Adding a 3rd battery and parallel the current 1 and 2

[peterjaw]

Good day, captains,
I have 2 batteries and the selector is 1/2/1+2.

I would like to add a 3rd battery as my starter battery, and paralle the current
battery 1 and 2 as domestic bank.

Is that a simple job? Just unplug the battery 1 and plug them back to the new (3rd)
battery; then connect the battery 1 with battery 2 in parallel way.

Battery 1 and 2 were installed in the same time, so I assuem they are of very similar
condition.

Any further recommendations and/or suggestions are welcome.

Thank you very much.

 

[dpb]

I've had a couple of boats which had a single battery connected to 'Battery 1' which I used as the starter battery and two batteries connected in parallel to 'Battery 2' which I used as domestic.
Its a simple set up that worked for me but I understand that there potential issues if the parallel batteries are not exactly the same (mine were not) and when one of the parallel pair start to fail.
But like I say, it worked for me.

 

[Rocksteadee]

Also take into account any charging system you have
Eg. Single engine or twin with alternators;
a shore supply battery charger;
Solar panels

I had a twin bank of doubles charged by a shore charger.
Charger “sense” was only on one pair (domestics), when they were being charged after use at anchor other pair were being boiled.
All rewired now

 

[peterjaw]

Also take into account any charging system you have
Eg. Single engine or twin with alternators;
a shore supply battery charger;
Solar panels

I had a twin bank of doubles charged by a shore charger.
Charger “sense” was only on one pair (domestics), when they were being charged after use at anchor other pair were being boiled.
All rewired now

Hi Rocksteadee,
I have a (single) outboard engine, no shore power system, no solar panel.

I don't quite understand you, "Charger “sense” was only on one pair (domestics), when they were being charged after use at anchor other pair were being boiled." Are you saying that the charger only charged one of the banks and the other was dead because the charger did not
know that there were 2 banks?

How did you rewire?

 

[Rocksteadee]

Hi Rocksteadee,
I have a (single) outboard engine, no shore power system, no solar panel.

I don't quite understand you, "Charger “sense” was only on one pair (domestics), when they were being charged after use at anchor other pair were being boiled." Are you saying that the charger only charged one of the banks and the other was dead because the charger did not
know that there were 2 banks?

How did you rewire?

Original wiring:
Twin inboards, each with own alt and own pair of batteries.
Domestics from one pair only but with a ‘jump’ solenoid should the domestic batteries run down, that side engine could be jumped from the charged side.
Shore battery charger through a diode splitter (two batteries can be charged ( but kept isolated) from a single channel charger.
The problem was the sense wire (on older chargers this senses the charge in the battery, modern ones this is now internal) was connected to the side providing domestics. When domestics were low on charge the charger kept its output high as the ‘sense’ wire was sensing low charge, ie doing its job.
However the fully charged side were still getting the high output from the charger and thus over charging and boiled.
Binned that charger and got a twin channel so isolation is done internally.

In your case,
You need to keep both banks (starter and domestics) isolated from one another, however an outboard does not normally provide a second charging output from its alternator. If it does, happy days, connect domestics to this as it is a twin channel alternator.
If not there are 3 ways of charging domestics:

Take output from alternator (after it supplies engine electrics but before it connected to heavy cable starter supply) and connect into centre connection of a diode splitter, then each side of splitter goes to each bank.
I don’t like these as the batteries never quite get fully charged due to 0.6 volt drop accross each diode;

Connect in a voltage sensing relay, so that after starting, once relay has detected starter bank is charged, ie voltage has come back up, it switches over to charge domestics.
Can be complicated if not an aux faux with electrics and expensive;

Use existing battery switch.
Start on 1, (assuming that this is the start bank), leave on 1 for half hour or so (this May need to be monitored to check starter bank has recharged)
Switch to 2 (assuming domestic bank) to then charge domestics.
1 and 2 gives a single bank as they are joined together.

Hope this is useful

 

[peterjaw]

Original wiring:
Twin inboards, each with own alt and own pair of batteries.
Domestics from one pair only but with a ‘jump’ solenoid should the domestic batteries run down, that side engine could be jumped from the charged side.
Shore battery charger through a diode splitter (two batteries can be charged ( but kept isolated) from a single channel charger.
The problem was the sense wire (on older chargers this senses the charge in the battery, modern ones this is now internal) was connected to the side providing domestics. When domestics were low on charge the charger kept its output high as the ‘sense’ wire was sensing low charge, ie doing its job.
However the fully charged side were still getting the high output from the charger and thus over charging and boiled.
Binned that charger and got a twin channel so isolation is done internally.

In your case,
You need to keep both banks (starter and domestics) isolated from one another, however an outboard does not normally provide a second charging output from its alternator. If it does, happy days, connect domestics to this as it is a twin channel alternator.
If not there are 3 ways of charging domestics:

Take output from alternator (after it supplies engine electrics but before it connected to heavy cable starter supply) and connect into centre connection of a diode splitter, then each side of splitter goes to each bank.
I don’t like these as the batteries never quite get fully charged due to 0.6 volt drop accross each diode;

Connect in a voltage sensing relay, so that after starting, once relay has detected starter bank is charged, ie voltage has come back up, it switches over to charge domestics.
Can be complicated if not an aux faux with electrics and expensive;

Use existing battery switch.
Start on 1, (assuming that this is the start bank), leave on 1 for half hour or so (this May need to be monitored to check starter bank has recharged)
Switch to 2 (assuming domestic bank) to then charge domestics.
1 and 2 gives a single bank as they are joined together.

Hope this is useful

Thank you very much.

These are so informative.

 

[EugeneR]

I would have gone with OP's original proposal i.e. smove 2nd battery in parallel with 1st, with new starter battery. We had this previously and I do not recall charging issues.

That said, another option assuming only one charging output, is a 12v to 12v charger. It will charge the house bank from the starter battery, but cut out before voltage drops below where engine cannot be started.

 

[Rocksteadee]

I would have gone with OP's original proposal i.e. smove 2nd battery in parallel with 1st, with new starter battery. We had this previously and I do not recall charging issues.

That said, another option assuming only one charging output, is a 12v to 12v charger. It will charge the house bank from the starter battery, but cut out before voltage drops below where engine cannot be started.

If connecting 2 originals in parallel and new starter:
Ensure originals are roughly same age, (old battery will reduce life of a new batt)
same electrical size (small batt will be charged first and could end up being overcharged or the big one charging the smaller one and neither being quite fully charged)
They are “Leisure” or duel use, indicated by an Ah (amp hour)value on a label. These can take a deep discharge cycle for multiple times but not a high starting current. As opposed to a cranking batt, indicated by a CC (cold cranking) value. These can take a high starting current but shortens life if deep discharged.

I hadn’t heard of the 12 to 12, sound a bit like a back to front charging relay. It would work, however not sure I would like to rely on a part discharged starter batt which has only just enough to start

 

[peterjaw]

If connecting 2 originals in parallel and new starter:
Ensure originals are roughly same age, (old battery will reduce life of a new batt)
same electrical size (small batt will be charged first and could end up being overcharged or the big one charging the smaller one and neither being quite fully charged)
They are “Leisure” or duel use, indicated by an Ah (amp hour)value on a label. These can take a deep discharge cycle for multiple times but not a high starting current. As opposed to a cranking batt, indicated by a CC (cold cranking) value. These can take a high starting current but shortens life if deep discharged.

I hadn’t heard of the 12 to 12, sound a bit like a back to front charging relay. It would work, however not sure I would like to rely on a part discharged starter batt which has only just enough to start

In fact, current battery 1 and battery 2 are exactly the same. They were inatalled when boat
dealer delivered the boat to me in last August.

The battery selector is 1/2/1+2 style, there is no so-called "starter" and "domestic" on the
selector's label.

Currnet batteries are regular lead-acid type, but the outboard manual requires to use AGM battery, therefore, I ask the dealer to replace one of the current battery with an AGM battery.

I think I might be able to connect the two lead-acid batteries parallely as my domistic bank.

 

[EugeneR]

I hadn’t heard of the 12 to 12, sound a bit like a back to front charging relay. It would work, however not sure I would like to rely on a part discharged starter batt which has only just enough to start

This is the one I used to charge a bow thruster battery in the bow. It saved me from having thick cables from the engine room.


http://www.xantrex.com/power-products/power-accessories/auxiliary-battery-charger.aspx

 

[AndieMac]

..............Use existing battery switch.
Start on 1, (assuming that this is the start bank), leave on 1 for half hour or so (this May need to be monitored to check starter bank has recharged)
Switch to 2 (assuming domestic bank) to then charge domestics.
1 and 2 gives a single bank as they are joined together.

........but not while OB engine is running, to avoid a voltage spike, damaging engine management system.

 

[Rocksteadee]

........but not while OB engine is running, to avoid a voltage spike, damaging engine management system.

Therein lies the problem of a 1,2,both switch.

Should the switch inadvertently go through the off position when switching from 1 to 2 there is also the possibility of damaging the alternator as momentarily it is disconnected.

https://www.force4.co.uk/blue-sea-add-a-battery-120a.html?sqr=Add a battery&

This product including the VSR does the job of switching for you. However there is no suppression for the possible voltage spike when connecting the 2 banks. The battery itself will, though act as a huge voltage sink and dampen any spike.
Long time since I have done any battery theory so happy to have my memory refreshed.

Talking of which, my statement of not liking charging both together seems to be flawed as this is the principle of the VSR.