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Old 27-07-2019, 09:26   #1
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LiFePO4 bank - adequate distance from compass?

Does anyone have guidance on the necessary distance for a LiFePO4 house bank from a compass to prevent compass distortion/deviation?

My 1980 Sabre 34 has the present house bank of two 12V group 27 batteries under the port quarterberth, next to the engine, which is under the cockpit. Pretty common layout.

I am planning to install a LiFePO4 house bank of about 600Ah @ 12V. One group 27 will be kept in the present battery location as a starter battery.

I am thinking of installing the new LFP bank near the base of the mast, under the bulkhead-mounted fold-down cabin table. It is a good spot in that it keeps weight low and centered. The bank would be easily accessible, and makes good use of an under-utilized space on the boat.

However, it is also less than two feet from the autopilot compass (on the bulkhead, under the starboard setee). Since the "Fe" in LiFePO4 stands for iron, I am concerned that proximity to the compass will cause an issue. And if so, then what is a safe distance as I think about a new location for the autopilot compass?
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Old 27-07-2019, 09:54   #2
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Re: LiFePO4 bank - adequate distance from compass?

Very unlikely to be an issue IMO.

If you're that concerned, design a rough test or two, but don't change things just because you think there might be a problem.
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Old 27-07-2019, 10:28   #3
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Re: LiFePO4 bank - adequate distance from compass?

Unfortunately, it is not just the batteries, but the wiring that needs to be considered. Electric current creates a magnetic field.

Less than two feet is likely to be too close to the main power wires and battery interconnect cables. These will be carrying considerable current at times.

Autopilots need all the help they can get.

Fortunately it is quite easy to move most fluxgate compasses. The wiring is not complicated although you need an alternative suitable location.

A Handbearing compass is helpful when selecting a location free of magnetic anomalies.
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Old 27-07-2019, 17:52   #4
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Re: LiFePO4 bank - adequate distance from compass?

Quote:
Originally Posted by KimSails View Post
Does anyone have guidance on the necessary distance for a LiFePO4 house bank from a compass to prevent compass distortion/deviation?

My 1980 Sabre 34 has the present house bank of two 12V group 27 batteries under the port quarterberth, next to the engine, which is under the cockpit. Pretty common layout.

I am planning to install a LiFePO4 house bank of about 600Ah @ 12V. One group 27 will be kept in the present battery location as a starter battery.

I am thinking of installing the new LFP bank near the base of the mast, under the bulkhead-mounted fold-down cabin table. It is a good spot in that it keeps weight low and centered. The bank would be easily accessible, and makes good use of an under-utilized space on the boat.

However, it is also less than two feet from the autopilot compass (on the bulkhead, under the starboard setee). Since the "Fe" in LiFePO4 stands for iron, I am concerned that proximity to the compass will cause an issue. And if so, then what is a safe distance as I think about a new location for the autopilot compass?
There are no iron bars in LiFePo batteries, the iron is part of a coating material. Your 2 ft should be fine but you can always re-mount the fluxgate farther. It's pretty common for them to be in a hanging locker for clothing.

As noelex77 wrote, the real issue and what you more want to avoid are electrical cables near the flux gate and to keep the compass-related wiring away from other power wires. But this is probably how it is mounted already. It's easy to check by simply watching the compass as power is switched on and off to each of the boat circuits.
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Old 28-07-2019, 05:10   #5
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Re: LiFePO4 bank - adequate distance from compass?

One common tool used for detecting magnetic fields is a handheld compass. Should the compass needle/card deflect as it is moved into the proximity an object, then the object may be considered to produce a magnetic field.
Similarly, a magnet may be used to test for or ferrous metal. Should a magnet be attracted to or be deflected from an object, then the object may be considered to produce a magnetic field.
One of the latest additions to tools for detecting magnetic fields is the Magnetometer function Gemeco's free iNstall app for smartphones. It is recommended that such testing be performed prior to permanently mounting the fluxgate. When seeking an installation location for a magnetic north seeking device such as a fluxgate compass, etc., it would be considered a best installation practice to test the magnetic field strength in the intended location while all onboard electrical devices are on (and cycling or switching if applicable).
While Raymarine, for instance, has not specified a maximum sensed magnetic field strength value, verifying the iNstall's app's magnetometer needle is in the green band (+/- 10 T) would be a good starting point.
Note that there is no form of screening (Lead box, wire mesh etc . .) that can be installed around a compass sensor to eliminate the effects of external deviating fields, in such cases where excessive deviation is encountered, relocation of the sensor is the only option.
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Old 29-07-2019, 10:22   #6
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Re: LiFePO4 bank - adequate distance from compass?

Thanks everyone for your input! This is very helpful!

I downloaded the iNstall app and also tested the battery bank a bit with a handheld compass and the autopilot itself. Curiously, the iNstall app gives a very high reading to the batteries (just batteries, no wires/current as yet). The handheld compass and the fluxgate compass did not seem to be impacted by the battery bank. I will have to check again once I get it setup with a current.

Thanks,
Kim
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Old 30-07-2019, 23:27   #7
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Re: LiFePO4 bank - adequate distance from compass?

I have mine lifepo4 bank approximately 2 feet away from the Ev sensor on my Raymarine autopilot. It works ok but when my relay cut charging when motoring then this obviously creates a current spike or something because then the autopilot turns a few degrees and sometimes looses its current course. Not a big problem if you are aware of it.
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