"Jump Starting" a Roomba NiCad

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"Jump Starting" a Roomba NiCad

Postby leonicholson » May 1st, 2019, 11:31 am

I know others caution about doing this, but I just had to try.

Received another $50 "Parts Only" 650 yesterday.
iRobot brand nicad battery completely dead and measured 0 volts.
Works perfectly with another battery - as usual.

Connected the positive to positive, negative to negative terminals from the 0 volt battery to the good battery for 20 seconds resulting with the 0 voltage gaining a fractional value, just enough to activate the charger.
Put in Roomba and charged overnight with the side plug.
Ran until exhausted today for an hour and 53 minutes.
It is now charging normally on dock.

I'm not recommending others try this due to possible charger damage, but it worked for me.
Doing this is not meant for lithium batteries, only nicad
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Re: "Jump Starting" a Roomba NiCad

Postby RTC » May 1st, 2019, 4:46 pm

NiCads have some peculiarities.

I had a pocket calculator in college designed for alkalines and I modified it to use NiCads with a homebuilt charger added to the calculator. However the 4 alkalines it was designed for provided 6V but the 4 NiCads only provided 4.8V, which I did not know was barely above the "low battery" detection point. Alkalines drop slowly in voltage as they discharge but NiCads hold an almost constant voltage as they discharge then abruptly "fall over a knee" dropping significantly in voltage. This difference in behavior ultimately caused the NiCads to lose much of their capacity and I was forced to operate the calculator plugged in all the time (or go back to carrying several replacement sets of alkalines). Trying to sit near an outlet in the college classes was not always easy!

There are also quite a few "tricks" one can pull on NiCads that can sometimes recover them (even from apparent dead cell conditions) that would only result in total destruction of other batteries. One is to back charge them very very briefly, which can burnout nickel dendrites shorting a cell and get it working again.
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Re: "Jump Starting" a Roomba NiCad

Postby mfortuna » May 11th, 2019, 7:50 am

I used NICAds and NIMHs for years while racing RC buggys/trucks with my son. The key is to take each cell down to 0V independently. I bought a jig that would do that with the common build your own side to side soldered packs.

We used to abuse NIMHs by discharging the 6 cell pack to 5.4V (0.9V per cell) and then discharging the cells to 0V and keeping them that way until recharge. The key was no reverse charging.

Good old days. LIPOs have a great power/weight ratio but they are lot easier to destroy.
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Re: "Jump Starting" a Roomba NiCad

Postby RTC » May 11th, 2019, 11:34 am

mfortuna wrote:Good old days. LIPOs have a great power/weight ratio but they are lot easier to destroy.


And they can abruptly catch fire when damaged, completely destroying the thing they are powering also! Few batteries are that nasty in response to problems or mistreatment.
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Re: "Jump Starting" a Roomba NiCad

Postby mfortuna » May 12th, 2019, 4:52 pm

I have posted this picture in the past but this was the end result of a LIPO self destructing! This happened as a result of a crash. There was no apparent damage to the battery.
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Re: "Jump Starting" a Roomba NiCad

Postby RTC » May 12th, 2019, 5:58 pm

mfortuna wrote:I have posted this picture in the past but this was the end result of a LIPO self destructing! This happened as a result of a crash. There was no apparent damage to the battery.


By "damaged" I was not referring to visible external damage.
QC problems in manufacturing have allowed batteries to be damaged in making them then sold that way (e.g. the many cases of metal chips inside the cells).
Minor internal overheating either due to draining or charging too fast, that eventually causes a sudden thermal runaway.
etc.

None are visible on the outside... until suddenly the battery bursts into flames! Then its very visible, but too late.

The damage is usually internal and frequently subtle.
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