What voltage should 24 volt inverter shut off at?

[belfert]

I have a Prosine 3000 24 volt inverter.  What voltage should the inverter be shutting down at?

The default is low voltage alarm at 21 volts and shutdown at 20 volts.  Those values seem way too low to me and I have been starting the generator to charge the batteries when they get down to around 24 volts.  I've seen the batteries get up over 28 volts when fully charged, but they seem to drop down to 25 volts or less in just a few hours with nothing except the fridge and a few 12 volt loads running.

[Jerry Liebler]

Belfert,
     To avoid shortening the life of your house battery it should be charged whenever it's 'resting voltage falls below 24.0.  The inverter can only sense voltage while under load, not resting voltage.  As a compromise I'd suggest you set the alarm volts at 24 and shut down at 23.8.  This will give some small margin for voltage drop due to load.  BTW 21.0 volts resting is a completely discharged battery and should never be allowed to occur.
Regards
Jerry 4107 1120

[Jerry Liebler]

Belfert,
     It sounds like your batteries need an 'equalising' charge.  If they are not conventional flooded cell batteries you should consult the maker's data for how to 'equalise' them.  If they are 'flooded cell' types that you have to add distilled water to then the instructions in http://www.rollsbattery.com/Bulletins/605.htm   should be used.
Regards
Jerry 4107 1120

[Tony LEE]

"BTW 21.0 volts resting is a completely discharged battery and should never be allowed to occur"

You sure that it is this low? Figures I have say that this would be totally dead and buried  at that voltage.

[DrivingMissLazy]

"BTW 21.0 volts resting is a completely discharged battery and should never be allowed to occur"

You sure that it is this low? Figures I have say that this would be totally dead and buried  at that voltage.

1.75 volts per cell is the low voltage cut off point for all UPS battery systems. this is equal to 21 volts for a 24 volt system. Every system I have ever been involved with uses this as the low voltage cut off point and it will not damage the battery.

Richard

[Jerry Liebler]

Richard,
     I don't agree with the definition of damage that you must be using.  To me, substantial reduction of useful life IS damage.  A good deep cycle battery will have it's life reduced by more than 1/2 if it is routinely discharged to 1.75 volts/cell  (visit any Deep cycle battery manufacturer's site to verify this).  A start type battery may be destroyed by 2 or 3 such discharges (this came from the Trojan site).  Perhaps a UPS really only needs to work 1 or 2 times and the designers allow such abuse of the battery to give their customers better looking numbers.  Since most busnuts, and alternative energy users, are trying to stretch the life of their battery rather than optimising ONE TIME performance (which is what the UPS makers are surely doing) we are well advised to limit discharged voltage at 2.0 volts / cell (which corresponds to a 50% DOD).  At 2.0 volts/cell most battery makers say we'll achieve over 80% the best life of our batteries.
Regards
Jerry 4107 1120

[DrivingMissLazy]

UPS batteries are typically rated for several hundred deep discharges to 1.75 vpc, and of course many hundreds more at less than full discharge. At least that was my experience in building and selling large UPS systems for many years.
Richard

[Jerry Liebler]

Richard,
     Those same batteries will live through thousands of cycles if only discharged to 2.0 volts/cell.  A UPS has different goals than a bus's house battery.  Since we are able to control when we recharge and would rather have longer life, 2.0 volts/cell is a much better compromise.  Do it your way.
Regards
Jerry 4107 1120

[DrivingMissLazy]

Richard,
     Those same batteries will live through thousands of cycles if only discharged to 2.0 volts/cell.  A UPS has different goals than a bus's house battery.  Since we are able to control when we recharge and would rather have longer life, 2.0 volts/cell is a much better compromise.  Do it your way.
Regards
Jerry 4107 1120

Jerry, you are absolutely correct and I am in no way disputing your information.

I am only trying to inform, based on my actual experience, that occasional deep discharges do not really do much if any permanent damage. Several hundred deep discharges is typically a lifetime for many busnuts. .Also, occasional deep discharge with an attendant equalize charge is considered beneficial to the battery system.

Richard

[DrivingMissLazy]

Richard,
     Those same batteries will live through thousands of cycles if only discharged to 2.0 volts/cell.  A UPS has different goals than a bus's house battery.  Since we are able to control when we recharge and would rather have longer life, 2.0 volts/cell is a much better compromise.  Do it your way.
Regards
Jerry 4107 1120

Jerry, you are absolutely correct and I am in no way disputing your information.

I am only trying to inform, based on my actual experience, that occasional deep discharges do not really do much if any permanent damage. Several hundred deep discharges is typically a lifetime for many busnuts. .Also, occasional deep discharge with an attendant equalize charge is considered beneficial to the battery system.

Richard

An update on battery end cell voltage. I was just dis-assembling the scooter we are planning on shipping and I noted that the batteries are rated as deep cycle. This is the information I copied from one of them:

Interstate SPCS-33
Sealed Deep Cycle
12 volt, 33 AH
20 hour rate to: 1.75 VPC

Although these batteries indicate they are sealed, the fill covers can easily be removed for checking liquid level. They should absolutely be removed anytime they are put on a high rate of charge or being equalized.

Richard

[Tony LEE]

"1.75 volts per cell is the low voltage cut off point for all UPS battery systems. this is equal to 21 volts for a 24 volt system. Every system I have ever been involved with uses this as the low voltage cut off point and it will not damage the battery."

That may be, but a UPS system is nothing like a normal RV system. By definition, a UPS is for emergency use and needs to hang on for as long as possible in the event of a power failure. Most UPS would spend 99.999% of their life on float and maybe come into use a couple of times a year. Battery life  and therefore costs -- is never a consideration and the once or twice it is flattened is not going to significantly reduce life anyway, especially since it will be recharged within hours at the latest.

A RV deep-cycle system is for everyday use and must be much more conservatively managed to get decent life out of the batteries.

I suggest using UPS parameters for UPS systems and RV parameters for RV systems.

The thing that is causing extra confusion is that some posters are talking about discharging to a certain resting voltage and others are using voltage under load and while 1.75V under load voltage may be appropriate if the batteries are being discharged at a C10 or even C20 rate, to go that low at a C100 rate too often will significantly reduce life.

[Jerry Liebler]

Tonylee,
WELL SAID!
      Regrettably only resting voltage is even an approximate indicator of state of charge.  For my battery bank, consisting of L16s, discharge at C/10 results in only a 0.04 volt/cell drop across the rated internal resistance.   My inverter cut out is set at 1.95 volts/cell and my generator start is set at 2.0 volts/cell.
As always, do it your way.
Regards
Jerry 4107 1120