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Battery backup for a 12VDC device

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tribat:
I have a little Intel Atom based PC that uses a DC to DC converter board as its PSU. The inverter draws 3.5 amps from a 12VDC 5A switched-mode power supply.

Since the power requirement is so low compared to a desktop computer I thought that it would be neat to have this platform protected against power failures with an internal SLA battery and I happen to have an extra 12V 5Ah SLA battery that fits in the case. My first idea was to attach the battery in parallel with the 12VDC input, but after looking at the specs of the battery I noticed that in order to keep the battery fully charged you'd need to have more voltage, something like 14V.

The next idea was to replace the switch-mode PSU with a car battery charger and just hook everything in parallel. But I looked at the specs of the DC to DC converter and the specs say: "Input: 12V DC +-5%, MAX 10A", with a battery and a charger in parallel it would be somewhere around 20%, so I'm guessing that's not gonna work.

I really was expecting the DC to DC converter having a wide range of input voltages, but I guess not?

Any ideas how I could do this? I know about PicoUPS and that it's the JUST right tool for this, but somehow I find the prize a little too salty... I mean compared to the fact that my first two ideas would have cost me nothing but some wire and connectors.

The DC to DC converter I have in the Atom machine: http://linitx.com/viewproduct.php?prodid=12066



Mechatrommer:
same with me. i'm still trying to figure out how to power my Atom with a 12V LA Batt and at the same time without disconnecting the batt, charging it with external circuitry powered by AC Mains.

TechGuy:
What you need is a DC to DC converter. Something like this:
http://www.mouser.com/ProductDetail/Cincon/CHB75-12S12/?qs=sGAEpiMZZMtwaiKVUtQsNZk%2fyhBUJKktPl0OLLVpKt0%3d
DC/DC Converters & Regulators 37.5-75W 9-18V 12V 6.25A

This until is fairly pricy ~ $85 USD, but with some searching effort, I think you will be able to find something much cheaper. Perhaps used from ebay or Craigslist will turn up a really good deal.

Basically you would connect this between your battery and the PC. it will regulate the voltage so it remains at a constant 12V out, as long as the input voltage is more than 9 volts.

You can roll your own using a Simply Switcher such as the LM3478 or LM3481 controllers. Although these would be non-isolated DC to DC converters, The unit from Cincon is isolated, and would offer better protection against voltage spikes or input noise. The charger might introduce excessive noise when the battery is near full charge as it continully adjust the voltage supplied to the battery. How much noise depends on the design of the battery charger.




tribat:

--- Quote from: TechGuy on September 13, 2010, 10:16:31 pm ---What you need is a DC to DC converter. Something like this:
http://www.mouser.com/ProductDetail/Cincon/CHB75-12S12/?qs=sGAEpiMZZMtwaiKVUtQsNZk%2fyhBUJKktPl0OLLVpKt0%3d
DC/DC Converters & Regulators 37.5-75W 9-18V 12V 6.25A

This until is fairly pricy ~ $85 USD, but with some searching effort, I think you will be able to find something much cheaper. Perhaps used from ebay or Craigslist will turn up a really good deal.

Basically you would connect this between your battery and the PC. it will regulate the voltage so it remains at a constant 12V out, as long as the input voltage is more than 9 volts.

You can roll your own using a Simply Switcher such as the LM3478 or LM3481 controllers. Although these would be non-isolated DC to DC converters, The unit from Cincon is isolated, and would offer better protection against voltage spikes or input noise. The charger might introduce excessive noise when the battery is near full charge as it continully adjust the voltage supplied to the battery. How much noise depends on the design of the battery charger.






--- End quote ---
It's even more expensive than PicoUPS =D

Zero999:
I would've also expected the DC-DC converter to have a wider input voltage range than that: 10V to 15V at least, otherwise it's pretty pointless.

5% could be a mistake, I'd send them an email, asking for clarification.

Perhaps the 12V output is directly connected to the 12V input?

Looking at the board, there are only three large energy storage inductors, probably one for +5V, 3.3V and -12V so I suspect that's what's going on, unless the little one on the right is for -12V, although I suspect it's just an EMI filter.

Try disconnecting the power from it, waiting a couple or minutes or so for the capacitors to discharge and measuring the continuity between the 12V output and the 12V input using a DVM, if the resistance is very low, (<0.1 Ohm) then you have the answer: there's no 12V DC-DC converter so the 12V input has to be within the range specified by the computer.

If this is true, the DC-DC converter isn't much use as few 12V supplies will be that well regulated and I imagine that the main use for such a supply would be to run a PC from a car battery which it can't do. If so, see if they except returns and look for a better PSU, otherwise look for a 10V to 15V to 12V 5A DC-DC converter.

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