I have a drill and impact driver with a 20/40V 5.Ah battery that has had little use. I am very pleased with the drill and driver which replaced ryobi nimh powered ones that I had re celled already once. Brushed motors but very cheap compared with Makita brushless ones and good enough for my domestic use.
However I put the battery on charge yesterday as it was down to 1 bar and the charger red LED indicated a faulty battery. It is past the 24 month warranty period.
From what I have read the problem is likely to be with the bms rather than the cells so I am tempted to dismantle it and charge the cells individually, any pointers?
It's possible the BMS puts some parasitic drain on some of the cells - eg if it tries to power itself off a single cell rather than the whole pack, the pack can get out of balance.
If the pack doesn't give you access to the individual cell terminals, dismantling them and constant-current trickle charging each up to about 3.6v would be what I'd do. Then put it back on the charger and see if it'll charge.
Charging each lower voltage cell individually would be my first suggestion, but mighty tedious and possibly difficult getting to them. Hopefully that brings everything back to life. Otherwise it's a cell problem: identify and replace bad cells, or it's a charger problem: get a new one if there's no obvious damage to fix in the current one.
Actually change that - first step is to make sure the contacts between the battery and the charger are good, before delving any deeper.
That sounds like a duff cell or cells, and one supposes that the monitoring circuit has this on continuous monitor Are the the Lithium sort, that may be why the system is ultra sensitive, since reverse charging is prevented I'd guess. Brian
They aren't actually, but who is keeping track :-)
Try running a digital camera, which has a single cell in it. It runs all the way down to zero, then you can recharge it. The camera does not try to run it down to zero, but it will self discharge that low.
I don't think single-cell digital camera battery at precisely zero volts is good for it, but at least the charger logic does not care. It may shorten the life or reduce the capacity, for them to go that low.
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OK, let's draw some packs.
3.7 3.7 3.7 0.0 3.7 3.7 3.7 3.7 3.7 3.7 3.7 3.7
OK pack Want to detect 22.2V when one cell is reverse-biased 18.5V/6 cells = 3V approx.
While it looks like I am "stopping the thing from going below 3.0V", that is not the case. The protective feature is attempting to tell when a single cell has discharged before its brothers did. It just happens to work out to 3V.
When a cell goes into reverse bias, metallic lithium plates out and this is hazardous.
This is why you don't discharge the *pack* by very much, because you are wanting to keep any single cell from being reverse biased.
If the BMS could monitor each cell individually, the rules could be quite different then. Like, if all the cells tracked exactly, you could continue to draw energy from the pack when at (6)*3.0v. Versus not drawing juice at (5)*3.7+0.0. the thing is, no pack will ever track to the (6)*3.0v state -- not gonna happen.
In the case of a digital point-and-shoot camera with a single lithium cell, there is no possibility of reverse bias. And consequently, the single-cell wall charger has no "rule" against charging a perfectly flat battery. Whereas the charger for a sic-pack battery, has rules about "oh, some battery has gone into reverse bias and is a fire hazard".
This is also why you (partially) recharge batteries in the off-season. To keep them out of the too-low voltage zone (the zone where the BMS approximation does such a shitty job). Check the battery every three months, to spot any "trends". A 70-80 percent charge is a good maintenance figure for Li. Leaving them at 100% is not recommended. "Voltage stress". BatteryUniversity has details.
Once a cell hits the knee, there is not a lot of left-over energy to extract, so chasing the pack into the noise doesn't really help. Drawing finer distinctions when the pack is almost drained, is a waste of effort. The not-very-low threshold is functionally good enough for operating the pack. As long as you keep them out of reverse-bias country, they will last through quite a few cycles.
I split it open and measured the two strings of cells, One string averaged out about 2.8V but with one cell at 1.6V.
The other string was around 3V.
I ordered a usb charger from ebay which took a while to arrive (actually
5 little printed circuit boards with a micro USB input) and I soldered on the two charging leads. I then proceeded to charge individual cells.
After a while I noticed not only did the cell I was charging increase voltage but so did the other cells in the string. The BMS must have a cell balancing function but it wasn't perfect.
The upshot is that the charge indicator on the battery pack shows it is full, the Aldi charger still shows a fault but the battery appears to be usable.
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