Two way battery switches

Modulation

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I have a fairly standard 2 way switch - connects battery 1, both or battery 2. I heed the general advice that if you link two batteries together, the lower-charged one draws from the other.
My question is how long does this take? I sort of assume that it's instantaneous. However that would mean that every time I switched between battery 1 and 2, the discharge would occur when the switch was - momentarily - on "both". I can understand that the reason for the the switch always connecting to a battery is to prevent damage to the alternator, but is the price I pay the fact that I part-
discharge one of the batteries.
Or does the whole process of equalisation take much longer than I believe?

Brendan
 
G

Guest

Guest
You are correct that the lower battery takes power from the other one but it is not instantaneous. I have the same type of switch and have no problem with it. This type of switch allows you to manually select which battery you are charging, instead of a blocking diode.
What I do is, number 1 is my engine battery and 2 for services. I start my engine on 1 and then I switch to BOTH for charging. If I'm only running my engine to charge I often let it run on both for half an hour or so, then I go to 2 so my domestic batteries get all the charge. When I'm finnished with the engine I switch to 2 for domestic use. Then I don't use any of the power in 1 and can start my engine.
Hope this helps answer your question
 

pvb

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Throw it away!

These switches have no place on a boat. They are confusing and offer lots of ways for owner or crew to set them wrongly - potentially allowing the engine starting battery to be discharged (ever tried hand-starting a diesel?). So throw it out and fit a diode blocker instead (with a battery-sensing lead to the alternator, or a smart regulator).
 

ccscott49

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I ahree thats the right way to do it! If you have one of those switches. I dont agree with the other answer, how do you start your engine form the domestic battery in an emergency? I am probably missing something!
 

pvb

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Yes, you\'re missing something...

If your batteries (or battery banks) are properly isolated, there's no reason for your engine starting battery to fail. If it does fail, and this is highly unlikely, surely it's not beyond the ability of most boat owners to link the wires up to another battery?
 

ccscott49

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Re: Yes, you\'re missing something...

In that case I did not miss anything, tried dis-connecting, connecting batteries in a seaway? and why should you, batteries go flat for all kinds of reasons and it will always happen when you least expect/need it, Sods third law I beleive! Switch and splitter pherhaps?
 

ccscott49

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Re: ? Pherhaps?

Perhaps, ferhaps, pherhaps, happen, furcaps, who gives a shit, the message was the same!
 

jollyjacktar

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16 May 2001
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Better Still Keep Switch and Blocking Diodes

Keep the double battery switch and fit blocking diodes in parallel as well. That way, even if the battery switch is in the off position, by accident while the motor is running you will not internally haemorage and will still be charging both batteries at a slighlty lower voltage as the diodes cause a voltage drop[ 0.6V if my memory is right]. The selection switch then will allow selection of whichever battery [or both] for whathever purpose with the motor stopped and while the motor is running it will allow full charge, over riding the diode voltage drops, to the selected battery [or both], due to the voltage drop function of the diodes. Further the diode will prevent discharge of one battery into the other. A case of the two systems producing more than the sum of the parts.
 

paulrossall

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ROYG gives a very sensible answer. One problem you get when asking a question like this is that there are numerous correct answers/solutions. My system works like royg`s and I have never had a problem. The more complicated you make the system the more there is to go wrong. I am fortunate in having a Yanmar 18hp engine which has decompression levers and is very easy to hand start should the batteries fail (they never have failed)
 
G

Guest

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When you switch from 1-both-2 (or vice-versa) while the engine is running, there is nothing to worry about because the alternator is charging.

When you switch from 1-both-2 (or vice versa) when the engine is not running, it is true that one battery may discharge momentarily into the other, while the switch contact is passing through the 'both' position. The amount of charge transferred will be relatively small, and nothing to worry about.

The battery changeover switch is simple, cheap and reliable. Its only drawback is that it susceptible to human failure.

The most common human failing is to forget to switch from the 'both' position after stopping the engine. The next most common failing is to switch from 1-off-2 (or vice versa) while the engine is running. This causes the production of a reverse current spike which may be at a high enough voltage to damage the regulator diodes. Modern regulator diodes can usually cope with such a spike, provided it occurs only infrequently.
 

kingfisher

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Freedom of choise

1) Smart regulators can and do break down

2) I want a regulator that charges with priorities: I want 100% engine battery and only then can it charge the household bank.
I don't want two batteries at 80 %

With a switch, I decide which battery gets the charge.

Obi-Wan
 

Sammy

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Re: Better Still Keep Switch and Blocking Diodes

Dont fit a blocking Diode unless you fit a smart regulator which is uneccesary. Fit a Relay cost about £20.00 should be wired into the alternater warning light circuit but you can use starter switch. Blocking diodes can drop the voltage by up to 2volts which will prevent getting a good charge.
Chris
 

jollyjacktar

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Re: Better Still Keep Switch and Blocking Diodes

Just checked my tech data, voltage drop on a silicon diode is 0.65V! not 2V. I can live with this slight loss esoecially as I have a more foolproof system using a two battery switch and blocking diodes in parallel. But as per normal, each to his own.
 

ccscott49

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Re: Better Still Keep Switch and Blocking Diodes

I tried to say this in the beggining, but was basically told I was wrong! Oh! well, I hope the guy gets the point! In the end.
 
G

Guest

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Re: Better Still Keep Switch and Blocking Diodes

just a quick note. The 0.65v drop actually fulls the altenater into thinking the battery is lower charged and gives more amps before dropping off. So charge time is reduced. As a liveaboard that helps on running costs.
 
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