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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

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Power Tools Off-Grid

Corded or Cordless Tools – Which are Better for Cruising?

Before we went cruising on our catamaran “Unbound,” I shopped around and bought a set of cordless power tools to bring along. I thought this would be the greatest thing since sliced bread. Like many people, I had worked on projects away from convenient power. You know the projects – building the kids’ gym set in the back yard, attaching a bracket to a wall from a ladder, hanging a mirror. Many of us have done these things.

One morning, after a couple weeks aboard “Unbound” I took on the project of mounting a GPS display near the helm. I needed to drill four holes. I grabbed my trusty cordless drill and … it was dead. I plugged it into the charger and turned on the inverter. Fifteen minutes later the inverter control panel was yelling at me (with yellow and red lights) that the batteries were too low! Now to crank up an engine to charge the battery bank, so I can drill four holes.

According to the label on the drill’s one-hour battery charger, it uses 65 watts at 120 VAC. The charger puts out 2 amps at 16 VDC. That’s 32 watts, so the charging efficiency is less than 50%. Our Xantrex inverter/charger is about 90% efficient at supplying 120 VAC, so to charge the drill’s battery for one hour uses about 6 amp-hours of house battery current.

Now let’s look at drilling those holes with a regular, old-fashioned corded drill. My Milwaukee drill (in storage at the time) is rated at 3.5 amps at 120 VAC, under full load. Now, I was drilling fairly small holes through fiberglass, so lets say it would draw 2 amps (240 watts). Each hole took about 15 seconds to drill. That’s one minute at 240 watts, at 90% inverter efficiency – 22.2 amps at 12 VDC. The total battery drain would have been only 0.37 amp-hours! That’s only 6% of the power used for the cordless drill! And, I wouldn’t have had to wait an hour to do the job.

If your getaway plans include a tool kit with cordless power tools – don’t waste your money! Their batteries are almost never charged when you need them, so you have to run your inverter or generator for an hour to charge them up (if you remembered to get the fast charger) before you can get started with your project. A cord-type power tool is ready all the time, and you only need to run your inverter while you are using it. Corded tools also take up less space (no bulky batteries or chargers). Saves time and house battery amps!

True off-grid life is different! Whether on a boat or in an RV, or in a mountain cabin, electricity usage off-grid is totally different. The number three priority for any cruiser (after safety and water) is battery power. Without it we have no communications, navigation, lights – or engines!

Update:

Battery technology for power tools is constantly changing. The latest Lithium Ion batteries hold their charge for a very long time. With this in mind, cordless tools get a second chance on board. I love the power and consistent tool speed that the new batteries offer. However, I don’t like the surprise when the battery is discharged – the tool just stops without warning!

The charging efficiency of Lithium Ion batteries is similar to the NiCd battery example above. If you are seriously pinching amp-hours, you may still want to use your corded tools.

Happy Cruising!

Tim Allen