micro UPS

mctrivia

I want to power a bunch of 12V devices from both the mains(120V AC) and a SLA battery(12V) I found the following 2 products which would work but is very expensive.

http://www.mini-box.com/DCDC-USB
http://www.mini-box.com/picoUPS-100-12V-DC-micro-UPS-system-battery-backup-system?sc=8&category=980

This got me thinking though it should not be hard to do if I use a 24V transformer as my starting point.

Anyways my idea was to use a LTC3703EGN#PBF to regulate the output voltage to 12V @5A but was wondering what the cheapest way to safely charge a 12V SLA off a 24V transformer would be. It does not need to charge quick since it will not get used often but the battery does need to last a long time.

Any ideas?

Franklin

SLA batteries are pretty forgiving if not over stressed so I'd say 14.4v @ 2A should work. You could build a charge regulator with a stamp (or less) to make sure the battery charge shuts down and starts up when needed. You don't say what the battery specs are but if you look on the internet you can find plemty of charging circuits and probably UPS circuits too.

mctrivia

the reason no battery specs have been given is I have not purchased the battery yet.

I think I may have come up with a really simple setup.

5 to 1 center tapped transformer with fullwave rectifier
or
10 to 1 transformer with bridge rectifier

will give me 15.6V peak.

Add an extra diode in series with a 200 ohm 2W resister to charge the battery with a shotkey backwards to allow high current discharge.

This would give a charging voltage of 14.9V peak and charge current will be 75mA max and drop down as the battery charges.

Simple and cheap. Question is will the battery take a constant trickle charge of 14.9V @ 15mA. And would the battery go from 0% to 100% in 30 days? Assuming a 10Ah battery.

Tracy Allen

A lot depends on the amp-hour capacity of the battery. For small SLA batteries, there is the PB137, a "bulletproof" battery charger IC available from Mouser for $1.30. It is a linear charger and requires only capacitors at input and output, like any other voltage regulator. It regulates the voltage at 13.7 V, which is perfect for float charging a 12V SLA battery at up to 1 A (heat sink required). Take the power for your devices directly in parallel with the battery, through a fuse for safety. You're transformer scheme to supply about 16 V at the input to the regulator would be perfect.

Batteries for backup should be charged from a voltage source at the float voltage, 13.7 ± 0.2 V at room temperature. Do you really need 5A of current to run the devices? Then make a 13.7 V regulated supply capable of supplying that current. At 13.7 V a good SLA battery will regulate its own charging current. Be sure the battery current can not flow backward into the charger if the main supply is disconnected.

You can buy "smart chargers" for SLA batteries, but they are mostly not designed to both charge a battery and run a load at the same time.

mctrivia

Thanks I will look into the chip. The charging circuit will not be providing power to the loads. Will be either mains or battery. I will have a regulator after the battery/mains combining point to make sure output is 12v no matter what the input voltage is.

wrightme

If you want simple and cheap, run your 12V devices directly from the battery, and charge the battery. As mentioned, if you are running all your devices at the same time, spec your charger to handle that load.


Mains to charger.
Charger to battery.
Battery output to regulator.
Regulator output to device bus.

mctrivia

Then you only get the first 20% of battery and waist a lot more going through the regulator. If I put reg after I can draw battery down to 7v

wrightme

Um, in my list, I did put the regulator between your battery and your load. That is what you described earlier.

In my post, the flow is from mains through each item to the load. When disconnected from mains, the battery powers the regulator, which powers your load.
If you want simple and cheap, that is what I see fitting that specification.

mctrivia

With the high unpredictable current needed by load inline does not make sense. Besides I only need a diode to combine the 2 sources without having the normal operating power run through the charger.

Tracy Allen

Never run a 12V SLA battery down to 7 V! Cutoff should be no less than 10.5V. That covers 90% of capacity.

I think you can do it either way. One beefy 13.7V supply for both charging and load, or a small 13.7 supply for backup battery charging, plus a second beefy supply for the main load. To use diodes to gate the supplies to the regulator-->load, the main supply will have to be greater than 13.7V. The diode "chooses" the higher supply.

mctrivia

Yes I know dropping below 10v is bad for the battery but this battery should never run in a perfect world. Still I should build a cut out at 10.5v

The difference is if I regulate before to floating voltage then output voltage will be greater then 12v while on mains and below 12v when on battery. Neither of these are what I want.

If I put reg after output will always be 12v. When on mains it will reduce voltage from 16v source when on battery it will increase voltage from battery.