1,2 Both Switch - When on Both do the 2 batteries even out on charge?

[Jaguar 25]

All in title so when I move the battery switch to 'both', presumably the batteries are in parallel and therefore, if one battery is more charged than the other, does the lower charged battery top itself up from the higher charged battery?

We tend to always use the boat with the switch on 'both' and never seem to have any issues. However, we have minimal electrical equipment - VHF radio and rarely used auto-pilot, navigation lights and cabin lights (all day sailing last year). On a swinging mooring last year without a solar charger and batteries always started outboard quite easily with charging only coming from outboard when motoring (and we were only out 5 times last year!).

 

[lpdsn]

If you mainly keep the switch set to "both" then the two batteries will have been through more or less the same cycles so will have reasonably similar characteristics when being charged (I'm assuming they were the same type and installed at the same time when new).

If they've had different cycles of load and charge then they will behave differently when being charged and won't necessarily ever even out.

And yes, when you switch to "both" if one battery is at a higher voltage than the other current will flow from it into the other until the voltages are equal.

 

[Spyro]

The danger of having it on both is when not being charged if a battery suddenly goes duff then that will draw from the good one and you will soon have two flat batteries.

 

[neil_s]

I try to switch to 'both' only when the engine is running.

 

[PaulRainbow]

1-2-both switches belong on the seabed.

 

[pvb]

1-2-both switches belong on the seabed.

That's my line!

 

[PaulRainbow]

That's my line!

Was waiting for you to show up

 

[Jaguar 25]

1-2-both switches belong on the seabed.

So, what's the best alternative?

 

[PaulRainbow]

So, what's the best alternative?

Two totally separate circuits, engine and domestic, a switch for each. A 3rd switch can be added for emergency use. Then a split charge arrangement, a dual sensing VSR is a good option.

 

[sailorman]

I have a 1 2 both + a (sensing both banks ) vsr,.I can then easily access both battery banks at the turn of a switch & i am the only one allowed to touch the 1 2 both.
when i bought the boat she had a simple relay for charging both banks with no way to access the domestic bank if there was a prob with a flat / duff engine battery.

 

[NickRobinson]

I spent the first season on Rivendell cycling the 1-2-both switch before realising it has a diode and Sterling smart regulator (battery sensed) set up so batts charge whatever the setting.

I can just leave the switch on 'house' with 'engine' as the emergency setting. No chance of disconnecting a working alternator either.

 

[Ceirwan]

A voltage sensing relay can work. However I know a fair few people that think they have no place on a boat. If not sized and fused correctly its very possible to burn them out, I was going to type an example and then I remembered that Sterling had already written a good one:

The bad side (and the very dangerous side) is that a relay is prone to over loading. Say, for example, you have a 70A relay on your system and a 55A alternator, all seems great, but if you fit a 1500W inverter which can draw 150A and one morning the domestic battery is flat. So, you start the engine to charge the domestic batteries, the 70A split charger relay will come online to enable the alternator to charge the domestic battery bank. Then you load your inverter to 150A, the 150A will not be drawn from the domestic battery because it is flat but can be drawn from the engine battery (which is full). That means you will draw 150A up the split charge cable and through the 70A relay. If you are lucky you will destroy the relay, if you are not so lucky then you will set fire to the cross over cables, hence the dangerous aspect,

 

[PaulRainbow]

A voltage sensing relay can work. However I know a fair few people that think they have no place on a boat. If not sized and fused correctly its very possible to burn them out, I was going to type an example and then I remembered that Sterling had already written a good one:

But that applies equally with everything electrical on the boat. The Sterling quote is mostly likely some of Charlies sales guff to put people off of buying a VSR and to encourage them to spend money on something he makes

 

[William_H]

A voltage sensing relay can work. However I know a fair few people that think they have no place on a boat. If not sized and fused correctly its very possible to burn them out, I was going to type an example and then I remembered that Sterling had already written a good one:

Re Stirling blurb. That would be a very remote possibility assuming that one would be drawing full power from the inverter at engine start up. Even then it is very unlikely the wiring would get too hot because the inverter would be struggling with insufficient voltage so almost certainly switch itself off.

Re the OP post. It obviously works for him to operate at all times on both. Gives a good engine start from both batteries.
However the idea of 2 batteries is to give a back up battery in case one battery has been discharged or as eventually will happen one battery drops dead. Even if both are not heavily discharged one is going to drop dead ie won't start the engine. This will not be identified as a problem until the other drops dead. So by using both at all times you effectively have just one battery with all the dependence on that. Like your car and you know you will eventually be caught out there.
So Op should at least try an engine start occasionally on each battery alone to confirm they are good. olewill

 

[vyv_cox]

1-2-both switches belong on the seabed.

I have been using 1-2-both switches on all my boats since 1988. I never had the slightest problem with any of them and they provide the ultimate versatility. There is nothing wrong with these switches, it's the owners who need to be changed

 

[sailorman]

I have been using 1-2-both switches on all my boats since 1988. I never had the slightest problem with any of them and they provide the ultimate versatility. There is nothing wrong with these switches, it's the owners who need to be changed

Some 1 2 B are better than others, some are very poor quality indeed

 

[Minchsailor]

I have been using 1-2-both switches on all my boats since 1988. I never had the slightest problem with any of them and they provide the ultimate versatility. There is nothing wrong with these switches, it's the owners who need to be changed

+1.

I've had no problems in half a century. KISS.

 

[Vegable]

+1.

I've had no problems in half a century. KISS.

+1 also.
However if the op would like to read a bit more about it here's a link to look at
https://forums.sailboatowners.com/index.php?threads/1-both-2-off-switches-thoughts-musings.137615/
Mike

 

[Bru]

1-2-both switches belong on the seabed.

That's my line!

And mine!

 

[lw395]

I think what matters on a boat wiring system is whether it works reliably and whether it meets the needs of the user.
I don't think having a 1-both-2 switch matters one way or the other.

Every alternative system seems to have potential problems when its not really well matched to how the owner wants to use it.

To answer the OP, if one battery is at a higher state of charge, yes the two will equalise, but surprisingly slowly. If you have a fairly flat battery at say 12.1 volts and try charging it from 12.6 volts, the current will be very low and it will take a long time to equalise.
If one battery is past its best and has a self-discharge issue, it will drag the other down though.