Hello everyone,
this is my first post here, very glad to have found what seems to be a helpful and caring community for electronics.
I'll try and stay on point: I need help in my second project, which is repairing an 18v vacuum cleaner.
https://www.pccomponentes.com/rowenta-air-force-extreme-18v-aspirador-escoba-sin-cableWhat's wrong with it?? Almost certainly just the dead battery, and no way to charge it. (A light comes on dimly when switch is moved into "turbo" position)
Why not just buy a charger?? It's 25€ which I don't want to spend without knowing if it is worth repairing/modding (was thinking of trying to upgrade it to Li-ion if it all works as the unit actually got really good reviews when it was released and wasn't particularly cheap.)
So here is my thought process and where I need help: I'd like to try and charge the battery (which has been dead for about two years, its NiMH, sitting at 10v) by whatever means possible, and then test the hoover to make sure the rest of it works as it should, before committing money to it.
As I understand, some people have had good success reviving NiMh packs using very slow charging and doing a few full cycles - that would be the most cost effective and simple solution.
The problem is [almost entirely due to my noob ignorance for sure] - the hoover is marketed as 18v. However the battery pack is 20s. I'm assuming this as no other configuration makes sense, there are definitely twenty cells, and NiMh has a nominal voltage of 1.2v and a max of about 1.4v, per cell. There is no data sheet online for this particular pack, and the only marking is a sticker which says 18v 2000mah (again making sense of the 20s configuration assumption with the 2000mah rating).
https://www.electrotodo.es/bateria-aspirador-air-force-extreme-18v-rh5233 - the replacement part.
So assuming my assumption is correct, the max voltage for the pack should be 28v, and nominal 24v.
To make matters more confusing [for me at least], the replacement charger is 36v, 250mA. Now this does actually make some sense to me, since the only PCB in the hoover was clearly designed for 6 different configurations, 12v, 18v, or 24v NiMh, or 14.4v, 18v, or 25.2v Li-ion. (here is the 24v version
https://www.electrotodo.es/circuito-aspirador-air-force-24v-rowenta-rh5313) so I guess it just steps it down as per each configuration.
Now I don't have a 36v adaptor to plug into the hoover board, nor a 28v to go directly into the battery, but I do have a 24v AC/DC adaptor. The adaptor I have spits out 600mA and actually measures 23.5v on my [poor quality] multimeter. I figured that I could use it under observation to get the batteries somewhere near 24v in short goes (as really most manufacturers seem to recommend 0.1C long charging as the easiest for NiMh) but it got pretty hot pretty quick so I haven't tried again.
Now that adaptor is not usually meant to charge a battery, it usually just runs a device which is on or off - so I figured [afterwards] that it would be missing a diode (or some more advanced circuit) to stop reverse charging?? Could I put one on one of the output terminals to avoid this? Is it that simple?
Also would I be OK to put a resistor also in series, to get that current down from 600mA to 100mA (which I figured would be perfect for a very slow charge to try and revive it)?? I'm hoping my awful maths skills are OK for the calculation haha, 24v/0.1A = 240ohms resistor?? I'm a bit embarrassed to say that I don't feel confident in applying ohms law, since I'm not sure if I'm putting in the right values!! My logic is that I want 0.1A from the 24v supply so that's what I calculated. If I got this wrong and there is an easy way to correct me, it'd be greatly appreciated!!
So I guess that's my questions for now. Sorry for the super long post, I'm trying to be as clear as possible. Thanks in advance for any help - there will undoubtedly be more questions in the future. And any suggestions/tips for changes of direction/learning resources etc. are super welcome!!
(I am starting to study via the book Make: Electronics, I hope to one day be able to help other people too, rather than asking so many questions!)
edit: typo