Battery backup for a 12VDC device - Page 1
Forum section: electronics / projects

author: tribat
author_rank: Contributor
timestamp: September 13, 2010, 08:43:50 pm
content: 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

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 13, 2010, 09:33:12 pm
content: 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.

author: TechGuy
author_rank: Regular Contributor
timestamp: September 13, 2010, 10:16:31 pm
content: What you need is a DC to DC converter. Something like this:http://www.mouser.com/ProductDetail/Cincon/CHB75-12S12/?qs=sGAEpiMZZMtwaiKVUtQsNZk%2fyhBUJKktPl0OLLVpKt0%3dDC/DC Converters & Regulators 37.5-75W 9-18V 12V 6.25AThis 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.

author: tribat
author_rank: Contributor
timestamp: September 14, 2010, 12:07:50 am
content: Quote from: TechGuy on September 13, 2010, 10:16:31 pmIt's even more expensive than PicoUPS =D

author: Zero999
author_rank: Super Contributor
timestamp: September 14, 2010, 07:00:15 am
content: 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.

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 14, 2010, 11:31:04 am
content: This is my Atom's PSU, cant tell the spec, i guess this is China made.

author: Zero999
author_rank: Super Contributor
timestamp: September 14, 2010, 12:56:46 pm
content: Attachments don't seem to be working, try tinypic or photobucket.

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 14, 2010, 03:19:29 pm
content: sorry. i keep getting database error just now while posting. seems ok now, here again the picture.btw, i'm not so masterfull enuf to design such thing. can someone do me a favor to tell what are the big coiled inductors sitting and doing there?

author: TechGuy
author_rank: Regular Contributor
timestamp: September 14, 2010, 05:19:21 pm
content: Quote from: shafri on September 14, 2010, 03:19:29 pmMost likely converting an input voltage into multiple lower output voltages (ie 5V, 3.3V, 1.8V, etc). Inductors are commonly used in DC to DC converters. They are also used to filter out noise as an LC filter. Its difficult to say exactly what each Inductor is doing in that Picture without seeing the circuit schematic

author: tribat
author_rank: Contributor
timestamp: September 14, 2010, 07:27:29 pm
content: I just found out that mini-box has a new PicoPSU variant called PicoPSU-WI. It has a variable input voltage of 12-25V and It costs around $50 in eBay.If it turns out the DC-DC converter I currently have just passes the 12VDC line trough, this new PicoPSU might be a good choice. It wouldn't be much more expensive than the PicoUPS and as a bonus it would free up the old converter, which is probably gonna come in handy on a PC test bench.

author: Zero999
author_rank: Super Contributor
timestamp: September 14, 2010, 07:34:46 pm
content: Quote from: tribat on September 14, 2010, 07:27:29 pmIdeally you want one that can boost lower voltages as well as step down higher voltages, ideally it should work down to 10V.QuoteYes, this is what you get with the cheapest 12V PSU you can get hold of.It's possible that the other outputs will give the correct voltages, even if the power supply voltage is slightly higher or lower than 12V. The 5% specification is probably for the 12V output. You might be able to connect the PSU directly to the battery, tap of the 12V output and pass it through a 10V to 15V to 12V DC-DC converter.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 15, 2010, 04:31:46 am
content: How about a 24V (2x12V) or 18V (3x6V) battery pack and a buck converter?

author: tribat
author_rank: Contributor
timestamp: September 15, 2010, 03:59:40 pm
content: Quote from: Hero999 on September 14, 2010, 07:34:46 pmVery good point, when using battery power the voltage will eventually drop well bellow 12V. Looking at the specs of an inverter, it uses the battery until the voltage is 10.3V, then it shuts down. So indeed a DC-DC converter that can work down to 10V would be needed. And also a circuit to switch the converter off at certain input voltage would be required?Quote from: Hero999 on September 14, 2010, 07:34:46 pmYea, that's also how I would expect it to work. I should really yank the thing out and do some testing with it but I don't like powering down my server, so I'm just gonna try email the seller and ask them about it first :pHacking the inverter and passing the 12VDC line trough a little DC-DC converter board sounds like a very interesting idea. It's sure gonna be cheaper than anything else :pLooking at the specs of my current converter it says 12V - 2A so Maybe this could work? :http://cgi.ebay.com/LM2596-DC-DC-Step-Down-Adjustable-Power-Supply-Module-/120606895831?pt=LH_DefaultDomain_0&hash=item1c14bb2ed7Quote from: NiHaoMike on September 15, 2010, 04:31:46 amThat would indeed allow you to use one of those new PicoPSU-WI's. But I don't have 2 SLA batteries or a 24V battery charger.

author: Zero999
author_rank: Super Contributor
timestamp: September 15, 2010, 05:23:46 pm
content: Quote from: tribat on September 15, 2010, 03:59:40 pmYes, an under voltage cut-out would be a good idea and is easy to implement with a voltage reference and a comparator.QuoteDo that if you like but I don't think they'll know. The chances are they'll just err on the side of caution and tell you not to exceed 12V by 5% and if the voltage is 5% below, it won't work properly. I think testing and examining the board components is the only way to go.QuoteUnfortunately that DC-DC converter is not suited to what you want because it's step-down only (buck) so won't regulate properly when the battery voltage drops below 12V plus the drop out voltage. You need a step-up/down converter (SEPIC or transformer) so the input voltage can be both lower and greater than the output voltage.

author: tribat
author_rank: Contributor
timestamp: September 15, 2010, 06:35:30 pm
content: Quote from: Hero999 on September 15, 2010, 05:23:46 pmSo maybe this would be more suited?http://cgi.ebay.com/3-5V-28V-1-25V-26V-DC-Automatic-Step-Up-Down-Board-/190443941626?pt=LH_DefaultDomain_0&hash=item2c5757eefa

author: Zero999
author_rank: Super Contributor
timestamp: September 15, 2010, 09:19:39 pm
content: That's better, it's probably a good idea to get a heat sink, in case you need the full 3A output current.

author: expitaxial
author_rank: Newbie
timestamp: September 22, 2010, 12:01:58 pm
content: Quote from: shafri on September 14, 2010, 03:19:29 pmcan someone do me a favor to tell what are the big coiled inductors sitting and doing there?Hi I'm new to this forum.. The PSU looks like four individual supply's on the one PC board. The ferrite 'coils' would be the 'magnetic storage' device in the switching supply.