6 | SAMLEX AMERICA INC.
SECTION 3 | Description and Principle of Operation
ExternalpowersupplyisconnectedtothePositiveandNegativeinputterminals.Thebattery
isconnectedtothebatteryterminalsthroughFuse1andtheloadisconnectedtotheload
terminalsthroughFuse2.
ThevoltagefromthepowersupplyisfedthroughtheisolatingSchottkyDiodeD1to
theloadandtothebatterythroughthechargingcurrentlimitingresistorsR1andR2.
DiodeD1providesisolationbetweenthepowersupplyandthebatterytoprevent
thebatteryfromfeedingbackintothepowersupplycircuit.D1hasaforwardvoltage
dropofupto0.5V.Hence,thevoltageavailabletotheloadandtothebatteryfor
oatchargingwillbeupto0.5Vlowerthantheoutputvoltageofthepowersupply.
In a battery backup application, the battery will normally remain in a fully charged,
standby condition most of the time. It will discharge to the load only when the power
supply fails. Under this standby application, the battery should be charged at the
recommended oat voltage. Assumingthestandbyoatchargingvoltagesof12V/24V
SealedLeadAcid(SLA)Batteriestobe13.5Vto13.8Vand27Vto27.6V(at60ºFto77ºF)
respectively,the output voltage of the power supply should be adjusted as follows:
• Power Supply Voltage for 12V SLA Battery Backup: Adjust to 14V
Theoutputvoltageofthisunitfortheloadandforoatchargingofthebattery
willbe14Vminusdropofupto0.5VacrossdiodeD1i.e.around13.5V.
• Power Supply Voltage for 24V SLA Battery Backup: Adjust to 27.6V
Theoutputvoltagefortheloadandforoatchargingofthebatterywillbe27.6V
minusdropofupto0.5VacrossdiodeD1i.e.around27.1V.
Aslongastheexternalpowersupplyisoperatingnormally,itwillsupplytheloadas
wellaschargethebatterythroughresistorsR1andR2,each1.2Ohm,25W(R1andR2
areconnectedinparallelandwillprovideanetseriesresistanceof0.6Ohm).
NOTE: As the external power supply is required to feed the load as well as charge the
battery, the current rating of the power supply should be at least 20% more than the
sum of the maximum load current and the maximum charging current (4A for 12V and
8A for 24V).
Ifthepowersupplyfails,theloadwillbesuppliedbythebatterythroughtheSchottky
DiodeD2.AsinthecaseofSchottkyDiodeD1,therewillbeaforwardvoltagedropof
upto0.5VacrossD2.Thus,whenthebatteryissupplyingtheload,thevoltageseenby
theloadwillbeupto0.5Vlessthanthebatteryvoltage.NotethatSchottkyDiodeD2
isconnectedinparallelwithresistorsR1anR2andisusedtobypassresistorsR1andR2
whenthebatteryisdischargingthroughtheload.
Maximum Battery Charging Current
LeadAcidBatteriesshouldnotbechargedatveryhighchargingcurrent.Themaximum
recommendedchargingcurrentofabatteryisnormallyrelatedtoit’sAh(AmpereHour)
capacity,designatedas“C”(basedon20ADischargeRate).Themaximumcharging
currentforanSLAbatteryisnormallyspeciedwithinarangeofC/5(0.2C)toC/10(0.1C).