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Air Conditioning without a Generator

Have you ever wished you could sleep comfortably without the droning of a generator keeping you awake? It is now possible thanks to the high storage capacity of Lithium (LiFePO4) batteries. Our service department recently completed a new system installation on a Leopard 440 catamaran that allows the cruisers to run the entire electric service panel from two inverters. Now, the owners can run both air conditioners, water heaters, microwave oven, 12 volt battery charger, and more, without running the yacht’s generator. Here is how we accomplished this:

We chose Victron Energy’s Quattro inverter/chargers as the heart of the new system. We added 48 volts worth of Lithium batteries (total about 20 kWh capacity), along with 2.2 kW of solar so the owners can keep generator use to a minimum.

It is getting crowded in here!

Since this particular Leopard 440 is equipped with twin 30 amp shore power connections, we opted to use two 5000 watt inverter/chargers – one for each shore power leg. We configured the Quattro Inverter/Chargers in a master/slave setup that allows the battery management system to handle the power flow. We chose Victron’s 24 volt Smart Lithium batteries to keep connections to a minimum. The 48 volt battery system allowed us to keep cable sizes down, which allowed us to run all cables through the factory conduits – a big help with keeping everything secure!

One BIG caveat with choosing a 48 volt battery system is the availability of switches, fuses, and breakers that can handle the voltage. Don’t make the mistake of using equipment rated for just 48 volts! While the NOMINAL VOLTAGE of the system is 48 volts, the ACTUAL VOLTAGE is between 53 and 58 volts!

Rather than upgrade all the boat’s existing 12 volt systems to 48 volts, we chose to add monitoring to the 12 volt house bank. Now, all the systems are monitored in one place! We kept the installation simple by using the existing 12 volt battery charger to keep the house bank full.

The Victron Cerbo GX with touchscreen display provides clear, graphical information on what is happening with the system.

We selected LG 375 watt panels for the solar arrays. We designed the solar strings in series for maximum charging efficiency, and separated the port and starboard panels into two strings. A custom stainless steel frame keeps panel shading to a minimum.

Please contact us if you have questions, or would like a quote for going silent.

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Battery Bank Sizing for Your Inverter

How to choose the ideal battery bank size for your inverter

The Battery Bank Sizing Guide from Xantrex is a very useful tool to determine the size of your inverter and battery bank. However, some additional information is always helpful. Here is what I have learned from personal experience:

  • Your inverter should be sized based on the total simultaneous load to be applied (including motor start loads)
  • Your battery bank should be sized based on your total daily amp-hour demands (inverter and DC loads)
  • Your battery bank should never be discharged by more than 50% of it’s rated amp-hour capacity, or your batteries won’t last long!
  • Your battery bank will be TOO SMALL!

My reason for this harsh assessment is based on the fact that static (unloaded) battery voltage is entirely different from dynamic (loaded) voltage. This is due to internal resistance of batteries, and the surface area of the plates. The voltage supplied by any battery is reduced as the current draw increases. Additionally, the amp-hour capacity of any battery drops as the current load increases.

There is one more important factor for battery bank sizing:

  • Your battery bank must be sized based on the maximum expected current draw, and depends on the battery type. The maximum charge/discharge rate for various deep-cycle battery types is:
    • Traditional lead-acid batteries: 20-25% of amp-hour capacity
    • Gel cell batteries: 30-35% of amp-hour capacity
    • AGM (absorbed glass mat) batteries: 35-40% of amp-hour capacity (check your cable sizes!)

As you can see, the battery type can make a big difference in battery bank sizing based on maximum current load. Try out our Marine Battery Load Calculator to help determine your ideal battery bank size.

A fully charged starting battery loaded at half its rated CCA capacity (load test current) will only put out about 9.7 VDC at 80°F. The voltage of a deep cycle battery will drop even more under similar load conditions. Keep in mind that most inverters will trip off to protect the batteries when their voltage drops to 10.5 V or less.

Xantrex provides a fine example of using a circular saw that uses 1500 watts of power. Their example indicates that the saw would only use 2 amp-hours at 12 volts if run for one minute. (Xantrex’s example ignores the inefficiency of the inverter). The point made by Xantrex is that while the saw uses 1500 watts, the total run time is short, so it uses very few amp-hours. If your battery bank is sized based on using this current draw for just a few minutes at a time, you appear to be in good shape. This may not be the case. My point is that the saw uses 1500 watts!

A 2000 watt inverter powering the circular saw will draw about 1667 watts (at 90% efficiency) from the battery bank. At 12 volts, the current draw is 139 amps. Using our Marine Battery Load Calculator, you would need a 700 amp-hour bank of deep-cycle flooded batteries! A battery bank with 400 amp-hours capacity doesn’t stand a chance of supporting a 2000 watt inverter load without help, but it can be done.

Don’t let me scare you away from an inverter just because the battery bank sizing can be complicated. A modestly sized battery bank will work great to power an inverter under most conditions. If you have an occasional need for high-wattage power, you can meet the current demand by simply running your engine – even if your alternator is not rated for the total current draw. The current supplied by your alternator will make your battery bank appear much larger to the inverter, and be enough to run a substantial load for a short time.

In the circular saw example above, your power supply (battery bank and alternator) needs to provide 139 amps of 12 volt DC current while the saw is running. This can be accomplished with a 400 amp-hour flooded battery bank assisted by a 60 amp alternator.

The lesson learned is that a large inverter is a very useful piece of equipment. Your battery bank does not need to be huge if your highest loads are only used occasionally. Base your battery bank size on the loads that will run on a regular basis.

Battery Bank Sizes Based On Battery Type

Flooded (Wet) BatteriesFlooded (Wet) BatteriesGEL BatteriesGEL BatteriesAGM BatteriesAGM Batteries
Inverter SizeMax Amps @ 12VBatt OnlyBatt +100A AltBatt OnlyBatt +100A AltBatt OnlyBatt +100A Alt
3000 W278 A1390 A-H890 A-H1120 A-H720 A-H840 A-H540 A-H
2500 W231 A1160 A-H660 A-H930 A-H530 A-H700 A-H400 A-H
2000 W185 A930 A-H430 A-H740 A-H340 A-H560 A-H260 A-H
1500 W139 A700 A-H200 A-H560 A-H160 A-H420 A-H120 A-H

As you can clearly see from the table above, using your alternator to help your battery bank under peak loads makes a huge difference in the size of battery bank you need. Please note that the battery bank sizes above were calculated based on powering the INVERTER ONLY. Your battery bank size should be based on your total power usage and your charging schedule.

Happy Cruising!

Tim Allen