1 or 2 batteries?

...hopeful;ly the engine starts quick so it will be OK, the VSR or a simple 30A relay does sound like the best idea (I use VSR cos I get them at the right price).

 

He wants bilge pumps , nav lights , anchor lights VHf, GPS and even ETC ---no chance.

 

Depending on the age of the 50 it might only be an unregulated 6 amp output and that will be at 4000rpm, like Frosty says thats not much oomph.

 

Hi there,
Like CDK has said many times, use a second battery paralleled to the main battery, with a Schottky diode in between the 2 plus terminals and with the minus connected to each other, this to keep the second battery charged at all times. I just came out of the hospital and when chatting to the surgeon again, he mentioned that he was an addicted fish-angler. But he always switched the diesel engine off and used an electric motor only, when getting close to the fishing grounds. He always forgot or had an half charged battery. I made the attached unit for him and maybe it will help you.

You need 3 hours of your time.
a piece of double sided 70 micron copper printed circuit board
2 Vishay double 20 Ampere Schottky diodes (cost approx 1,5 USA dollar each) type 40CPQ100 (or 60 for 60 Volt or 80 for 80 Volt or 100 for 100 Volt. I just happen to have the 100 Volt at hand, you can use anything from 50 Volt upwards.
a small box ( cost +/- 0.8 USA dollar)
2 pieces of appropriate cable with slugs, connections.
You drill 4 holes in such a way that the two diodes 4 x 20 = 80 Ampere cannot make short circuit between the two copper plates. i.e. the one side you drill with a flat point the copper away. i.e. if you have a 1,5 mm drill for the diode legs, you use a 8 mm drill to scrape the copper away around the 1,5 mm hole.

You mount and solder the 2 diodes
You drill two mounting holes for 3mm screws and nuts
You cut from a NESTLE hot chocolate tin plastic cover a small insulation plate and ensure that the two screws do not shorten the two sides of the printed circuit board.
Thereafter you solder the two leads onto the copper sides, close the box and connect the 2 batteries between the plus.
The surgeon was impressed; he no longer will have flat spare battery on his boat.
Bert

P.S. the loss over both sides is 340 millivolt at 3 Ampere, at 10 Ampere 430 milivolt and at 60 Ampere, theoretical 700 milliVolt.

 

I just got informed, that the Nestle plastic melts by higher charging currents greater than 30 Ampere. This special when the battery has been used extensively. The best is to replace this piece of plastic with a piece of fibre glass printed circuit board of which the copper is removed with some sandpaper.

 

If the second battery ends up at 14.4v or slightly more fine . if it does not then like other diode system dropping 0.7v it is no good. A voltage sensitive relay would do the same job and no voltage drop

 

What charge is a 50HP--- 6 amps ?

Then a 60 amp batt bank is all you can go.

You cant keep piling on batts to cure a problem --they have to get charged sooner or later.

I would practice rope starting your 50.

 

One has to understand that a battery when nearly full, the charging current will drop, whereby the voltage drop over the diode falls to 0,35 Volt. This is absolute not an issue, this little bit of waste. My experience is that the battery even last longer. Voltage sensitive relay needs energy and is also a waste. It will be a personable view in what to select, a sparking paladium contact with also energy losses (No contact is zero Ohm! You will allways have a contact resistance, how small that may be) or a Schottky diode.

 

Fully agree with you, hope this fellow will follow up your advise.

 

with a small boat with one engine its is better to have two batteries with on for the starter only. It does not matter if the engine can charge the whole lot because if you have any sense you will fully charge both at home before you go on a trip ....