A Look at Voltage Transients in Marine Electrical Systems
I had been meaning to put this together for a while and assembled a system for a training class I was doing. Once that was assembled I realized I could simply add a starter motor, an oscilloscope and a Fluke 289 & Fluke 376 and then would would was able to create a video on voltage transients.
In the video below I’m walking through the very typical, though not necessarily ideal, marine battery system which utilizes the venerable 1, BOTH, 2, OFF battery switch. The typical 1, BOTH, 2, OFF switch is configured for both bank charge selection and bank use selection.
With the switch configuration in the video, identical to most factory systems, the battery bank selected, via the switch, serves as both a house bank and starting bank as well. It also selects which bank is getting charged either bank 1, bank 2 or BOTH.
This image was taken on a customers boat who was complaining of poor windlass performance. The windlass on this vessel is connected to the common post of 1/2/BOTH switch, like it is on many boats. This owner had been told it was best to use his windlass on his “starting battery”. With a 1/2/BOTH switch you really just have two battery banks not a dedicated starting battery. His Group 27 West Marine starting battery was passing a Midtronics CCA/MCA test but it failed to drive his windlass motor, in the manner he expected, for the duration needed to raise the anchor.
This image, showing the voltage transient created by a windlass, is a prime example of why merely creating a dedicated & isolated starting bank will not remove all potential for voltage transients in a marine DC system.
There are basically two scenarios related to battery switching which folks often discuss;
- DC Brown-Outs
- Voltage Transients
Electrical brown-outs can happen when the bank is under sized, unhealthy or over discharged for the loads applied to it. It can also occur on an incorrectly wired system due to voltage drop in the system wiring. If the DC electronics are on the same bank being used for high loads then the electronics may drop out when the starter, windlass or other high draw item is called upon. Brown outs, while annoying, rarely cause any damage to equipment. On a cruising boat with a decent sized house bank, that is properly wired, electronics drop outs should not occur even when starting the motor.
Voltage transients are another story and can be damaging to some electronics. I say some because any good electronic design, from the last 30 years, should already have transient voltage suppression capabilities on the input path.
It’s Not just Starting Motors:
The vast majority of windlass installations, on sub 45′ vessels, are wired to the house bank and there is nothing wrong with this. The house bank is almost always considerably larger and in almost every situation can drive the windlass motor far better than a typical starting battery will.
Starting batteries are rated for 30 seconds use with a terminal cut off voltage, at 30 seconds, of just 7.2V.. Raising 200-300′ of chain is not a 30 second operation. Most automotive engines start with a cranking duration of about three-quarters of a second. Cranking duration is; starter loaded to unloaded. A starter battery, intended for 3/4 of a second of cranking of auto motors, is really a poor choice for windlass use and leads to anemic performance.
As can be seen in this image the voltage transient created by the windlass was over 20V. While I was not there to test for voltage transients what I did find was that the owners starting battery was totally inadequate for driving his windlass and his windlass wire was grossly undersized leading to a low voltage at the windlass end of just 4.40V.
“Ah yes, you do have poor windlass performance.”
The “fix” on this boat was easy, I then switched over to the house bank, which consisted of four GC2 6V batteries wired series then parallel, and the peak low voltage at the windlass end jumped to 8.92V. Still, the owner had bad wiring but, his windlass performed considerably better on the larger bank rather than the smaller one.
The video below will walk through this and you’ll be able to see this, with instrumentation connected.