My 535 NiMH Battery gets hotter that 120 Degrees F

vic7767 · Joined: 14 Jan 2006 Posts: 3287 Location: Louisiana

Attached is the actual charging data captured via Procomm Plus and Rootooth after a 2 hour cleaning cycle.

Gordon · Posted: June 3, 2008, 10:53 pm Post subject:

The attachment shows what the plotted data looks like.

As you say, this charger finishes with a really HOT battery!

Only about 2.7 Ahr was added to the battery. I'm guessing that battery is not yet fully charged, because charging must have been terminated early due to high-temperature. Perhaps, if left connected to the charger, the charging would have been picked up again once the battery had cooled.

The MCU programmer has a weird way of tallying charge added to the battery. Maybe we will understand it someday, but not now! Our numbers match closely enough, but how's that final step-function rationalized?

igors48 · Joined: 17 Apr 2008 Posts: 61 Location: Ukraine

PLEASE!!! What does it mean for ordinary Roomba owners? :D

vic7767 · Joined: 14 Jan 2006 Posts: 3287 Location: Louisiana

If this type of charging of the NiMH battery is not just ocurring on my 535 then it could possibly indicate a shorter life than anticipated. Further testing will begin with a model 510 I now have. More data to follow.

wsngmartin · Posted: June 4, 2008, 7:44 pm Post subject:

vic
would the shortened battery life be due to running a partially-charged battery as well as the higher temps?

glo69 · Posted: June 8, 2008, 10:20 am Post subject:

If this charging cycle is typical, 5xx NiMH batteries are going to have short life spans. Here is a quote from Isador Buchman's book, BATTERIES IN A PORTABLE WORLD. Quote: The NiMH chemistry degrades rapidly if cycled at higher ambient temperatures. Optimum battery life and cycle count are achieved at 20°C (68°F). Repeated charging and discharging at higher temperatures will cause irreversible capacity loss. For example, if operated at 30°C (86°F), the cycle life is reduced by 20 percent. At 40°C (104°F), the loss jumps to a whopping 40 percent. If charged and discharged at 45°C (113°F), the cycle life is only half of what can be expected if used at moderate room temperature

One point that jumped out at me in the chart Gordon posted is that the battery was over 40°C at the beginning of the charge. I would assume that is the residual temp from running the Roomba, which means that the battery is always charging and discharging at over 40°C. Not good.

One point that I would disagree with Gordon concerns the battery heating stopping the charge before the battery was full. I think the sudden increase in the rate of battery heating was caused by the battery getting full. Here is another quote from Buchman that seems to fit. Quote: .... sense the rate of temperature increase, defined as dT/dt, or the change in temperature over charge time, rather than responding to an absolute temperature (dT/dt is defined as delta Temperature / delta time). This type of charger is kinder to the batteries than a fixed temperature cut-off, but the cells still need to generate heat to trigger detection. To terminate the charge, a temperature increase of 1°C (1.8°F) per minute with an absolute temperature cut-off of 60°C (140°F) works well.

It looks like I need to design a 5xx Li-ion battery. Does anyone have a dead 5xx battery they want to let me play with? :)

vic7767 · Joined: 14 Jan 2006 Posts: 3287 Location: Louisiana

Gary, here's a 3600 mAH LI-ion that seems to charge and work on a 535 but when charged using a 560 you get the err5 condition. Experimentation continues.

bob_ninja · Joined: 17 Apr 2006 Posts: 174

Regarding heat, it is difficult to avoid it.
NiMH cannot handle big loads very well or fast charging. I am not sure about the total load. I imagine Scooba in particular would be pulling 1C if not more.

That is why I am particularly interested in the new "safe" Lithium chemistries designed for abuse, such as A123 cells (A123 Systems Nanophospate lithium ion). I am experimenting with B&D VPX tools that use A123 cells and so far they are handling abuse very well.

So what do you battery geeks thinking about a new battery using A123 cells, either 18650 or 25600 size? These would solve many problems:
- heat is gone
- handles many C charge/discharge
- well over 1000 cycles

I have links, so can provide you with a lot of info from RC people.
For instance, they are pulling 10A and more from a smaller 18650 1.1Ah cell without any problems. Therefore, even Scooba could easily pull as much current as it needs without damaging a battery.

Now this sort of battery would cost more. However its longevity would more than pay for the extra cost. Even using it every single day you'd get at least 3 years.

Let me know if you are interested and I'll post some links, maybe a new thread in hacking.

(most of my limited knowledge is in NiMH so I am not brave enough to attempt this hack on my own)