Nav instruments turn off when engine starts

[William_H]

re Volt drop when starting the engine. The previous answers correctly suggest that the nav gear be running on a separate battery to avert the problem.
Just as a matter of interest perhaps to those with only one battery. The sudden volt drop to the nav gear when engaging the starter can be avoided by fitting an isolating diode (schotky type is best) in the supply to the nav gear. Then fit a very large electrolytic capacitor like 10,000 microfarrad rated at 20 volts or more across the input to the nav gear. The capacitor will act like a battery for the short period the volts have fallen. The size of the capacitor will dictate how long the voltage can be suppoerted ont he nav gear while cranking so can be chosen by experiment. (add more capacitors in parallel if needed) The diode stops the capacitor feeding its power back to the starter when the volts drop. You can indeed use a small battery in place of the capacitor. olewill

 

[PaulRainbow]

Not on my boat, it doesn't! If the engine battery is low it takes a short while for the relay to cut in (as shown by the VSR indicator light).

According to Merlin, who make and sell the things: 'VSR's initially stay open (off) when the engine is charging the engine start battery, therefore, the engine battery is priority charged. Once the engine battery has reached 13.7V from the alternator charge, the relay will close allowing both engine and domestic battery banks to be charged simultaneously. When the engine is stopped, the voltage returns to normal levels allowing the relay to open again - separating the engine and auxiliary batteries. '

Link: http://www.merlinequipment.com/markets/group.asp?groupid=37

If you start an engine that has a flat battery and then measure the voltage at the battery terminals you will get a reading of whatever the alternator is outputting, typically 14.4 - 14.6v with a modern alternator. As a VSR is connected to the battery i cannot see how it can possibly tell the difference between battery voltage and alternator output.

Merlin say

The M-Power VSR is self contained, enclosing the contactor and control electronics. Unlike most other VSRs a short delay timer between battery sensing and switch activation prevents damage to unit from relay ‘chatter’.

This is where your delay in seeing the light come on comes from.

 

[lw395]

An alternative to messing with the heavy current side of things is to run the plotter from a small 12V battery of its own. This can be charged with a low current DC-DC converter from the main system.
On my old boat, this small battery could also power the VHF. I think this used to be asked for by some racing regs?
Anyway, it worked for me and is a potentially much cheaper solution if you are generally happy with the current set up.

A 1.2Ah alarm battery would run my old GPS128 for many hours. Modern plotters use a lot more current I fear. But coping with a few seconds cranking outage will be no problem.

 

[howard easton]

An alternative to messing with the heavy current side of things is to run the plotter from a small 12V battery of its own. This can be charged with a low current DC-DC converter from the main system.
On my old boat, this small battery could also power the VHF. I think this used to be asked for by some racing regs?
Anyway, it worked for me and is a potentially much cheaper solution if you are generally happy with the current set up.

A 1.2Ah alarm battery would run my old GPS128 for many hours. Modern plotters use a lot more current I fear. But coping with a few seconds cranking outage will be no problem.

Thats arguably the wrong way round. Your boat ought to have all domestic services wired to the domestic bank via an on off switch with the engine seperately wired to its own fairly light battery. The only interconnection is the charging wire from the alternator to the domestic bank but that needs a switch in circuit to prevent the domestics draining your engine battery. When the engine isnt running, the switch is open. When it is running, the switch is closed. You can do this exercise with a simple switch ( cheap) or a relay ( bit more) or a VSR ( most expensive option).

My Prout cat came from the builders with a simple relay built into the charging circuit that made the connection between domestics and charging when the engine ignition was operated. That relay was the ubiquitous headlamp relay used on BL cars. Cheap, capable of 40 amps and worked reliably. If you have a big alternator and a big battery bank you likely will need something more beefy , but this approach has the advantage that its automatic. You dont have to remember to switch over.

Amazon list an 80mp waterproof relay at £6.

The only difference between this approach and a VSR , apart from £70 or so, is that the VSR waits for the alternator output to run up to the threshold whilst a simple relay open the circuit when you operate the ingition switch. So a delay of maybe 10 seconds. Even if your domestics are flat , the engine battery wont become flat in 10 secs.

 

[lw395]

Thats arguably the wrong way round. Your boat ought to ......

I was approaching the problem as the simplest, cheapest change from what the OP has, to achieve avoiding the problem he has.
I was also describing an actual installation which has some merits.
I'm well aware there are alternative systems.
Each system has different pro's and con's.

Using a 40A relay to connect the domestic and engine batteries while cranking is not the greatest idea, what prevents a large fraction of the cranking current from going through the relay?
PArticularly on a modern boat where the domestic batteries may be at a higher charge state than the cranking battery, due to solar charging or shore power.

 

[Dipper]

My instruments used to cut out when starting the engine. I fitted a new battery which solved it.

 

[lw395]

re Volt drop when starting the engine. The previous answers correctly suggest that the nav gear be running on a separate battery to avert the problem.
Just as a matter of interest perhaps to those with only one battery. The sudden volt drop to the nav gear when engaging the starter can be avoided by fitting an isolating diode (schotky type is best) in the supply to the nav gear. Then fit a very large electrolytic capacitor like 10,000 microfarrad rated at 20 volts or more across the input to the nav gear. The capacitor will act like a battery for the short period the volts have fallen. The size of the capacitor will dictate how long the voltage can be suppoerted ont he nav gear while cranking so can be chosen by experiment. (add more capacitors in parallel if needed) The diode stops the capacitor feeding its power back to the starter when the volts drop. You can indeed use a small battery in place of the capacitor. olewill

These modern plotters seem to take 400mA or more.
A lot seem to need over 10V to work.
So a capacitor charged to 12V (healthy battery less diode drop) would need 0.2F for every second of cranking time. That's a pretty stout capacitor bank.

 

[GHA]

These modern plotters seem to take 400mA or more.
A lot seem to need over 10V to work.
So a capacitor charged to 12V (healthy battery less diode drop) would need 0.2F for every second of cranking time. That's a pretty stout capacitor bank.

Big 12v capacitors available on ebay for car hifi buffs.

 

[lw395]

Big 12v capacitors available on ebay for car hifi buffs.

At about £20 a farad?
How many seconds of cranking time do you think is reasonable?

Caps are a good solution when you have a GPS that only draws a few mA and will work down to 9V or something.

 

[GHA]

At about £20 a farad?
How many seconds of cranking time do you think is reasonable?

No judgement was made, just showing a source should anyone be interested.
I'll submit anything for checking by you next time to make sure any posts made are suitable

 

[lw395]

No judgement was made, just showing a source should anyone be interested.
I'll submit anything for checking by you next time to make sure any posts made are suitable

My post does read as more terse than intended. Sorry!

 

[GHA]

My post does read as more terse than intended. Sorry!

Thinking about it, is the voltage drop momentary as the inrush current peaks or is it severe all the time the charger is running?
I have no idea but maybe a smaller Capacitor might work?
Dunno.

 

[pvb]

Surely a capacitor is simply papering over the cracks?

 

[lw395]

Thinking about it, is the voltage drop momentary as the inrush current peaks or is it severe all the time the charger is running?
I have no idea but maybe a smaller Capacitor might work?
Dunno.

The dip as the starter motor starts will be perhaps down to 8V?
While cranking it won't be so low, but is very likely to be lower than the 10V + 1 diode drop needed for the plotter not to be running off the cap.

It could be that the plotter will run on a lower voltage than spec'd in reality?
But I'm not a fan of things that rely on that kind of margin.
The day you have a rough sail and the engine is slow to start is likely to be the day you want the plotter to keep working.

 

[bedouin]

Have 2 domestic batteries (115Ah each) and also a separate engine starter battery.

Are you saying we should only use the engine start battery to ever start the battery, and use the 1-2-both switch to facilitate this? As usually we start the engine at the marina using the engine start battery and then we switch to the 2 x domestic batteries after about 20 mins. When we have then decided to start the engine later have not turned the switch and let it on the domestic batteries.

paul, as you say it may be easier to just switch to the engine start battery every time we want to start the battery as the nav instruments are always powered from the domestic batteries even when all are turned off using the switch.

If I read that correctly then you are saying the instruments in question are always powered from the domestic batteries. In that case the problem must be caused in the part of the circuit common to both engine start and domestic - probably the negative return to the battery.

Taking the negative feed for the instruments directly from the battery ought to fix that.

 

[Ruffles]

My GPS resets when I start the engine. Not quite annoying enough to make me fix it.

In my case it's the voltage drop across the 1-2-B switch that's the majority of the problem. If you have a multimeter with Max/Min connect it across the switch, set it to max and start the engine. I get around 2 volts which combined with the internal resistance in the battery and the cables is enough to reduce the switchboard supply to below the magic 10 volts. I think most modern marine equipment is rated 8 to 32 volts?

 

[RichardS]

Surely a capacitor is simply papering over the cracks?

When I was a kid I used to use paper capacitors in my home-built circuits ..... which makes you one seriously smart cookie. :encouragement:

Richard

 

[PaulRainbow]

Surely a capacitor is simply papering over the cracks?

Is that a polite phrase for bodging a problem with a shite setup rather than installing a superior one ?

 

[pvb]

Is that a polite phrase for bodging a problem with a shite setup rather than installing a superior one ?

I think you win the (notional) prize!

 

[PaulRainbow]

I think you win the (notional) prize!

I still have the old 1-2-both switch onboard, not sure whether to go for a ritual burial at sea or put it on Ebay