1 or 2 batteries?

Hi all,

Just bought my first boat, a Shetland 535. It has an electric start Johnson 50 outboard and I will be running the usual electrical suspects - bilge, nav lights, anchor light, deck lights, horn, GPS, fish finder, VHF etc.

As the outboard has an alternator output is it worth having 2 batteries with a split charge system and running a "start" battery and a "leisure" battery setup or just run with 1 larger battery for everything.

The obvious con to 1 battery is when its flat, Im not starting the engine, but what have you guys done - think is best??

Thanks in advance

 

Well....when you crank an engine it " spikes " the 12v systems voltage. Electronics do not like this voltage spike, so two battery banks... a house bank for electronics that is not influenced by the start crank load and a starter bat is certainly a good idea.

Ive never liked charge splitters. One alternator for two bat banks. In your case you have no choice.

Ask your electric supplier for advice on installation , particularly the output amps of your outboard and its well worth a small marine battery charger that you can energize when you have shorepower to keep your house bank topped up.

I see this BLUE SEA splitter in most marine shops. Dont know anything about it.

http://www.uship.fr/bateaux,blue-se...zoom,mod,boutique,voilerie,35118-3381.gb.html

 

A 1-2-all battery switch is the simplest way to charge both batteries and use whichever you want for house loads. The outboard may charge 15A or so. They are usually not enough for all the electronics but help some. Most people keep the batteries on a charger when they get back to the dock or trailer.

 

I would go with two batteries, a starter and a deep cycle to run the electrics and electronics. I agree with Gonzo. KISS. keep it simple. A 1-2-all battery switch, and a battery charger when you have shore power.

 

The most cost effective solution involves no switch and no charge splitter.

Buy a small battery capable of starting your engine, in this case anything over 25 Ah will do. Place it next to the existing battery and connect the negative poles using a piece of starter wire.

Buy a small Schottky diode like an SB520 or SB530 (about the price of a can of beer) and use that to connect the positive poles. The side marked with the ring is connected to the new battery.
Finally, make a short positive bridging wire with heavy duty alligator clips on both ends, spray those with WD-40 and put it in a plastic bag.

This gives you a spare battery that will always be fully charged in case you ever need it. Because no power is ever drawn from it, there is no wear: it will outlive your engine and probably the boat as well.
In case your battery is flat, use the bridge wire, start the engine and put it back in the plastic bag!

My boat has twin engines, two batteries and a big switch, so I would not need it if I paid attention and turn the switch when anchoring. But with guests and cold drinks I tend to forget such things, so I have a 3rd battery wired this way for over 15 years now and needed it twice....

 

I always want to agree with CDK but not in this case ..the use of a diode for charging will reduce the voltage to the second battery by in this case 0.55v which is bad for battery life ... AND as batteries start to die the minute they are built I think after 15 years you are carrying a lump of lead and thats all .

For the original UK poster ..if you are going to sea and it does not have a pull start then yes you must have two batteries ...the big red switch or a VSR from e bay ..dont pay a fortune for it .

 

That's also what I would think at first glance.

When I installed the battery I didn't think of it as a permanent solution because of the reasons you gave here. But the reality is different.
The battery is an ordinary black Exide 70 Ah with a "magic eye" which is a simple SG float showing a green side when it is near full charge. And after all these years it still does! Even more significant: its voltage at rest is higher than that of the two starter batteries that have been replaced twice already.

This is what happens:

With the engines started, the alternator voltage slowly rises to slightly more than 14 V when the starter batteries are topped up. The spare battery gets just a trickle charge because no current was ever drawn from it and has 13.5-13.6 V at the poles, chemical activity is almost nil.

At rest, there is a constant 33 milli-Amps drain from various devices like the radar, depth sounder, VHF radio, a clock and an alarm unit from the starter batteries. That amounts to 5.5 Ah each week and lowers the voltage to nearly 12 V. where the spare battery still has almost 13 V.
Also the self discharge rate from the starter batteries is much higher than from the spare because it still is as good as new.

Even in the winter, when a small solar panel tries to compensate the steady discharge, the spare battery maintains a higher voltage.

 

Std lead starting batteries charged to 13.6v tend to have a life of only 6-12 months but thats when used for starting....still dont see after 15 years it can be any good

 

I'm in year 10 of my auto battery and it's a starting and house batt.

-Tom

 

That is exceptionally long for a battery that is actually used. I assume it is over-dimensioned for its task and your engine is a rapid starter.

The traffic of ions between the plates is what wears out a battery. I use a small one (38 Ah) to power the electric gate, cameras and outdoor lights, charged by a solar panel. The daily charge/discharge cycling limits the battery life to 4 - 5 years, after that the capacity has dropped so far that the gate doesn't work properly before sunrise.

 

tom ..they dont build them like they use too ......!!!

 

It was standard dealer issue here in Victoria, nothing special. I do run 5w50 synthetic oil however in a 2 litre four cylinder with instant start ups and a head light alarm, so I've never left them on.

It is exceptional life, I agree.

Every "winter" I expect it to die...

-Tom

 

Install the start and deep cycle batt , the rotary switch and leave it in #1 engine start position.

Wire all the DC fun loads to the house deep cycle battery.

Purchase a $17.00 RV combining solenoid and wire it to operate from the "ON" terminal of the key switch.

Best of all worlds , turn ON the engine the house is automatically charged , no thinking.

Turn OFF the engine the house is automatically disconnected , no thinking.

Problem , the rotary switch can be used.

FF

 

I know this is an old thread but 50HP is not going to have much ooomph to charge one battery let alone 2.

 

and the solenoid pulls about 10 watts. Small 30A relay would have a much smaller holding current........10/10 frosty