DPDT relay from two SPDT relays. Will this work...?

Thread Starter

ihsan.duet11

Joined Aug 10, 2021
12
I am dealing with old fashioned MPPT charge controller which is not smart enough to decide whether to run the Load from PV(Solar Panel) or battery. I need a DPDT relay (12v, 20-30A) that switches the load between MPPT & Battery. But dpdt relay in this rating is not available in local market so I have to use two spdt (JQX-15F(T90)) relays to do the job.
Following is the schematic I have designed to switches automatically between mppt and battery. Will this work or I need to modify it??? Can I use it without diodes D1 & D2 ?SPDT to DPDT with diode.JPG
 

crutschow

Joined Mar 14, 2008
27,956
which diode will work fine...? load is between 150-260watt i.e (12-22A).
The diode only has to conduct the coil current of the two relays momentarily until the magnetic field collapses when the relays are turned off.
It has nothing to do with the relay load current.
 

Ramussons

Joined May 3, 2013
1,095
I will not suggest this. Replacing 1 Relay with 2 relays with half the Contact Current Rating is a No No. The relays will not make / break contacts at the same time and 1 relay will have to take the full load for an instant during the transition.
The problem is compounded during the "break" operation when it will be 1 relay whose contacts will disconnect the full load. The arcing may be sufficient to do a lot of damage.
 

michael8

Joined Jan 11, 2015
205
questions, questions...

Why does the MPPT controller need to know? Does the MPPT controller have two outputs?

Do you need to switch both + and -? If not one SPDT relay is all you need.
(Are there other connections between the battery and the MPPT controller or panels?)

What is the load? Is it going to be upset with the switching (power glitch)?

Not knowing the load or the MPPT controller (or panels? driving the MPPT controller) this just doesn't sound like a
good solution.
 

michael8

Joined Jan 11, 2015
205
People on #5 & #6 -- look at his diagram: the relay contacts are NOT in parallel but series. They each switch one side
of the load between the battery and the MPPT output (this would be clearer if RY3 wasn't upside down).
 

eetech00

Joined Jun 8, 2013
2,611
People on #5 & #6 -- look at his diagram: the relay contacts are NOT in parallel but series. They each switch one side
of the load between the battery and the MPPT output (this would be clearer if RY3 wasn't upside down).
Then the current capacity will be reduced by roughly 50% because two voltage drops are in the series circuit instead of one.
 
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crutschow

Joined Mar 14, 2008
27,956
Then the current capacity will be reduced by roughly 50% because two voltage drops are in the series circuit instead of one.
What does the total voltage drop have to do with the relay contact rating?
Each relay will still have the same contact current rating whether in series or not.
 

michael8

Joined Jan 11, 2015
205
mppt take input from solar panel. mppt output goes to zero but gradually as the sun is setting...or there is clouds.

So the MPPT isn't connected to charge the battery, just runs the relay(s) and the load when the relay is switch on?

Anyway, looking at the relay specs https://cdn.sparkfun.com/datasheets/Components/General/Relay.JQX-15F.pdf
pickup voltage 9V, dropout voltage 1.2V, what will your load do when the relay switchs from the battery to the MPPT
when the MPPT reaches 9V or stays switched to the MPPT down to 1.2V?

I'd need more information on all the connections between the panels, MPPT controller, battery and more
characteristics of the load (what voltages it likes, current usage, what about power interruptions?) to say more.
 

eetech00

Joined Jun 8, 2013
2,611
What does the total voltage drop have to do with the relay contact rating?
Each relay will still have the same contact current rating whether in series or not.
Yes....but there is twice as much contact resistance when they are in series. For that much current, it will make a difference.
 

crutschow

Joined Mar 14, 2008
27,956
.but there is twice as much contact resistance when they are in series. For that much current, it will make a difference.
Difference how?
A relay's contact resistance is normally very low, and thus has only a very small voltage drop.
Two in series will not significantly affect the TS's circuit.
 

Thread Starter

ihsan.duet11

Joined Aug 10, 2021
12
I'd need more information on all the connections between the panels, MPPT controller, battery and more
characteristics of the load (what voltages it likes, current usage, what about power interruptions?) to say more.
[/QUOTE]
The whole system is 150-240-watt which means max current upto 20A, and includes mppt (14.2V output), AC to DC battery charger (14V output), Battery( 13.5V), Inveter (500watt) all in a box.
First relay to decide whether to charge battery from mppt or ac to dc charger, 2nd relay to decide whether to run DC load from battery or mppt, 3rd one to decide whether to operate inverter on battery or mppt (with mppt at first priority in all three cases).
 

michael8

Joined Jan 11, 2015
205
Is there an input to the MPPT controller? Solar? Wind? Hydro?

What size & type of battery? If AC is out, it's night (no solar) then they battery would have to put out 20A all night.
That takes a huge battery bank and would then imply a larger solar so that the solar could run the load and charge the
battery at the same time. Can the load deal with brief power outages?

Is there a path to run the load directly from AC too or is the AC too poor quality (spikes, low voltage, etc)
to run the load and only useful when it's there to be used via the battery charger?

First relay to decide whether to charge battery from mppt or ac to dc charger, 2nd relay to decide whether to run DC load from battery or mppt, 3rd one to decide whether to operate inverter on battery or mppt (with mppt at first priority in all three cases).

So there are 2 loads, the "DC load" and the input to the inverter? What's the power level and what voltage
stability do each need?
 
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