I recently built a couple of battery packs out of recovered laptop 18650 LiPos.

While this has no doubt been done a thousand times before, I thought it worth a quick write up here. The hard work is being done by one of these modules, take care when looking for these to ensure you get the protected version.

The objective is to build a safe 3.6 V approx. supply out of unprotected 18650 cells. In theory we can parallel up as many as we like, but in practice probably 4 or 6 cells would be about as far as we can push the little charger board we are going to use, without modification. More cells would give us a longer running time, but would also proportionally increase the charge time.

I'm using my packs to power a Canon P&S camera running CHDK, but you can obviously use the method to power anything that will operate on 3.6V. Long timelapses and long duration motion detection are two of my goals. So far I have managed to power the camera for more than 24 hours at a stretch.

We also need to be careful with recovered cells, often one or two of them may be unwise to re-use, if they have been allowed to discharge below the safe limit. Discard anything which reads less than 1.5V, it is almost certainly not worth the risk of recharging, cells left at or below this threshold for prolonged periods can fail dramatically when you attempt to recharge them.

Of the two dozen or so cells I have recovered recently, only one was unserviceable. It had failed open circuit, and this failure may well have been the reason the entire pack no longer took a charge. It is probable that single cell failure is one of the conditions that the inbuilt charge protection controller in laptop packs can sense. It is therefore entirely plausible that the charge controller in the pack deemed the pack unsafe to recharge as a result of this condition.

You could of course use new 18650 cells, as they are available reasonably cheaply from all of the usual sources (often with wildly exaggerated capacities, and potentially of dubious quality). I would suggest however that recovered cells, make for a more interesting build, and are usually high quality cells.

Recovering them involves quite a bit of genuine "hackery", while extracting them from the failed laptop battery. Watch your fingers while you attempt this, the tabs on the cells (which you need to carefully detach and remove with a pair of pliers) are razor sharp.

There are several youtube videos showing what is involved, so I wont go over that here. Use suitable gloves to protect you from the sharp edges, and take care not to short out anything. Work in a location where you will not be too worried about throwing the hot, hissing firecracher that is an errant lipo out of harms way. Take care with sharp knifes and conductive screwdrivers, a stabbed or shorted LiPo is a dangerous one.

Everything described here, you do it *entirely* at your own risk. Don't forget your safety neck tie .

Laptop battery packs are "intelligent" in the sense that they contain a charge monitoring circuit, which will kill the battery (usually permanently) if it detects any problem. Often a single cell failure is all it takes for the charge monitoring circuit to deem the battery beyond its serviceable life, and under these circumstances, all of the remaining cells are usually perfectly usable (assuming they have not been allowed to discharge below 1.5 V).

Unprotected LiPo cells are also slightly more dangerous than some other battery technologies, in that if we short them out, things can quickly get out of hand. Melted wires, smoke, flames, potentially even an explosion can result, so care must be taken when constructing this build. Having said all that, lead acid, Nicad, NiMh .. in fact pretty much any battery technology can set things on fire, so LiPo's are not unique in this resepect.

If you would like to learn a bit more about cell and battery technology, one of the best battery related resources on the web can be found at batteryuniversity.com and there is a good writeup about LiIon/LiPo technologies and charging here (http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries).

Treat things with care, and you will avoid any nasty surprises..

One point I will keep coming back to is never, ever, even for the entertainment value of it, parallel up a discharged lipo cell with a fully charged one. Always measure the voltage of cells and ensure they are within a few tenths of a volt before strapping them together in parallel.

The reason is simple. LiPo cells have a fairly strict charging envelope, and relatively low internal resistance, of the order of 50 milli-ohms. If one cell is at 2.5V and the other is say around 4.2V, then we have a voltage difference of about 1.7V and a series resistance for the two cells of around the 0.1 Ohm. marker. This will allow a current of around 20A to flow through our 0.1 Ohm resistance, producing about 400 Watts of heat.... All of these figures are approximate, and things get worse of we parallel up 3 charged cells with one discharged one as the resistance of the charged cells will effectively be in parallel. Parallel = lower resistance = more current..... and since the power in Watts varies with the square of the current... under these conditions we are risking entering big bang territory.

Genrally this kind of mistake wont explode instantly, but will cause the cells to vent, blow their internal protection, smoke and possibly eventualy burst in to flames. This doesn't look good if you are working on the kitchen table, so don't say I didn't warn you.

Having said all that, some cells contain a PTC device to limit the current, but with recovered cells, this may or may not be the case. In this build I have used a 2.6A polyfuse, the camera I intend to use the pack with, will not draw more than about 2A. If your cells can handle more current, and you need the extra grunt, you can use a higher current rating fuse, up to the maximum discharge rate of your LiPo cells.

A polyfuse is a form of resetable fuse. Resetable fuses are as "cheap as chips", and may save you from some un-needed excitment of the sort caused by accidentally placing a screwdriver or other metal object across the output of a high current source. Its surprising how quickly things spot weld themselves in place, leaving you panicing and wondering what to do next as they start to glow cherry red hot and the battery pack starts to sizzle. The few cents they cost are well worth it for the additional protection. Their resettable nature makes them more convinent than convential fuses, but you could obviously sunstitute a conventional wire fuse (a 20mm slow blow of say 3A would be a good choice, the current, again depending on your application)

Wire fuses may be preferrable as these may be easier to source locally. One tip I would suggest is to put the fuse in a fuse holder if you think there is a risk you may end up having to replace it a few times.

Additional protection (depending on your application and level of p

aranoia) can be added if necessary. Laptop batteries often include a mechanical thermal protection device (similar to the switch in a kettle), for example something like this 90 Deg C miniature thermostat (an example of which I recovered from a Dell LiPo pack). Similar devices are available online from the usual suspects, for example here.

As with the wire fuse, these devices are pretty cheap, but unsurprisingly this is where the cheaper manufacturers tend to cut costs. If you buy a no-name unbranded replacement battery for your laptop, there is a good chance that the reason it is cheaper is not only because it doesn't bear the glittering logo of bigcorp, but also because the components it contains cost less. That in turn means that the cells are likely to be lower capacity, and corners may have been cut on the protection circuitry. Quality costs more I'm afraid.