12 V spikes

[mikenda - delete]

I know that if I run from an independant battery I can avoid spikes on my Garmin ( switching off on engine start). Is there a electronic device I can put in line to avoid spikes and leave my 2 battery switch intact?

 

[Bergman]

Are you sure it is a "spike" causing the problem. If it only happens on engine start it could be that the voltage is being pulled down by the load taken by the starter motor. A check with a voltmeter would probably show this.

If it is then you may have a tired battery or a bad connection somewhere in the wiring from the battery to the starter, most likely on the battery terminal.

 

[LadyInBed]

This problem is normally caused by a large load on the battery causing a voltage sag.
I had this on a previous boat causing the autohelm and Decca to reset, when starting a 7.5hp engine, even with a good battery.
I solved the problem by running the above kit from a small 12v NiCad battery that was float charged via a diode from the main battery.

 

[philip_stevens]

Snap! I did exactly the same when I had my Decca installed for exactly the same reason - only I used a Schottky diode with a lower forward resistance for a smaller volts drop to the small "start-up" battery - not that it was that critical for the short duration it was used.

A better way to do it (my hindsight) without a diode is to put a small relay in the circuit with normally closed (NC) contacts supplying the small battery, and energised from the engine start circuit. This way, when you start the engine, you will isolate the small battery. As soon as the engine has started, the contacts will close again - restoring supply to the small battery.

regards,
Philip

 

[LadyInBed]

Thought about using a relay, but it was too much fuss wiring it in, and its easier and cheaper to keep a couple of spare diodes in the boat 'odds & sods' kit.
I worked on the principle of keep it simple, and I reckoned that when the diode (it was a Schottky) was reverse bias, it was as good as o/c to the main battery.

 

[bryantee]

have a look at http://www.sto-p.com/ on the marine electronics page.we use these sto-p units commercially to screen out welding currents and protect data in the computers on start up and over/under voltage spikes.a small 25 a/h battery looks after the electronic power supply.

 

[oldharry]

Its far more likely your Garmin is switching off at engine start because of the voltage drop caused by the starter motor. I do not know the minimum voltage required to keep the Garmin going, but it is quite normal for 12 volt electronics to cut out if the supply voltage drops by much more than around 20%.

As a starter motor demand can pull battery terminal voltage down by 25 - 30% on iniitial engagement any voltage sensitive electronics on the same circuit will drop out.

Early teens sons were computer addicts (now professionals earning more than I ever did!) so an essential adjunct to late 80s crusing was the old Amstrad 6128 and a portable telly running off the boats battery. Howls of rage from the forecabin if I started the engine without 'permission' - 'Dad you've crashed the computer AGAIN!' Amstrads needed 10.5 volts minimum supply or thereabouts. Peaceful relations between 'fo'csle and quarterdeck' were restored by the installation of a service battery and blocking diode to preserve the computer supply voltage.

 

[rogerm]

As others have said it is almost certainly a 'dip' rather than a spike. While a Ni-Cd is fine a small gel type lead acid would be just as good. As long as the 'support' battery is only for the Nav instruments the size can be quite small. Just think each nav instrument only takes about 50ma, perhaps 100ma if backlighting is on. I don't suppose you have more than 6 nav instruments (log/depth/wind/navtex/gps/??) so the load is only going to be 0.6A with backlighting. The time to start an engine normally is measured in seconds. Even a 1Ah battery will hold the nav instruments going for an hour or so and is so small that you can lose it in the space behind the switch panel.
Roger