Help with ideas for LiPo PSU (based on Daves PSU) - Page 1
Forum section: electronics / projects

author: PeterF
author_rank: Contributor
timestamp: October 07, 2012, 07:00:45 pm
content: I want a small, portable PSU, so I figured - why not power it from a LiPo?I want to be able to go from at least 0-15V, or at least up to 12V, and get about 3-5A out of it.Figured I'd base it on LT3080's , will be ripping alot of the design from Dave's PSU for that part (paralleling a few 3080s).The part I haven't really found a neat solution for yet is the charging of the LiPo. I haven't found a simple enough IC for charging 4-5 cells directly ( I don't want any switching, I don't mind powering the charging from a sufficient wall-plug).The following options are the ones I've come up with so far:* Run a 4-5 cell pack, design my own charger circuit, with constant current/voltage balancing the cells etc. Since I'll have a MCU and probably a separate 12bit ADC, making the charging precise enough can't be that hard I imagine? I'll have to be able to discharge each cell through the balancer-connector anyway, which would make sure that every cell stops at 4.2V with enough precision.* Run 4-5 separate single cells, each with a separate single-cell charging IC (such as the MCP73811).Disadvantage (besides the "ugliness" of running 4-5 separate cells) would be that while charging, the cells need to be disconnected from each other meaning you cannot use the PSU while charging.Any ideas or thoughts about charging the LiPo, or the idea of running from a LiPo in general?I thought I'd run my idea through you guys because I can't really find a neat solution for my LiPo charging in my PSU project.

author: yanir
author_rank: Regular Contributor
timestamp: October 07, 2012, 07:09:47 pm
content: I would recommend using an ic to handle the charging of the lipos. Mutli cell chargers are available. From past research there are devices that will manage charging your cells from an input voltage (usb or regulated wall plug 5VDC) and will handle all the switching for you. And they provide an output voltage rail for your application. I've seen maxim ics' for 1 and 2 cell charging. I don't know if there are parts that will charge 5 cells. Which may mean you need to develop the circuit your self. I would use caution however as lipos can explode if not charged properly.

author: muvideo
author_rank: Frequent Contributor
timestamp: October 07, 2012, 07:16:27 pm
content: Quote from: PeterF on October 07, 2012, 07:00:45 pmRun time, i.e. total capacity?Lifetime (how many cycles before changing battery)?Quote from: PeterF on October 07, 2012, 07:00:45 pmNot necessarly, if you power up the chargers isolated from each otherfor example from a multiple secondary transformer,you can charge the cells while they are still connected in series.Also Atmel has some battery management AVR.Fabio.

author: PeterF
author_rank: Contributor
timestamp: October 07, 2012, 07:24:44 pm
content: Thanks for your reply!I first thought that I would like to be able to charge the thing through USB or a wall-plug, but I ditched the USB charging idea now. Given the low maximum current from USB, and all the extra fuzz to switch up the voltage, I made the decision that I preferred a single 24V (or whatever value I deem neccesary) charge input instead.I haven't found any 4/5 cell chargers that werent SEPIC or the like, costed quite a bit, and in effect required quite many extra components and boardspace.In order to charge the LiPo safely you need to make sure that the maximum charge current doesn't exceed whatever specs your LiPo has, and then you want to settle at 4.2V with a certain accuracy. Individual cell balancing is easily managed by discharging each cell separately during charging, this is required even if I get a 5-cell charging IC.Can it really be that hard to manage this?* Limit charging current can be easily done with a constant current stage* Setting the voltage to 4.2V*5 with a certain accuracy. Haven't given this enough thought, but making a constant voltage stage with say 10 mV accuracy doesn't sound like rocket science to me?* Monitoring each cell with the ADC that I will use for other purposes as well, discharging individual cells as needed to maintain balance within a certain rangeHave I missed something?Edit:muvideo, thanks for your reply!I haven't decided how much capacity I want yet, depends on how big I want the thing to be.Ah, yes I guess there are ways to overcome that disadvantage, good idea! I will check out those AVRs, thanks!

author: PeterF
author_rank: Contributor
timestamp: October 07, 2012, 09:15:05 pm
content: Since cost is no issue to me, I might consider slapping an isolated dc/dc on each rail to the individual cell chargers.. This way I could charge through the balancer port of a standard 5 cell pack...Sent from my Lumia 800 using Board Express

author: TorqueRanger
author_rank: Frequent Contributor
timestamp: October 10, 2012, 01:08:07 am
content: What about using a cheap R/C lipo charger???

author: PeterF
author_rank: Contributor
timestamp: October 10, 2012, 08:23:47 am
content: well, I want everything built into the box.Plus, I want to learn as much as possible, so no shortcuts for me!Haven't had time to spend on this project any more yet, so the solution I'm considering is powering each charging IC with a separate isolating 5V DC/DC.