Battery Charger receiving two power sources

[Bouba]

This is an old chestnut, but I would like to ask what everyone’s latest thinking is.
If the engine and generator (or even engine and shore power) are on at the same time, the battery chargers (I have two) are receiving power from the engines alternator and the 220 volts from the generator/shore power.
I have no problem with the idea that batteries ‘take what they need’ but how do the chargers know where to source their power?
What gives way first (I suspect the alternator)?
And does it matter if the charger is on bulk, absorption or float?
Lots of questions, thanks
Ps. Oh dear have I got this wrong? Does the alternator bypass the battery charger for ALL batteries (house and thrusters etc) so the batteries choose between the alternator AND the chargers? The more thought I put into this the more possibilities open up

 

[Portofino]

You decide .
Your AC panal will have breakers for the two chargers .
When we are boating ( engines on ) we turn the chargers along with all the AC off .

So alternators only feed via a split diode ( eng 1st ) the two bat banks , eng and house .
Occasionally especially this year we geny up to run the Aircon while motoring to chill the cabins down .
Here I will elect to leave the two chargers off as the engine alternators are running and just flick the Aircon on .

At anchor with the geny running or dock with shorower with the engines off then elect to turn more AC stuff .
I never run the engines and AC chargers together as there’s no need really .

Somthing to do with alternators excitation or lack of ( someone will be along soon to give detail ) resulting in them knackering or a part knackering ? - I think .
There will be an alleged all singing and dancing “ unit “ that some one will propose - if you want to buy another gizmo to go wrong ,which will auto switch , until it packs up taking Somthing else expensive down with it .

 

[jrudge]

How it works does not really matter!

Many of us run the gen whilst moving and there are no issues.

So it works fine unless you can find someone to tell you different.

As such either the chargers and alternators don’t care or they do care and all chargers etc have the means to deal with it.

 

[kashurst]

When a battery is charging it acts as a resistor whose resistance steadily increases as the charge in the battery increases (think of it like a water tank being filled with water with a floating ball valve controlling the water level)

An alternator is a fairly simple device - it will try to maintain an output voltage of 14v ish, regardless of how much current is required, up to its maximum output current capability - on a boat engine that could be 140 amps.

Connect a battery to an alternator and it will charge up to 14v ish and hold it there - pretty quickly, If the battery is not being discharged by something, then once the battery is fully charged it effectively becomes a high resistance and the alternator pretty much stops outputting current. There is no longer potential difference between the alternator output and the battery terminal. But the alternator does maintain an output voltage - there's always something else needing electrical power

A modern battery charger is a bit smarter - it monitors the rate of current going into the battery and the batteries voltage. Again once full charge is reached it will maintain the output voltage but very little current will flow into the battery - its full! Some battery chargers go up to 14.5v ish for short periods (absorption mode I think) then drop back to float mode at @ 14v. Then at some point effectively turn off and let the voltage drift down to @ 13.5v ish. I believe all this switching voltage levels helps give long life but I don't know why.

So if your charger is on and your engines are running both voltage sources want to push charge into the battery if it needs charging. For a while both charger and alternator will be providing charging current. Think of it like a tank of water with two float valves - but one float valve is bigger than the other and can pass more water.

They can't push current into each other because of the way they work internally. Once the battery voltage gets to 14v ish, all current flow into the battery stops because its full. Which charge source stops first depends on the specific design of the alternator and the charger.

On a boat though it will never be quite full as other stuff will be drawing current from the battery, As the charge in the battery reduces, it will be replaced by either the alternators or the battery charger or both if using lots of electricity.
For all practical purposes it doesn't matter.

 

[BartW]

You can have as many alternators and battery chargers in parallel connected to the batterys as you like, nothing will go wrong, the charger with the highest charging voltage will deliver most of the current, the system will ballance over the internal and external (cabling) resistance, so not a problem.

 

[baylabayla3288]

On my Ferretti I have noted that sometimes when I have the batterycharger on at the same time as I start up the engines the rev-meters seems to be confused and stay at zero rpms. This problem goes away as soonj I switch off the the charger. Othervise no problems but due to this I tend to swich off the charger if I run the genset when on the move.