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Reason for Voltage Level for Off Grid Inverter System

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sabu31

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Hi all,

I would like to what is the standard/limitation that governs the battery voltage (nominal ) for off grid inverters. For example 3kVA systems have 48V Battery. 6kVA or more have 72 V and so on. Why cant we use higher voltage to reduce conduction losses. Thanking you.
 

there are inverters that run off 110VDC ( 121.5V float ) and 220VDC and 380 - 420VDC .....
 

Easy peasy said:
there are inverters that run off 110VDC ( 121.5V float ) and 220VDC and 380 - 420VDC .....
Click to expand...
Yes . But is that related to power level of inverter and is there any standard governing that
 

sabu31 said:
Yes . But is that related to power level of inverter and is there any standard governing that
Click to expand...
Yes ..It is related to power level mostly. As the power rating goes up ,generally battery voltage is kept high to reduce the current & hence reduce the cable length, cable loss & copper weight(Both in wire & busbar etc). In the inverter design also, higher battery voltage(DC Link voltage)calls for special attention for isolation etc which will add to the cost.
 

biswaIITH said:
Yes ..It is related to power level mostly. As the power rating goes up ,generally battery voltage is kept high to reduce the current & hence reduce the cable length, cable loss & copper weight(Both in wire & busbar etc). In the inverter design also, higher battery voltage(DC Link voltage)calls for special attention for isolation etc which will add to the cost.
Click to expand...

is there any standard governing this.

Why not use 120 V input for 1kVA system instead of 24 V typically available in commercial system
 

Its a costs thing.
Look at the cost of high capacity batteries and the cost of inverters. Higher voltage may mean less losses but the battery cost is higher and maybe not cost effective.

Brian.
 

betwixt said:
Its a costs thing.
Look at the cost of high capacity batteries and the cost of inverters. Higher voltage may mean less losses but the battery cost is higher and maybe not cost effective.

Brian.
Click to expand...
Hi Brian,

Thanks for the reply. But still its not clear

For example batteries are rated based on energy rating so if 120V 10Ah and 24 V 50 Ah should cost similar due to similar capacity.
 

Hi,

you may reduce wiring loss...

but then you need higher voltage transistors, usually they have higher conduction loss.
And when you connect batteries in series, then the weakest cell determines the overall capacity and the overall lifetime. When you parallel cells then the capacity of all cells are added, independent of weak ones or healthy ones.
For batteries in sereis one should concider a balancing circuit. --> additional cost.

Klaus
 
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    sabu31

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sabu31 said:
is there any standard governing this.

Why not use 120 V input for 1kVA system instead of 24 V typically available in commercial system
Click to expand...
Not in my knowledge. You can of course use 120V for 1 KVA system
It all boils down to several factors such as efficiency of the system, Isolation requirements (In case of higher battery voltage)which also calls for essential safety circuits & cost etc.
The system designer considers all the above to select the optimum voltage for a particular power level.
 

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    sabu31

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When you have 200 Amperes going to your inverter, the wiring and connectors become unwieldy and hard to handle. Clean contacts are vital.

It makes a good reason to double the voltage, thus you halve the current.

Besides which there is a problem called the Peukert effect which is unavoidable when you try to get by with a lower voltage battery. Losses go up as the square of the Amperage (expressed in the formula I*I*R). So a 12V battery lasts less than half as long as the 24V battery, even when using the same wattage.
 

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    sabu31

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there is no standard, it is just easier to use 12 or 24 volts up to 1 kw or so, 48 volt above that and so on ....
 

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    sabu31

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So Basically what I conclude is that there is no standard as such but decided based on power levels.
 

Primary determining factor for power limit at a given DC voltage of a converter is DC current. Large current requires large gauge wire. Connections resistance matters more. The MOSFET switches and their leads have to have low resistance. It quickly becomes impractical at lower DC voltages.

12v < 1.5 to 2 kW
24v < 3-4 kW
48v < 8 kW.
 
sabu31 said:
For example batteries are rated based on energy rating so if 120V 10Ah and 24 V 50 Ah should cost similar due to similar capacity.
Click to expand...

Theoretically perhaps, but large series strings of batteries
become a "weakest link" proposition with usable charge
limited by the lowest capacity one. Cost may be the same,
life expectancy won't be once random bad luck has its
time to accumulate.
 
Slightly related issue...
12V is practically universal in appliances made for recreational vehicles and home backup power. Lamps, pumps, refrigerators, battery chargers. The vast majority take 12V supply.

A higher system voltage requires sufficient knowledge to operate and maintain equipment and instruments, with proper caution.
 
If you compare 2 nos 50AH battery and 1 number 100AH battery always 2 numbers will have higher cost.
The cost of the control circuit and driver section also increases as you increase the voltage. Even the MOSFET's
DC caps are also costly as the voltage is higher. Better efficiency and lesser gauge of conductors will be the
only advantage if you go for higher voltage for lower capacity.
 

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