Discovery-Trickle Charge issue

dermbrian · Joined: 14 Jun 2008 Posts: 14

I've been trying to get my Discovery back to working order, and the info on this site has been very valuable.

Symptom: After charging overnight or longer, unit begins to clean and goes straight to red LED and halts in about 5 seconds. When placed on base or connected directly to the power supply, unit will have glowing/throbbing red POWER LED for about 30-40 minutes followed by solid GREEN power LED....so it looks like it is charging.

I replaced U2 and U4 on the main circuit board with equivalent, but more substantial, MOSFET's. I then charged it until the light turned green and the little guy ran for about 25 minutes. I assumed I had fixed it.

Well, the real issue is that it does charge while the light is RED, but then slowly discharges instead of trickle charging while the light is green. I can always get 20+ minutes of operation if I remove it right when the light turns green. I always get an immediate fail if I leave it on the charger for several hours to 3 days. It is weak at that point, too....so I am certain that the stoppage is due to low battery.

The battery appears fine. Individual cells all measured within .05 Volts of each other in a no-load situation, typically 1.3V when I tore it apart and checked them.

Has anyone had this issue and solved it with a component replacement other than a board? I believe I can get a rapid charger off of Ebay, but would prefer one more try at a cheap repair.

Brian

glo69 · Posted: June 14, 2008, 1:43 pm Post subject:

Hello Brian

I can't help you if there is a problem with your electronics, but I can warn you not to dismiss the battery from further testing.

The facts you have listed could very well be the battery. When a battery gets old and weak it will give up very little charge when used and will take very little to appear recharged. The fact it only takes a short time for the green light to appear, and then the battery only runs for 20 minutes would fit a very weak battery.

Second, a very weak battery can have a very high self discharge rate, potentially much higher than the Roomba trickle charge rate, which is not very high when you deduct the Roombas standby power drain.

Next would be the battery charge. A well charged battery should be close to 17V, or 1.41V per cell. A battery that is only showing 15.6V actually has very little charge in it.

Finally, a no-load voltage test on a battery is mostly meaningless, although a low charge on a supposedly fully charged battery, such as you have, is not good. On the other hand, you could show 17V and still have a weak battery. Put a ~10-20W, 12V load on the battery and test the voltages.

Gordon · Posted: June 14, 2008, 2:00 pm Post subject: Re: Discovery-Trickle Charge issue

Brian, I'll apologize in advance for brevity, I must spend only a few minutes on this post.
dermbrian wrote: ...Symptom: After charging overnight or longer, unit begins to clean and goes straight to red LED and halts in about 5 seconds. When placed on base or connected directly to the power supply, unit will have glowing/throbbing red POWER LED for about 30-40 minutes followed by solid GREEN power LED....so it looks like it is charging. I would say that amount of charging is insufficient, unless the battery has been charged in the last few days, but not used. Quote: I replaced U2 and U4 ... then charged it until the light turned green ... Don't assume that green LED means the battery is charged. There is no wiring between the Power-button LEDs and the charging controller -- its a software connection. Just assume the switch to green means a process has completed. Quote: Well, the real issue is that it does charge while the light is RED, but then slowly discharges instead of trickle charging while the light is green. I think that remains to be proven. The only thing that changes between high-rate charging and trickle-rate charging is: the controller sends a different waveform to the gates of the MOSFETs. IOW, if those items work at high-rate, they ought to also work at a lower rate. Quote: ...The battery appears fine. Individual cells all measured within .05 Volts of each other in a no-load situation, typically 1.3V when I tore it apart and checked them. "Appearing" fine and being fine are not the same. You have given no age info about the battery, or reported what its voltage is when loaded and delivering an amp or two.

There is a graph here of a worthless battery's performance, that you can examine. Notice how it begins work with a high terminal voltage, but, then droops so quickly that it is only able to do a 45-minute mission. Also observe how the terminal voltage 'recovers' upon removal of the load.

You might benefit by doing a load test on your battery. If you can do it safely with the cell pack removed form the case, do so, and measure individual cell-voltages once you notice a decrease in terminal voltage.

Talk later.

dermbrian · Joined: 14 Jun 2008 Posts: 14

I charged it until it turned green again and have taken the battery out. I want to see if the battery does discharge itself over the next 24 hours before I go through the process of getting a 12V bulb and testing it under load.

I guess I was expecting one or two bad cells to be more obvious.

The battery is the original and we've had the Roomba for about two years. I'd say during that time we've used it maybe 30 times, tops. We don't use it in the carpeted rooms but instead only on laminate flooring or vinyl, with a small low-nap area rug in the living room.

Thanks for the feedback. I'll post tomorrow afternoon with the results of letting the battery sit for 24 hours disconnected.

Brian

Gordon · Posted: June 14, 2008, 4:21 pm Post subject:

dermbrian wrote: ... I was expecting one or two bad cells to be more obvious. They are most obvious when you monitor them during a trial discharge. While they may charge up to normal voltages, those voltages fade away faster than *normal* cells' volts. The duff cells are a bit tricky too, because they can exhibit different turn-off points during the discharge time period! Quote: The battery is the original and we've had the Roomba for about two years. Say 'goodbye' to that battery! Quote: ...I'd say during that time we've used it maybe 30 times, tops. ... No matter, its two years old, and duff.

dermbrian · Joined: 14 Jun 2008 Posts: 14

You're both right about the battery. After pulling it out yesterday afternoon immediately after the power light turned green, the Roomba didn't even get two inches before shutting down this morning.

I guess I'll be ordering a battery. It's amazing you can't just pick one up locally in a major city, with how many must go bad every year.

Thanks.

Brian