Optimized NiMH charger, including d2V/dt2<=0, temperature and dV/dt<0 checking - Page 1
Forum section: electronics / projects

author: F1GWR
author_rank: Regular Contributor
timestamp: November 14, 2024, 08:35:29 pm
content: After reading many advices:https://www.eevblog.com/forum/projects/diy-ni-mh-battery-charger-no-delta-v-and-no-delta-t/msg3864785/#msg3864785https://www.eevblog.com/forum/beginners/need-help-designing-a-charger-for-nimh-batteries/msg1259231/#msg1259231and some literature:https://www.analog.com/media/en/technical-documentation/application-notes/an64f.pdfhttps://www.analog.com/media/en/technical-documentation/data-sheets/lt1325.pdfhttps://pe2bz.philpem.me.uk/pdf%20on%20typenumber/A-C/BATTERY.pdf(unfortunately Temic as well as the said IC's disappeared)I need to charge occasionally, with very infrequent use, a device powered by 2x2 6V (i.e. symmetrical +/- 12 V) button cell battery (type SAFT or Varta). Therefore the degradation risk of self-discharge with reverse cell charging, and/or the questionable trickle charge with overcharge would be very likely.Now my question, am looking for charger bricks that incorporates ALL these features:d2V/dt2<=0with filtering,andtemperature increase,anddV/dt<0 detection for overcharge prevention, either:- a stand alone integrated circuit that is not obsolete- an open source micro-controller software and circuitpreferably not too expensive.A batteries conditioning procedure would be a plus.Thanks,Christian

author: shabaz
author_rank: Frequent Contributor
timestamp: November 15, 2024, 04:07:01 am
content: I think that with all those features you list, it becomes so particular that you may have to trailblaze and custom develop it.There are some Arduino-based chargers on Instructables, but I have no idea how good or bad they are.An alternative approach: you could simplify with a trickle charge at 0.1C (check your battery datasheet if this is supported).I recently built myself a 9V NiMH charger.Reading aGP battery technical PDF paper, it's OK to charge at 0.1C, for a full charge at 16 hours according to that doc. It also states that indefinite charging at 0.1C can be endured for a year (for GP batteries).Based on that, I simply charge at a constant current for 0.1C, and cut-off after 19 hours (this was easier for my circuit than 16 hours). The charging is inhibited if the battery gets too warm (I mounted a thermistor close to the battery positive contact on the PCB), although that should never occur at 0.1C, but it's a safety feature.I didn't use a microcontroller, just a personal choice since it was a hobby project and I just wanted a break from coding.I used 74HC counter/divider ICs to time the 19 hours, and a MOSFET to switch it off.All jellybean components, no microcontroller or specialist charger IC that may become obsolete.With this approach, I didn't use any dV techniques (although they are excellent, when charging at a more normal rate). I just didn't have a need to charge the battery within an hour or two; I was happy to wait a day for the battery to charge (I can always keep a spare battery charged and ready to go).Is this approach any good? No idea. I'm merely mentioning it, in case you wish to rule in/rule out some sort of charging method at 0.1C instead, if you have not already considered it.