# Off-Grid to motor

Solar Expert Posts: 141 ✭✭✭✭✭
Have one Berkeley motor and want to have an off-grid for it. input voltage is 240/480 3 phase and Amps: 18A . What size of system do I need and how many batteries?

Do Trojans are the best batteries??

thanks

Re: Off-Grid to motor

Trojans are probably middle of the road batteries.

Top of the line are going to be Rolls/Surrette and various industrial Forklift/Traction batteries.

Sizing the battery bank depends on two things... Watt*Hours (average watts * hours of operation per day) and maximum surge current which may be 5x the motor's full load current, or it may be much less (if using some sort of soft start motor controller like a VFD--Variable Frequency Drive).

When sizing the Inverter itself--you need to take into account not only the Watt rating, but the VA rating too... And any power factor control that may be added to the system (motor run caps, VFD's, etc.). A plain induction motor may be around 0.6-0.7 power factor, which would increase the current require by 1/0.7 or 1/0.6 times (1.43 to 1.68 increase of current).

And--What is the current rating at XXX VAC? You gave two voltages but only one phase current rating. This sounds like a 5-10 HP motor or so...

The problem with large motors running heavy loads is that it take a huge battery bank to "keep it happy"...

Lets assume that it takes 5kW and runs 8 hours a day--An 85% efficient inverter and 50% maximum discharge (lead acid battery):
• 5,000 watts * 8 hours per day * 0/0.85 inverter eff * 1/0.50 maximum discharge = 94,000 WH battery bank...
Assuming 48 volt battery bank (probably wrong--large inverters are going to run much higher voltages):
• 94,000 WH / 48 volts = 1,958 AH at 48 volts
At retail, a pair of:

Crown Industrial Battery - 24 Volts, 1875 Amp-hours
Price: \$6,892.00

Is going to cost around \$14,000 just for the battery bank.

Also, remember you are going to need time to recharge the battery bank too (run 8 hours, charge for 8-16 hours) if time shifting your electrical loads.

Also, there are other losses... Battery charger will 80-90% efficient (if powered from AC mains), battery bank will be roughly 80% efficient. And you will have to replace the battery bank every XXX cycles.
Specifications
• 24 Volts
• 1875 Amp-hours at the 20 hour rate
• Dimensions: 38.63"L x 19.19"W x 31"H
• Weight: 2844 pounds
Warranty: 1500 cycles to 80% DOD for five years full replacement.
Assume "summer cycling" (peak power season is 6 months in our area--typically) or ~130 cycles assuming 5 days per week.
• 1,500 cycles / 130 cycles per year = 11.5 year cycle life?
Note, I have used 50% maximum discharge level--You might use the 80% maximum discharge level to reduce your costs (smaller battery bank than my calculations estimated)--May be more cost effective.

All this is based on paper guesswork... If you need a multiple HP AC Inverter--You will need to find a company that specializes in those guys.

And, for large industrial battery installation--working with somebody who has more knowledge than I would be a big plus too...

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 3,009 ✭✭✭✭
Re: Off-Grid to motor

Definitely looking at one MONSTER of a system that's going to require very, very deep pockets to put together!