Battery Power Supply - Page 1
Forum section: electronics / beginners

author: bgsteiner
author_rank: Contributor
timestamp: December 03, 2013, 03:25:42 am
content: So I am designing a battery power supply for my speaker project. I am looking to get a LiFePO4 20Ah battery like thishttp://www.batteryspace.com/High-Power-LFP-Polymer-Cell-3.2V-20Ah-100122200-2C-64Wh-40A-rate.aspxas the main battery.To power the unit I need 12v so I will need a boost converter to get the 3.2v up. Currently from my 12v source I also have a 5v rail from a buck converter to power a SD/USB reader.My current design will now include a raspberry pi as a media server and a TP-LINK TL-WR702N in bridge mode so I can have it link up to any wifi and be controlled by phone since android phones cant connect to ad-hoc networks.Overall I would say that the system should draw no more that 4 amps. 2A on 12v for the amp and I have to confirm the draw of the router and pi.So I guess what I am looking for is a boost circuit the rest (charging, control) I can handle Having 2 rails from the boost circuit or having two circuits is an option if It will help efficiency or increase battery life.

author: peter.mitchell
author_rank: Super Contributor
timestamp: December 03, 2013, 06:20:17 am
content: Boost converters are less efficient than buck converters due to higher switching current, additionally, they are pretty noisy so not very good for the audio, I would suggest NOT using a single 20AH cell ( a number of reasons, including but not limited to; awkward shape and size, difficulty to attach permanent terminals, expensive, will require multiple converters, fragile cell), instead use somethign similar to 4-8xhttp://www.batteryspace.com/lifepo426650cell32v3300mah19.8arate10whunapproved.aspx(4x will give about the same or slightly less life, 8 will give significantly more) the amplifier should be able to run off the unregulated battery voltage, run a 5v buck converter to the pi and the mini router.

author: bgsteiner
author_rank: Contributor
timestamp: December 03, 2013, 03:24:36 pm
content: What about balance charging LiFePO4 batteries. should I look into a charger that can balance a pack or not.

author: peter.mitchell
author_rank: Super Contributor
timestamp: December 05, 2013, 03:07:07 am
content: Quote from: bgsteiner on December 03, 2013, 03:24:36 pmIt depends, with a small number of cells in series (4), if they are quality cells, it is unlikely they will go out of balance with your prescribed usage. However, hobby balance chargers for small packs are cheap and do a sufficient job, so i see little reason why not.Also, mind linking the amp you wish to use?

author: bgsteiner
author_rank: Contributor
timestamp: December 12, 2013, 05:04:48 am
content: Quote from: peter.mitchell on December 05, 2013, 03:07:07 amOk thanks, and here is the amp.http://www.parts-express.com/dayton-audio-dta-2-class-t-digital-audio-amplifier-module--300-385Sorry it took me a while work has been having me on 12 hour days and i just want to sleep at 11pm

author: minime72706
author_rank: Frequent Contributor
timestamp: December 12, 2013, 03:50:43 pm
content: Dayton makes decent stuff. My ex boyfriend was one of those awful audiophile types and even he approved of the quality of their cheap drivers.

author: peter.mitchell
author_rank: Super Contributor
timestamp: December 12, 2013, 05:14:24 pm
content: Quote from: bgsteiner on December 12, 2013, 05:04:48 amNo worries;The amp uses this chip;http://audiolabga.com/pdf/TA2024.pdfThe ABSOLUTE MAXIMUM input voltage is 16v, and the maximum recommended voltage is 13.2v, so technically 14.4v (which will drop to 9.6~ when flat) from 4x LiFePO4 cells shouldn't hurt it, but i wouldn't normally recommend it. HOWEVER, at that price, i'd give it a whirl, you don't really have much to lose.Using the unregulated output of 3 cells wouldn't be sufficient and would require a boost converter, so i'd try with 4.

author: bgsteiner
author_rank: Contributor
timestamp: December 13, 2013, 12:03:12 am
content: Quote from: peter.mitchell on December 12, 2013, 05:14:24 pmNy plan is to get four and put a pcm on it to regulate the voltage and current during discharge and charging.

author: peter.mitchell
author_rank: Super Contributor
timestamp: December 13, 2013, 05:51:31 am
content: Quote from: bgsteiner on December 13, 2013, 12:03:12 amWhat protection module specifically?Protection modules are usually designed as a failsafe, not as a normal means for charge/discharge management (eg, you don't turn your TV off at the breaker box when you're done, you put it in standby, or you turn it off at the wall maybe).In commercial devices, there are usually multiple layers of protection, for discharging; software to put device into standby/shutdown, then if the device continues to draw power beyond a charge threshold there is a charge controller to disconnect the load, then over time the battery will self discharge and also discharge slightly from the charge controllers circuitry and other attached leakage, this is where the protection module cuts off everything from the cell.For charging; there is usually some software to set the charge rate in the charge controller, then the charge controller charges the battery according to its specs (at the current set by software, with failsafe hardware limit, to a specified voltage according to spec), if the battery continues charging beyond that voltage, there is a protection module to cut the cell off from the charge controller.For this application, that is not necessary though. First off, the software level can simply be thrown out. Secondly, charge controller chips (as far as i have found) rarely ever support cell balancing/equilibrium which, i think you will need, so I think they're out. Then there is the PCM, the PCM doesn't really offer what you need all in one product, but it sure is nifty, so it's a maybe.I'd get a "hobby" battery charger, like this one:Here. It will charge and balance your batteries safely, but it does require a 11-17v input.I'd get a PCM that suits your needs.I'd also build a small "primary" low voltage cutoff circuit myself, this would be the primary low voltage cut off; it would allow you to get good cycle life out of your batteries, which you wouldn't if you used the PCM as your primary low voltage cut off.Also, a PCM doesn't regulate the output voltage of a battery pack in the typical sense, so the output voltage of the PCM should be equal to the voltage of the battery throughout its charge life (2.7~v flat to 3.6~v full charge), which would be 10.8~v flat and 14.4~v full charge.The voltage of the cell looks something like this;So you'll need a 5V 2A~ buck converter for the pi, the tplink (and maybe phone?).

author: owl_uk
author_rank: Contributor
timestamp: December 13, 2013, 09:48:15 am
content: QuoteI'm not sure this adds up. Lets do the maths. I looked at the amplifier datasheet and it can drive into a 4 ohm driver. We will assume you like listening to sine waves, because it makes the sums eaiser.Your amplifier chip has an h-bridge type output thing (someone who knows the right terminology will probably correct me). My point is that it can connect either end of the speaker to vcc or ground, so it can drive up the page on the datasheet when it wants to make a positive voltage in the speaker coil, and down the page on the datasheet when it wants to make a negative voltage. So by this magic you have 24 volts peak to peak. Which is 8.48 volts RMSr= (v*v)/r= 25.4 watts on each speaker= 50.8 watts totalThat gives you just over 4 amps on your high voltage side and 15 amps on your battery side. And all of that is assuming 100% efficiency in the amplifier and switching, which is not going to happen.Your speaker project sounds like quite a monster

author: peter.mitchell
author_rank: Super Contributor
timestamp: December 13, 2013, 11:37:08 am
content: Quote from: owl_uk on December 13, 2013, 09:48:15 amYes, the math clearly does not add up, however; I find it unlikely he will listen to a sine wave at full volume continuously for a given time period, personally, I have made a few similar projects using a TPA3116D2, and for loud but still comfortable listening, in an enclosed area, it only used ~25w total. However if it is for use at say... a picnic or the like, i'd highly suggest more batteries (maybe 4s3p).