Do I need Protection?

Thread Starter

siringo

Joined May 16, 2023
4
Hello, just trying to learn here.

In my 4x4 I use solar (an MPPT charger) and a DC-DC charger to recharge my aux batteries.

The MPPT charger only operates when the vehicle is stationary and the DC-DC charger only operates when the vehicle is moving. The outputs of both these chargers join at a bus bar and carry on down to the aux battery.

One thing that has been bugging me for a while is, do I need any protection on the outputs of the chargers?

EG: Charger A is running and outputting 14v and say 20 amps. Could this 14v and 20A damage the output circuitry of Charger B? Or, should these devices be designed with that sort of thing in mind and should be able to handle it?

I'm just speaking generally as I'm aware every brand will be different and so on.

Should I put something in place to block any incoming charge? If so, can anyone suggest something?

Thanks for any help.
 

Ian0

Joined Aug 7, 2020
9,671
How does the MPPT know that the vehicle is moving?
The MPPT will probably be OK, because it will shut off when the voltage at its output terminals exceeds the set point (i.e. the voltage that it is programmed to charge the battery to), and it is designed not to allow power to flow back to the solar panels.
The DC-DC converter is less predictable. Does in increase or decrease the voltage? (i.e is it a buck or a boost) Is it isolated?
If it is Isolated, you will probably be OK.
If it is a boost converter, there will probably be a blocking diode as part of the boost circuit.
A buck regulator might cause trouble as there is a path through the output MOSFET from the output back to the input.
I use the word "probably" a lot, because I don't know the voltages involved, or the makes and models of converter.
 

Thread Starter

siringo

Joined May 16, 2023
4
How does the MPPT know that the vehicle is moving?
The MPPT will probably be OK, because it will shut off when the voltage at its output terminals exceeds the set point (i.e. the voltage that it is programmed to charge the battery to), and it is designed not to allow power to flow back to the solar panels.
The DC-DC converter is less predictable. Does in increase or decrease the voltage? (i.e is it a buck or a boost) Is it isolated?
If it is Isolated, you will probably be OK.
If it is a boost converter, there will probably be a blocking diode as part of the boost circuit.
A buck regulator might cause trouble as there is a path through the output MOSFET from the output back to the input.
I use the word "probably" a lot, because I don't know the voltages involved, or the makes and models of converter.
Thanks Ian0.

The MPPT knows when the ignition is on as there is a sense wire which tells the charger, but what I meant was, it doesn't operate, or output anything when moving, because the solar panels sit on the ground, so they are only put out when we are stopped :)

I don't know if the DC-DC charger is buck or boost, but I'll try to find out.

What do you mean by isolated? Sorry, not sure what you mean?
 

Ian0

Joined Aug 7, 2020
9,671
Thanks Ian0.

The MPPT knows when the ignition is on as there is a sense wire which tells the charger, but what I meant was, it doesn't operate, or output anything when moving, because the solar panels sit on the ground, so they are only put out when we are stopped :)

I don't know if the DC-DC charger is buck or boost, but I'll try to find out.

What do you mean by isolated? Sorry, not sure what you mean?
I imagined that the panels might be permanently fixed to the roof.

Isolated - no electrical connection between input and output. (same as there is no direct electrical connection between the mains live and the output terminal of a battery charger)
I doubt it would be isolated as it costs more to make it isolated.
 

Thread Starter

siringo

Joined May 16, 2023
4
I imagined that the panels might be permanently fixed to the roof.

Isolated - no electrical connection between input and output. (same as there is no direct electrical connection between the mains live and the output terminal of a battery charger)
I doubt it would be isolated as it costs more to make it isolated.
Regarding buck or boost, I was measuring what was going in and coming out just the other day, 12x volts was going in and 14.x was coming out (I had the charger set to output 14.xV out), so I wonder if that means it's a Boost charger? Does that help at all?

And let's go with it not being isolated, it wasn't an expensive charger.

Thanks for the continued help.
 

Ian0

Joined Aug 7, 2020
9,671
Regarding buck or boost, I was measuring what was going in and coming out just the other day, 12x volts was going in and 14.x was coming out (I had the charger set to output 14.xV out), so I wonder if that means it's a Boost charger? Does that help at all?

And let's go with it not being isolated, it wasn't an expensive charger.

Thanks for the continued help.
What is the spec for the input and output voltages?
if it is non-isolated input- is likely to be directly connected to output-: you can check that with a meter.
Do you have a meter with a diode test facility?
If so, connect red lead to output+ and black lead to input+ and see if you get a reading.
If it says OL then you are probably OK to connect it all as you describe.
 

Thread Starter

siringo

Joined May 16, 2023
4
What is the spec for the input and output voltages?
if it is non-isolated input- is likely to be directly connected to output-: you can check that with a meter.
Do you have a meter with a diode test facility?
If so, connect red lead to output+ and black lead to input+ and see if you get a reading.
If it says OL then you are probably OK to connect it all as you describe.
Thanks Ian0 I appreciate the help.

I think my meter has a diode on it, I'll take a look tonight. So if the meter reads OL I'm assuming that means the charger is isolated as the input is isolated from the output, correct?

That makes me think of another question.

If the +ve output of the charger is isolated, should the -ve output of the charger be connected to the same -ve as all the other (I have 2 other batteries which get charged from the vehicles alternator) batteries, which is the chassis of the vehicle?

Thanks for your help.
 
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