battery switch

dennyl · Mar 8, 2015

I just bought a used boat that has 2 batteries with a switch to go from one battery to the other. How do you no what electronics to connect to what battery ? Or do you connect to both and when one goes dead you switch to the other ?

alldodge · Mar 8, 2015

dennyl said:
I just bought a used boat that has 2 batteries with a switch to go from one battery to the other. How do you no what electronics to connect to what battery ? Or do you connect to both and when one goes dead you switch to the other ?

:welcome: to iboats

There would be a house (for boat electronics, bat 1) and the other used to start the motor (bat 2). Never move the switch when the motor is running. Do you have a dual battery charger or an isolator?

dennyl · Mar 9, 2015

Thanks for your reply. No. I do not have either on. I do have a basic battery charger.

Grandad · Mar 9, 2015

Depending upon how your system is wired, one battery may not be dedicated to either house or engine. It may be that the setup is like mine where either battery can be called upon by using the selector switch to be in use or be in reserve. This way, as long as you rotate their use regularly, your engine charges each one periodically. What you need depends upon how you use your boat. - Grandad

Scott Danforth · Mar 9, 2015

follow the wiring from the back of the switch my boat has battery 1 as the motor and battery 2 as the house batteries.

alldodge · Mar 9, 2015

dennyl said:
Thanks for your reply. No. I do not have either on. I do have a basic battery charger.

Here is an examples which could be happening

Now with a battery charger it could be just going to the one battery. As others have mentioned need to trace the wires. There are several other configurations which could be designed.

2-bank w/ ACR
2-bank w isolator
2-bank w/ either the above and switches
dual bank chargers
Etc

dennyl · Mar 10, 2015

Thank you for the info

NYBo · Mar 10, 2015

The usual wiring scheme is to hook everything except the bilge pump to the common terminal on the battery switch, not directly to either battery. The bilge pump is connected directly to one of the batteries so it can't be turned off accidentally. However, as noted by others, there are other ways to wire things up, depending on what you want to accomplish.

muskyfins · Mar 11, 2015

NYBo said:
The usual wiring scheme is to hook everything except the bilge pump to the common terminal on the battery switch, not directly to either battery. The bilge pump is connected directly to one of the batteries so it can't be turned off accidentally. However, as noted by others, there are other ways to wire things up, depending on what you want to accomplish.

When wired this way, if the switch is in the both position, both batteries would charge from the alternator correct? Also, if anchored and on house battery all day running electronics, what would the difference be between starting from batt 1 position or both? In other words why ever have switch set to batt 1?

alldodge · Mar 11, 2015

muskyfins said:
When wired this way, if the switch is in the both position, both batteries would charge from the alternator correct? Also, if anchored and on house battery all day running electronics, what would the difference be between starting from batt 1 position or both? In other words why ever have switch set to batt 1?

If you left the switch on "both" then you basically have two batteries in parallel. You would charge and drain both batteries at the same time. If you left it on 1 then you can drain the house battery and it would not effect the start bat. Once the engine is running you do not want to switch the switch unless it has the since lead micro switch provision, or you could damage the ALT

muskyfins · Mar 11, 2015

sorry. My batteries are batt 1 is start and batt 2 is house. The question is why would you ever be strictly on start battery? The way I see it, if you drain the house battery you could just as well switch to both for starting. Then when running you'd be essentially charging both on your way back to the marina or next anchorage. Or is there a reason to start from only the start battery? Thanks for the help. Like the OP, I also am on my first boat with two battery system.

alldodge · Mar 11, 2015

muskyfins said:
sorry. My batteries are batt 1 is start and batt 2 is house. The question is why would you ever be strictly on start battery? The way I see it, if you drain the house battery you could just as well switch to both for starting. Then when running you'd be essentially charging both on your way back to the marina or next anchorage. Or is there a reason to start from only the start battery? Thanks for the help. Like the OP, I also am on my first boat with two battery system.

You have a battery which is fully charged and if you put the drained battery in parallel with it the good battery voltage starts dropping by the weak/drained battery. Instead of having 12V to start your motor you now may have 9 or 10V depending on the state of the weak battery. Switches are installed so IF you fail to be able to start your engine on the start battery, you can switch to the other or both so you can start and get home.

Getting off the OP's topic but here are other ways of fixing the 2 battery charge issue