How do I use two batteries?

[Grahamdownunder]

My 33 foot cruising yacht has two batteries and the usual 3-position switch. Battery 1 is a new deep cycle battery, whilst battery 2 is a couple of years old 'starting' battery that's fine still. I hear conflicting opinions about how to use them. One electrician says that the 'All' position is for emergency use only to start the engine (2-cyl Volvo) if neither battery alone will start it and I should use each battery on alternate days - battery 1 on odd days, battery 2 on even days. Then, when arriving at an anchorage, I should use that battery for lights and instruments. And if it's flat, well, the next day is an alternate day, so use the other battery to start.
What happens then to the flat battery?
Another electrician says to use the 'All' switch at all times when running, then use the deep cycle battery for lights at anchorage. Start the engine on the starting battery next morning, then whilst it's running, switch to All to charge the flat battery. Doesn't this action risk blowing the alternator in the case there's a dead spot (open circuit) in the switch? And doesn't this risk lowering unnecessarily the good battery?
I'm puzzled.
Anyone with a good suggestion?
Regards from downunder
Graham

PS I have a small solar panel that I disconnected recently - the electrician said that it 'sulphates' up batteries because the charge isn't big enough.

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[oldsaltyfish]

Hi Graham, I'd be looking for a new electrician if I were you. I would like to be a bit more constructive but I am too dumbfounded by the advice that you have been given to think of a good reply yet.
regards

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[vyv_cox]

I believe that most people have separate and dedicated service and starter batteries. The service battery(ies) is often a deep cycle type whereas the starter battery can be a normal automotive one, as in general the requirements of small yacht auxiliary engine starter motors are not that high. Start the engine on the starter battery, run the engine for a time to replace the charge consumed, not actually all that long, then switch to the service battery. This seems to concur with the advice of your second electrician. In the correct type of switch there are no dead spots, the transition from one setting to another is carefully designed to avoid this.

Using "All" can give some problems. Unless the batteries are identical, the charging is not equally distributed across them which may give problems such as overheating. Running the boat on "All" without the engine running risks not being able to start it on the following morning.

The solar panel advice is nonsense. A panel up to about 10W is reckoned to not require a regulator and can be connected directly to the battery, with no risk of damage. Above 10W there are plenty of options for regulation. My solar panel has been in service for 8 years now, with the same batteries. The regulator is a 'smart' one that charges service and/or starter batteries, dependent upon which has the lower charge.

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[ccscott49]

ok, The starter battery is just that an engine starter battery, use that to start the engine only, it is not designed to run lights etc, when the engine is off, think car. The deep cycle battery is not designed to start engines, only use it to start the engine in an emergency. Use the deep cycle for the lights etc which is what it is designed for. When you start the engine, leave it running charging it's own battery, for 1/2 hour, then switch over to "both" to charge the "domestic" deep cycle and the engine start batteries. The switch should only be in the both position, when charging or in an emergency. The best way is to fit a charge splitter, but not essential also an advanced alternator regulator would be a good idea, but again not essential. The solar panel, will be fine, as long as it has a diode to stop the batteries discharging through it at night, a trickle charge is fine for the batteries, your electrician needs re-educating. IMHO

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[bedouin]

In your case the second case seems more sensible; that is the mechanism I used before fitting a smart alternator controller and diodes. Although the first regime can work, it is only really appropriate when using identical battery banks.

I think all "1-both-2" switches have a Make Before Break action - so there is no risk of damaging the alternator. With the engine running, there is no problem having the switch in the both position, since the voltage output by the alternator is higher than either battery. Leaving the switch in both once the engine is off does risk lowering one battery into the other.



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[brianrunyard]

The system I use is similar to 2.
I start on the starting battery (No1) and run on that for approx half an hour, which fully charges it, I then turn over to battery 2 so it receives a charge I don't switch back to Bat 1 until I next start the engine.

<hr width=100% size=1>Brian
http://homepage.ntlworld.com/brunyard

 

[brianrunyard]

I know there will be objections to my system, but it works for me (last 10 years).
The system I use is similar to 2.
I start on the starting battery (No1) and run on that for approx half an hour, which fully charges it, I then turn over to battery 2 so it receives a charge I don't switch back to Bat 1 until I next start the engine. This way my starting battery is always fully charged.

<hr width=100% size=1>Brian
http://homepage.ntlworld.com/brunyard

 

[Rosa]

"Leaving the switch in both once the engine is off does risk lowering one battery into the other."
Please can you explain why?




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[bedouin]

If the batteries are at different states of charge (i.e. different voltages) the higher charge (=higher voltage) one will try to charge the lower charged one.

Probably not too much of a problem while both are new, but as soon as one battery gets older, and will not hold it's charge, that will have the affect of discharging the 'good' one as well.

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[roger]

I dislike those changeover switches intensely. To accept them you have to know that nobody, ever, is going to make a mistake and switch off the batteries when the engine is running. I believe sincerely in human stupidity - particularly mine. The two orthodox ways of dealing with the situation are:-
a. Fit split charge diodes and make the corresponding changes to the charging syatem so the diodes dont effectively prevent any charge happening and use alternator or regulator that senses battery voltage.
b. fit a split charge relay. Betamarine provides that system for its large alternator option on the BD 722. It worked well for me.
With either of these systems fit 2 simple on off battery switches and leave them on until you leave the boat.
Incidentally opinion is moving to the view that you need a large fuse in each battery lead.
As far as batteries are concerned I like the idea of having 2 large service batteries for powering all the domestic stuff and a small battery only for engine starting - it neednt take up much space and gives you a much greater reserve when not using the engine.
Also for using a rapid shore based charger Sterling prefer you to use ordinary cheap car batteries rather than the deep discharge ones - mainly because you can charge them faster.

<hr width=100% size=1>Roger

 

[vyv_cox]

I believe that every boat I ever owned had that type of switch and nobody ever turned it off by accident. There is always the type that has a removable key that would presumably prevent this. Or I guess it wouldn't be a massive engineering task to add a small stop to prevent total rotation of the switch.

On a slightly different tack, I am led to believe that it's not turning the switch off that will cook the alternator diodes, but turning it back on again in an "oh, sh*t" reaction.

Personally, I have never seen the need to reduce my charging voltage by adding a split charge diode.

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[snowleopard]

g'day. is it true you have to reverse the polarity in the southern hemisphere?

seriously though, looking forward to my first visit to oz next month. where are good places to see in victoria?

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[ccscott49]

All of it!

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[beneteau_305_553]

The big advantage of using splitter diodes is that you can leave all batteries turned on whether the engine is running or not and all batteries are charged automatically when the engine is running.
The diodes ensure that the domestic circuit can only draw power from the domestic batteries leaving the engine starting battery fully charged.
Diodes do reduce the charging voltage by about 0.5 V but I find this ample to recharge the domestic battery quickly.

<hr width=100% size=1>Richard

 

[Dave_Knowles]

Have you seen this split charging and switching system <A target="_blank" HREF=http://www.merlinequipment.com/prod_range.cfm?cat_id=85>http://www.merlinequipment.com/prod_range.cfm?cat_id=85</A>
It looks interesting at around £80.

<hr width=100% size=1>Take care.

Dave

Dave Knowles
Southampton - UK

http://www.MyCleopatra.co.uk

 

[andyball]

I made a voltage dependent using split-charge system for my van using a cheap durite regulator (sealed) + 80Amp relay.....the relay is normally off, isolating the two batteries, until the voltage on the starting battery exceeds 14.4V.

Though far from ideal, ie the starting batt could still be a long way from charged when the terminal voltage reaches 14.4V....it has worked perfectly well since 1987, even though the unsealed relays are just behind the front grille, getting lots of spray when roads are wet.

With yacht sized banks , 80Amp is probably undersized for the initial surge, and a higher rating/more relays would be good.

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[roger]

Idiocy on boats

I get very tired sometimes on my boat and do not always have a fully trained crew. I reckon that the"normal" battery changeover switch is a disaster waiting to happen.
If you switch off the batteries when the engine is running I think you will immediately get a very very high voltage developed in the alternator which is likely to blow the alternator diodes.
Depending on whether they fail open circuit or shut you might get no poer or no power plus an electrical fire.
Thats what I think Ihope I dont make the experiment!

<hr width=100% size=1>Roger

 

[ccscott49]

Re: Idiocy on boats

You can wire in a wire and a fuse, that allows the switch to turned off, but still leaves the alternator connected, so as to stop this happening, although most of the 1,2 both switches, make before they break, between banks. The fuse is wired in to the off position. I cant remember offhand how you wire it, but I will, I will.

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[roger]

Splitter diode voltage loss

I had experience long ago of a split diode system which had voltage losses that ensured that the batteries never fully charged. This I cured by fitting a higher quality regulator that sensed the actual voltage of one of the batteries. These days you can achieve the same end by using very low loss diodes, a voltage sensing alternator, a split charge relay or a smart alternator regulator. Only the split charge relay is cheap.

<hr width=100% size=1>Roger

 

[mikewallace]

G'Day Cobber,I suspect the bulk of your replies admirable as they are, are from Brits. Ive sailed rag boats offshore competitively all my life but to keep her indoors happy run a 35 stink boat with a Perkins 6 Turbo intercooler.
Battery management has been the bane of my life. More so because I think in most places in Australia the standards of the "professionals" in the trades we owners have to deal with is woeful & I think not so much from lack of skill as the personalities that are attracted by the munyana marina attitudes.
I run a pair of deep cycles for house connected to their own 3 way & a pair of starters also connected to a 3way with the whole 4 coming on-line with switches to both when the last resort is required. I drew up the circuit before hand & faxed it off to a re lative in the 12v auto game who rang me back and said it was not quite right & he'd get back to me. 3 years later ??? What hope have we got.
I'd put your situation to one of the alternative energy outfits preferably located as far from the coast as you can get.

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