Samlexpower PST-1000-12 Owner's manual - Section 7 |
22 | SAMLEX AMERICA INC.
Intheexamplegivenabove,the10.5VLowBattery/DCInputAlarmwouldtriggerat
around80%dischargedstate(20%SoC)whentheC-RatedischargecurrentisC/5Amps.
However,forlowerC-RatedischargecurrentofC/10Ampsandlower,thebatterywillbe
almost completely discharged when the alarm is sounded. Hence, if the C-Rate dis-
charge current is lower than C/5 Amps, the battery may have completely discharged by
the time the Low DC Input Alarm is sounded.
Low Dc input Voltage Shut-down in inverters:Asexplainedabove,ataround80%State
ofDischargeofthebatteryatC-RatedischargecurrentofaroundC/5Amps,theLow
DCInputVoltageAlarmissoundedataround10.5Vfora12Vbattery(ataround21V
for24Vbattery)towarntheusertodisconnectthebatterytopreventfurtherdraining
ofthebattery.Iftheloadisnotdisconnectedatthisstage,thebatterieswillbedrained
further to a lower voltage and to a completely discharged condition that is harmful for
the battery and for the inverter.
Inverters are normally provided with a protection to shut down the output of the
inverteriftheDCvoltageattheinputterminalsoftheinverterdropsbelowathreshold
ofaround10Vfora12Vbattery(20Vfor24Vbattery).ReferringtotheDischargeCurves
giveninfig7.1(page19),theStateofDischargeforvariousC-Ratedischargecur-
rentsforbatteryvoltageof10Visasfollows:(PleasenotethattheX-Axisofthecurves
showsthe%ofStateofCharge.The%ofStateofDischargewillbe100%-%Stateof
Charge):
- 85%StateofDischarge(15%StateofCharge)atveryhighC-ratedischarge
currentofC/3Amps.
- 100%StateofDischarge(0%StateofCharge)athighC-Ratedischarge
currentofC/5Amps.
- 100%discharged(0%Stateofcharge)atlowerC-rateDischargecurrent
ofC/10Amps.
ItisseenthatatDCinputvoltageof10V,thebatteryiscompletelydischargedforC-rate
dischargecurrentofC/5andlower.
In view of the above, it may be seen that a fixed Low DC Input Voltage Alarm is not
useful.Temperatureofthebatteryfurthercomplicatesthesituation.Alltheabove
analysisisbasedonbatteryelectrolytetemperatureof80°f.Thebatterycapacityvaries
with temperature. battery capacity is also a function of age and charging history. older
batterieshavelowercapacitybecauseofsheddingofactivematerials,sulfation,corro-
sion,increasingnumberofcharge/dischargecyclesetc.Hence,theStateofDischargeof
abatteryunderloadcannotbeestimatedaccurately.However,thelowDCinputvoltage
alarmandshut-downfunctionaredesignedtoprotecttheinverterfromexcessivecur-
rent drawn at the lower voltage.
Use of external Programmable Low Voltage Disconnects
Theaboveambiguitycanberemovedbyusinganexternal,programmableLowVoltage
Disconnectwheremoreexactvoltagethresholdcanbesettodisconnectthebattery
based on the actual application requirements.
seCtIOn 7 |
General Information on Batteries for