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# Battery Ah demand calculation & Questions

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#### Andrew Leigh

##### Member
Hi,

I am using an old PSU as an inverter. I would like to calculate the Ah rating of battery required to give a certain life.

From what I have read the fridge being powered (electric element) has a duty cycle of 50%. The power consumption of the heater is 160W so in theory this means I will be consuming 80W per hour on average? The other knowns are that the inverter is fed by two 12V 11Ah batteries in series and that the no load current is 180mA and an output of 240V. The Fridge will need to operate for the duration of a 6 hour road trip.

Question 1 - Two 12V 11Ah batteries in series = 24V @ 11Ah or 24V @ 22Ah?

Question 2 - I would like to establish the efficiency of the inverter. I am not looking for an nth degree calculation here but ± 5%. I was thinking of connecting a load, measuring the voltage and current on the LV and HV sides then calculating the power. The difference between the power in and power out I was thinking would be what is being consumed internally and therefore the inefficiency?

Question 3 - I do not think that I will connect 220V to the PSU input when power is available. The batteries I plan using are going to be of sufficiently larger capacity and therefore may not charge correctly. Will need to measure the charge current before a final decision is made. If I do not connect the 220V supply this should not cause any damage?

Question 4 - When 220V power is not available at the input to the PSU, the PSU has a standard feature which is a buzzer that sounds at a too regular frequency. I have not located this yet but would think that it could be desoldered from the board without any negative effects?

Finally would the rough formula for the Battery required look something like this;

Ah = Current at 24V side when under load * Time Required

So assuming a 15% inefficiency then the current draw will be 7.84A * 6h = 47Ah

Thanks
Andrew

Question 1 - Two 12V 11Ah batteries in series = 24V @ 11Ah or 24V @ 22Ah?

In series it would be 24V @ 11Ah, and in parallel it would be 12V @ 22Ah, either way you can only get out 264W for one hour.

Question 2 - I would like to establish the efficiency of the inverter. I am not looking for an nth degree calculation here but ± 5%. I was thinking of connecting a load, measuring the voltage and current on the LV and HV sides then calculating the power. The difference between the power in and power out I was thinking would be what is being consumed internally and therefore the inefficiency?

most inverters your lucky to see 75% efficiency from what i hear, but i dont get what your saying about using an old PSU as an inverter.

Question 3 - I do not think that I will connect 220V to the PSU input when power is available. The batteries I plan using are going to be of sufficiently larger capacity and therefore may not charge correctly. Will need to measure the charge current before a final decision is made. If I do not connect the 220V supply this should not cause any damage?

huh?

Question 4 - When 220V power is not available at the input to the PSU, the PSU has a standard feature which is a buzzer that sounds at a too regular frequency. I have not located this yet but would think that it could be desoldered from the board without any negative effects?

Should be easy enough to desolder it, and there wouldnt be any negative effects unless the buzzer is hooked up in series with some other safety sensor built into the PSU, but probably not.

To calculate your battery life you take your wattage draw on the output and divide it by your battery voltage on the input, which will give you the amps being drawn from the battery.
Then 11Ah/amps being drawn, will give you how long you can use it for.

That calculation doesnt count your inverters efficiency though.

Last edited:
Thanks,

dyslexia rules ... not a PSU but a UPS.

Cheers
Andrew

A lead-acid battery is destroyed if it discharges too much.
A lead-acid battery has its Ah rated at the 20 hours rate. Your 24V/11Ah battery can supply only 0.55A for 20 hours. It might deliver 3A for only 1 hour. 3A at 24V is only 72W so your battery is much too small to power your fridge.

If your inverter does not have voltage regulation then its voltage drops as the battery runs down. The fridge might have a voltage that is too high when the battery is charged then have a voltage that is too low when the battery is discharged.

Hi,

thanks for the help boys.

I never intended using the original batteries and was going to connect 2 x 60Ah. I am just trying to establish if the 60Ah batteries will be sifficient. The point being made was that I was not going to use the exisiting UPS charging citcuit as it would be woefully inadequate. I will charge the batteries independently.

Did some research, there is voltage regulation and therefore I assume tnhat the batteries will cut out before discharging too deeply.

Thanks again
Andrew

your not using deep cycle batterys?

Most UPS devices are cheap and nasty and not meant for continuous use (not meant to be used as inverters). They have low duty cycles and are only meant to be used for a few minutes (to back up your data etc).

Hi,

you make a good point there regarding duty cycle.

I have no clue of what a good UPS would be, mine is identical to this one but with a differnt paint job.

The consumption will be 350W max, I need it to run for 6 hours continuous at a 50% duty cycle and once every three months when I go away.

Yes probably will use deep cycles.

Whatchya think?

Cheers
Andrew

Hi

What is a UPS if not an inverter with some fancy other features. This is full sine wave and free? Or am I missing something here? I just could not justify spending money on such an infrequent usage pattern.

Cheers
Andrew

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