Charging after NiCad=>LiIon swap

[llama]

For my Black & Decker cordless drills I've changed the original 10x NiCad cells for 3x LiIon cells. It would be a right pain to extract them to put them individually into my pukka LiIon charger so I'm wondering if I could get away with using the original charger.

I could take them out every half hour and measure the voltage. Or should I make up a new charger or modify the original in some way?

Any ideas gratefully received.
Graham

[The Philpott]

I have done this, i must admit (and seemed to get away with it) but it's not recommended and i think i cut off quite early, 3.9v per cell or somesuch.

Is this one of the B&D's where the charge point is a socket in the drill body? (ie the battery doesn't have to be detached for charging?)

In my experience Li-ion chargers are much much cheaper than the battery packs themselves, so you should be able to cobble something together. Li-ion chargers turn up second hand on ebay quite cheaply sometimes.

Dave

[Nickthedentist]

Be aware that house fires are not unheard of when lithium batteries are left charging from dubious chargers. Seems to be up there with fags, deep fat fryers, candles and tumble dryers!

[paulsherwin]

The problem with them is they don't tolerate trickle charging as NiMHs do. If you're going to charge 3 Li cells in series then the simplest arrangement you can get away with is a 12.6V fixed voltage with a current limiting resistor. As Nick says, err on the side of caution - don't charge somewhere where a small fire would be a disaster, and don't leave it charging indefinitely.

[Nickthedentist]

https://www.heraldseries.co.uk/news/...se-near-thame/

[jamesperrett]

You would need to remove them much more regularly than every half an hour to check - they really need constant monitoring on charge and they must not be allowed to go to more than 4.2V per cell if you want to avoid a fire. If using a simple charger I wouldn't go higher than 4V per cell as you don't lose that much capacity by not having that last 0.2V.

[llama]

Thanks for all the great knowledgeable input.
The battery pack comes off the drill body - in fact, I have two and the pack is about the size of your fist.
I won't use the NiCad charger but might well build a suitable charger into its case. I have a place where charging can take place.
Thanks again.
Graham

[cmjones01]

Originally Posted by paulsherwin

The problem with them is they don't tolerate trickle charging as NiMHs do. If you're going to charge 3 Li cells in series then the simplest arrangement you can get away with is a 12.6V fixed voltage with a current limiting resistor.

That will work to begin with, but it's not safe in the long run. The three cells will each have slightly different capacities, so one of them will always end up getting fully charged first. If they're connected in series, that one will get over-charged while the others are still charging. Likewise during discharge, the weakest cell will get over-discharged. There are circumstances in which attempting to charge a LiIon cell which has been over-discharged can cause a fire.

These cells need to be closely monitored during charge and discharge to make sure that no cell in a series-connected pack gets over-charged or over-discharged. That's what battery management systems (BMS) do. Basic ones are available cheaply on eBay and seem to work OK, but it's still important to make sure that the charger voltage is exactly correct otherwise the charge states of the cells can end up very unbalanced. Though not harmful to the cells, this will spoil the performance of the battery pack.

Chris

[Ed_Dinning]

Hi Folks, an interesting topic, can anyone point to a definitive guide to charging the many varieties of cells now available?

Ed

[The Philpott]

The (name brand) Li-Ion power tool battery packs that i've looked inside have certainly had the BMS that Chris refers to, babysitting each cell. I recall the JCB one had either a thermal fuse or a thermistor strapped to the outside of the cells. It refused to charge on one occasion and on checking what the LED's were telling me- it confirmed the ambient temp. was too low. So that would be another significant change from Ni-Cad systems.

Dave

[paulsherwin]

While a proper BMS is certainly desirable and best practice, lots of cheap consumer devices do apply a simple fixed voltage to the string of cells without individual cell management.

[llama]

I suddenly had a thought - my "WorkZone" multi-tool has a simple 3-cell battery with charger so if I take the connections from the charger pod to my newly made battery pack the job's done.
However, on the base of the WZ pod it says "output 10.8 v". Makes no sense - when I had the WZ battery pack to bits it has 3x 18650s in it, simply in series.
So I think my favourite option at the moment is to feed a well regulated 12.6v via a series resistor, as suggested by Paul.
WZ still a mystery though!
Graham

[paulsherwin]

The cells will be undercharged, but that may not matter for this application, and it's a bit safer.

[stuarth]

As Chris says in post 8, simply charging the string of 3 cells to 12.6V is not without risk, and BMS modules are cheap on eBay. Even a simple module just to control charging will be better than nothing, especially since the requirements for monitoring charging are quite onerous. What these modules do is shut off the charging as soon as one of the cells reaches 4.2V. (You mustn’t overcharge lithium ion cells).

A module which also prevents over discharging and protects against short circuits on the packs, would be better but more complex, and hopefully you’ll stop using the drill if it starts to slow down. If you get a module with short circuit protection, you’ll need to make sure it’ll handle your drill starting current.

Modules with cell balancing are a further level of complexity, but could be helpful if you’re using a bunch of mismatched cells.

The difference between 10.8 and 12V battery packs if largely down to marketing. Most Lithium ion (not to be confused with Lithium iron) cells are nominally 3.6 to 3.7V per cell on load, depending on the state of charge, and can be charged to 4.2V per cell. So you could justify a claim of either 10.8 or 12V for a 3-cell pack. Lidl changed the labelling of one of their 3-cell drills from 10.8V to 12V. I had access to both “types”, there were no differences in on load voltages or charge voltages, and the battery packs were interchangeable.

Stuart

[G6Tanuki]

I personally would not charge Li-Ion batteries without a BMS; there is one hell of a lot of energy stored in a fully-charged Li-Ion cell, and if this is subject to an uncontrolled release due to inappropriate usage-regimes the results can be somewhat messy.

If a major aerospace-manufacturer like Boeing can get it sufficiently wrong to have its newest airliner grounded worldwide as a result.....

https://www.scientificamerican.com/a...he-dreamliner/

[The Philpott]

-Agree with Stuart's last para., it's down to the manufacturer being unable to decide whether they badge the thing based on 3.6v per cell, 4.2v per cell,or whatever else.
Dave

[trsomian]

I would really not suggest that anything other than a current limited supply of the correct voltage be used for charging lithium ion batteries. Anything else is inviting disaster sometime. Cell balancing is very desirable also, though obviously irrelevant for single cells.

[GMB]

Li ion batteries with built-in BMS are best for sure. Anything else and you are inviting serious trouble. Don't forget that there are several significantly different types of Li ion battery so any external charging system must be designed for the correct type.

The difference between 10.8 and 12V battery packs if largely down to marketing.....So you could justify a claim of either 10.8 or 12V for a 3-cell pack.

YES. Even the same product in the same shop! We bought a couple of ultra cheap power drill/screwdrivers that said 10.8V in big letters - for a sinlge job where we could chuck them after abusing them. But they turned out to be so good and robust that I went back to buy another only be see it a different colour plastic and saying 12V. It turned out to be completely idenitcal with the parts and batteries interchangable. I think there should be a standard for voltage descriptions. Trading Standards - are you reading this?

[duncanlowe]

The voltage thing is definately about marketing. More volts musty be better, no?

Ferrex (Aldi's in house Brand) and Worx (who actually make Aldi tools as well as selling them under the worx name) re-branded their tools from 18v to 20vmax. They are still 18v nominal just that when just off charge they will be sitting closer to 20v. Bosch still market their tools with the same nominal voltage as 18v.

Incidentally the Ferrex and Worx batteries won't actually fit the other brand as the identical battery connection plates have little coding tabs selectively moulded in. Selectively removing these coding tabs with a sharp craft knife solves this!

[llama]

Time for an update.

I mounted the 3x 18650s in one of the battery packs and put a WW resistor in the original charger with 4mm sockets to connect my PK55 PSU to, set to ~12.4 Volts. I steadily reduced the resistor value until successful charging was achieved at, I think, 10 Ohms. Plugged into the drill it worked fine.

But all the above words-of-wisdom kept nagging at me and what I had was hardly convenient. More words of wisdom were to be found in the Sept 2022 RadCom.

I ordered a few "10A" BMSs off ebay and added those to the mix. What I didn't realise was that the input and output terminal are one and the same. Re-reading the RadCom article (a few times) showed me that this must be the case however. Meanwhile I'd taken the output directly off the top and bottom of my battery pack and the drill worked superbly.

But having realised I should connect the drill to the + and - terminals on the PCB and not use the battery directly (as over-discharge and over-current protection seems to be built into the BMSs) I again rewired the battery pack correctly. The battery charges perfectly but no joy in getting the drill to turn. I tried floating my charging arrangement across an on-charge lead-acid battery but results were the same. After much head scratching I realised that the over-current protection must be the problem.

So now I've ordered some "20A" and some "40A" BMSs and will swop those into my battery pack and hope they do the trick. Happy to learn but a lack of documentation and my lack of awareness of some of the edge conditions are proving "interesting"!

Graham