DIY battery pack Li-polymer 14.8v 8ah

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krupted

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for $50!

just to make sure, if i wired those two in series to get the 14.8v, then two more of those in parallel for 8ah, that pack would be chargeable with something like this, right?- http://www.batteryspace.com/smartch...polymerrechargeablebatterypack--ullisted.aspx

just making sure it will be fairly safe. i know theyre probably not-great batteries, but i need to feed about 2a @ 12v for as long as i can so i figured this might be the best route, especially since weight is a major concern. and size.

its for a diy portable windows boombox... will draw up to 4a at full volume, full system load, but mainly i think itll be around 2a on average (atom ion setup). nimh would be the better/safer route, but the price on these polymer packs is good enough that i think theyre worth considering. i would imagine around a 6 hour runtime at moderate listening levels... thats what im aiming for anyway. i dont care if it takes 5 hours to charge either, if rather have the charging safe then fast.
 
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Li-Ion ussually needs some sore of control circuits as far as I am aware. If they are built into the battery packs already then you are probably ok.
 
Discharging them in a series/parallel combination is fine. I would strongly recommend that you charge each battery individually, or at least in series pairs, however. These batteries should have the necessary protection circuitry (should...this *is* ebay) so that they won't burst into flame and burn your house down, but lithium chemistry batteries are quite finicky about charging and shouldn't be charged in parallel unless the whole pack is specifically designed to be used that way (tighter voltage tolerances, often cell-balancing circuitry). It's not quite as dire as the charging situation, but you should also try to avoid over-discharging the batteries as well, this can damage them permanently. Some batteries (and hopefully these) will have a low voltage cutoff, but if they don't it would be a good idea to add one yourself.

There's no datasheet on those batteries so I would be especially careful about these things. Batteries of this type should have the necessary protection circuitry, but no datasheet or details is provided and those extra two wires hanging off in the picture are suspicious - possibly an unconnected temperature sensor? The best idea would probably be to add some protection boards, like the ones here: http://www.batteryjunction.com/pcb.html, but you'd need to break the batteries apart to get at the individual cells, so you'd see if a similar circuit were included anyway. If you use this circuit it should be *safe* to charge them in parallel, but the charger may stop early and not fully charge both. With an unknown protection circuit I probably wouldn't try it.

Do not underestimate the danger of lithium chemistry batteries. They can, and will, start nasty, violent fires if charged or discharged improperly. Use them in the recommended way and be very careful with your wiring, shorting these batteries without a protection circuit will start a nasty fire surprisingly quickly.

Offtopic, but do you really *need* Windows? You could probably half your current power budget (and double your runtime) by going with Linux on something like a BeagleBoard. Easy, cheap, capable, much simpler (less wiring, much smaller, simpler power requirements, etc.) than a full Windows PC, and there's lots of documentation. For what it sounds like your goals are, it would be perfect. Add a 5V DC-DC regulator, USB hub, whatever display you were planning, and SDHC cards or USB HD's for storage.
 
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its gotta run windows. my real goal is to replace my laptop. i bought a 9" usb monitor, and a touchscreen kit for it too. the monitor runs off 500ma, and im going to hinge a small keyboard to it, hard to explain but basically the end result will be pretty much like a netbook, except that its tethered to the toolbox by one usb cable (and the guts of course will all be in the toolbox.) i figure this cord wont really get in the way, i can always stick the screen back inside the toolbox for portability, and the best part is 50w of car stereo audio :D laptop speakers have pretty much done this to me alone... i like to take my laptop and play music with it in different places but the sound just doesnt cut it. also, the screen is very lightweight, even with the keyboard attatched it will be much lighter then any netbook, only minor catch will be the usb cord.

im well aware of what li-ion batteries can do, thats why i was originally going to go nimh.... but for the same weight you can pretty much double your capacity, and if i want anything close to 6 hrs runtime ill need li-ion. its also why i posted to ask a few questions, because i know this is potentially dangerous.

i might just spring for a balancing charger, maybe if nothing else just to charge the 8-cell series/parrallel pack the first couple times to even out the cells. then use the regular "smart" (it seems more dummy then anything) for daily use. i wouldnt mind using the digital balancing charger all the time but they doesnt appear to be 'plug and forget', as i would need. http://www.batteryspace.com/intelli...e64v-192vsla2v-20vbatterypackwoacadaptor.aspx
 
heres another one- http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=6443 but i think youre right keenan, these chargers dont seem to claim anything other then series-only packs. i might just have to make a few smaller packs i guess

you know what screw it. i want to see if i can make an 8cell work the way i want. i see in that link, keenan, that you can get 14.8v pcb protection chips for 4 cell packs. ill just make two of those and wire then in parallel, using that $30 "smart" charger to charge the whole pack. i imagine itll still act like two seperate chargers. forget about balancing the cells, i dont think itll be an issue at all to be honest.

parts ordered.
4x7.4v lipo battery packs (will be taken apart for rebuilding) - $52
2x14.8v protectice pcb circuits - $12
fuel guage- $7
ul listed smart charger- $29

totaled ~$105 shipped (battery junction had a warehouse fire, they are currently suspended for business!). but if you dont include the charger, thats just $70 for a 8ah 14.4v battery pack with LED life guage. its not a crazy good savings, but similar packs do sell for around $130 so the savings is there.... the batteries are coming from china so i have time to check all the fire extinguishers in my house.
 
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Yeah, you can never be sure about people on the Internet, so I'm always careful to warn people playing with LiIon batteries - glad you did your research first ;). It's actually a bit scary how many unprotected lithium rechargeables are sold as finished products, really dangerous if people go out and expect them to work like their good 'ol lead acids, but anything to undercut the competition...

Looks like a good plan you've got there. I'd probably still prefer to charge the batteries individually, but depending on your construction plans that might not be possible (or convenient). You should get close to a full charge in both batteries but the charger will probably cut out early, depending on how close the batteries are in characteristics. If you want maximum runtime out of that kind of setup what you might do when constructing your packs is charge all the cells you've got, discharge them for a few minutes to get them down to working voltage and then measure and bin them and try to balance the batteries as closely as possible to get close to ideal even discharge/charge between the parallel strings. Not necessary though...

Another suggestion in the battery life department would be to use a class D amplifier - most car decks are just standard linear class AB and will be quite inefficient (though that might've changed in the past few years - class D is everywhere now) - the additional efficiency should buy you another 5-10W in power budget. Easy to use modules are available on eBay - seller sureelectronics is popular and I've bought stuff before - e.g. http://cgi.ebay.com/2-25W-4-TPA3123...emQQptZLH_DefaultDomain_0?hash=item3a578e25ba would be perfect. Too bad about Windows :(
 
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yeah youre right there is nothing wrong with warnings... thousands of people will be reading this thread (its already first result on google if your search 14.8v 8ah. i guess it is a bit of an odd amperage rating though)

i was going to use a t-amp, but that d amp looks nice especially because of the price. i guess sound quality wont matter much.. anything is going to kill my laptop speakers (and theyre altec lansings... great for voice but anything else, its clearly a laptop). i plan on using JL audio 5 1/4" coaxials.... theyre 30w rms so they should be perfect.

does linux, or whatever type of os youre talking about, have a touchscreen music player like albumplayer? not for this project, but i have a VIA 800mhz mITX computer thats currently running XP just for album player, but its a little slow for my taste. it does work well though.

also, i think i might charge and cycle each cell a couple times individually just to check for bad cells. i imagine the quality control on these packs is basically nill.
 
Sure electronics sells some Tripath amps as well. As far as I can tell, class-T is basically the same as class-D, it's just a Tripath trademark. Either way, take a look at the Sure offerings, even if you wanted a T-amp they have some of those as well. Also if you're after SQ check out diyaudio.com and search for sure electronics there, there are threads for most of the amps with a bunch of mods you can do to fix design flaws or soup them up for sound quality. For the price, SQ on at least the Tripath models is pretty much unbeatable and with a few bucks put into simple mods, they can be very good.

I haven't seen Albumplayer in action, but it does look pretty slick. I'd suggest you take a look at gmpc + mpd as a Linux alternative. Turn up the system font sizes a bit and it looks like it similar display modes, though you might find it's not quite as good.

Edit: All you need for volume control is a log pot (10K or 47K is usually suitable), I believe Sure provides a schematic - or you could just use the PC volume control, but it may not be sensitive enough. A headphone voulme control will probably be the wrong impedance to work properly with an amp.
 
gotcha. i think ill just buy an amp with a nice volume knob already on it, that way i just punch a nice round hole in the toolbox and mount the amp so the knob sticks through a little. will save time and make things easy to work on (interchangeable components is kinda what i was shooting for anyway, as thats the other thing i dont like about laptops- you cant upgrade them later on).

overall, the fab skills required for this project should be very low. punch a few holes, cut out a pair of 5" circles and rivet in some mounts inside for all the parts. this is the toolbox i plan on using- if i can find a 16" model or even a 12" i would rather do that though. smaller the better!

http://www.waterlooimages.com/CategoryResults.cfm?div=32&cat=940&cat_parent=1&pack=4870
ImageDisplay.cfm


all that will be visible on the outside will be the two speakers... and im debating on punching out a bunch of holes instead of cutting out the whole speaker. depends on how nice i can keep the paint job i think... but when closed up, its secured nice and there should be enough air volume in there to make the speakers have some nice full sound. hopefully.
 
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Little red box of fun, I like it. Though it looks too small to cram what you're planning in there! Fun little project, good luck with it.

You should put some internal baffles to isolate the two speaker's air volumes, if you hadn't considered that already. And seal each compartment as well as possible. Sound quality might also benefit from some dampening. That thin sheet metal is probably going to resonate quite a lot.
 
i thought it was too big! i found this (though i havent even looked locally yet, i know ill find something at farm and fleet i can use). mITX boards will sit flat in the bottom of these though, at least it appears so by the measurments

http://www.canadiantire.ca/AST/brow...6 Mastercraft+Maximum+Tool+Cabinet++56-In.jsp - this one is better then the waterloo because the center latch... itll allow the speakers to be spaced out much better.

i was going to work on sound quality once i got most of it together (because it will be harder to just get it all to fit, sq is not the most important) but as far as the chamber, i planned on modifying that tool tray that sits int here to seal the top off once you put it in place. i can use dynamat to line the insides (i agree, it will sound tinny if i dont) but i dont think ill have room to seperate each speaker. i dont think that will matter much anyway.
 
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Hehe, I didn't really look at the measurements, it just *looks* really small. I was thinking it wouldn't be too difficult to stack the electronics and batteries in the centre and leave the edges for the speakers. What size speakers are you planning?

I think isolating the speakers if possible will be worth it. Even if it's just a stiff piece of card glued in to give it its own airspace. Putting two drivers in the same un-engineered enclosure will set up interference between the drivers that will upset sound quality. I'm not sure how severe it'll be, but you'll definitely see frequency nulls formed from driver crosstalk. But then it seems to work okay in the trunk of a car, but a car isn't exactly an optimal audio environment to begin with.
 
yeah a tunk is a lot bigger airspace then what ill be working with too. but i also dont want to lose any airspace either by creating separate chambers... maybe i can put a divider right in the middle, and then in one speaker chamber can reside the motherboard, harddrive and power supply... and the other speaker chamber can house the lipo battery pack. as long as the weight distribution is close it should be feel OK to carry around.

if this turns out like im planning, i could make the same thing but with an h55 chipset and i5 processor, with some decent power drawing video card. a portable gaming box like this would be killer! even if battery life would be about 0.5hrs :D

these are the speakers i think ill go with... http://www.sonicelectronix.com/item_20033_Pioneer+TS-G1343R.html most 5 1/4" speakers are about 30w rms or more, and these are rated 25w. furthermore, pioneer is always very efficient in their speaker designs... i would really like to put some polk's in there but from my experience you gotta feed them more power then you would for a similar pair of pioneer.
 
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Well, that's why there's an efficiency specification :p

I wouldn't worry too much about power handling, unless you're using a car amplifier or some other amp with a dc-dc boost converter to get rails higher than 14.8V. At that voltage with a 4 ohm speaker a square wave is 57W, a pure sine is only 27.3W, program material will be less - and any amp at that level would be totally unlistenable, the distortion would be gross. Even a 15W speaker should be perfectly fine as long as you don't drive it deep into clipping; that's about the most clean power you can expect out of a system with a limited rail voltage like that.
 
youre a lot smarter then i am though, ive been through trade schools because i didnt want to go to college :D

ill keep looking for speakers up until the box is nearly done. something that sounds nice and deep at low volumes and still can get "freakin" loud is what i want. its party supposed to be a jobsite radio... when the lid is closed... to hide the porn on the monitor.... ..... i see what you mean though by speaker power. i didnt think about how these little tamps work, but i get it now. partly why theyre power efficient i guess. but that is why i like those pioneers, because theyre only designed to handle 25w, and i have always noticed to get a budget speaker to really "sing" you have to give them close to their rated power... if you grossly oversize the speaker it just doesnt rock the same. i figured 15w was about what each of these speakers would actually be seeing at a hefty volume, so i really dont want a 35w rms speaker... but in 5" size its hard to find a low-wattage coil, unless you buy a shit brand. i really should go 4", but then i lose bass
 
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sure enough, farm n fleet had the 16" waterloo, and it was on sale this week for $17, go figure.
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im measuring somewhere between 1/16th and 1/8th inch clearance for a 6.75" mITX motherboard to lay flat in the bottom. so this project has officially taken off in grand fashion (which i like!). it has to be 4" speakers too, 5" is not going to fit right. there is a pair of focal 4" on sonicelectronix for $100... very tempting. the only part i wish this toolbox had was the humped lid, but thats just because they look a little more classic.

everything is going to fit easy though. im a little surprised how perfect the dimensions of all this is... all i need is a 3.75" metal hole saw, and a good day's worth of patience. oh yeah, and about $600 :D but this contraption is supposed to replace my tx2z tablet pc, so hopefully selling that will offset the cost of this project completely.
 
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There shouldn't really be a relationship between the power handling rating and anything else, usually if speakers sound poor in a certain system but better in others it's because the amp's underpowered, not because of the drivers. But if you've found that the real world disagrees, I'll defer to your experience, I haven't worked with these type of small coaxial speakers aside from the 2 5-1/4" Infinity's in my car doors :p.

Might take a look at what Parts Express has on offer for drivers in your size/price range (you obviously need 4 ohm ones with such a weak amp though). Might be something more efficient or more cost-efficient. They're the go-to shop for a lot of folks building stuff like you are.
 
ive been ordering from those guys forever. my early 90's pioneer HTiB, which is more like a real component home theater then what you see today, has been replaced with speakers from parts express... $200 for cones and line filters and i have amazing sound.

i could do 4x8ohm but i dont see any reason to, it would look ugly. i prefer less speaker in higher quality anyway.

i think were talking the same thing about amp power... i dont mean for it to matter a whole lot, but i generally find speakers like these to sound best when they are fed ample watts. only certain brands retain their full sound with very little wattage on them (like pioneer, and infinity/jbl/klipsch are like that too). rockford fosgate is another good choice, but their drivers have a very distinct sound to them and i generally prefer a more neutral output like pioneers always have.

edit- these are my newest favorites. i didnt see these earlier, theyre 4 ohm full range and only 20w, so they should match up perfectly. from the user reviews on the 8ohm version, these drivers have a long throw for their size and produce really good lows... http://www.parts-express.com/pe/pshowdetl.cfm?&DID=7&Partnumber=290-208&ctab=1#Tabs theyre also a lighter weight then others i was looking for, will be easier to shield if i need to
290-208_s.jpg

a guy in the reviews posted avid showing this little thing on a t-amp looking like the subwoofer in my car hahaha- http://www.youtube.com/watch?v=QybUWAXDz9c i think these will be the perfect drivers.

i have this amp coming too, found from the diyaudio forums- http://www.41hz.com/shop/item.asp?itemid=43
showgraph.asp

will do 15w @ 4ohms cleanly off 12v (<0.1%THD), and a max of 25w at 10%THD for when im feelin it.
 
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I built 41hz's 100W kit a couple years ago. Great sounding amp, but the build instructions left quite a bit to be desired. I knew enough to figure it all out easily enough, but it took some poking around on my own to get it working right. Great stuff.
 
yeah, i blindly ordered the kit last night, then realized you can buy them pre-assembled. i emailed them, hopefully they cancel the first order. or i just get two amps... oh well! :)
 
i was aware! i was going to research the volume thing later. the email i sent them about the first order bounced back, i just might end up with two amps. hopefully not, because even though i could figure out the kit assembly im not so sure i can physically get it all together proper. i learned my lesson the last 5 times i tried to build a kit- i just dont have the shop, time and patience for it. my hands are big and a lot of other excuses.
 
i have a question about incorporating both a/c power and battery power...

instead of building a "fancy" ups circuit, would it be feasible to just use a relay and put a large capacitor on the input for my dc-dc converter? im thinking it would be a safe route sine when the device is plugged into the wall the battery will be completely physically disconnected... which will allow the li-ion charger to work properly. best i can figure im always going to need a diode and that will lose me a good 1.6v at the worst, but the dc-dc converter only needs 12v to operate efficiently and when needed it continues to work down to 6v... even though the li-ion battery cuts out at 11v, and on top of that, the dc-dc converter automatically sends a shutdown signal to the atx pico psu when a certain voltage is reached, which can be say 11.5v. that would give it 3 layers of protection for the battery....

mechanicalups.png
 
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You don't really need the relay, just put a diode on both input sources and connect them together after (diode-OR) and make sure the AC adapter outputs a higher voltage than a fully charged battery. You might add a small cap at the junction to minimize any switching transients, but most diodes should be fast enough. Use schottkys if you're worried about the voltage drop, shouldn't be hard to find them at the current rating you need.
 
ok i see what youre sayin... just use at least an 18v ac/dc adapter then.....you sure thats still safe for a li-ion battery though? there must still be some sort of reverse current going through a diode, and this radio will likely be plugged in most of the time- likely for months on end... i really dont want any sort of trickle charge or drain to happen on the battery, for safety reasons. which is also why i wanted a 15v or close to it input, so it doesnt cause any weird voltage spikes on the battery... and i know the reverse current is in the uA's, i just dont know if that can make a difference on the battery if left like that for weeks...
 
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It'll be fine. The leakage current should be less than the self-discharge of the batteries. The spec is typically made at the maximum continuous reverse voltage, and goes down from there. I'd expect most suitable diodes should have a reverse current at your working voltage (which will only be a couple volts) of well under a uA. Taking a look at MUR820, the spec only goes down to 20V, and at that reverse voltage it looks like the reverse current is 0.05uA. Seems small enough that I wouldn't be concerned.

If you're worried about it you can stick with the relay. With a simple comparator circuit you could make it switch automatically.
 
keenan said:
The leakage current should be less than the self-discharge of the batteries.
Click to expand...

this is exactly what i was thinking/hoping... after looking through mouser too i see they do make some very efficient schottky 10a diodes, seemingly made for this purpose. 50 cents a piece too, instead of the $10 it would cost to use the relay with ultracapacitors. after doing more research too, it seems like most people just buy good diodes and thats it.

so i thank you for saving me some money and making it a little easier!
 
received two basic-6 amps today, i guess they never got my email asking to cancel the first order. so one is already assembled, and if i feel like ruining the other one someday ill use the pre-assembled one for guidance. it looks like something im capable of doing, but that tripath chip does not look easy to solder in.
 
batteries are finally in! and pci wifi

IMG_1866.JPG


those arent temp sensors- theyre actually just balancing leads... meaning i dont even have to take these packs apart to make a pack, i can just solder those leads right to the pcb's!

and the batteries are THIN. 3/8" at best. there is no question now- i will be buying 4 more and doubling my amp-hours. for sure! i can see now, that i could actually put all 8 pre-made packs in the lid of the toolbox- leaving the entire left side of the toolbox open. i dont know if ill actually do that, but my options just got wide open.... if the batteries do go in the lid, the power circuitry can fit on the left side- which would leave a ton of airspace on both sides for the speakers to work properly....
 
Got a sketch of the proposed final build, or are you figuring it out as you go along?
 
Izza said:
Got a sketch of the proposed final build, or are you figuring it out as you go along?
Click to expand...

you mean... like... a schematic? a symbol diagram? im a residential electrian, we dont use no farkin drawings:D:D

i suppose i could make a sketch, but ideas are flowing faster then any pencil could right now. just last night i realized i can arrange for enough room to add FM/AM/HD radio to this, so it will be much more of a proper 'boombox'. besides, from the outside its just going to look like a waterloo toolbox with a speaker grill on the front, and a wrapped up a/c cord on the back. i am attempting to not even drill any mounting holes in the exterior- i want to use the pre-existing rivet spots as my only anchor points... if i can...

i promise the final project will be kick ass. originally i thought this was going to be a slap-together project, but it costs too much and im too involved now. time to start looking for color matching red paint ;)
 
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