Lithium tool battery failures

[dnix71]

I have 2 neighbors that support themselves metal recycling and working on cars, lawn equipment and off road motor bikes. If they find a battery powered tool or batteries and chargers they try to fix them and use them in their trade. I have a tab welder with built-in variable voltage charger so I get the problem cases.

The 20v and 40v Ryobi One Plus lithium packs are popular here, so I get to practice repairing those a lot. There seems to be a common mode of failure that points to something simple I haven't figured out yet. Most of the packs will appear to work in the manufacturers charger, but only half of the cells inside are actually charging. The charger shuts off long before the pack is full or properly balanced. If I open the pack and manually charge the depleted cells the pack will come to full voltage and work again - for one charge cycle. Then the pack fails to charge the same way.

If I open a pack, and hold it with the contacts facing me, the center cell always fails first, then the next cell back towards the contacts, then the last one at the edge of the pack. The two on the far side and both still fully charged.   1-2-3-4-5 cells in a 20v pack. 3 fails first, then 2 then 1. The plugin contacts are above 2 facing 1. That's the pattern for at least half the packs we get.

I have an SMD handheld resistor/diode/cap tester, but I don't know what to look for and probably couldn't fix the smd board inside a battery pack. They look like they must have been assembled by industrial robots. The cells are series tabbed but one end is wired through a triac circuit that turns on when the charger energizes the third external contact. The only way to brute force charge the string easily would be to open the pack and tab a connector to opposite ends on the battery string. Then you would need a variable voltage regulated charger that would trickle charge the string until it was full.

Does anyone have an idea what is the original point of failure?

[Mary B]

Sounds like a balancing failure... does each half have its own board or just one board?

[joestue]

two separate strings of 5 cells in series and one of them fails leading to reduced capacity?

or is this one of those switchable 20/40v packs that has a set of contacts in the case.

someone gave me a 20v/60v battery and the cells are good but the switches failed and it shorted out the pack blowing a few links. i have not gotten around to finding a new use for the cells yet.

[dnix71]

joestue the middle cell in almost every pack is the first to go, whether it is a 20v 5 series or 40v series of 10. I have seen the pattern of failure in packs that slide in and packs that have a tower that stabs in. The only common factor is Ryobi made and 4.2v lithium cells.

Nothing looks or smells bad, none have swelled or look like they got wet. Rebalancing works if done manually. All the cells then seem to be well matched. These cells in the middle are discharged hard dead, but almost all come back with an hour or so of trickle charging at 1 to 2 amps.

Either something it damaging the center cells or the assembler is deliberately putting the weakest cell in the middle of the pack.
All of these packs have temp sensors on the #4 cell.

[SparWeb]

Could the middle cell the the hottest one during recharging?

[dnix71]

SparWeb That was my first thought, or overheated during use. The thermal cutoff on the packs are all on cell #4. Number 3 is in the middle.

But last night I got another pack to work on. The whole pack was 0.3v. Stone cold dead. The charger blinked red "checking out the pack" but never decided anything. It didn't go pulsing red "bad pack" or switch to blinking green "charging."

So I opened up the pack and forced the it up to about 17v with my Sunkko tab welder/battery charger, then plopped it in the charger and it immediately went blinking green and charged for over an hour. When it was done the pack had 20.7v and all the cells were within 0.1v of each other and NONE were even warm. I took it back to Sid and put it in a drill and it worked fine.

So the charger is okay. There is definitely something going wrong with the packs, or Sid. Discharging a 5 cell lithium pack to zero volts should not even be possible. Below 3.6v the cells are basically dead and it damages the cells to take them below that.

[Mary B]

Could be Ryobi's charger topology not balancing the cells properly.

[dnix71]

Mary B The charger uses a 3rd battery post to turn on the pack's triacs to allow charging. The balancing is done on the battery circuit itself. There is a complex board with separate wires feeding each cell. The cells are wired in series but the positive post goes into the triac circuit and then to the positive exteral battery post. Discharging through that post does not require turning on the triacs, but charging backwards does. The triacs are dual diodes.

[Mary B]

I will have to take one of mine apart and look at it... maybe I can spot something...

It could be the way they are balancing cells then... charger turns off for one cell on the factory charger... to low of voltage? Not enough current? Mine are just wall wart power supplies. And those chargers are universal for 12-20 volt packs

[dnix71]

Mary B I got another battery to work on a couple of days ago. This one looked fine, but every cell in it was open, as in mOhms impedance. On one level this can't happen. After the first cell goes open it should open the series and stop whatever is going on.

I am beginning to suspect heat alone is capable of trashing these packs. The dash of your car can be 170F or higher when parked in the sun with the windows up. If someone was careless with a tool or battery pack it might melt something inside but not be obvious from the outside.