How to rebuild your BP51 battery

The Problem

After three years I was finally fed up with my PCGA-BP51 battery that came with my 505FX. While I never got the advertised 2 hours of use out of it, no matter which energy saving settings I used, I could finally get 20 minutes of use out of it, which was ridiculous.

The Cause

I found some interesting theories on LiIon batteries while surfing the web. Apparently LiIon cells are not as sturdy as the manufacturers want us to believe. The life expectancy is around 500 charge cycles, sometimes considerably less. They don't seem to like it when you power your notebook through mains all the time. They also don't like deep discharges, like you had to do for NiCd cells. And still, some people have bad batteries that hold only a 15 minute charge with under a hundred charging cycles on them. So while the LiIons don't have a memory effect, they have something similar that Sony doesn't like to tell us.

There is a layer (oxide or something) that builds up on one of the poles when the battery is not in use. This layer is intentional and prevents the cell from discharging itself, which is one of the good features of LiIon cells. This layer grows when you don't use the battery, and it is reduced, when the battery is discharged. So far, so good. The problem starts when the battery is left in the notebook while you're on mains power and the battery is full. If you draw current from the battery, this layer has a resistance, and the more power you draw from the cells, the higher the voltage drop over the resistance. And the thicker the layer, the higher the voltage drop. This voltage drop reduces the usable voltage at the cell's terminals. Now, today's LiIon battery packs are smart, and they monitor cell voltage. If the cell voltage is too low, the battery pack shuts down, and you think it's empty. But in reality the cell is still charged, you only can't use the remainder in the cell. Now you recharge the cell, which is really still pretty full, and the layer grows. This is where the vicious circle begins. The battery never really discharges, because the control circuitry shuts the cells off before they are really discharged, because they look discharged. But discharging is the only way to reduce that layer, which is the reason why it looks discharged. And charging (which you do because it looks empty) will make the layer grow and worsen the problem.

Some people have reported good results from discharging LiIons manually, i.e. with a resistor across the cell terminals. The resistor should have a current draw that is in the range of the device being normally operated by the cells. A few manual charge/discharge cycles has yielded great results for some people. If you draw too much current, you either damage the cells, or blow the internal fuse in the battery pack, and then you have to open them up anyway. I decided to go ahead and replace the cells and see if that cures my problems.

Rebuilding the battery pack

I did some research on the web, and found that the BP-51 is made up of three LiIon cells internally, type US18650GR. This seems to be a standard battery type, since Panasonic makes a compatible battery under the designation CGR18650A.

I ended up buying replacement cells at www.onlybatteries.com for $9.99 a pair. That's $20 to refurbish a PCGA-BP51, and under $30 to rebuild a PCGA-BP52. So I went ahead and ordered five pairs, enough to rebuild my BP51 and BP52. But the BP52 is still good, so I'll have cells ready when the time comes.

These pictures show what you'll get, the first is the unopened battery pack, the second shows where the cells are. I hope this will help you decide where you can open the pack without damaging the cells. The cells are glued to the back of the case with silicone, but you can carefully remove them from the case and discard the case. Next you have to desolder the center contact and the other contacts from the circuit board. You can throw the circuit board away too. Now you have two pairs of cells, but where do you put them? Exactly, we have to take apart the BP-51. The first thing you'll notice is the lack of screws. You can look high and low, there are none. And it's worse. Not only are there no screws, the battery cells are glued into the case halves. And the two rubber feet are held in place by that same glue. So, when you remove the cells, you lose the rubber feet too. Unless... you cut them off with a sharp knife. You can glue them back in place with super glue when the whole process is over.

Now the feet are off, now what? Well, there is really nothing holding the top and bottom half of the case together, except for the cells that are glued into them. Sony used black silicone for this purpose (as you can see in the picture), and it comes off cleanly from both the cells and the case, but it is tough to get to in the first place. You have to insert a thin blade type tool like a knife in the seam that runs around the case. Try to distribute the force evenly along the edge, or you'll end up leaving marks. Start at the inside, because that part is hidden when the battery pack is on your VAIO. There is no real hint here, except to be careful, and work yourself around. The material is quite flexible, but I'm sure it will break, if you force it too much. Also, make sure you don't damage the circuit board that is mounted along the inner edge (also seen in the picture). Unsolder the original cells from the circuit board, and discard them.

This has been the hard part. Now you have to cram three new cells in there. That is not the problem. The problem is that the soldering tabs don't have the right orientation to solder them directly to the tiny circuit board. And unless you have a laser soldering system you can't attach new tabs. Forget soldering them, the battery will absorb all the heat and you'll end up destroying the cell. But, one tab is always useable, and what I did was put solder on the other soldering tab, heat up the tabs on two adjacent cells with the soldering iron put some connection material (recycled from the discount battery packs) between there as well, then pulled out the iron while pushing the cells together. Repeat this for the third cell, and if you didn't waste too much space, you can solder the whole trio to the board, and it will still fit into the case.

When I was this far, I reattached the pack to my VAIO, only to find out it wasn't recognized anymore. Had I broken something? I don't know. So I unsoldered the cells again, powered up on mains, with only the circuit board without cells attached. Batteryscope found the BP51, but correctly reported a 0% charge. I disconnected the BP51 brain, soldered the cells back, attached it again, and it showed some 40% or so charge (batteries aren't shipped fully charged). Funny, huh? I don't know what had happened the first time, but don't be alarmed if things don't work the first time. Maybe the charging circuitry on the battery pack had to be tricked or something. When putting the pack together again I put a drop of super glue on 6 locations around the case. Enough to keep it together, but easy enough to break open again if I have to. Finally glue those feet back to the bottom of the pack, and you're set.

Of course the charging circuitry still thinks you have the same number of charges on the cells you had before the rejuvenation. I don't know if there is a way to reset the counter. But it doesn't matter anyway. After charging the pack was as good as new, and I get 90 minutes of charge from the pack again, which is what I got out of it when it was new. I don't leave the pack on the notebook when it's on mains, unless it's charging. And I kept the old cells I pulled from the battery pack, they have a shelf life of 10 years, maybe the resistor discharge method will help when I need to put those back into the pack in a few years...

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