Adding 2nd battery to boat with outboard?

[SnaxMuppet]

I have a Merry Fisher 625 which has an outboard charging a 110AHr battery. I have added a small Waeco fridge and clearly I am at risk of running the battery flat and not being able to start and so I would like to add a 2nd battery.

I have already got a suitable smaller battery that I would like to keep as a starter only and use the existing one as a domestic. Is this possible with an outboard and if so what would I need to do to add the 2nd battery?

The engine is a Suzuki DF115 with a 40A alternator.

I am not very good with boat DIY so is this something I should perhaps get a professional to do or is it completely dead easy?

 

[breezer5012]

We'll do you a deal - you gut the many mackeral we are bound to catch tommorow and Paul will sort your battery /forums/images/graemlins/wink.gif.

 

[Birdseye]

Its simple. You need to be able to separate the engine starting circuit and the domestics. Then wire the large battery to supply the domestics and the small one for engine starting.

Have either a simple switch or a relay energised by the changing circuit so connect the domestic battery to the laternator (ie in parallel with the starter battery) when the engine is running so that the alternator charges the domestic battery, The engine starter battery is permanently connected up to the alternator

You can use the common 12v 30 amp relays available from any car electrical shop. best do it that way because you will forget a manual switch.

 

[William_H]

Birds eye there are problems with this simple relay approach.
If the starter battery is low (or old) it will try to start the engine with current coming mostly from the domestic battery via your 30 amp relay which will melt. On the other hand if the domestic is low it will tend to drag down the starting performance of the start battery (and again melt)
You need to put the rleay from a circuit which is not powered when the engine is starting (cranking). This may be possible if the start switcgh has an accessory position like a car which is de-energised when starting but this is unlikely on a outboard start switch.
This is why we go to a Voltage Sensing Relay which will only connect when the alternator is charging. (or an oil pressure switch circuit.)

You keep a jumper lead or seperate switch (high current) for emergencies when you need to start on both batteries.
olewill

 

[andyball]

Hi Arny

I did just the same thing with a DF115 on a Crestliner 21' - I added a 1-2-both battery switch + appropriate cables & used the smaller ( about 80AH I recall ) battery for starting, switching to just the larger one for extended use w/o the engine running.

In practice, I mostly left the switch in the both position for starting from the marina.

Should you get a pro to do it? I'd say no need to - but do get the cables made-up & sized properly + buy a decent quailty switch ( perko are fine in my exp, and they do a lockable one ). Ensure both batts are secured well & boxed or at least protected at the terminals.

extra=flexible batt cable makes the install easier.


Get all the bits together, Perko switches come with a handy diagram/info - & site the battery attempt to fit yourself... you can always call in someone just to make the connections - but really, it's very simple indeed. Tinned batt cable is nice on a boat where spray may reach the battery - tho I used std cable with glue-filled heatshrink sealing the lugs.

some nice ( the £32 + vat ones ) switches here


For nottalot more cash than a decent switch, you can buy an automatic relay - like this . which will let you fit/forget it. You'd then use the starter batt for just the engine etc & the domestic gear from the 2nd battery - the relay will charge both batts, but isolate them until the engine is charging the starter.




Personally, I liked having the switch, just in case the starter battery gave problems - soo easy to change to the other or both for starting.... but eeven if you go the charge relay route, you could easily add a simple cheap switch to let you start from the domestic batt when needed.

 

[jakeroyd]

Hi Arny , like you I have only a single battery and have just gto a waeco 18ltr fridge. Although this is said to use an average of 0.3AH , I also worry about running down the battery too far.
Incidentally , very approximately an exhausted battery will give 12V at the terminals and a fully charged one about 12.7 so you can get an idea measuring with a multimeter.
In the sort term I have bought a veap bootst charger from Maplin (about £40) so I could start if the fridge too too much out of the battery. My engine has a 55A alternator , so I chrage fro about an hour a day at the moment.
Like you I am going to have to think about a second battery.
I may but a single deep cycle battery and try that.
I can hand start my engine (a Yanmar 1GM 10) easily when it;s semi warm , but have not been able to when it's cold (ie fisrt time you start it for any trip)
However , having been once caught out in another boat on a lee shore , I know desperation gives you tge strength of ten men , so maybe in extremis I would start the engine cold !

 

[Gunfleet]

French's, the E coast Yanmar dealer, do a pulley you can bolt on to the front of the 1GM10. Then you can start it with a cord, as if it were a big mower engine.

 

[SnaxMuppet]

Thanks everyone...

I like the simplicity (and cheapness /forums/images/graemlins/smile.gif ) of andyball's 1-2-both switch. Having heard the options regarding relays and switching boxes, now that I have thought about it a bit more, I really don't think any more complication is needed other than the ability to switch over to another battery instead of just taking a spare battery along and physically disconnecting the old and reconnecting the new. This is an option in the short term but a 1-2-both switch and housing the spare battery in a suitable battery box and with suitably sized cables seems like a simple solution for this boat.

Sure, I can forget to switch it over but for a boat like this, where there are few other domestic loads, I think that the simple solution would be the best one. It is only an mergency measure after all /forums/images/graemlins/smile.gif

I will report back once it is all done.