Alternator and Solar living in perfect harmony!

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seafox67

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I've googled this question around the net but opinions vary! This forum has pointed me in the best direction in the past so I thought it would be worth asking here (Sorry for another solar question).

I have a new 115 amp alternator and some solar panels controlled by a victron MPPT.

When I turn my sailboat into a motorboat, I like to put my 1-both-2 switch to both and have the alternator charge up both my batteries.
Since installing my first panel, i've noticed that the alternator may not be charging a quickly as it did before adding the solar!

So... and finally the question ;)

Will the alternator stop charging if it thinks the battery voltage is high enough to indicate a fully charged battery?

I'm wondering if the charge voltage coming from the MPPT (even though its probably only 15W) is tricking the alternator into thinking the batteries are fully/nearly charged


Cheers guys
Paul
 
seafox67 said:
So... and finally the question ;)

Will the alternator stop charging if it thinks the battery voltage is high enough to indicate a fully charged battery?

I'm wondering if the charge voltage coming from the MPPT (even though its probably only 15W) is tricking the alternator into thinking the batteries are fully/nearly charged


Cheers guys
Paul
Click to expand...

Yes, it will. The alternator will sense the voltage from the solar controller and reduce its output. Solutions vary. A cheap solution is to fit a switch and turn the solar panels off when you start the engine. A better solution would be to fit a normally closed relay to the solar controller and have it opened by a power feed from the engine "ignition" switch, only cost a few quid and easy to wire up.

There are more complex/expensive solutions, but if you only have a small panel and 115a alternator, there is nothing to be gained by trying to harvest a small amount of solar energy while the engine is running, the alternator will take care of charging the batteries.
 
PaulRainbow said:
Yes, it will. The alternator will sense the voltage from the solar controller and reduce its output. Solutions vary. A cheap solution is to fit a switch and turn the solar panels off when you start the engine. A better solution would be to fit a normally closed relay to the solar controller and have it opened by a power feed from the engine "ignition" switch, only cost a few quid and easy to wire up.

There are more complex/expensive solutions, but if you only have a small panel and 115a alternator, there is nothing to be gained by trying to harvest a small amount of solar energy while the engine is running, the alternator will take care of charging the batteries.
Click to expand...

Paul

Thanks for your quick reply and solution. I'll make the modification this weekend while the sun is still here in the Clyde ;)


Cheers
Paul
 
The small solar output will only raise the battery voltage significantly if the batteries are near full. If the batteries are near full it is appropriate that alternator output is reduced.
 
seafox67 said:
I'm wondering if the charge voltage coming from the MPPT (even though its probably only 15W) is tricking the alternator into thinking the batteries are fully/nearly charged
Click to expand...

If the current coming in from 15W of solar is enough to push the battery voltage up high then there's no tricking going on. Your batteries are nearly charged.

Difficult to see any reason to switch out the solar, the batteries don't know or care what is producing the current, they'll take what current they want depending on state of charge with the voltage limited by the solar/alternator regulators.
 
GHA said:
If the current coming in from 15W of solar is enough to push the battery voltage up high then there's no tricking going on. Your batteries are nearly charged.

Difficult to see any reason to switch out the solar, the batteries don't know or care what is producing the current, they'll take what current they want depending on state of charge with the voltage limited by the solar/alternator regulators.
Click to expand...

Read posts 1 and 2
 
GHA said:
If the current coming in from 15W of solar is enough to push the battery voltage up high then there's no tricking going on. Your batteries are nearly charged.

Difficult to see any reason to switch out the solar, the batteries don't know or care what is producing the current, they'll take what current they want depending on state of charge with the voltage limited by the solar/alternator regulators.
Click to expand...

Unless I'm missing something, then I think you are right. If the "battery voltage is high enough to fool the alternator into thinking that the battery is fully charged" then the battery is, indeed, fully charged and the alternators output, and/or the solar panel's output, will be politely shown the door. :)

Richard
 
PaulRainbow said:
Yes, it will. The alternator will sense the voltage from the solar controller and reduce its output. Solutions vary. A cheap solution is to fit a switch and turn the solar panels off when you start the engine. A better solution would be to fit a normally closed relay to the solar controller and have it opened by a power feed from the engine "ignition" switch, only cost a few quid and easy to wire up.

There are more complex/expensive solutions, but if you only have a small panel and 115a alternator, there is nothing to be gained by trying to harvest a small amount of solar energy while the engine is running, the alternator will take care of charging the batteries.
Click to expand...

Assuming it's a normal simple alternator, it will attempt to regulate at about 14.5V or so. That is all a simple alternator does.
It will have some output impedance so the actual voltage at the batteries will decrease slightly as they draw more current.
If less current is flowing, it is because the batteries are more charged and take less current to stay at 14.5V or whatever.

You only get problems when two 'smart' chargers confuse each other.
E.g. solar charger puts in a small current which puts the battery at 13.9V. The mains charger sees 13.9V and thinks it is looking at a fully charged battery floating, so switches off.

If the alternator has a multi=stage boost charging regulator this could happen.
 
RichardS said:
Unless I'm missing something, then I think you are right. If the "battery voltage is high enough to fool the alternator into thinking that the battery is fully charged" then the battery is, indeed, fully charged and the alternators output, and/or the solar panel's output, will be politely shown the door. :)

Richard
Click to expand...

How do you measure the battery voltage when it is connected to two charging sources ?
 
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