Battery suggestion for a backup system

[pradhan.rachit]

Hello Experts.

I have a load requirement for 12 V/0.9 A and 5 V/0.2 A which will be powered by the mains. In case of mains failure, the unit will switch to the auxiliary power provided by a battery.

The backup time requirement is 3 hours at the required voltages and currents.

I wanted some suggestions for suitable batteries of the smallest form factor to suffice the required need. I was thinking about battery voltages of ~ 6V to be fed to the converter to derive the 12V and 5V supplies.

The product needs to be as small as possible. Kindly suggest some suitable standard batteries and packages that could be used for this application.

 

[KlausST]

Hi,

Where is your calculation?

With a 12V battery:
12V, 0.9A load and 3 hours obviously makes 0.9A x 3 h = 2.7Ah
5V, 0.2A is 1W. Use a DCDC converter, then 1W from 12V makes 82mA.0.082 A x 3h = 0.246Ah.
Makes a total of about 3Ah.
Maybe you need an extra SEPIC converter for the 12V output... and the battery becomes weak with time, then calculate with 5-6Ah.

Klaus

 

[pradhan.rachit]

Sorry for you have to perform the Ah calculations.

If I consider using a 12V/5Ah Li-Po battery, what would be a safe depth of discharge in order to increase battery life?
Also, what is the expected battery life of Li-Po batteries?

 

[KlausST]

Hi,

For these questions you should contact the manufacturer or the datasheet. Only he can give you valid informations.

Klaus

 

[D.A.(Tony)Stewart]

Battery choices are made from specs for;

Cost, watt-hour capacity, size, weight, safety, performance, and life-span

If size and power fail capacity is your only requirement, this gives many choices.

As you think about the other factors, you will graduate towards a limited number of options.
There are too many to discuss unless you add more requirements.

Most often the battery voltage is chosen to match the primary load if the range intended state of min/max state of charge matches performance requirements.

For Laptops there are many outputs, but the battery size is often increased by voltage from series strings as efficiency tends to be improved for regulating down with lower currents than the load.

For emergency floodlights wet-cell SLA or open LD batteries were the norm. but cycle life and frequency of power failure may dominate your choices and ease of replacement and a means of detecting when battery life warrants replacement.

 

[chuckey]

I have been involved with debugging set ups like this. The normal problem is that the battery fails when called upon after a couple of years or so. Test the set up by taking the mains of the voltage detection circuit AND the battery charger. I have been in a situation where the added volts from the charger gave the impression that the battery was OK.
Only use a fully regulated and smoothed charger. Any ripple voltage will shorten the battery life. I would use a charger that can carry the load and charge the battery at its maximum rate.
Keep the batteries cool.
Frank

 

[Warpspeed]

The product needs to be as small as possible.

I have been involved with debugging set ups like this. The normal problem is that the battery fails when called upon after a couple of years or so.

Yup, that about sums it up.
If you design it so it only just works, it will do so for a limited time as the battery loses capacity.
You really get to choose between planned obsolescence or very conservative over design.

The harder you work the battery, the faster it will age.