battery selector position

[Massey]

My arrangement, common to most I think, is position 1 for engine start battery and 2 for domestic batteries . I know diesel engines like a "good wellie" for starting in which case should one always select "All/both" when starting? If the domestic batteries are significantly discharged is this still the case or will they have the effect of flattenning the engine start battery - in which case presumably position 1 would be better in this situation?

There seem to be different views on this, but there myst be a "right" answer?!

 

[Malcb]

If you have a 1-2 Both switch, then there is no 'right' answer to suit everyone. It will depend on each boats owners coinvention.

On my boat, I have 2 x 85Ah combined domestic/start batteries so I can use them for either startng or domestic. So my convention use the batteries 1 or 2 for domestic on odd or even dates, thus using the other battery as the engine start battery. Once the engine is started then the switch goes to 'Both' for charging. I find that this way stops continually running the same battery down more than the other.

Other boat owners will have a different convention, they may have a dedicated starting battery, say as battery 1, but a bigger Ah bank on battery 2 for domestic. They can always be combined together for emergency starting.

What is 'right' for one boat owner may not always be 'right' for another.

 

[Phoenix of Hamble]

I only ever start on Engine battery, unless I need a bit of support from the domestics..... and if they are heavily depleted, i'd suspect that it would drag the engine battery down, so wouldn't use 'both'

 

[oceanpilgrim]

Hi i use one bank for starting on one day,the uther the next and so on,MOST BOATS ARE NOT WIRED TO SWITCH OVER WHEN THE ENGINE IS RUNNING,this can damage the alternater,the spliter should charge both banks no-mater what is selected

 

[mikehibb]

I thought that the "1 both 2" switch was a make before break switch, which is designed to allow switching whilst running.

 

[oceanpilgrim]

can not see a name on mine it's red with a black plastic nob but when i tested it ,it broke before makeing the next cnnection,

 

[Malcb]

The battery selector switch can be moved from 1 through Both to 2 (or 2 through Both to 1) while the engine is running as the contacts maintain a battery in connection with the alternator.
What you shouldn't do is turn the switch to the 'Off' position while the engine is running.

 

[oceanpilgrim]

methinks time to buy a new switch, more cost,back to beans again "o"poo

 

[VicS]

1, 2, both, off switches have a poor reputation!!

Separate on/off switches for each battery bank have their merits even if a little more inconvenient and less impressive appearance.

Buy nothing but the best if you fit/replace a 1, 2, both, off switch.

Sorry even more beans!

 

[Refueler]

If 1 battery is not enoughy to start your engine - then a) your battery is not big enough or duff or b) your engine is.

I never start on Both ... If battery goes down while churning engine over such that it fails - I switch to other battery ON ITS OWN ... I would not put both together - that is suicide for the good battery.

Only time to use both together is from initial so that both hopefully are full charged etc. But as I said - I don't do it.

Think on this ... average boat diesel is either a taxi engine or land stationary engine based ... and usually has a smallish HD cranking battery to start it. Why should a boat with same engine be different ?

Today my Perkins 4-107 after 5 months sitting through winter etc. started on 1 battery, with no throttle or cold start - took about 10 secs turning over before it fired. (I do not like the full throttle version of starting .... and avoid all start additions if possible). I like to have engine turning over a bit before starting after winter lay-up - IMHO it helps get some oil round systen before real demand comes on.

 

[Refueler]

[ QUOTE ]
I thought that the "1 both 2" switch was a make before break switch, which is designed to allow switching whilst running.

[/ QUOTE ]

It is most certainly IS ... unless of course a Mr. Cowboy has produced own version ... unlikely nowadays !

 

[Refueler]

[ QUOTE ]
can not see a name on mine it's red with a black plastic nob but when i tested it ,it broke before makeing the next cnnection,

[/ QUOTE ]

Some cheap ones reportedly do that .... If it has - then a) UK law is on your side especially if it damages your alternator diodes, b) if outside UK - then ??

 

[shmoo]

What "Both" will not do is to partially flatten a fully charged battery as it tries to charge a flat one. This is becuase the voltage across a fully charged (but not actually on-charge) battery is lower than needed to charge the other battery.

A fully charged (but not on-charge) battery will be 12.6V - 12.7V. The minimum voltage needed to charge a flat battery, and thus take current from the fully charged one, is about 13.5V.

 

[Refueler]

[ QUOTE ]
What "Both" will not do is to partially flatten a fully charged battery as it tries to charge a flat one. This is becuase the voltage across a fully charged (but not actually on-charge) battery is lower than needed to charge the other battery.

A fully charged (but not on-charge) battery will be 12.6V - 12.7V. The minimum voltage needed to charge a flat battery, and thus take current from the fully charged one, is about 13.5V.

[/ QUOTE ]

Which flies in the face of electricians and many peoples advice that I've had and many others for years.

In fact the full charge battery does not really CHARGE the flat one - it equalises out in effect, or just loses 'energy' split of to the flat battery. The flat battery does not suddenly become a non-conductor.

Love to know where the info you give here comes from - I'm ready to be corrected if source is viable ... there are many areas that have been accepted for years that turn out later to be in error. (But I doubt this one - having had battery go down quick in past with others I've been with that have done "both" .. it's why I will not do it.)

 

[shmoo]

[ QUOTE ]

The flat battery does not suddenly become a non-conductor.


[/ QUOTE ]

Actually it does more or less just that for voltages under about 12.5V

For all practical purposes the charge voltage and discharge voltages don't overlap.

<edit>BTW C/3 etc means capacity/3 Amps or 40A for a 120Ah battery</edit>

www.arttec.net/Solar_Mower/4_Electrical/Battery%20Charging.pdf

 

[halcyon]

The graph is the principle that a split charge relay works on, this voltage differance stops the engine battery discharging into the service battery.

The load voltages look a little high, but that can depend on the battery type, and a number of other points.

Brian

 

[William_H]

One battery should start the engine happily. If it won't you need a new battery.
You should at least occasionally try each battery individually to ensure they can start the engine alone.
When you do paralell the batteries of course there will be sme current flow from a higher voltage battery to the lower voltage battery. However the voltage difference will be quite small so that if you start the engine immediately after paralleling the batteries little will be lost. Of course once the engine starts cranking the voltage falls by a few volts on each battery such that each will deliver what it can in proportion to its charge state.
IMHO olewill