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An Uninterruptible Power Supply (UPS) is an electrical device that provides backup power to critical equipment in the event of a power outage or other power-related issues.

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How long does a UPS supply power

Hello, I recently got an Infosec E4 LCD LT UPS, unfortunately the two batteries that came with it were dead, so I replaced them with new ones same capacity and voltage (12v & 7Ah), then I downloaded the software to tweak and monitor the UPS, the temperature is normal 18°C, the output power is 220v 0.5A at 50Hz, which only 12% of the load capability of the UPS.

The batteries take a very long time to get to 100% but when I remove the plug from the outlet to test the UPS then it can only supply power for a maximum of 5mins .

If I’m not mistaken, the batteries voltage is 24v and their capacity should be 3.5Ah since they are in serie, which means 84Wh and I’m consuming 110W (assuming the load as DC current), so it should last at least 45mins.

I didn’t find any approximations in the internet to have an idea on how much time a UPS should take so I hope I don’t have bad batteries, but if so then the fault alarm must be triggered, like when I test it the first time with the dead batteries.

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Hi @vertinhol

I think your calculations are a bit incorrect.

If the batteries are in series the voltage is 24V but the current is still 7A. It doesn’t halve, it is the same. It doubles if you connect the batteries in parallel i.e. 14A but then the voltage stays at 12V.

If the load is 110W @ 24V this equals 4.58A so theoretically it should last for about 1.5 hours. But in real life the voltage starts to drop as the batteries are being used and it may get to a point where there is insufficient voltage not current.

It may be that you have 1 faulty battery that is failing under load or is faulty but usually most UPS run for only about 5-10 minutes so yours is probably OK They’re primarily designed to give enough time to save any work that is open and to shut down safely or to cover unstable mains power supplies not to keep working for hours on end.


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Thank you for your answer, I was talking about the capacity and not how much current the battery can deliver (7Ah), and my multimeter cannot measure over 10 amps, also I don't have a shunt resistor, so my idea is to charge both batteries at 100% and discharge them using a car lightbulb while measuring the time it takes, I'll monitor the voltages during the discharge.



They're related.

Ohm's Law : Power = Voltage x Current so if you know the values in theory you know the duration but it doesn't happen this way because the voltage begins to fall as the battery discharges so less current is available because the load stays the same so something has to give

Again Ohm's Law: I = E/R when I = current, E=voltage and R = resistance (or load) If the voltage drops and the resistance is the same, the current drops.

So either the UPS stops because there isn't enough voltage detected by it from the battery or there isn't enough current being supplied to it from the battery.

According to this for an E4 LCD LT 2000 (1600W) the backup time for a half load is only 7 minutes. What is the full model number of your UPS just to see what the manufacturer states regarding expected backup times?


IF the batteries were fully charged they would produce 84WH. That spec is based on an ideal load and will decrease with increasing load. IE, if the ideal load is 1A and you draw 7A it will not last for 1 hour.

Then you must consider the UPS efficiency of 88%. That gives you only 74WH.

According to this site, https://www.xpcc.com/selector/ the selected UPS units are at approx 20% load to yield 45min for 110W = looking deeper, it becomes more like black magic because it says 2 units rated at 600W will yield 45 and 84 min.

I would start by insuring the batteries are fully charged. Then try (not always possible) to find the load values behind the 7AH rating. Also insure that your load is what you think it is. You say the batteries were dead, so I assume it was used. It may have some defects, like bad caps, that has a serious impact on its efficiency. Maybe with all that you can determine where the problem is.


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