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I have a basic curiosity question while I trying to learn/figure electrical "things" while I trying to size my UPS.

I know that an UPS will convert power from mains (240v in my case) into 12v battery and then convert back to 240v when in use.

1) May I know why is it 12v is the most "Standard" for batteries? Why not just 240v or 110v? That will do away all all those Ah conversion and inefficiency during conversion right?

Is the capacity stored inside a 12v 100ah battery is actually lesser (due to conversion) than a 120v 10ah battery if the output is 240v?

One of the reason that I could only thing of is probably safety? If safety is not an issue, then this will not happen right?

Out of so many voltages, why 12v is the chosen voltage?

2) May I know the conversion formula and explanation on each value and meaning on the conversion? I found many formula but having a hard time comprehend those values.

Thank you very much!

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  • \$\begingroup\$ Do you have any idea how heavy a 110/240V SLA battery would be? \$\endgroup\$ – Ignacio Vazquez-Abrams Jul 20 '16 at 2:43
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    \$\begingroup\$ You realize batteries are DC and mains is AC, right? Also, a battery is a 'pile' of sandwiched materials. One such sandwich has a fixed potential difference based on it's chemical makeup - generally somewhere between 0.5 and 3 volts. To make a higher voltage battery, you make a stack - a 12v lead-acid battery stacks 6 'sandwiches' of 2V each - making 12v. To make a 240 V battery, you'd have to stack 120 - which would make the battery far too large. \$\endgroup\$ – RJR Jul 20 '16 at 2:50
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    \$\begingroup\$ Highly depends on the chemistry of the battery @ejp \$\endgroup\$ – Passerby Jul 20 '16 at 3:06
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    \$\begingroup\$ 1.5V things are CELLS (not "batteries"). 12V things are BATTERIES (a series combination of CELLS). Depending on the chemistry a cell can range from less than 1V up to over 3V. High-current batteries (like car batteries, and UPS batteries) tend to be made from six lead-acid CELLS which are 2V each. \$\endgroup\$ – Richard Crowley Jul 20 '16 at 3:40
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    \$\begingroup\$ @ejp actually, you don't see any 1.5V batteries. Those are cells. Multiple cells combined are a battery. But to reiterate, Nickel (NiMH and NiCad) 1.2V, Lithium Primaries (Li-Mn02) 3V, Li-Ion/Li-Po/Li-Thionyl can be 3.6/4.1 or 3.7/4.2 depending on chemistry, Lead Acid are 2V, The phosphate-based lithium-ion has a nominal cell voltage of 3.20V and 3.30V; lithium-titanate is 2.40V \$\endgroup\$ – Passerby Jul 20 '16 at 3:40
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Batteries are DC, not AC. When 240VAC is rectified to DC, the resulting voltage is higher by a factor of sqrt(2). So 240VAC becomes 340VDC after rectification. There is no way to make a battery that stores AC voltage.

A 12VDC Sealed Lead-Acid battery has 6 cells connected in series. If one cell fails, the entire battery fails. A high voltage battery is possible, but impacts reliability because more cells means more possible points of failure. For an Uninterruptible Power Supply, the big customers demand high uptime / long Mean Time Between Failures. More cells would also mean more weight. (Lead and Sulfuric Acid are some of the densest, heaviest commonly available materials.)

As far as capacity and runtime are concerned, the typical consumer-grade UPS is intended to run for 5-30 minutes to give enough time for the computer to finish shutdown or hibernate. Commercial UPS may work in conjunction with diesel generators, and only need to hold the load steady until the generator stabilizes

Another feature often found in UPS intended for data center, is multiple hot-swappable batteries. So if one battery fails, the operator can swap in a new battery without the need to power off the system or disconnect the load. This is practical with 12VDC SLA as they are lightweight enough for one person to safely pick up and move around.

High voltage circuits require stronger insulation, longer creepage clearance around the conductors. This adds to cost and weight.

Economy of scale is important. 12VDC SLA Batteries are widely available from many manufacturers, well characterized and with safety lab approvals (UL, CE, etc.). The motor vehicle industry and the home security industry use a lot of 12VDC SLA batteries, so this is already a commonly available voltage. Using a bespoke battery system incurs much higher NRE engineering costs, plus regulatory testing. Unless there is a compelling benefit, standard components are preferred.

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  • \$\begingroup\$ Hi MarkU, thanks for the explanation, your 1st paragraph says that batteries are DC and not AC. Though I know the most basic of AC and DC but I thought the "voltage" as in Potential Difference is the same between both? for example 240v is 240v no matter in DC or AC. But from your answer it seems that in order to achieve 240vAC, the equivalent DC is 340vDC. Any links/guides I can get more info from? I am very interested and appreciate it. Thank you very much! \$\endgroup\$ – KokLiang.LIM Jul 20 '16 at 4:35
  • \$\begingroup\$ The difference is that 240VAC is the Root Mean Square voltage (RMS), the actual sinusoidal amplitude is higher. If the AC waveform is a pure sinewave, then the "crest factor" is the square root of 2. If a 10ohm heater load was connected to a 240VAC power source, the power delivered to the load would be equivalent to a 240VDC source. This is only true for ohmic loads, that follow Ohm's Law; not LEDs or transistors. Diode rectification catches the peak voltage instead of the RMS average voltage. \$\endgroup\$ – MarkU Jul 20 '16 at 5:39

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