Ryobi CB120L repair (again) — UPDATED 5/10/20
By: Cameron Mcewen
Tags: Ryobi Cordless Tools, Repair, Battery

Update TL;DR: rebalancing didn’t work at all, Bought a refurb CB121L and it’s nicer, see below.

The ryobi 12v line is still soldiering on, even though it’s been mostly replaced and eclipsed by the 18v tools. This one in particular is still running because new ones cost money. I’d re-celled the batteries about two years ago with new Samsung 18650 cells, but recently when I plugged it in to charge it started giving me the “defective battery” indicator. I hadn’t beat on the thing that hard, so I decided to open it up and see what was going on (again).

I neglected to take pictures of a fair amount of this, but the problem turned out to be a seriously unbalanced battery pack. Cell 3 (most positive) was at 2.65v and the other two were at 3.96v. I figured the first thing to try was just rebalancing the pack by charging cell 3. I started cautiously with the bench supply, and ramped it to 500 mA and then 1 amp once it was over 3v.

Hey Mikey…

With the pack rebalanced, the internal protection chip and the charger have no issues with it at all. The drill spins and the lights blink to indicate the battery is charging. So what went wrong here? Acute failure of the cell is possible, but it behaves like a healthy li-ion cell. The rebalancing process was split across two days and the cell voltage didn’t drop at all overnight, a cell damaged enough to have that low of voltage in a pack likely would.

Please excuse my dear aunt I don’t have a macro lens

This little guy labeled U3 is the BMS chip for this board. It’s an Ablic 8254AA, produced from 2002–2016. It’s a fairly simple protection IC for 3 or 4 cell lithium ion batteries which can detect the voltage of each cell and provides over charge and overdischarge protection. Critically the IC does NOT offer any cell balancing, but with those C1 and C2 connectors on the charge terminal surely this must be managed by the charger itself, right?

There’s only the one FET on the board, between the negative terminal and the battery negative. Application notes for the 8254AA show two JFETs on the positive, so not sure what’s going on there.

Apparently some fairly large inductive spikes are expected from the motor. Pretty sure the thing under the pink tape is a thermal fuse.

Clearly there also isn’t any provision for passive balancing on this board either, I’d expect to see some big ole resistors if that were the case.

It’s a good thing they put these security torx in here, otherwise I might have assumed that it was totally safe to open under any conditions. Interestingly there are no warning labels about no user serviceable parts inside or whatnot. Maybe those are in the manual it says to read.

Well there’s your problem…

The charger itself runs on a real actual ATMEL branded microcontroller, this really must have been early days for lithium tools. Anyway those two big pins to the right of the inverted T shaped isolation slot are the wires from C1 and C2 on the charging connector. Via some resistors, they go basically straight in to the microcontroller. There don’t appear to be anything other than passives nearby enough to do anything useful, and those are far too tiny to be used in any significant passive balancing. It’s certainly possible the balance charging is being driven direct from the micro at an unreasonably small current, but given the other evidence of the state of my battery pack, I think it’s safe to assume this charger performs no cell balancing whatsoever. Woops. Kinda makes you wonder how many of these ended up in the landfill with 2 or even 3 good cells in them.

I guess a future project will be building an adapter board to plug this in to the hobby charger…

Oh if anyone wants to poke their battery to test it without taking it apart, here’s my terrible hand drawn pinout of the connector. The unlabeled pins appear to go straight to the thermistor, which was around 18k in my 75ish F room. You’ll need some super narrow tips, my normal meter leads wouldn’t fit.

UPDATE:

I bought some macro tubes too

Lets just get down to it. There’s been quite a few changes between the CB120 and the CB121, both in the plastic molding and the board.

The bolts are up top now. And one has a warranty plug

fixed

Anyway lets talk about the board. From usage, I know this battery is much better at staying balanced than the older 120. Too much better to be an accident, and there’s a reason for that.

remember those big ol resistors I mentioned the old board was missing?

There’s a few important additions that have been made. One is this little chip here, which I can’t find on google at all. Presumably it’s some kind of battery balance IC because those big 51 ohm resistors are each connected between one of the cells and one of those transistors. There’s an oddly placed third one up near the cell 1 connector. The conformal coating on the board is very thick for too much poking around, but from the amount I have done, and due to the fact that the cells stay rock stable balanced to within 10 mV, I’d say it’s a pretty safe bet one of the major revisions to the CB121 was the addition of balancing.

At 4.2v, the 51 ohm resistors would dissapate about 346 mW each, which should be sufficient for relatively fast passive balancing, and within the 400 mW spec for a 1206 smd resistor.

Other revisions of note: Although it’s kinda in shadow up there, hopefully you can see the flyback diode is much smaller, 1 amp instead of 3 amp by the looks of things. I couldn’t read the part number off the larger chip due to the thick coating, but due to it’s size and what appears to be some sort of serial interface nearby, I’m guessing it’s a full microcontroller, possibly with an integrated gate driver. The funky metal strip on the positive terminal is likely a makeshift fuse, possibly they had some issues with the packs being able to deliver more power than the board could cut off. They’ve also added a big ole electrolytic capacitor, they even had to add a slot in the board for it. I didn’t bother checking, but I’d guess this has to to with power supply for the large micro. The hand soldering on the larger joints is pretty ugly but I’m not concerned about it’s durability. I’m sure it will last at least the life of the cells.

To sum up, if you’re interested in re-celling your CB120 batteries… Don’t! For just a little more than the fresh cells (refurbished), you can get one of the newer CB121 batteries complete with cells and ready to go, and it’s got balancing in it. Get one of those and re-cell it in a few years. Although that being said, if you’ve got the cells already and just need the drill to spin NOW, the re-cell on the CB120 did last me two years, which isn’t abjectly awful for a tool battery. I’ll be using the CB121 from now on though.