Battery Switch

[viva]

Just bought a boat - a 1996 Norfolk Smuggler and I’m getting conflicting info on the battery set-up. It has two batteries and the standard isolator switch with “Off” “1”, “2” or “ Both”. The boat has a charge regulator and shore power facility.
Question 1 : The boat manual says always start on “Both” and don’t alter switch while engine is running” and switch to Domestic battery once the engine is off

Question 1: If switched to “Both” wouldn’t the batteries equalise their charge so that a discharged domestic battery will pull down the charge on the starter battery? Or is it possible that a diode or something in the circuit prevents this. How can I check?
The previous owner is no longer with us but a friend says he used to start on 1 - the starter battery and leave on 1, citing the equalisation of charge issue if “Both” were used.

Question 2: OK but how does battery 2 get charged?

I am weak on boat electrics but I do have a volt/ohmmeter are there any checks I can make to ascertain the best procedure when being reliant on batteries and away from shore power?.

 

[NormanS]

I'm no expert, but it seems reasonable to me. If "Both" is only on immediately before starting, and while the engine is running, it should be OK.

I felt that my charge splitting diode was costing me voltage, so bypassed it, and do something similar to you, except that Instead of "1, 2, Both", I have three separate isolators. One of them joins the Domestic and the Engine batteries. When the engine is running, the Domestic batteries are always on charge. If I want to charge the Engine batteries, I close the joining isolator.

 

[macd]

If switched to “Both” wouldn’t the batteries equalise their charge so that a discharged domestic battery will pull down the charge on the starter battery?

Not unless one was completely knackered: the alternator voltage will be way above resting voltage, so both sets will charge. If you're worried about it, just leave the switch on 'both' for a short while: the starter battery, assuming it started out fully charged, will need very little to get it full again. This...switching while the engine is running...assumes that your main battery switch is of the 'make before break' variety.
Overall, I'd say you're best starting with the starter battery, then switching to both when the engine is running, later switching to domestic-only if you wish.

 

[ghostlymoron]

I'd suggest that you install a changeover relay (vsr). That way you'll always keep the start battery fully charged for starting. The domestic battery will receive charge as soon as the start battery is up to snuff. You can rewire to the 1-2-B switch to select which battery is being drawn from or replace with individual isolators. A vsr is only about 50 quid and eliminates the risk of a flat start battery. There's a good circuit diagram on a site run by a chap called tony. I'll put a link up when i can.

 

[Jabamusic]

Oh good, that would be useful for me as well.

 

[ghostlymoron]

http://www.tb-training.co.uk/MarineE09.html it's worth reading the whole of this site if you're new to boat electrics.

 

[ditchcrawler]

I have had 1/2/both switches on all my boats and never had a problem.As I understand it it is usual to designate one battery to use as the starter and the other to use as domestics.I always start my engine on no1 leave it on 1 to charge for about15mins(assuming that it was charged before I started the engine) and then switch to no2 via the 1/2 position to carry on charging.A boat electrician told me that it only requires a few minutes to put back in no1 battery what you have taken out whilst starting providing the engine starts straight away.I have a specific battery for starting i.e. a Red Flash AGM high cca battery.You may have general leisure dual function batteries.When i want to start the engine I switch back to no1.In emergency i have occasionally started on no 2.
I fitted a switch over VSR but abandoned it because it kept switching in and out and when the starter battery is charged it charges both batteries.With the switch you can just charge no2 if you want to.
There will be people who will rubbish 1/2/both switches and say you may forget to switch it over.In circa 20 years of using them I have never yet forgotten.It just becomes automatic.

 

[Pye_End]

Question 1 : The boat manual says always start on “Both” and don’t alter switch while engine is running” and switch to Domestic battery once the engine is off

Question 1: If switched to “Both” wouldn’t the batteries equalise their charge so that a discharged domestic battery will pull down the charge on the starter battery? Or is it possible that a diode or something in the circuit prevents this. How can I check?
The previous owner is no longer with us but a friend says he used to start on 1 - the starter battery and leave on 1, citing the equalisation of charge issue if “Both” were used.

If the engine is running the voltage from the alternator is higher than either battery, so both batteries will charge regardless of whether one is weaker than the other. When the engine is off the advice to change to one bank is good. It will leave you with a battery that will be in a position to start the engine again of the other bank becomes low, and also stops the problem you describe - ie equalisation down to the worst battery.

 

[xeitosaphil]

I'd suggest that you install a changeover relay (vsr). That way you'll always keep the start battery fully charged for starting. The domestic battery will receive charge as soon as the start battery is up to snuff. You can rewire to the 1-2-B switch to select which battery is being drawn from or replace with individual isolators. A vsr is only about 50 quid and eliminates the risk of a flat state battery. There's a good circuit diagram on a site run by a chap called tony. I'll put a link up when i can.

This is exactly what I did at the start of the season, put two new isolators in one for each battery bank and a VSR to charge both banks automatically. I used the 1-2-Both switch ( as I didn’t want to get left with a 100mm hole in the bulkhead showing) for a emergency combiner if deemed necessary to join both banks together.

Hope the Diagram helps.

Note : This wiring diagram is to allow me to Isolate either battery bank individually, and also start from either bank, or combine both banks for any reason. If using this diagram and using Domestic Battery Bank for engine starting, be sure to make sure your domestic wiring will take starting current or you may be in trouble!

 

[viva]

Many thanks for your help and the diagram

 

[William_H]

Firstly I think the boat manual is wrong in saying always start on both batteries. It is very important tha tif you have a dual battery system that either battery be able to start the engine. hence it is important to at least occasionally and regularly you ensure that either battery alone can start the engine. If you always use both then one can die as it inevitably will bu the other battery will mask the failure until it too dies. Interesting also that some modern starter motors are designed to run at about 9 volts cranking. if you have a very big battery or 2 batteries the higher voltage sustained on cranking can exceed the design rating of the starter motor. (doesn't seem to matter though)

It is a bit difficult to say if there will be any damage in changing the selector to "both" once the engine is charging. Certainly do not go via "off". Many modern 1,2,both switches have contacts to ensure make before break so there is not time between 1 or 2 and both when there is no load on the alternator. No load has been known to cause diode failure in the alternator.
So a simple solution is to fit a bidirectional VSR between each battery positive such that you can start run and drain on one battery while the other battery will always get charged. You would need more isolation to disconnect the +ves from the VSR when you leave the ship.
Currently especially when buying new batteries it is common to have a dedicated start battery generally smaller and cheaper designed for large CCA (cold crank amps) and a dedicated service battery larger and designed for deep discharge high capcity. (large Amhour rating.)
You then wire the engine battery as if it were alone with an isloator switch. The domestic circuit is similarly wired in isolation. The VSR connects the 2 +ves only for charging. You carry a jumper lead or fit an emergency engine start switch for using the domestic battery to assist the start battery if needed. good luck olewill

 

[Jabamusic]

So a simple solution is to fit a bidirectional VSR between each battery positive such that you can start run and drain on one battery while the other battery will always get charged. You would need more isolation to disconnect the +ves from the VSR when you leave the ship.

Is that not what Post No:9's diagram illustrates or am I missing something (again)!
Jaba

 

[Spi D]

A ready to go solution that will separate batteries and optimize charging for each battery, used on many trucks, ambulances busses etc. with demanding consumption:

Septor, manufactured by Tystor

http://www.tystor.com/index.php?option=com_content&task=category&sectionid=4&id=18&Itemid=27

SEPTOR make it possible to charge banks of batteries from a single generator or charger.
During load the bank is divided and only desired battery is possible to empty.
SEPTOR is used for applications such as boats, mobile homes, lorries, busses rescue vehicles and also for
charging of lift fork while driving the lorry.