Samlexpower BBM-1225 - Page 6

6 | SAMLEX AMERICA INC.
SECTION 3 | Description and Principle of Operation
ExternalpowersupplyisconnectedtothePositiveandNegativeinputterminals.Thebattery
isconnectedtothebatteryterminalsthroughFuse1andtheloadisconnectedtotheload
terminalsthroughFuse2.
ThevoltagefromthepowersupplyisfedthroughtheisolatingSchottkyDiodeD1to
theloadandtothebatterythroughthechargingcurrentlimitingresistorsR1andR2.
DiodeD1providesisolationbetweenthepowersupplyandthebatterytoprevent
thebatteryfromfeedingbackintothepowersupplycircuit.D1hasaforwardvoltage
dropofupto0.5V.Hence,thevoltageavailabletotheloadandtothebatteryfor
oatchargingwillbeupto0.5Vlowerthantheoutputvoltageofthepowersupply.
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. Assumingthestandbyoatchargingvoltagesof12V/24V
SealedLeadAcid(SLA)Batteriestobe13.5Vto13.8Vand27Vto27.6V(at60ºFto77ºF)
respectively,the output voltage of the power supply should be adjusted as follows:
• Power Supply Voltage for 12V SLA Battery Backup: Adjust to 14V
Theoutputvoltageofthisunitfortheloadandforoatchargingofthebattery
willbe14Vminusdropofupto0.5VacrossdiodeD1i.e.around13.5V.
• Power Supply Voltage for 24V SLA Battery Backup: Adjust to 27.6V
Theoutputvoltagefortheloadandforoatchargingofthebatterywillbe27.6V
minusdropofupto0.5VacrossdiodeD1i.e.around27.1V.
Aslongastheexternalpowersupplyisoperatingnormally,itwillsupplytheloadas
wellaschargethebatterythroughresistorsR1andR2,each1.2Ohm,25W(R1andR2
areconnectedinparallelandwillprovideanetseriesresistanceof0.6Ohm).
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).
Ifthepowersupplyfails,theloadwillbesuppliedbythebatterythroughtheSchottky
DiodeD2.AsinthecaseofSchottkyDiodeD1,therewillbeaforwardvoltagedropof
upto0.5VacrossD2.Thus,whenthebatteryissupplyingtheload,thevoltageseenby
theloadwillbeupto0.5Vlessthanthebatteryvoltage.NotethatSchottkyDiodeD2
isconnectedinparallelwithresistorsR1anR2andisusedtobypassresistorsR1andR2
whenthebatteryisdischargingthroughtheload.
Maximum Battery Charging Current
LeadAcidBatteriesshouldnotbechargedatveryhighchargingcurrent.Themaximum
recommendedchargingcurrentofabatteryisnormallyrelatedtoit’sAh(AmpereHour)
capacity,designatedas“C”(basedon20ADischargeRate).Themaximumcharging
currentforanSLAbatteryisnormallyspeciedwithinarangeofC/5(0.2C)toC/10(0.1C).