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[Gaffers]

I have just forked out for a 12V trolling motor for the boat I am building and I am looking at getting some car batteries (sealed) to run the motor off of.

My question is how can I rig this up.  I have the plans for the connection to and from the motor to the battery well in the middle of the boat (helps with ballast) even thought about the waterproof terminals for near the seat.

I have heard that instead of having a battery switcher to go from one battery to another that you can put them in a parallel curcuit and it wont affect the motor.

Is this true?  :-/

P.S. once I have jotted down the curcuit diagram I would be obliged if someone could help checking it over....  

[Entwood]

Battery "switcher" ... will run one battery to "flat" then you change and run second battery down ..

Advantages : You know you are on your last reserve of power
Disadvantges : 1st battery "sits" in a discharged state for a length of time which is NOT good for the battery; You have to physically change the power over - if the 1st battery dies at an inconvenient moment you could be in trouble ...; one battery can be used far more than another unless a systematic useage is ensured

Batteries in Parallel ... both batteries discharged at the same rate (assuming batteries are identical)

Advantages: Continuous power throughout; no battery ever sits in discharged state; both batteries are "cycled" regularly; no worries about trying to switch power at a critical time.
Disadvantages : No warning of running to reserve, you have to know how much charge is left.

My quick thoughts on the subject .. probably wrong ...

[Mr Skrunts]

Just to add a slight twist to entwoods post.

I personally would consider a more powerfull battery as the second one, thery being if traving with the flow on the way out, then extra juice for return journey.

Also, allthough not very powerfull and have only seen them at 600Ma so far, maybe a solar charger for the 1st battery as it went flat.

I dont know if either would work but just two penny worth.

[Gaffers]

I have thought about the advantages of both setups but the idea about having a more powerful one for the return is a good one!

Most of the time I hope to be under sail so I shouldent use the battery too much unless it is close to port or the Baltic is as flat as a sheet of ice!  I will in the longer term be forking out for the solar charger too, theres a 5W one for about 50 quid that should help 'save' a flat battery once it is discharged.

But if the two batteries are the same I can parallel them! Cool

I am working on two curcuit diagrams right now and will post them in due course

[Markjay]

On another note.... you could get a Gel battery e.g.:

http://www.carbatteryshop.co.uk/sc-Varta-Gel-AGM-Ultra-Dynamic-Batteries-I619.html

Now they are very expensive, but the theory is that the electrolyte contents is not liquid does not splash about from side to side.

Just a thought.....

[Gaffers]

Each one of those is more than the motor cost! lol

Thought bout it, a normal sealed battery in a waterproof enclosure is the way forward  

[Kevin Wood]

Yep, a Gel or AGM battery would be a safer bet I reckon. They need more care than a liquid battery with respect to charging though and will cost more.

If you're going for a liquid battery get a "leisure" style battery rather than a car battery. They are designed with deep cyclic use in mind rather than delivering huge cranking current but not ever being deeply discharged.

I'd be happy to look over the circuit diagram.

I doubt a solar charger would do much with batteries this size other than keep the batteries topped up when the boat is parked up. Depends if you envisage taking them home to charge or mounting them permanently in the boat.

Kevin

[Kevin Wood]

Reember that batteries vent hydrogen and oxygen when charging so a totally sealed enclosure is a bad idea. You'll need at least a vent pipe to allow the gases to disperse. Some batteries have a connection for a vent pipe built-in.

Kevin

[Gaffers]

No worries the waterproof compartment is removable with battery inside so I can remove it and recharge at home in the cellar.

WRT the solar charger if I do deplete to say 50% wouldent the solar panel trickle charging it help save the battery from the deep drain?  (I am assuming 50% reduction would potentially damage the battery... :-/)

[philhoward]

I agree with the battery discharge factor - but you'd surprised how much trickle you can get from a solar charger - definately enough to keep the cells happy.  A lot of these mains "battery savers" only kick out a couple hundred mA, whereas the solar ones will make 150-200mA easy.

Venting is needed on a Lead Acid one at all times, really - plus there is the splashing issue on a boat - inside a box or not.

You might want to look at something based around a split charger circuit, so the one battery won't drop below "death" levels.  I've seen something for car batteries when the car is left laid up for a long time - will self disconnect when down to a preset level (something like one last start) - google for "battery brain" - basic unit is from about £20-odd.

How much current does this motor draw?  Seen some cracking little devices based on the old fag lighter socket, but only allow 10A..

[Bandit127]

200 mA will charge a 70 Ah battery in 350 hours (more actually). So if you take the boat out out once a month, you will be ok with solar charging.

Batteries need to vent while in use. I was in a car that blew a cell from a blocked vent once. Explosion is not an understatement. Even if pressure build up doesn't get you, oxygen and hydrogen gasses built up in an enclosed space will do just as good a job with the aid of a small spark.

If it was my boat, I would switch between the two. You definately know when you have used 50% this way...

[Mr Skrunts]

Totally naive on battery technolgy.  Are these any good.

http://www.halfords.com/webapp/wcs/stores/servlet/product_storeId_10001_catalogId_10151_productId_264255_langId_-1_CarSelectorCatalogId_13502_CarSelectorGroupId_120816_varient_117993_categoryId_117993_crumb__parentcategoryrn_117993

[Markjay]

otally naive on battery technolgy.  Are these any good.

http://www.halfords.com/webapp/wcs/stores/servlet/product_storeId_10001_catalogId_10151_productId_264255_langId_-1_CarSelectorCatalogId_13502_CarSelectorGroupId_120816_varient_117993_categoryId_117993_crumb__parentcategoryrn_117993

I suppose so, but quite pricey though.. a GO battery from Vx will cost half that on TC.

[Leomas]

Have you considered Motorcycle batteries? They are made to cope with being in a much more hostile environment and at a variety of angles (60 degree lean around a corner?). They are smaller but you may work out cheaper per battery.

[Gaffers]

arent they 6V?

Would they work ok in a series curcuit?