I recently bought second hand Roomba 581. It is in excellent condition - I assume somebody didn't use it much because everything is clean and only with minor scratches on bumper. I even recieved replacement brushes and I assume the installed brushes are originals (never replaced).
Everything is good but battery - I tried everything but 2cells are dead (0Ah capacity), two of them are good (2.5Ah) and the rest is at 0.5Ah.
I was thinking about these options:
1) Buy genuine battery
2) Buy aftermarket battery
3) build battery from good cells.
I decided to go with 3) bucause I don't like aftermarket batteries (you don't know what cells are inside) and genuine battery is no fun
Here I could just buy 12 SC Panasonic 3000mAh batteries and be done but in many my projects I use Eneloop batteries so I decided to go with them. Roomba has relatively low discharge current (2A max?) compared to cordless drills or even handheld vacuum cleaners - they go up to 15A from battery and you really need high current cells for them which Eneloops are not as far as I know. But for Roomba they are absolutely fine.
The only problem is AA cells are longer than original cells and I could squeeze only 12of them resulting in only 1900mAh capacity which is not so great. So I decided to go little further and modify roomba battery housing to fit 24 of those AA Eneloops resulting in capacity around 3800mAh.
First I desoldered battery spring contacts for +, - and thermistor. I tried several orientation how to place the batteries to fit and the best one seems to be 3 rows of 8 in lying position. But I was missing around 1-2mm of space. So I decided to thinner the plastic wall between the battery compartment and front of the robot. I used dremel with abrasive disc and also brush and paper wipes with acetone to dissolve the plastic away. Actually the acetone gave it nice glossy finish so it looks like it is factory made
Next I was going to make the battery pack - I placed 8 eneloops in row and gleud them together with neutral silicone
After these 3 packs cured I glued them together to form one big pack
Then soldering comes in. I know that soldering is supposed to be bad for batteries but I think I can do it safely without any damage to the cells and I have done it many times (even with lithium batteries) and the batteries worked fine for many years. Trick is to use good flux and big soldering iron and keep the time it is heating to minimum. I use flux for pipes (agressive! must be washed away!) and I am done with soldering in 3seconds.
The connecting bars were tinned only from one side to keep the thickness to minimum - this is why it looks like it is not soldered properly. But trust me - it is.
I then Installed thermistor in the middle of the pack. I was thinking about using the original one from old battery pack but I had another one with long wires already around. I checked they have very similar resistance at various temperatures. I simply sticked it between the batteries in the middle of the pack.
This is how it looks like inside the robot:
Oh and final adjustment had to be dome to bottom cover - I had to remove these pieces of plastic to allow more room for batteries:
After full charge and full discharge I get 3562mAh (read from serial port in roomba during charge). Before this pack it read max 2696mAh (something around this number).
Summary - I get better capacity for even lower price compared to genuine battery (at least where I can locally buy it)
PS: I know there is topic about battery rebuilding but I think mine modification is different and deserves its own topic because it is not just about battery but about roomba mod.