Battery Selector Switch

[gavin]

Nice clean set up in the engine bay, what about the extra push if you are asking the alternator to deliver current to two flat batteries with the sw in both position, I have seen this only last week where a flat battery in a hard boat jumped from an adjacant battery which became flat also.When the motor was boost started from a pack, it couldnt idle until one of the dead batts was removed from the circuit, the alternator might not be able to deliver current to 2 flat batteries gavin

[chrisallse]

am i being thick or are both ends of the diode connected to the same terminal?

[jwalker]

Yes, the blue wire carries the charging current unless/until the blue knob is turned to connect the main ground.
Chris, yep, you are. This terminal is normally open. The connecton is made by tightening the blue knob when the battery is required. Look at the circuit diagram.

A normal diode can be used but these have a voltage drop of about 0.6 - 0.7volts. This will ensure the battery is not fully charged. The schottky type has a drop of about 0.4v so the battery charge will be pretty well complete, depending on the voltage regulator output.
There was a time when schottky diodes with a high current capability were not available. I've used ordinary diodes with complete success.

Gav, the problem you describe is simply because the engine is not able to produce enough power to spin the alternator at tickover. A well designed modern engine should be able to cope with the extra load. I have seen a situation where a slightly slack alternator belt slips when charging a flat battery but is ok when the battery is charged. Also, this system is supposed to prevent you ever having 2 flat batteries!

Edit.. forgot to say. The thinner black wire is, indeed, the negative for the unswitched supply.

[chrisallse]

see what you mean now...didnt realise the blue knob isolates the neg.....what happens to the diode if you forget to close the isolator and switch to the backup batt?

[jwalker]

Nothing.

[chrisallse]

Have you tried it? wouldnt the load current try and flow through the Diode?

[jwalker]

Yes, of course it would. But diodes block current from the reverse direction. That's their job.

[gtflash]

what sort of current does the schotty handle rms?

If you use just a 1, 2, and both switch then disimilar batteries then surely a less than adequate battery could be over pushed and cook, batteries arent like resitors they dont always draw just what they need gradualy dop they?

What do the standard marine split chargers do?

[Pete7]

Err, forgive for saying but isn't this a bit OTT. The boat is a 6.5m rib with an outboard and its a Yamaha, so its bound to start even with the starting rope they all come with, which should satisfy the surveyor when she is coded.

Peter

[jwalker]

Originally posted by gtflash
what sort of current does the schotty handle rms?

If you use just a 1, 2, and both switch then disimilar batteries then surely a less than adequate battery could be over pushed and cook, batteries arent like resitors they dont always draw just what they need gradualy dop they?

What do the standard marine split chargers do?

The package I used contains two of, 15amp diodes.

As a battery charges, its voltage rises and this controls the amount of current which flows into it. Eg. if the battery is at 12v and the charger is at 14v then there is 2v pushing the charge current. When the battery has reached about 12.8v (max.), then there is only 1.2v pushing the current through it. So, the batteries look after themselves depending on their state of charge.

I've never used a marine split charger but I guess a relay connects the charge current when the ignition is switched on or, perhaps, clever ones sense the charge voltage and switch the relay when the voltage reaches a level which indicates the alternator is running.
I'm sure someone can tell us.

[scm]

Peter,

I can only agree and do not forget this is for INLAND WATERWAYS, cat D water. I have also got to have a lifreaft amongst other things.

All this just to code a boat in the Solent

Guys,
Many thanks for some very helpful advise indeed. Just got to get round to doing it now!!

Regards
Stuart

[robin]

Hello Stuart,
what made you change your mind about the coding?

Disregard that...just read your link.

[Paul F]

JWalker
They are actually know as Battery isolators and do basically as what you have done.
Without going into the nitty gritty, diodes(depending on how many batteries being charged) and sensing unit housed in heatsink box, 3 poles on top 1 going to each battery, the other is the main charge line from the alternator(normaly goes to live side of start soly) smallest unit rated at 75amps and up to 36v. Have a look at EC smith & sons or aquamarine etc
Paul

[Pete7]