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roomba doesn't sense the right battery voltage level

PostPosted: May 29th, 2018, 9:18 am
by 170watt
I have a 500 series (model 552 pet) Roomba that has the following issue: it works well for about 5-6 minutes, then suddenly stops, with the central led flashing red; it also sends an audio message: “please recharge Roomba” (something like that). A few things to point out: it had this issues since I bought it (used), but used to work longer, for about half an hour before getting to a full stop. Important: the central (clean) button didn’t get amber, it was green all the time, until it switched to flashing red, with Roomba stopping and issuing that message. I tried a couple other working batteries, same issue, it is not the battery. When docked and charging the battery, it seems OK, except the charging time is small (it seems to “sense” correctly, that the battery is not fully depleted). Also, it can return to the base by itself, if the cleaning area is small and if finishes before it wrongly “senses” that the battery is depleted.
I have basic soldering skills, but don’t know much about the inner working of this robot. I was wondering if you can point to some specific issue that may be fixed. Is it something specific around U8 chip that needs to be checked? By the way, before it got worse (while it was still working for about 30 min before stoped), I replaced the U8 (LM324) chip, but it didn’t seem to make any difference. I remember I checked the voltage divider resistors R207 and R208, thinking that maybe these gave the wrong voltage, but were OK.
Anyway, any suggestion?

Re: roomba doesn't sense the right battery voltage level

PostPosted: May 29th, 2018, 12:37 pm
by a1robotrepair
May be a charging problem. Your roomba charges to change in battery temperature or max temperature.
1st try cleaning the battery terminals and bot battery spring connectors. Pull down lightly on connectors before reassembling bot.
Next, check out the thermistor circuit. Read this thread:http://www.robotreviews.com/chat/viewtopic.php?t=17151&p=139276#p139276

Re: roomba doesn't sense the right battery voltage level

PostPosted: May 29th, 2018, 3:22 pm
by 170watt
Thanks so much for your reply! I tried the easiest part, i.e. cleaning and pulling down the connectors; no difference.
However, I am not sure that it’s a charging problem. The reason I am saying that is that it performs the same with a different battery fully charged from another Roomba, and also, if I use the battery charged by this defective Roomba with another (similar) Roomba, that one performs well – meaning the battery is “fully” charged. When working, it seems like it does not measures correctly the battery voltage. Anyway, will look later (weekend) into the thermistor circuit, and report back.

Re: roomba doesn't sense the right battery voltage level

PostPosted: May 30th, 2018, 11:59 am
by a1robotrepair
All the issues you describe is probably a problem with the battery sensors circuit.
U8 conditions the main battery sensors analog inputs to the MCU.
Built In Test #8 could tell you which sensor circuit is faulty.
Cleaning around U8 with 99.9% alcohol solves battery sensor problems 75% of the time.
Pervious link in thread has a good explanation of U8.

Re: roomba doesn't sense the right battery voltage level

PostPosted: June 4th, 2018, 4:10 pm
by 170watt
I cleaned the area with isopropyl alcohol as indicated, no effect.
I measured the voltages at all the U8 IC pins. While all are the same as showed in the table indicated above, I noticed that pins 5, 6, and 7 were around 2.82 (with pin #5 fluctuating between 2.82 to 2.84). This seems to be OK for charging mode values, except that measurements were done with the unit battery powered. As per table “should” have been a bit lower. Not sure if 0.1 V difference is significant or not.
Next step would be to check the value of resistors 207 and 208 (the voltage divider), but for that I would have to de-solder them first; I am waiting first to purchase new resistors just in case.

Re: roomba doesn't sense the right battery voltage level

PostPosted: June 4th, 2018, 11:24 pm
by a1robotrepair
No problem with voltage divider. 2.82 sense volts = 2.82 X 5.75 = 16.2V Battery voltage. Perfectly normal charging or charged battery voltage.
You need to do the built in tests.
At this point I would suspect an H-Bridge circuit failing, drawing the voltage down with high current.

Re: roomba doesn't sense the right battery voltage level

PostPosted: June 5th, 2018, 4:52 pm
by 170watt
I think you are on to something. I’ve done some repairs previously at the H-bridge (same board). I had to replace Transistor Q83 (B772) and Q68 (D882), all shorted. I didn’t have the H-bridge schematic, except a hand written one for left side, found on this site. The weird thing is that I had to solder a shunt (small conductor) between the Q68 collector and R249, otherwise it didn’t work. I couldn’t figure out if the collector was supposed to be connected directly with R249, but anyway that’s the only way both motors would have worked correctly).
So yeah, I am guessing there is some current loss somewhere.

Re: roomba doesn't sense the right battery voltage level

PostPosted: June 5th, 2018, 6:47 pm
by a1robotrepair
This thread has a good drawing of the right wheel H-Bridge circuit.
http://www.robotreviews.com/chat/viewtopic.php?f=1&t=16618
The left,right wheel and CHM H-Bridges are nearly identical except device numbering.
Running the BITs and monitoring with terminal software using a SCI cable will probably show the problem.