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Voltage dependent switching relay

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I appreciate all your effort, I need to remember that the switching of various loads up to as high as 150 amps may prevent me using a Voltage sensitive device. My highest load (150 amps for 5 minutes) will drop battery voltage by an apparent 1V due to internal battery resistance. I say apparent because I believe that in the milli-second range on first connection it may be higher than 1V but too fast for my meter to read.

With this in mind I shall try the timer option first, by the way I now know that the controller if set to switch off at 12.1V does not switch back on until 12.6V so the hysteresis is good I think.

On another note (perhaps it needs a new topic?) and hopefully a simple question; If I wanted to regularly (say 5 times a day) dissipate say 300W for a few hundreths of a milli second (Guessing a 1/4 second) could I use a tiny 1500W TVS Zener? They seem so tiny for such a high load and cost just a few dollars!
 
A power resistor would seem a better choice, as it would pass a known current.
 
Iwas trying to keep it simple; I meant a TVS Zener in series with a Power resistor such that the combo would only conduct above the zener breakdown voltage.
 
Rethinking this project, it seems right to disconnect the battery when its voltage under load drops below a set point; but shouldn't it only be reconnected once it has been fully recharged by the PV system? In other words the 'reconnect' signal should come from the charger (possibly in response to the battery voltage, without any load, exceeding a reset point)?
 
Re the TVS, I can't believe a 1500W rating one would take that power for very long. I'm guessing 1/4 second would blow it. Have you checked the datasheet?
 
I checked the specs on it; 22.5V max and 67 amps max would appear to be 1500 watts but the fine print states for 10/1000 of a micro second so unbelievable small in reality.

To answer your first point needs a little thought; Charge is a function of I input to system and I output from system, hence 25A into the batteries from the PV array and 15 amps out to a load still equals a net charge of 10 amps. In reality battery voltage determines if the system is charging or not. My system uses a complicated PWM and Maximum Power Point Tracking (MPPT) charge controller. If all battery load were disconnected the controller output would exceed 12.8V (100% charged battery state for a battery at rest) in a heartbeat.
 
Alec; Thank you so much. Your asking a query about the charging caused me to fish out my very thick user manual for the controller. In it I found that an auxiliary output could be set to activate/dis-activate at what ever voltage I chose and for what ever time period.

Initially I set it for 12V off and 12.8V on with a time delay of 10 seconds. The whole thing is now functioning perfectly for example with a heavy load such as a microwave and with low daylight it switches to shore power as soon as the light level increases it switches back to battery and with no relay chattering at all. After a days test I'll tre-set the timje delay to 15 minutes or so.

PROBLEM SOLVED. Thanks Guys.
 
Always good to hear of a successful outcome.
 
Indeed. Presently for example it is 8am here so the PV's are only producing 150W and the batteries are as low as I ever want to deplete them (50% or 12V) Switch on a 750 watt toaster and the load shares between battery and shore power.
 
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