BMS for replacement Li-ion battery 800/900 series

[Fixit4cv59]

I have two 805s and two 960s that have served me well for quite a few years. I wanted to upgrade the batteries to 3300 mAh cells. I rebuilt one battery with new Sanyo cells, but I cannot get the bms to turn on. The fuse that looks like a tuning fork was blown, but I soldered a wire across it as a temporary test. It did not work, so I will need a new bms. I could just buy a new high capacity battery, but I don't trust Irobot to sell me a good quality part at this point. Lithium Power no longer sells roomba batteries, so I don't know of any reputable aftermarket replacements. Here are my questions:

Can I install a new generic bms for the pack and have it work in the roomba?

If so, would I just connect the thermistor to the other two terminals?

Where would I get a good quality bms? Some of the ones on ebay might be good, but which ones?

[Fixit4cv59]

I purchased a 30A BMS with balancing and it seems to work well. I know I don't need 30A, but the capacity is probably overstated. The battery will charge in the roomba (cells reach a max of 4.1V), but after a while I get a charging error (error 5 I think). I tried sending the 960 out to vacuum my second floor (all carpet). It ran for 90 minutes, then returned to the home base to charge. After charging, it finished the job. 30 minutes or so after reaching a full charge, I got the error again. That sounds similar to what happens when a Li-ion battery is charged in a 500 series. Maybe the roomba can't recognize the new BMS as a Li-ion battery and thinks it is Ni-mh.

Since I don't schedule my roombas, can I just ignore the error? I use my roombas once or twice a week.

[CodaCM]

I purchased a 30A BMS with balancing and it seems to work well. I know I don't need 30A, but the capacity is probably overstated. The battery will charge in the roomba (cells reach a max of 4.1V), but after a while I get a charging error (error 5 I think). I tried sending the 960 out to vacuum my second floor (all carpet). It ran for 90 minutes, then returned to the home base to charge. After charging, it finished the job. 30 minutes or so after reaching a full charge, I got the error again. That sounds similar to what happens when a Li-ion battery is charged in a 500 series. Maybe the roomba can't recognize the new BMS as a Li-ion battery and thinks it is Ni-mh.

Since I don't schedule my roombas, can I just ignore the error? I use my roombas once or twice a week.

The 980/960 was shipped with a LI-ION battery so I don't know why it would not recognize it. If you have the money to spare I would recommend the OEM batteries. I know they are expensive, but for me, they worked well. The X-Life batteries last longer than the Norm Ni-MH Batteries and should have a longer life in them so if you want to downgrade that would be the best option for Ni-MH batteries.

[Fixit4cv59]

I agree that the Ni-MH are a downgrade. Just as a point of reference, I know the 800 series will also work with NiMH batteries, but Irobot doesn't make it clear if Ni-MH is compatible with the 900 series.

I recently purchased two used OEM 3300mAh Li-ion batteries. They are in great condition and have long run times. If I had thought of that first, I wouldn't have embarked on changing cells. Irobot is such a mess now that I don't trust them to send me a good quality battery.

Someone said (on a facebook group) that cells could be replaced if the bms is kept hot - meaning that power is supplied in parallel with each cell at the same voltage. I did this with new cells that were charged to the same voltage as measured on the old ones. It worked just fine, but I don't get the full run time expected from the new cells. The original BMS may tell the roomba what the battery capacity is and will only allow a certain wattage discharge. I got a longer run time, but probably not more than the original 1800 mAh cells when new. Definitely not worth the work, but interesting. Still testing.

Does anyone know if it hurts to keep the roomba on the dock after an error 5?

[CodaCM]

I agree that the Ni-MH are a downgrade. Just as a point of reference, I know the 800 series will also work with NiMH batteries, but Irobot doesn't make it clear if Ni-MH is compatible with the 900 series.

I recently purchased two used OEM 3300mAh Li-ion batteries. They are in great condition and have long run times. If I had thought of that first, I wouldn't have embarked on changing cells. Irobot is such a mess now that I don't trust them to send me a good quality battery.

Someone said (on a facebook group) that cells could be replaced if the bms is kept hot - meaning that power is supplied in parallel with each cell at the same voltage. I did this with new cells that were charged to the same voltage as measured on the old ones. It worked just fine, but I don't get the full run time expected from the new cells. The original BMS may tell the roomba what the battery capacity is and will only allow a certain wattage discharge. I got a longer run time, but probably not more than the original 1800 mAh cells when new. Definitely not worth the work, but interesting. Still testing.

Does anyone know if it hurts to keep the roomba on the dock after an error 5?

From what I have dug up the 900 series does not support Ni-MH batteries but anything between the 800 series and 600 (Specific models) will work with Ni-MH and Li-ION batteries. For the shorter runtime, it could be that the charging system just is not working right. I have a Neato XV Signature that thinks it's charging all the time so it never gets on the dock and stops a few inches away from it. It (from what I found) has a Diode (D14) that is malfunctioning causing a voltage making it think it charges. I ended up getting a new pack of batteries as the old ones lasted around 30 minutes but one day it was not working so I opened up the battery doors and the heat shrink plastic around it has shrunken a broken from heat. Sorry about that little rant, I have some very bad luck with Neatos.

[Fixit4cv59]

I took the battery with new cells out of the 960 and ran two missions in an 805. I got nearly two hours with the 805 and over an hour and a half when I put it back in the 960. That means the BMS board doesn't really know the original capacity of the battery. I just finished my second (and last for now) cell replacement while keeping the BMS hot.

The generic BMS board works well, but gives an error 5 after reaching full charge. For now, that board is just a spare for another project. All of my original roomba Li-ion batteries were made before 2019. They are conservatively rated and charge the cells only to 4.0V. I measured the cell voltage after the roomba stopped due to low battery and all were above 3.0V. This explains why the batteries last so long and why the nominal cell rating is so much more than the battery rating. I have no idea what the current batteries contain, but would expect cost cutting measures to be apparent.

My old batteries:
1800mAh battery - cells are Sanyo 2150mAh
2130mAh battery - cells are Sanyo 2500mAh

All of these batteries were still giving reasonable run times, but not quite as good as new.