Ever since I've had my current boat, the Navman Chartplotter has always switched itself off when I started the engine, whereas the Garmin Fishfinder and Navman VHF, connected to the same auxilary fusebox (but different fuses) remain on. I concluded a long time ago that this must be due to the Navman Chartplotter being more sensitive to voltage drop than the other two units. A digital multimeter shows only quite a small voltage drop as the engine is cranked, in the order of 1.5V for either 110 A/H battery if I recall correctly.

Initially that fusebox was fed from the ignition switch (permanently +ve side).
I checked all the ground wires; all sound.
Then I took a feed instead direct from the battery switch to the fusebox; still the same problem.
Next I took a pretty thick cable (20mm2 conductor area) from the battery switch to the fusebox; same problem.
I disconnected all the other units fed from that fusebox; same problem.
I tried starting on the "Both" battery setting; same problem.

I really do not know if the Chartplotter is more sensitive to voltage than is should be, due to an internal issue, it's outside warranty so I am reluctant to take it to a dealer as it works OK otherwise.

I finally sorted it; I bought a small 12V lead acid battery, the sort that is designed to be used as a back up in domestic alarms. The -ve terminal is connected to the ground busbar at the helm. The +ve feeds the auxillary fusebox. Another +ve feed is taken from the ignition switch and connected to the fusebox, but wilh a 6A diode in line. So, the small battery and the boat batteries are wired in parallel. When the engine is started now, the voltage the chartplotter "sees" does not drop beyond the voltage of the small battery, due to the diode. That small battery is charged up during normal running, but not fully due to the voltage drop over the diode. It seems to work fine though!

Total cost was quite low, especially as the "6A" diode is actually four 1.5A diodes in parallel, these were free as they were "salvaged" from a cheap mains stereo!

Graham

My Raymarine Fishfinder does that sometimes.

Nice fix!

silly question but unless the unit is also a fishfinder, why would it be on before starting the engine?

For mine, I have a separate switch on the helm for the fishfinder that I also wired the GPS to. When this switch is on, the fishfinder and GPS are on regardless of key position or even whether it is present in the ignition switch.

Bruce,

If we are fishing on the drift, we switch the engine off. The chartplotter (and fishfinder, for all the use it is at finding fish I can catch!) stays on when the engine is switched off, because they are not connected to the switched side of the ignition switch, but to the permanent +ve feed to the ignition switch. So,it still plots our position/ course as we drift. Then, when we started the engine again, it used to switch off.

Graham

You basically proved the problem. I think the reason you did not see the voltage drop is that the typical digital multimeter works by sampling the voltage, and therefore is not fast enough - or in synch to see fast transients that might be enough to shut down your device.

There are commercial items to prevent this occurrence, such as the Newmar Start-Guard, but you solved the problem less expensively.

http://www.newmarpower.com/DC_Power_Conditioners/DC_Power_Conditioners.html

The only thing I might consider adding is maybe a 1uF or 10uF capacitor from the positive side of your supplemental battery to the negative side. This will prevent RF transients from possibly damaging your equipment.

As devices switch on and off - even DC devices, the leading edge of the transient from off-to-on is rich in harmonics - all the way up to the RF range. So while the battery can be considered a low-frequency filter, there could be a damaging-to-microelectronics RF component that, with the power on, could possibly (and I mean only possibly), damage equipment. A lot depends on how good of a design the equipment is.

The value is a bit of guess, because you would have to use an oscilloscope to see if you have RF transients, and which value of capacitor is the most effective. But 10uF would be a good compromise.

I also noticed on the web page that I posted, you can buy just a filter if you don't want to mess with installing a capacitor. I am sure that the filtering in that device is probably a multi-stage filter, whereas the capacitor is a single stage, and won't filter the powerline as well. But the capacitor is better than nothing.

Al,

I confess to knowing very little about electronics. Thanks for the advice re the capacitor; as a matter of interest, the boat already has 2 batteries, when the battery switch is in the "both" position they are connected in parallel. Why would the 3rd battery, also connected in parallel, possibly need a capacitor, whereas the other two do not? Something to do with the voltage drop across the diode maybe? In fact, on my boat even when the battery switch is in the "off" position, the +ve terminals of the 2 main batteries are permanently connected together via a diode for each. That circuitry provides permanent power to the bilge pump and active corrosion protection unit. I presume it's there to prevent a potentially weaker battery causing the other to discharge.

Thanks for the advice about voltage drop detection; next time I am checking for voltage drop I'll use my trusty old 1980 "Radio Shack" analogue multimeter, rather than the digital youngster that pensioned it off!

Graham

How much are those power conditioners? My 450W RMS UPS for my home computer was under $100 including tax and shipping and protects everything in my home office except the laser printer.

Graham;

The capacitor idea is my recommendation due to you wanting to turn the chartplotter on before starting the engine. This is true whether or not you have two or three batteries.

If you leave the chartplotter off, then the open switch will prevent the transients from hitting the innards.

I recommended the capacitor next to the battery as I assumed you put the battery next to your chartplotter, and the battery leads would be a convient spot for the capacitor.

But if the battery is located a distance from the chartplotter, then the capacitor should still be located close to the chartplotter.

In reality, you could also use a RF Choke rather than a capacitor if it is easier to install. They basically accomplish the same thing, and in fact, high quality filters use both components in tandem, a PI filter being a good example of this.

In a PI filter, two capacitors and a choke make up the filter, and it is called a PI filter only due to its shape in the schematic, rather than anything mathematical.

The RF Choke suppresses high frequencies by acting as a series impedance, and the capacitor shorts the high frequencies to ground. Two different approaches to the same thing.

An analog meter won't fully "see" the transient - only something like an oscilloscope will. But the meter will at least deflect when you turn power on, so you will be able to tell something is going on.

By the way, the diodes in your batteries form a Battery Isolator, and you are absolutely correct. If you follow the concept that a diode will only conduct electricity one way, it prevents one battery from discharging into the other, but allows both batteries to supply the current to the load, and be charged by the alternator.

Deepv; I believe the conditioners (with the rechargeable battery) is about $150 or so. Not bad for marine pricing.