I'm guessing the math above was based on the batteries you mentioned as well.

James

BB.

James,

Lithium Ion batteries are a horse of a different color... LiFePO4 are usually used for off grid power systems. They are "safer" than many other Li Ion chemistries (should not catch fire if overcharged, physical damage, etc... Unlike many other Lithium battery chemistries which can, if misused or mishandled can be quite spectacular.

However, you need to be aware of LiFePO4's limitations. Typically cannot cycle below freezing, and usually use a BMS (battery management system) to ensure that the cells are not stressed beyond specifications (usually "per cell" voltage monitoring, and other parameters too such as max current, min/max temperature, etc.).

In this case:

Max Continuous Discharge Current Amps:	150
Max Continuous Power Watts:	7500
Max Discharge Peak Current Amps:	350 (6 Seconds)
Max Charge Current Amps:	90
Charge Temp Range (°C):	0°C (32°F) / 55°C (131°F)
Discharge Temp Range (°C):	-30°C (-22°F) / 55°C (131°F)
Optimal Discharge Temp Range:	15°C (59°F) / 35°C (95°F)
Storage Temp:	-5°C (23°F) / 35°C (95°F) (Max 6 Months)

A single battery is limited to 150 Amps / 7,500 Watts max continuous, and 350 Amps peak for 6 seconds.

And if you are in New Hampshire, ensure that the battery bank does not freeze during normal operation.

I would look into where your battery bank should be installed... If it ever caught fire (as always. follow all codes and safety requirements), one of the products of combustion (+water from fire fighting) is Hydrofluoric Acid--Very toxic stuff (and not something easily cleaned up from site of fire):

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5577247/#!po=0.781250

Anyway... Still do not have a definition of how much energy per day you want to use. That is really critical for a successful design and installation.

For small appliances (i.e., 120 VAC plug-in things)--A Kill-a-Watt meter (or equivalent) is great for figuring out how much energy per day your smaller devices are using:

https://www.amazon.com/s?k=kill-a-watt+meter

For larger hardwired AC loads, a whole house energy monitor is something to look at.

https://www.theenergydetective.com/ (whole home, and optional per branch circuit monitoring)
https://www.amazon.com/s?k=whole+house+energy+monitor

I have not used the above monitors (other than a Kill-a-Watt)--The links are intended as starting points for your research.

-Bill