My DIY USB external battery pack

[yorker]

How about using a Altoids tin can to hold the batteries?

[raspabalsa]

Ah, yes, I've seen many DIYs with those cans. Trouble is, I haven't seen Altoids mints sold in my country . The problem with the tin cans show in your pic is that the corners are too rounded, meaning you're wasting a lot of space.

But it's a good idea, I think there are some other products that come in similar tin cans. And a metal can has the advantage that it can withstand a lot more harsh treatment and better dissipates heat. I'll have to look into metal cases for the next implementation. Thanks, yorker.

I've seen some "project boxes" at the local Radio Shack. But the ones I've seen are too squarish, and I need an oblong box, to avoid wasting inner space. In this sense the plastic case I have is ideal, so I'll try to find a similar, slightly larger one to fit the extra components (LM317's resistors, SOC circuit, etc).

[yorker]

Ah, yes, I've seen many DIYs with those cans. Trouble is, I haven't seen Altoids mints sold in my country . The problem with the tin cans show in your pic is that the corners are too rounded, meaning you're wasting a lot of space.

But it's a good idea, I think there are some other products that come in similar tin cans. And a metal can has the advantage that it can withstand a lot more harsh treatment and better dissipates heat. I'll have to look into metal cases for the next implementation. Thanks, yorker.

I've seen some "project boxes" at the local Radio Shack. But the ones I've seen are too squarish, and I need an oblong box, to avoid wasting inner space. In this sense the plastic case I have is ideal, so I'll try to find a similar, slightly larger one to fit the extra components (LM317's resistors, SOC circuit, etc).

Some RC cars have large battery compartments, you could try those. Radioshack is certainly the best place to go!

[Ashton]

I dont suppose you'd consider selling completed circuit boards? I'd *LOVE* one of these for my Axim (which EATS batteries whole in <2 hours ) but I've never used a soldering iron and have no clue what I'm doing... I also have a laptop I may be selling soon that has a spare battery that I coudl keep and use for this.... (Dell Inspiron 8100, if anyone knows the capacity of the battery off-hand)

[raspabalsa]

Sorry, I'm not in for a mass production thing. I don't have the resources, supply of parts or time for this. Besides, the charger as it is is right now is a bit on the crude side: it dissipates a significant amount of power, it does not have all the safeguards that are required of Li-Ions, and does not have a state of charge indicator yet. All these things will come later, but still I have no plans to produce more than I will use.

But I'd happily help you if you do want to try somtehing like this. This is why I'm posting as much info as possible in this thread, so anyone can use this basic design and tailor it for any particular needs. Using a soldering iron is pretty much like riding a bicycle: once you burn your fingers for the first time you'll never forget to check which end of the iron you're attempting to grab ... ok, this has no relation to bikes, but what I mean is that assembly is not that difficult, all components are relatively large and easy to handle. And I'm sure you'll have the time of your life assembling one of these monsters yourself.

If you have a spare laptop battery then you already have the most important and costly component: the Li-Ion cells. I don't know what cells are used in the Inspiron 8100, but you can check the battery decal for specs. There should be the voltage (V or VDC) and the capacity (mAH or AH), from that it's easy to figure out the capacity of each individual cell. For example, the original HP battery I used was rated at 11.1V, 3000mAH. From the voltage you can deduce how many cells are connected in series, since each Li-Ion is rated at 3.7V. Thus, 3.7V x 3 = 11.1V, so the pack has three cells connected in series. As for the capacity, 3000mAH most probably means two 1500mAH cells connected in parallel, so the final pack is made up of six cells, that is, cells connected in parallel in pairs, and each pair connected in series to the next. From this you can figure what's the ideal arrangement for your DIY charger. In my case I arranged the cells connecting three of them in parallel, and the two sets of three cells in series. This gives me 3.7V x 2 = 7.4V, and 1500mAH x 3 = 4500mAH total rating.

[Ashton]

I just checked the battery (which, as per the on-battery indicator, sadly is 100% drained, so probably if I keep doing this wont be useable... btw, can this be left in to display charge in this project?) and it says "14.8V" and a line with dots under it then "3800mAh" it also says "charging current 3.5A"

[raspabalsa]

I just checked the battery (which, as per the on-battery indicator, sadly is 100% drained, so probably if I keep doing this wont be useable...

Probably, but not likely, unless you've left it drained for a long time. Li-Ions like to be kept in a charged state, leaving them fully drained reduces useable lifetime. How long has it been since you last used this battery? In any case, what the indicator shows probably translates to a lot more power than "drained". Most batteries are designed to show "discharged" when in fact there's still a lot of juice in them. This is a safeguard to protect users' data. In any case, if you plan to use this battery, be sure to charge it, this way it's health won't suffer.

btw, can this be left in to display charge in this project?)

Depends... I'm not sure. I made the foolish mistake of discarding my battery's indicator, and now I don't have any to check its thresholds. Your indicator could be used, but it will be necessary to analyse it and identify components and layout.

and it says "14.8V" and a line with dots under it then "3800mAh" it also says "charging current 3.5A"

Let's see.... 14.8V equals 3.7V x 4, meaning that there are four cells or four sets of cells connected in series. 3800mAH probably means 2 x 1900mAH (likely cell capacity). So this means that your laptop battery ismade up of eight Li-Ion cells. Each cell rated at 3.7V, 1900mAH. Cells connected in pairs in parallel, then the four pairs connected in series. Or, four cells connected in series, then the two sets of four cells connected in parallel (easier to wire this way). Remember that in series connection the voltage is added and the capacity (or current) remains unchanged. In parallel connection the capacity (or current) are added, while voltage remains unchanged. So your battery looks more or less like the first schematic in the attached picture. The second schematic is how I'd connect them forthis project. You'll get a huge 7600mAH capacity <uncontrollable drooling>

EDIT: Forgot to say, the "line with dots" after the 14.8V means it's DC voltage. AC voltage is symbolized with a "~" symbol.

[Ashton]

*jaw hits the gound* sssssevsevsevseventy-six hundred!!!??!!!

geeze, even if I dont sell this laptop it's worth buying an aftermarket battery for $25 off ebay to make this thing! (now I understand more than before why dell had problems with laptops catchign fire... this is 1 battery and my laptops takes _2_ of these at a time!)

As for how long? um... I've not used hte laptop in a few weeks (see why I want to sell it?) and the indicator does nto light up at all when I press the button... and I'd bet $$$ that the lappy wont boot off it either, these cells seem to discharge worse than most I've seen... hell I lext my Axim X5 off for about 3 months and it only lost like 10-30% of it's battery! (and I remidn you that there are _2_ of these in the laptop) something in there eats these things alive when it's not plugged it!

I think I'll chekc ebay and see what these batteries are going for... with that kind of pwoer, I'd never have to worry about charging up my devices before leaving home ever again!

[raspabalsa]

*jaw hits the gound* sssssevsevsevseventy-six hundred!!!??!!!

Heh heh.. yes, that's an amazing capacity, 60% more than what I got with my design. The problem with using such large batteries as yours is that you will need a higher current to charge them in an adequate time. For example, my charger is designed to be charged overnight (8 hours) with a current of about 600mA, which is not too much. Many adapters can be found with this amperage rating. But to charge your pack within 8 hours you would need almost 1000mA, which translates to a large, heavy, adapter. Plus, you would need a larger heatsink for the regulator. All this can be worked out, you just need to carefully design to avoid future problems (overloading the adapter, overheating the regulator, etc).

geeze, even if I dont sell this laptop it's worth buying an aftermarket battery for $25 off ebay to make this thing!

At $25 it's very cheap if of the same capacity as yours. I'll PM you my address so you can send one down here

(now I understand more than before why dell had problems with laptops catchign fire... this is 1 battery and my laptops takes _2_ of these at a time!)

I didn't know about that... probably overheating caused by keeping the laptop always connected to the wall outlet?

As for how long? um... I've not used hte laptop in a few weeks (see why I want to sell it?) and the indicator does nto light up at all when I press the button

A few weeks is not too much. I don't remember the exact figure, but Li-Ions have a very low self-discharge. The Li-Ions I used were in a drawer for months and still had about 3.9V when I assembled the device. Best thing to do is open the battery's case and measure with a multimeter directly on the cells.

these cells seem to discharge worse than most I've seen... hell I lext my Axim X5 off for about 3 months and it only lost like 10-30% of it's battery!

Strange, as I said above, Li-Ions don't suffer from severe self-discharge, quite the opposite of NiMHs, or worse, NiCds. Maybe the battery is old? Do you know year of manufacture?

The figure you quote for your Axim is consistent with a Li-Ion or Li-Poly self-discharge. A good battery if stored in charged conditon should be ready for immediate use even after a few months.

I think I'll chekc ebay and see what these batteries are going for... with that kind of pwoer, I'd never have to worry about charging up my devices before leaving home ever again!

Heh heh... that's absolutely true. With my TX's 3400mAH battery I need to charge only once every week (and I watch lots of movies and listen to lots of music). Adding my charger means that I can go for maybe a month without plugging in the AC adapter.

[Ashton]

I was wrong, looking on ebay the cheepest battery is $40 (including shipping) though that's still not a bad price for the capacity I'm getting... I wonder if this coudl be used to power a UMPC...

The computer was shipped with win2K on it, I got it about a year or two ago, and it was a year or two old then, and I think it was already passed on from off-lease or seomthing... I coudl look up the laptop's service ID if you'd like, since the batteries arent newer than the laptop.

I'm on the fence about selling hte laptop, ebay has a perfect-condition 8100 WITH printer and dock for $122 with like 2 hours remaining... and mine is not perfect (damaged screen in fact, though still viewable) hense why I'm reluctant, since it still works, and has 2 batteries that, if I dont keep killing them, hold good charges. (5 hours light work, 3 hours with cellular over USB or BT) If I can get a really good price on it, without the battery, I'll keep it and use it for this, selling the laptop and probably buying a wibrain UMPC to replace it. But, right now, I'm not seeing a good price... I'm not even seeing getting my original $200 back...

I know a buy that rips apaprt comptuers and batteries, I'll ask him how much he'd let some Li-ion cells go for, might be worth it, I know he has 6 lieing around on his desk...