Roomba Batteries

It means nothing that the battery shows 14V into a multimeter's multi-million-ohm impedance.

That is not an indicator of its capacity, nor of its ability to supply current.

How about this test: can that battery light a 12V bulb rated for several watts? And for how long?

One reason would be that the replacement battery is no longer new; you've had it in there for several years already. These things don't last forever.

Batteries are actually complicated and fascinating. There is a lot of info on the "Battery University" website, if you're interested:

Read this:

Especially, find the table of Advantages/Limitations of NiMh batteries. Among the limitatons of NiMh:

- "Limited service life; deep discharge reduces service life" - "Requires complex charge algorithm"

Regarding the last one, what are the odds a device designed for NiCd batteries does *not* implement the "complex charge algorithm" required for NiMh?

Contrast that with NiCd:

- "Fast and simple charging even after prolonged storage" - "High number of charge/discharge cycles; provides over 1,000 charge/discharge cycles with proper maintenance."

You do realize that "I intend to throw it away if it actually needs repair" is hardly in the right spirit for an electronics repair newsgroup.

Are you measuring the voltage "open-circuit", or with a current load similar to what the Roomba actually draws while it's operating?

You may find that the battery voltage "sags" badly when it's under load. If so, it's probably dying and will need to be replaced.

Try temporarily connecting a power resistor or other load across the battery terminals... something like a car tail-light bulb would be good... pull a half-amp or so out of the battery, and see what sort of voltage is delivered under those circumstances.

My understanding is that NiMH batteries are more easily damaged by over-charging than NiCd... even low-level "trickle charging" of a NiMH can cook it when it's full.

It's somewhat tricker to detect "full charge" in a NiMH than a NiCd. With a NiCd, the terminal voltage rises as you charge it, then peaks, and then begins to drop (this happens at full charge, when the current into the battery ceases to pump up the electrochemistry and just heats up the battery). NiMH cells don't show such a substantial drop in terminal voltage at full-charge... the terminal voltage stops rising ("zero delta V") but doesn't start falling ("negative delta V").

What this means, is that many devices which were built with "NiCd" charging circuits, will tend to over-charge NiMH batteries and appreciably shorten their lives.

On Friday, February 21, 2014 11:31:28 AM UTC-6, Charlie wrote:

l NiCad battery died and I replaced it with an aftermarket NiMh replacement that worked fine for several years. Now the robot fails after a few minute s with a red light showing that indicates insufficient power. The service i nfo says to remove the battery and to press a particular button to "reset t he battery". Of course, this procedure doesn't reset the battery, but reset s whatever sensing circuit checks the battery. I tried that. Still fails. T he NiMh replacement is labeled 14.4 volts. My cheap VOM show about 14-15 vo lts. Seems good to me. I sent an email to the robot company asking for thei r take on the problem. Their response was that buying a new NiCad from them would solve the problem. I am a skeptic. I think that customer service jus t has a checklist that is simple-minded and covers only what the average co nsumer can do. Along with the email response was a phone number if there wa s still a problem. I called and essentially got the same answer. I said tha t my concern is that a new battery will not solve the problem. I think that the voltage sensing circuit has failed. I asked if there wasn't someone wh o understood the inner workings of this box who could shed light on what is happening. I was put on hold and then was told that there was nobody else to talk to. What was offered was a discount on the replacement and that if it did not solve the problem the replacement could be returned for a refund . I bit at that offer and we will see what happens. I am open to thoughts t hat there is a reason that the replacement battery is at fault. If the faul t is in the robot, it is doomed to the recycle bin. There is not likely to be anything that I can do without a service procedure for guidance. I also suspect there will be a control board with non-standard components that are unobtainable. Charlie

We don't even know that the battery is being fukky or partially recharged, at all or maybe incorrectly since NiCad and NMh batteries are somewhat diff erent as other poster have noted.

have you lubed the motor and geartrain at all? roombas get all dirty and nasty inside and can grind to a halt.

A hard running powertrain could drain a questionable to start with battery or cause a low voltage problem.

What's the exact model number? I've fixed a few of these and found that each has its own idiosyncrasies.

Perhaps it would be helpful if you converted "several" into a number. How many years did the NiMH battery last? More specifically, approximately how many charge cycles? A really good NiMH battery will last for about 1000 cycles if everything is done perfectly, or about

50 charge cycles if you do everything wrong. Much depends on the detection of the EOC (end of charge) point in the charge controller. If it's a dozen years old and was originally designed for a NiCd, it is unlikely that the EOC algorithm is suitable for an NiMH battery. You can easily overcharge the battery, with will kill an NiMH battery rather quickly.

Test it under a load outside of the sweeper. Try to approximate the operating current of the Roomba with a resistor. For more accurate results, borrow a discharge tester such as a West Mtn Radio CBA:

You can get a NiCd replacement cheaper on eBay or Amazon. If you're not sure if the charger will work properly with an NiMH battery, then perhaps reverting to a NiCd might not be such a bad idea.

I am a cynic.

In my experience, that covers at least 90% of the questions. Answers like "is it plugged in?" are all too common. In the distant past, I spent some time on a support phone pool creating a database of boiler plate answers. I wasn't too happy with the software or my answers, but it was a start and worked tolerably well.

The only way you're going to know if the voltage sensing or whatever circuit has failed is to either buy or borrow a known working battery. Troubleshooting by replacment is an old and quite effective method of fixing things.

I used to design marine radios. The dealers knew the engineers by name and usually bypassed the normal support channels. They would spread the broken radio onto the table, and then have me walk them through the troubleshooting and repair. That saved them the cost of hiring a qualified technician. That might explain why there was nobody available with a clue.

I suspect a replacement battery would have been cheaper and faster.

You may have killed the NiMH with a charger made for a NiCd. Or, you overcharged it with a charger that didn't have a suitable EOC circuit.

About half of my repairs are mechanical, not electrical. A sweeper full of hair, dirt, and crud will bog the motor, causing the battery to drain or die rather quickly. Add those that like to fix things with far too much oil, which soon turns to tar. It's amazing how many things can be fixed by simply cleaning out the crud.

Good luck.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

"Charlie"

** They are probably right.

But NiCds are getting scarce these days and you might find replacement packs for appliances are rather old and hence not in top condition when you get them.

I bought some sub C cells for my Bosch screwdriver the other day and found that Sanyo no longer sell NiCd cells under their own name - but badged " Panasonic " instead. The fine print says made by Sanyo and they look exactly like the N-1700SCR cells I was after so it was all OK.

IME buying loose cells from a wholesaler is a lot safer than some dodgy after market or even OEM pack that could be ten years old.

... Phil

I recall reading this somewhere else, it involved removing the brush/rollers and literally chiselling out a couple of bearings made out of compressed, burnt (by friction) hair. Followed by some hacking to prevent it happening again, because -- it did keep picking up and storing more burnt hair!

It caused a "motor overload" situation, but that would have knock on effects for the battery getting overloaded too.

--
--------------------------------------+------------------------------------ 
Mike Brown: mjb[-at-]signal11.org.uk  |    http://www.signal11.org.uk 

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I no longer have the facilities to hang a load on the battery.

For all those who suggested cleaning out crud etc., this unit has reasonable sensors that tell you it needs cleaning. The biggest problem is hair that wraps around places. There are no consumer available places for lubrication.

For all those who remarked about the different charger requirements, I have found that the replacement coming from the mfgr is now a NiMH. I'll have to wait and see if they include specific charging requirements.

Charlie