Charging Boat Batteries via Engine

[Jason2tpa]

2017 F250XCA

Hey guys. My buddy just bought a Sea Hunt. The boat has 2 batteries, 1 for engine and the other is house. Both batteries are tied to a rotating battery switch (1, 2, All, Off). The dealer told him when the switch is set to ALL and the engine is running, the engines charging capacity will not only charge the cranking battery but will also charge the house battery. Could someone please explain to me how this is possible. Mind you, this boat is loaded with electronics that are pulling amps from the house battery.

[boscoe99]

Setting the switch to the both or all setting is one way to charge two batteries. Problem is that if the boat stops and the switch is left in that position then both batteries will be supplying current. Both can go dead. Not a good idea when 60 miles offshore.

Yamaha offers an isolator lead so that the motor charges both batteries when the motor is running but prevents the starting battery from being depleted when the motor is stopped. Run the house battery dead, use the start battery to start the motor, and the motor then begins to recharge the one dead battery.

A combiner/ACCR/VSR will do the same thing. Simpler and cheaper. Takes the worry out of not having a battery charged enough to start the motor.

The F250XCA model is capable of supplying more than 50 amperes of current. This should be more than enough to run the electrics when the motor is running and to recharge the house battery at the same time.

[Jason2tpa]

Setting the switch to the both or all setting is one way to charge two batteries. Problem is that if the boat stops and the switch is left in that position then both batteries will be supplying current. Both can go dead. Not a good idea when 60 miles offshore.

Yamaha offers an isolator lead so that the motor charges both batteries when the motor is running but prevents the starting battery from being depleted when the motor is stopped. Run the house battery dead, use the start battery to start the motor, and the motor then begins to recharge the one dead battery.

A combiner/ACCR/VSR will do the same thing. Simpler and cheaper. Takes the worry out of not having a battery charged enough to start the motor.

The F250XCA model is capable of supplying more than 50 amperes of current. This should be more than enough to run the electrics when the motor is running and to recharge the house battery at the same time.

Great explanation, thanks bossman. This isolator lead you mentioned, can you post a link for the part or provide the part # for this engine. Many thanks.

[fairdeal]

The dealer told him when the switch is set to ALL and the engine is running, the engines charging capacity will not only charge the cranking battery but will also charge the house battery. Could someone please explain to me how this is possible.

well, the engine can provide up to 42 amps toward charging the battery(ies)
(at higher rpms - not as much at idle)

Unless your friend has a "wall of sound" stereo system, his electronics are not using anywhere near that.

Also, "charging" is a function of not just amps, but also time. As is "discharging".

The amount of battery electricity used or replaced is measured in amp-hours.

If you use 10 amps, for one hour, you have used 10 amp hours - and that is what has to be "charged" back into the battery.
(plus a little more - charging is not 100% efficient)

But 40 amps - for 1/4 hour - is also 10 amp hours.
So theoretically the engine could recharge that in 15 minutes.

The engine starting battery typically does not require much "charging" -

although the starter motor might use well over 100 amps while it is running -

a modern EFI engine normally starts with barely one second of cranking

So the amp-hours used in starting the engine is miniscule:

1 second is 1/360 of an hour - multiplied by say, 120 amps - is 1/3 of an amp hour.

Charging at 42 amps - that 1/3 amp hour would (theoretically) be replaced in under one minute.

[throrope]

2017 F250XCA

Hey guys. My buddy just bought a Sea Hunt. The boat has 2 batteries, 1 for engine and the other is house. Both batteries are tied to a rotating battery switch (1, 2, All, Off). The dealer told him when the switch is set to ALL and the engine is running, the engines charging capacity will not only charge the cranking battery but will also charge the house battery. Could someone please explain to me how this is possible. Mind you, this boat is loaded with electronics that are pulling amps from the house battery.

That switch allows use of #1, #1&2 in parallel, #2, or no batteries. It's likely a Perko unit which makes before breaks connections for on-the-fly switching. The switch has one common output connection or load terminal and doesn't separate motor from house loads. Often folks run on both batteries and manually switch to one when stopped so the other is available in the event the "loaded" one drains. Even if you only have one battery, a switch is great for preventing parasitic losses during storage.

[boscoe99]

Great explanation, thanks bossman. This isolator lead you mentioned, can you post a link for the part or provide the part # for this engine. Many thanks.

Two part numbers. One with a fuse and one without a fuse. See bottom of page 4-15

https://yamahaoutboards.com/catalog/2017/MRP/index.html

[Jason2tpa]

Two part numbers. One with a fuse and one without a fuse. See bottom of page 4-15

https://yamahaoutboards.com/catalog/2017/MRP/index.html

Thanks bud. Appreciate all the input guys.