DC-DC PSU and Built-in Battery Backup

Larry · Post by **Larry** » Tue Sep 25, 2007 9:20 am

I've been contemplating building a system using one of the high efficiency DC-DC PSU from Mini-Box. I'm not exactly sure which of their products will meet my exact needs, but for this idea just assume one of them does the job and only requires a DC power input. I'm also going to use some micro ATX computer case that will have a huge cavity where a traditional ATX power supply would be installed.

It would be cool to implement a battery setup similar in concept to how notebook computers work. You could fit an inexpensive >10Ah SLA battery (~$45) in the space of an ATX power supply. You can also purchase relatively inexpensive NiNH AA batteries in bulk and create a 10Ah pack as well (~$80) but take up 8x less volume than SLA. I don't believe I need 10Ah of backup, but the little reading I've done suggests you don't want to discharge batteries at a rate >70% of the battery pack rating. I'd guess this would put you in the 10Ah range as a result.

It seems to make sense that this is where a battery backup should reside. A traditional UPS takes DC and changes it to AC after which the PC power supply takes the AC and turns it back into DC. Doesn't sound like an efficient way to make DC power for a computer. What is missing in this equation is a DC-DC charging circuit for the batteries assuming you might charge the batteries from the same power supply feeding the Mini-Box ATX power supply and logic for "UPS" operation. Would adding a battery pack charging circuit add some level of inefficiency that would negate all the attention being placed on a high efficiency ATX power supply in the first place?

Any ideas on whether this idea is valid and how to move it forward?

Flandry · Post by **Flandry** » Tue Sep 25, 2007 10:31 am

I like this idea (indeed, i've been contemplating it myself), but i don't know how hard it would be to implement. Using the wide input PicoPSU and a 16-19V battery setup would solve all the problems except the matter of charging the battery--that part i know nothing about.

Bluefront · Post by **Bluefront** » Tue Sep 25, 2007 12:11 pm

Here's an idea I've considered......Find a hi-capacity UPS. Most use a 12V battery inside (all the ones I've taken apart anyway). Tap into the 12v wires inside the unit, using this 12V to power the PICO. The UPS own charging circuit will recharge the battery.

This should work ok. The part I don't know about.......will the output to the PICO when the computer is running, be too much for the charging circuit of the UPS? Don't know....

Larry · Post by **Larry** » Tue Sep 25, 2007 1:00 pm

Bluefront wrote:Here's an idea I've considered......Find a hi-capacity UPS. Most use a 12V battery inside (all the ones I've taken apart anyway). Tap into the 12v wires inside the unit, using this 12V to power the PICO. The UPS own charging circuit will recharge the battery.

This should work ok. The part I don't know about.......will the output to the PICO when the computer is running, be too much for the charging circuit of the UPS? Don't know....

I've never taken apart a UPS but don't most of them pass-through AC to the attached outlets and monitor the power line to "switch" over to the battery? I'd guess the 12V DC portion of the circuit must implement some simple multi-stage charging setup, but would be suprised if most output more than an amp or two. I'd guess you'd have to have 6 or 7 amps of power available for a carefully crafted Core2Duo system.

Trekmeister · Post by **Trekmeister** » Tue Sep 25, 2007 1:38 pm

Bluefront wrote:Here's an idea I've considered......Find a hi-capacity UPS. Most use a 12V battery inside (all the ones I've taken apart anyway). Tap into the 12v wires inside the unit, using this 12V to power the PICO. The UPS own charging circuit will recharge the battery.

This should work ok. The part I don't know about.......will the output to the PICO when the computer is running, be too much for the charging circuit of the UPS? Don't know.... :?

I'd say it probably depends on the type of UPS. Offline and line interactive (the more common consumer type) UPSes might not be built to handle the heat output of constantly chargeing the batteries/powering the Pico PSU. After all they are supposed to charge the batteries once and then just keep them happy by trickle charging them.

On line UPSes on the other hand should have no problem at all with this since they always do AC->DC->AC conversion while running on line power.

Oh and another thing, I think high capacity UPSes usually has two or more 12V batteries in series. Drawing power from only one of them might cause problems since the charge level would differ between the batteries.

|Romeo| · Post by **|Romeo|** » Wed Sep 26, 2007 2:30 am

I've looked into this. The snag is that powering from a 12V SLA is not a particularly good idea as you can't guarantee the voltage. The solution is to use two giving you an input voltage of 24V and one of the PicoPSU family that allows you to use 12-24V input. SLA is a pretty good battery choice for it as you aren't worried about weight.

That hardware part is easy; however the hard part is battery monitoring and feeding it into windows power management. I say hard, it's not really that hard, it would just take time/money to do properly. Which is why I haven't got a prototype yet.

Obviously the battery technology you choose will dictate what battery charging/monitoring you use.

Flandry · Post by **Flandry** » Wed Sep 26, 2007 4:58 am

|Romeo| wrote:I've looked into this. The snag is that powering from a 12V SLA is not a particularly good idea as you can't guarantee the voltage. The solution is to use two giving you an input voltage of 24V and one of the PicoPSU family that allows you to use 12-24V input. SLA is a pretty good battery choice for it as you aren't worried about weight.

That hardware part is easy; however the hard part is battery monitoring and feeding it into windows power management. I say hard, it's not really that hard, it would just take time/money to do properly. Which is why I haven't got a prototype yet.

Obviously the battery technology you choose will dictate what battery charging/monitoring you use.

Yeah, a 12V battery alone would not be within tolerances for the 12V rail--it would require a higher voltage battery regulated by a 12V PSU like the variable input Picos. How would you manage the battery charging with SLAs? I wouldn't really care if there wasn't an interface to the OS.

|Romeo| · Post by **|Romeo|** » Wed Sep 26, 2007 6:43 am

Flandry wrote:
|Romeo| wrote:I've looked into this. The snag is that powering from a 12V SLA is not a particularly good idea as you can't guarantee the voltage. The solution is to use two giving you an input voltage of 24V and one of the PicoPSU family that allows you to use 12-24V input. SLA is a pretty good battery choice for it as you aren't worried about weight.

That hardware part is easy; however the hard part is battery monitoring and feeding it into windows power management. I say hard, it's not really that hard, it would just take time/money to do properly. Which is why I haven't got a prototype yet.

Obviously the battery technology you choose will dictate what battery charging/monitoring you use.
Yeah, a 12V battery alone would not be within tolerances for the 12V rail--it would require a higher voltage battery regulated by a 12V PSU like the variable input Picos. How would you manage the battery charging with SLAs? I wouldn't really care if there wasn't an interface to the OS.

My plan was to steal the charging circuit from another product I was involved in that used a 12V SLA (probably would have used more of that actually, it had RS-232 capability as well). Battery charging is relatively well documented, but I can't remember which way(s) SLAs are sensitive. I could look it up, but a quick google will probably do you just as well (probably find a schematic for it)

Solid Snake · Post by **Solid Snake** » Tue Oct 02, 2007 7:46 pm

I've looked into this. The snag is that powering from a 12V SLA is not a particularly good idea as you can't guarantee the voltage. The solution is to use two giving you an input voltage of 24V and one of the PicoPSU family that allows you to use 12-24V input. SLA is a pretty good battery choice for it as you aren't worried about weight.

Good thoughts. Any decent transformerless step-down DC-DC converter should be able to come up with the right voltages from a 24v line. I would think you'd need very low impedance devices and large capacitors on the 3.3v and 5v lines though. Two marine batteries in series will provide the right voltage and still be able to make use of most of their capacity unlike standard SLAs.

I wouldn't have current going through the batteries the whole time. Changes in power supply and demand will cause corresponding changes on the main DC line which will give your batteries grief over time. I'd have a main unregulated power supply somewhere in the 28-36v range powering the system's DC-DC converter. I'd use diodes to isolate the batteries from this higher voltage. This keeps the higher 28-36v from going into the battery pack and damaging it. When the AC power goes out, the unregulated output voltage will drop quickly and once it goes below 24v, current will flow out of the battery pack, through the diodes and into the converter keeping the PC alive. A 24v regulator on the mains line would help charge the batteries once the power comes back on.