Low Voltage shut off for inverter

jimmyaz

Hi Guys,

What is the best way to protect my battery from a low voltage situation?  Whether to shut off the DC side of my inverter or the AC side?  My inverter does have low voltage shut off, but it;'s way too low... 19-20volts for a 24V system.

My Classic 150 have the AUX channel to do 12volts control.  But where can I buy a 12V DC coil relay that can control 120v AC?  Need about 2500watts capability.

thanks

mike95490

You can get 2 different low voltages in a system !!
First, running small loads, slowly lowers your battery charge and it's voltage
Second, a large load kicks in and quickly, your battery voltage sags from the load, and then when the load is removed, the voltage recovers (internal battery resistance, cable resistance, chemical reactions inside the battery all effect this) Afterwards, the battery can still be >90% charged, but heavy loads lower the voltage, so inverters often have a lower LVD than you would expect

jimmyaz

HI Mike,

When the heaviest load come on, my battery never go below 23volts and I want to keep it that way.  Incase let say a PV failed or charge controller failed... I don't want it to kill my battery dragging my battery below 50% SOC.

vtmaps

If you don't want to go below 50% SOC, the LVD (low voltage disconnect) should be about 24 volts.   A heavy load will (as mentioned) cause the voltage to sag below 24 volts when the SOC is still above 50%. 

I set the LVD on my inverter to 23.8 volts... but it won't shut down unless the voltage is below 23.8 for a minute.  This allows for short (less than a minute) surges.

I don't recall if the classic AUX can respond to both voltage and time.  Do a search for "12 volt power relay" and you will find something you can use with the classic.

--vtMaps

jonr

I didn't like my inverter's LVD and power saving options, so I wired a relay to the inverter's on/off switch.  Then I connected an arduino to this relay.  And this arduino can read the battery voltage.   So the end result is that I can easily program any behavior I want.  For example, cycle 15 min on then 45 min off and only during daylight hours.     Or more complicated LVD algorithms.  No big, power hungry relays or SSRs.

jimmyaz

vtmaps said:

If you don't want to go below 50% SOC, the LVD (low voltage disconnect) should be about 24 volts.   A heavy load will (as mentioned) cause the voltage to sag below 24 volts when the SOC is still above 50%. 

I set the LVD on my inverter to 23.8 volts... but it won't shut down unless the voltage is below 23.8 for a minute.  This allows for short (less than a minute) surges.

I don't recall if the classic AUX can respond to both voltage and time.  Do a search for "12 volt power relay" and you will find something you can use with the classic.

--vtMaps

I believe the Classic does have the time delay thing you can set... the voltage must drop below the set voltage for X amount of time before it engage.

I have bought some 12V coil relay... 30amps rated... but they mention nothing about the connecting part can handle AC or not... can the contact handle 120V?  Does it matter AC or DC on the contact side?  I am confused....

vtmaps

jimmyaz said:

I have bought some 12V coil relay... 30amps rated... but they mention nothing about the connecting part can handle AC or not... can the contact handle 120V?  Does it matter AC or DC on the contact side?  I am confused....

It most certainly does matter whether AC or DC on the contact side.  I guess that the 30 amp rating is an AC rating.  What are the specs on the relay?

--vtMaps

jimmyaz

I bought this relay  a while back, but it mention nothing about DC or AC on the contact side.  Just say DC coiled.

http://www.ebay.com/itm/141401605272?_trksid=p2057872.m2749.l2649&ssPageName=STRK:MEBIDX:IT

Can you all tell me which relay I need to get to have DC coiled and AC contact?

Vic

Hi jimmyaz,

The relay in the ebay link IS a DC coil,  DC contacts type relay.  Most DV contact relays can be used for AC voltages on the contacts,  BUT,  they are often not rated for AC voltages.   The ebay relay probably is not rated for 120 VAC.

Here is a type of relay (some would call it a Contactor) that IS rated for AC voltages on its contacts,  and the coil runs on DC.   This relay's coil is 24 VDC,  so it could not be directly driven by the Aux output of a Classic,  of FM CC.   One could use a simple transistor driver for the 24 V coil:
http://www.allelectronics.com/make-a-store/item/rly-2425/24vdc-dpdt-25a-relay/1.html

12 VDC coil contactors are readily available,  but you will need to be conscious of the approximate 200 milliamp maximum current limit on most of the Aux outputs of most CCs

Here is the general listing for alllectronics.com's relays:
http://www.allelectronics.com/make-a-store/category/500700/relays/automotive/power/1.html

And,  while we are at it,  the general Relay listings:
http://www.allelectronics.com/make-a-store/category/500/relays/1.html

I have no financial interest in allelectronics,  but they are a very reputable vendor,  IMO.

Vic

jimmyaz

Thanks guys.

For the DC coiled 12v and AC contact, I found this one.
http://www.ebay.com/itm/201491081665?_trksid=p2057872.m2749.l2649&ssPageName=STRK:MEBIDX:IT

              Brand New Solid State Relay SSR-25A 25A / 250V 3 - 32VDC            
            - Long service life and high reliability            
            - Highly reliable and compact sized            
            - Designed to offer users maximum simplicity            
            - Fast switching            
            - Output Voltage: 24-380 VAC            
            - Output Current: 24A            
            - Input Voltage: 3 - 32 VDC            
            - Control Method: DC to AC            
            - Mounting Method: Bolts fixed            
            - Switch time: <= t10 ms

Vic

Hello jummyaz,

Had considered weather an SSR would be OK for you intended use,  before mentioning that relay Contactor.

SSRs use very little current to turn them ON,  and the control voltage to do this has a fairly wide range  --  both good things.

SSRs need a Heat Sink,  because there is some voltage drop across them  --  they use a semiconductor to do the switching.

Another possible disadvantage could be surge currents for some of your loads,  and possible voltage transients from switching inductive loads.

IMO,  it is simply NOT possible to make a high quality,  reliable SSR for the selling price that the vendor in the ebay link is asking.

If I were using SSRs,  personally would use a Crydom brand,  or some other USA manufacturer  with a long-term good reputation for quality.

Searched for    '  fotek  '     and did find this kind of mixed review:
http://www.instructables.com/id/The-inner-workings-of-Counterfeit-FOTEK-SSRs/

FWIW,  just my opinions,   Vic