How long will 50Ah battery run my computer?

Azel

I have a 50Ah AGM battery (12volt) in my computer room with an inverter that I use when the electricity goes off (happens too often). I would like to know how long it will run my computer, but am not sure how to measure it.

My computer and monitor together draw 1.35 ampere. So, how long will my 50Ah battery run my computer to a 50% discharge?

I divided 50Ah by 1.35A and got 37 hours. Now if I figure in that the inverter is only 80% efficient I will need to take 20% from 37 hours. That leaves me 29.6 hours. And if I want only a 50% discharge I should only run my computer 15 hours.

I don’t know if my answer is correct or not.

Another solution is to use a volt-ohm meter to measure my battery voltage as it discharges. I found a battery percentage-of-change chart that said 12.70V was 100% charged and that 12.20V was 50% charged. My AGM battery 100% charge reading is a constant 13.40V. I am not sure why it does not match the battery chart I found.

Do wet batteries and AGM have different percentage-of-change charts?

Cariboocoot

Re: How long will 50Ah battery run my computer?

First, you won't want to take the battery below 50% DOD if you plan on keeping it around. So right there you're down 25 Amp hours.
Second, any inverter consumes power for itself. This can be an appallingly large amount.
Third, your 1.35 Amps - is that at 12 VDC or 125 VAC? You should convert to equivalent Voltage: 1.35 Amps @ 120 is 13.5 Amps @ 12 (roughly speaking - not precisely).
So if the computer is sucking down 13.5 Amps and the inverter another 1.5 the rate of drain is up to 15 Amps.

Suddenly the battery isn't going to last very long, is it?

inetdog

Re: How long will 50Ah battery run my computer?

Azel wrote: »

Do wet batteries and AGM have different percentage-of-change charts?

Lets call them flooded and AGM, since both are wet. And if the plate chemistry is the same (which it may not be, depending on the brand, but it will be close), then the resting voltage at 100% SOC should be close. But the voltage measured while actually under charge or under discharge will be different because of the lower internal resistance of AGM batteries. And the setting for the Float voltage on the AGM charger has to be lower than the corresponding setting when charging FLAs.
Measuring the voltage either while current is flowing in or out, or too soon after charger or load is disconnected, will give misleading results.

Azel

Re: How long will 50Ah battery run my computer?

I used a kill-o-watt meter. It read 1.35A at 120volts. And 160watts.

Also, when I took my battery voltage reading it had been sitting for several hours without drawing any power, nor charging. As I understand it, you should take voltage reading off a battery after it has been sitting a few hours not being used.

I am not sure how to measure what power the inverter is using. My kill-o-watt meter will not work on it. Plus, I would probably have to have my computer and monitor drawing power to get a correct reading. I guess I would need to meter between the battery and inverter while it was ON and powering my computer and monitor, and then subtract 1.35A from my reading to find out what the inverter is drawing. But, I don't want to burnout my volt-ohm meter, so I don't think I will try that.

inetdog

Re: How long will 50Ah battery run my computer?

Azel wrote: »

I used a kill-o-watt meter. It read 1.35A at 120volts. And 160watts.

Also, when I took my battery voltage reading it had been sitting for several hours without drawing any power, nor charging. As I understand it, you should take voltage reading off a battery after it has been sitting a few hours not being used.

I am not sure how to measure what power the inverter is using. My kill-o-watt meter will not work on it. Plus, I would probably have to have my computer and monitor drawing power to get a correct reading. I guess I would need to meter between the battery and inverter while it was ON and powering my computer and monitor, and then subtract 1.35A from my reading to find out what the inverter is drawing. But, I don't want to burnout my volt-ohm meter, so I don't think I will try that.

A Kill-a-watt meter is designed for AC only. It will not tell you anything useful when connected to DC and you can easily burn it out with high DC current. Learn the limitations of your tools before using them, or risk losing them and more.

The 1.35 amps at 120 volts is the same amount of power as 13.5 amps at 12 volts. Unless your ammeter can safely measure more then 20 amps, do not put it in the circuit. And as a general rule any ammeter other than a clamp-on is a dangerous thing to work with around a battery. One wrong connection and boom, no more meter and parts of it in your face, literally.

nsaspook

Re: How long will 50Ah battery run my computer?

A typical home computer UPS is designed to ride out a small power bump and to provide maybe 15mins of power so you can have a orderly shutdown (automated with software usually). So if it can handle a complete shutdown cycle of your computer it's running long enough. My home server UPS is a old Toshiba 1400SE 2000VA on-line series (10 mins at full load, 30 mins at half load).

http://www.toshiba.com/ind/data/tag_files/1400s%20(Plus)%20Series%202000-2400VA%20Mar03%2044730-001%20-%20UC1G2L_3387.pdf

Theory of Operation
An uninterruptible power supply is a system that is installed between the commercial
power and the load equipment. It is used during short-term blackouts or brownouts. The
UPS provides steady ac output power during these commercial power interruptions.
This power is provided for a long enough time so that the load can be shutdown in an
orderly fashion. This prevents loss of data and possible damage to both hardware and
software.

Cariboocoot

Re: How long will 50Ah battery run my computer?

Azel wrote: »

I used a kill-o-watt meter. It read 1.35A at 120volts. And 160watts.

Also, when I took my battery voltage reading it had been sitting for several hours without drawing any power, nor charging. As I understand it, you should take voltage reading off a battery after it has been sitting a few hours not being used.

I am not sure how to measure what power the inverter is using. My kill-o-watt meter will not work on it. Plus, I would probably have to have my computer and monitor drawing power to get a correct reading. I guess I would need to meter between the battery and inverter while it was ON and powering my computer and monitor, and then subtract 1.35A from my reading to find out what the inverter is drawing. But, I don't want to burnout my volt-ohm meter, so I don't think I will try that.

This is what I was talking about: that 1.35 Amps @ 120 VAC is going to be ten times that to the 12 Volt battery, plus the conversion loss. The inverter should have a manual which contains information about how much power it draws and what its conversion efficiency is.

My previous estimates are apparently close to accurate so what you get is 25 Amp hours / 15 Amps total draw = about an hour and a half maximum run time.

Here's a suggestion: laptops draw far less power than desktops and have their own built-in battery to provide 2-3 hours back-up power. It might be cheaper to buy a laptop than to rig up the equivalent back-up power for a desktop using a stand-alone battery & inverter (if this is what you need).

waynefromnscanada

Re: How long will 50Ah battery run my computer?

I agree with Cariboocoot on t he laptop. Many laptops use less than 20 watts, a huge reduction from you're 160 watts, and that would allow nearly 10 times longer run time on you're battery. Not only does the lap top use less power, but the reduced draw on the battery permits more amp-hours to be drawn from the battery before it reaches that 50% discharged mark.