Got a Blaupunkt cordless tyre compressor. With a fresh battery it works for a decent time then stops. Leave it a few moments and it works again, but for a shorter time. Battery has capacity LEDs in it, and notice if I switch those on, they go out when the drill stops. But work with the battery in or out of the device. So guessing it is some form of protection within the battery? (I first thought it might be a over heat switch in the device) They aren't showing battery low when the device cuts out.
I can buy a new battery but pretty pricey. It's an 18v 2 amp hour Li-ion. Would new cells sort it? Or possible to test the individual ones?
Some Li-ion cells include their own protection circuit, to protect themselves from shorts/ overheat/ over discharge. Perhaps one is overheating or faulty.
Most of the time the protection circuit is in the pack, not individual cells. It sounds as if the pack is overheating - pulling lots of current will heat the cells up. Sooner or later it gets too hot and disconnects, which is why the capacity display goes dead.
It could be that one of the cells is faulty and getting too hot, as you say. Or that the pack management has allowed the cells to get out of balance and one of them is becoming overdischarged.
Open up the pack and feel the temperature under load. You can also probe the voltages across individual cells and see if any cell is either getting overly hot or overly discharged (<2.5V).
If you are interested in playing with such things there are some pretty fancy chargers out there that would allow you to measure / monitor the charge / discharge / capacity of various numbers of cells in a pack, but you may have to add the relevant connectors.
Many Li chemistry battery packs have their on BMS (battery management system) a PCB in the pack (if not individually protected) that monitor the voltage across each cell in parallel (the charger to battery) whilst the cells are charged in series, holding back the ones that are showing full charge to allow the others to catch up.
Therefore you can 'just' apply a pair of charging wires (+Ve / -Ve) to the pack and it looks after the rest.
Or, as with my RC model packs, the battery is just a connection of raw unprotected cells and the charger (whilst charging) protects / balances the battery using connections to each series cell connection.
I have such a charger that I think monitors up to 10 cells in series and various battery chemistry's. You can also hook it up to a PC to log the activity and it will even charge a battery of a higher voltage than the source PSU voltage and discharge one battery (doing a capacity test) using the load of charging another.
It's a parallel tool to your 12V battery capacity tester. ;-)
I guess you need to find out if the actual device runs within spec current wise when it works, Some kind of psu withth the correct voltage and a current measurement. I remember many moons ago now a cheap cordless screwdriver started to act this way after a couple of years, turned out that the motor comutator had worn out and either the brushes were not making contact or they were shorting out the supply momentarily, not good for battery or motor health. Brian
Just had a look inside. Each cell is connected individually to the PCB - as well as the obvious series connection. Although without dismantling, can't really see if those connections to the PCB (other than the main ones) actually go anywhere. There is also (I'd guess) a heat sensor glued between two of the cells. Oddly, one of those being the suspect one.
I know no load voltage tests ain't usually much good but these are interesting. Freshly charged battery.
4 cells read 4.19 or 4.2v. One reads 3.48v.
Looking on Ebay, 2000 mA 18650 seem rare. That's what they're marked as.
There seems to be a bit of an inverse relationship between ampere-hours in the capacity department, and the maximum discharge current flow rate. Not only are you looking for a 2000mAh cell as a replacement, but the "max current flow" spec (not printed on the sleeve), has to suit the application.
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"At the time of writing (June 2018), the current maximum CDR attainable in an 18650 battery is 38A at 2000mAh."
That's why a designer might select a particular 2000mAh unit, instead of a 3400mAh. The 2000 one may allow twice the peak current flow. The physical composition of the cells is somehow different.
3400mA are usually rated at around 20amp maximum discharge rate. Possibly the OP's cordless might be an older item, designed/ built when
2000mA cells were the highest capacity available.
I would want to replace the cell with the same type, because other characteristics may be different beyond the capacity and maximum current. When it's in a series pack any kind of discrepancy is more likely to pull it out of balance.
The cell should be identifiable from the writing and colour of the wrap - search for what's written there and compare photos. Then buy it from a reputable non-ebay supplier (nkon.nl is another one).
Although I imagine it would only be a stopgap solution and recelling the battery might be needed sooner or later.
The capacity is related to the area of the jelly roll. Larger area = more capacity. But to get larger area into an 18650 shell means making the layers of the roll thinner. A thinner roll has a higher resistance and is less able to deliver current. It also heats up more and is more likely to cook the electrolyte when a high current is drawn, which reduces the cycle life.
This battery appears to charge each cell individually.
It's not the sort of tool where vast battery life is needed anyway. Just long enough for the job in hand. My brother has the same tool, and it doesn't have this problem.
Are you sure? Usually packs have a chunky series connection, which is used to charge and discharge the cells. Then there's a connection to each cell - this is used to monitor and balance the cells. The individual cell connections are often fine wires and couldn't take the several-amps charge current.
Dumb balancing switches resistors across the cells - these are often quite low wattage because the balancing only needs to bleed off a few millivolts, but it does mean the balancing is not designed for high currents, both from a wiring and a thermal (where the resistor heat goes) point of view.
Smarter balancing transfers energy from one cell to another. That's mostly on bigger battery packs like EVs though.
They are extremely chunky individual cell connections to the PCB. The equal of the main output ones. Certainly not a thin voltage sensing wire.
However, it might just done like that for physical strength. Without a circuit, I'm a bit in the dark.
Thanks Theo. It's a black art to me.
I did do a quick check of voltage with about a 1 amp load across each cell. The 'good' cells hardly varied their voltage. The suspect one dropped considerably. I'm going to replace it only. Can't be worse than what I have.
The make is Blaupunkt - and I suspect obsolete. But nicely made. Think it may be a Bosch offshoot - since the batteries look quite similar to theirs. Dunno if interchangeable, though.
One last test you could try... it is possible the cell has just become discharged and so out of balance the BMS can't manage to keep it in sync. It might actually still be mostly healthy. The sweet spot for lithium ion cells is about 3.6-3.8V - for most of the voltage curve the cell sits there, higher and lower of that range it will swing rapidly. So a rapidly dropping cell might just be one that's charged to 10% not 70%, but not necessarily a sick cell.
So a quick and dirty hack would be to apply a constant current (maybe
100-200mA) across each cell until they all come up to the exact same voltage around 4.1 to 4.2V (making sure you don't go above 4.2V). A bench power supply with a current limit is an easy way to do this - set current to
100mA, voltage to 4.2V.
Once done to all the cells the pack will be balanced. Then use it and see if the suspect cell starts dropping out earlier, or if it stays in sync with the other cells.
Blaupunkt was part of Bosch until 2009, since then it's been a brand slapped on various Chinese stuff. It is very possible the battery packs are shared with another platform, especially in the Chinese phase.
Thanks Theo. Since my spare cell has yet to arrive, nothing to loose. So put that cell on charge at 100 mA at about 1400 yesterday. Checked the voltage a few times and it was going up. By this morning up to the same as the others.
Did the same discharge check as before on individual cells, and all seem to be the same now. So I'll put it back into use and see how it goes.
Looks to be a success. Let all the air out of the spare, and pumped it up. Before, it would have cut out long before achieving this. And needed charging to keep going.
Real test will be if the cells remain balanced after being run low, and recharged in the normal way.
That's good to know. As you say, putting it through a few charge/discharge cycles will tell you if it's just a one-off, the cell is weak, or whether there's some fault with the charging or balancing circuit.
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