Back-up battery. PMIC, what is the cleanest solution?

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world54
 
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Joined: Sat Mar 15, 2014 8:49 pm

Back-up battery. PMIC, what is the cleanest solution?

Post by world54 »

Hello,

I am new to the forums, but I have been lurking for a while :)

Before I get into the main issue I will discuss the context: I am building an Arduino based door lock powered via PoE (48v). I would like to have a back-up battery (lipo) so that if the power goes out the door can continue to operate for a short period of time. So I have two power sources, PoE and the lipo battery. If PoE is connected it should recharge the battery and when PoE is disconnected then the battery should kick in. I also need two power rails, one to power the Arduino (either 5v so that I can plug it into the Arduino's 5v pin or >7v so that I can connect it to Vin) and 12v @ ~500ma to power the door striker.

My main issue is finding an elegant solution to this problem. I would like a single IC that could take care of prioritising the inputs and charging the battery (safely) (cost isn't really an issue at this time). I am not 100% on using a lipo battery, a lead acid may be a better solution as it provides a convenient 12v and weight isn't really an issue for a wall mounted solution.

I have a couple ideas of how I could achieve switching between 48v PoE and a battery, its just that most of them are rather clumsy and involve multiple buck and boost converters.

Here is my latest idea which is still sorta clumsy:
Convert the 48v PoE down to 12v, use a 12v lead acid battery as back-up
Then I need an appropriate IC to take care of the recharging and power source selection, most of the PMIC's that I have found operate at 5v so this may be an issue
Then use the 12v output to power the door striker
Convert the 12v down to power the Arduino (either 5v and plug it straight into the 5v pin or down to >7v and plug it into the Vin pin, I'm not really sure which one is better, to convert it down to 5v and bypass the linear regulator will save power but the output wont be as clean).

Sorry for the long question, any help would be really appreaciated :)

Thanks,

Jack.

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Franklin97355
 
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Joined: Mon Apr 21, 2008 2:33 pm

Re: Back-up battery. PMIC, what is the cleanest solution?

Post by Franklin97355 »

One of the ways they build UPS systems is to have a power source charge a battery and have all the devices run from the battery. That way there is no switching at a power failure only a lack of charging. This seems to be the least likely to fail and there is no down time during the switch.

world54
 
Posts: 2
Joined: Sat Mar 15, 2014 8:49 pm

Re: Back-up battery. PMIC, what is the cleanest solution?

Post by world54 »

Thanks for the reply :)

I was reading that this can shorten the longevity of the battery. If the system goes out for a couple of seconds thats not really an issue. I will look further into that solution, however I think that an IC may be required to achieve the best solution.

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