Trailer Battery Charging Project Problem

dasintex

Joined Apr 18, 2016
4
Here's my project, I have a large utility trailer that has it's own 12 Volt Battery and a Auxiliary 12 Volt Battery, either can be charged by a AC Charger or Solar panel, all controlled by a panel. I have a two way switch that allows me to chose the Charging source either the AC charger or the Solar Panel; this feeds into a 2nd two way switch that allows me which Battery I want to charge. I have a voltmeter on the 1st two way or Charging source switch; depending on the selection either AC Charger or Solar Panel will show me the voltage that the selected charger is putting out. Here's the problem, the voltmeter works great, when I select the solar panel it will show the voltage (depending on the sun, 4 to 18V). However, when I turn the 2nd two way switch to one of the batteries, the voltmeter will show the voltage of the selected battery. How can I isolate the Voltmeter so it only reads the voltage of the Chargers, not the batteries. I might add, not shown on my diagram; each battery has it's own voltmeter showing what each battery's voltage is.

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Lestraveled

Joined May 19, 2014
1,946
@dasintex

OK, You will need another two way switch to connect the voltmeter to the charging source. But why?? You probably don't care about the AC charger but you are probably interested in the output of the solar panel when unloaded. Right? Each battery has its own volt meter, when connected to a charger, the battery voltage is the charger voltage. (Captain Obvious here)

dasintex

Joined Apr 18, 2016
4
Not sure about that, example, when I have the charger source switch open neither AC or Solar Charger then select one of the batteries, the meter Voltage may say 12.5Volts, then when I select AC Charger the voltage meter jumps up to 14 Volts indicating the Voltage the AC Charger is putting out; but when I do the same and select solar without selecting a battery the meter may say 8 Volts depending on the sun, but the minute I select a battery the meter jumps up to 12.5 volts, the voltage of the battery not the solar charger voltage; What about a Blocking Diode between the 2 switches, on the line or supply from the charger source to the battery selection?

ResFiber

Joined Mar 9, 2016
6
You could try a blocking diode as you suggest, but you may get a voltage drop across the diode which might limit the charging of the battery.

For example, if the AC Charger is connected to one of the batteries and you put a diode between the AC Charger and the battery, then I think you would have 14V on your charger, but only 13.3V on the battery (14 - 0.7) because of a 0.7V drop across the diode. However, it still might work because 13.3V may be enough for charging.

Kermit2

Joined Feb 5, 2010
4,162
Your meter is working perfectly. If the charging voltage is lower than the battery voltage. The meter will show the battery voltage. If the charging voltage is higher than the battery. The meter will show that voltage.

Your charging devices may both show a very high reading when not connected to anything, but that voltage reading means very little. The important voltage is the one that you have when your charger is actually charging something.

If a solar panel shows less voltage than that needed to charge a battery, it is pointless and might even damage the solar panel if you connect it to a battery.

Connect your volt meter to the battery. You can monitor the battery as it is working, AND see the charge voltage when they are connected to a charger. Knowing the open circuit voltage of your chargers is nice, but is not a piece of knowledge essential to your daily needs.

dasintex

Joined Apr 18, 2016
4
I guess it's more of general interest to see what the Solar Panel is putting out at any given time, the AC Charger not so much because that will always be the same, I am trying to figure out a way where I can accomplish this, as far as damaging the solar panel, what is not in my diagram is a regulator that is between the solar panel and the Charging Source selector switch which protects the solar panel not to mention the panel has blocking diodes in it already but the regulator protects the battery from over charging from the solar panel which can put out up to 18 volts

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wayneh

Joined Sep 9, 2010
17,406
Since the circuitry is protecting things, I agree with Kermit2 that reading the open-circuit voltage of the panel is nearly useless. It might give you some idea of the amount of light the panel is receiving, but you can tell that by just looking at it.

You might set up a comparator circuit that could tell you whether the panel is driving a charging current into the battery, if your controller does not already indicate that. Then you would know to switch to the AC charger.

dasintex

Joined Apr 18, 2016
4
That is what I want to see, if the solar panel is putting out enough charging voltage while it is connected to a battery, right now once I select a battery the meter shows the battery voltage and overrides what the voltage is for the solar panel

hp1729

Joined Nov 23, 2015
2,304
Not sure about that, example, when I have the charger source switch open neither AC or Solar Charger then select one of the batteries, the meter Voltage may say 12.5Volts, then when I select AC Charger the voltage meter jumps up to 14 Volts indicating the Voltage the AC Charger is putting out; but when I do the same and select solar without selecting a battery the meter may say 8 Volts depending on the sun, but the minute I select a battery the meter jumps up to 12.5 volts, the voltage of the battery not the solar charger voltage; What about a Blocking Diode between the 2 switches, on the line or supply from the charger source to the battery selection?
I think the suggestion Lestraveled is making is to remove the connection between the two switch commons and add the third two-way switch to select either of the commons connected to the volt meter.

SgtWookie

Joined Jul 17, 2007
22,230
You should really be more interested in what the charge current is, as that is what makes the difference to the battery during the "bulk charge" phase. The current should be maintained until a desired voltage is reached, say 14 to 14.5v. At that point, an "equalization" phase can be used to ensure the electrolyte is stirred up, holding the voltage constant for a period of time, perhaps 10 to 20 minutes. Following that, a "float charge" phase can be entered, where the battery is maintained at 13.4-13.7v @25°C/77°F adjusted for temperature. The battery can be on float charge indefinitely.