Roomba Li-Ion battery pack
After playing with a few dead laptop batteries and realizing that BatterySpace (www.batteryspace.com) had battery chargers and protection circuits for Li-ion cells, I've decided to rebuild the Roomba's battery pack by using Li-ion cells.
Unless you already have the cells, Li-ion is NOT a cheaper alternative to NiMH. And even if you do have the cells, you still have to buy the special charger ($25) and the protection circuit ($9). However, I wanted to buy the charger anyway, since I plan to use it on other projects, so for me it was not a big deal. If you manage to reverse engineer the protection in the battery packs that you recycle, you may save the $9, I didn't have the time and since my battery packs were dead, it was too much effort.
Clearly, you need some electronics skills to do this correctly (and safely), if there are things that are not clear, feel free to ask. Also, feel free to point out typos and problems with this document. And the warranty on your Roomba will be void after this.
- Li-ion cells have roughly the same capacity of NiMh cells, but they have higher voltage (3.7V), so you can use less cells and have a lighter battery pack.
- Recycling dead battery packs may be interesting and fun.
- You can reuse the charger (and what you learn) in other applications.
- Li-ion cells can explode (and according to http://www.batteryspecialist.com/Merchant2/panasonic.html, they can kill you in seconds. But this sounds strange to me) and it's always a plus to play with dangerous things :)
- If you don't use recycled cells, they are expensive.
- You have to buy the charger ($25) and the protection circuit ($9).
- If you have the Discovery or the Scheduler you can't use the docking feature.
- The Roomba doesn't seem able to sense correctly the battery charge, so it will suddenly die on a discharged battery, without any warning. No red light, no beeps, nothing. Not sure if the Roomba can still sense the battery temperature.
- 8 Li-ion cells small enough to fit the Roomba battery case (ex. 18650)
- 1 protection circuit ($9 from BatterySpace)
- 1 Li-ion battery charger ($25 from BatterySpace)
- Roomba battery pack
- The usual tools (wire stripper, soldering iron, screwdriver, multimeter, ...)
VERY IMPORTANT: You should NEVER charge or use Li-ion cells without a protection circuit. They can be easily damaged, explode, catch fire. Li-ion cells should not be overcharged or fully discharged. The protection circuit does exactly that (plus a few more things, like limiting the discharging current). Another cool feature of the protection circuit is that these functions are performed on each cell, individually. So even if the cells are connected in series, each cell is protected and charged independently from the others.
To open the Roomba battery pack refer to the thread http://www.roombareview.com/chat/viewtopic.php?t=231 . The 12 NiMh cells can be discarded or you can charge/test them independently and see which ones you can recycle, (usually only one or two are bad). Keep the metal contacts, the plastic casing, the PTC and the wires. I haven't reused the PTC resistor, but it could be a good idea to use it in the rebuilt Li-ion battery pack.
The next step involves opening the Li-ion battery pack(s). I've used three Dell battery packs kindly donated by the sysadm of the company for which I work (thanks again, Nathan!). They looked identical, only now I realize that they were instead P/N 75UYF and P/N 5081P. I expected to find the same type of batteries in all of them and collect at least 8 batteries in good conditions but each pack used a different kind of battery and mixing is not a good idea. However ALL cells in one of the battery packs tested OK (the electronics was probably faulty), so at the end, I've managed to get what I needed.
I have discarded the electronics. With some reverse engineering it should be possible to recycle that too and save the money of the protection circuit. The battery packs I've used also had a LED indication of the charge. Reusing that part could be a nice plus too.
I have no idea how to test a Li-ion cell properly, however I've noticed that my charger will refuse to charge some cells and happily charge others (the LED color changes from green to red when a cell is charging). This was an obvious criterion to select my cells. When testing the batteries with the charger MAKE SURE YOU SELECT THE CORRECT VOLTAGE. These tests were extremely short (a few seconds each) so I have performed them without the protection circuit.
The batteries I've used are Panasonic CGR18650HM (yellow). An alternative is to use Panasonic CGR18650HG (orange), but I couldn't find 8 good ones. Batteries like the Panasonic CGR17670HC (purple) won't work because they are a bit too long to fit the case (the ones in the picture were from a Sony Vaio battery pack).
If you are willing to spend some money on this project, you could get the LG 18650 2400mA/h cells ($9 each at BatterySpace) and build a 4800mA/h battery pack. That would keep the Roomba running for a while.
In my battery packs the batteries were connected in parallel, two at the time, to form a 3.7V, 3260mA/h (HM) or 3600mA/h (HG) cell (interestingly enough, the capacity on the battery packs was listed as 3360mA/h and 3800mA/h respectively). The four cells of two batteries each were then connected in series to get the 14.8V (see the picture with all the batteries on the table).
The protection circuit requires wires soldered before and after each cell (see schematic in the picture). So if you have 4 cells, you need to solder 5 wires (labeled B+, B1-, B2-, B3- and B- in the picture). Another two wires will go from the protection circuit to the metal contacts of the Roomba case (labeled P+ and P-).
The simplest way of connecting the cells seemed to be to cut the connections of the battery pack in the middle (four batteries on each side) and stack the cells in two layers. In this way you can reuse all the posts since soldering new ones can be tricky and adds thickness. Between the layers I've placed four plastic separators recycled from the packs. When taking the battery packs apart, try to salvage the insulation plastic/paper. Be extremely careful in isolating the contacts from accidental shorts (again, fire, explosions, etc...).
The Dell battery packs also had a black component with 2 pins, connected in series with the + wire. I believe this is a switch to limit the discharge current, for added safety (the instructions in the manual of the charger suggest to use one). I haven't used it since I was not sure of its function.
Four cable ties, some hot glue and two chopsticks later (I’ve been called Capt. Crude in another forum ;-) ), all the wires were soldered to the protection circuit and the battery pack was ready for a quick testing followed by the first charge. There is enough space left to insert the protection circuit in the case, actually, you have to fill up the gaps to avoid movements of the batteries. A cleaner alternative to cable ties, glue and chopsticks is to use PVC thermal shrink wrap (also from BatterySpace, I’ve mentioned them a lot… And I am not even associated to them).
I haven't timed yet the Roomba with the Li-ion battery pack but, it seems to be at least as good as I remember it was the fresh APS. It is not unlikely that the reduced weight makes the battery last even longer. I am going to rebuild another battery pack with 12 brand new NiMh cells I've just bought (guess where…).
I'll post some comparison soon...