DIY - NI-Mh battery charger, no delta-v and no delta-t. - Page 1
Forum section: electronics / projects

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: November 19, 2021, 08:56:31 am
content: Hello everyone.I'm starting a project for a charger, with characteristics very different from traditional chargers. I want the charge to be defined by the battery itself. Observe the circuit inimage 1- see through the calculations presented, that the charge decreases as the battery voltage rises. Seeimage 2, the behavior of the voltage and current I want to obtain. As the final charge decreases, there won't be the possibility to measure the delta V or the delta T. My real intention is that the battery doesn't heat up as it approaches the final charge. I intend to build a circuit in which the cut will be determined, when the battery voltage reaches 1.495v. I am aware that the final charge may not reach 100% but I want to reach at least 80%. My question: what kind of problem might occur with "non-linear" loading?Image 3: Reply #22 - Final project with excellent result!

author: TheMG
author_rank: Frequent Contributor
timestamp: November 19, 2021, 02:35:51 pm
content: In practice this is really not much different than charging the battery through a simple resistor from a regulated 1.5v supply. No need for the opamp transistors etc.

author: ogden
author_rank: Super Contributor
timestamp: November 19, 2021, 02:54:39 pm
content: Never ever charge batteries w/o current limiting/regulation

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: November 19, 2021, 05:01:54 pm
content: Quote from: TheMG on November 19, 2021, 02:35:51 pmExactly! The idea applied here is very simple. I will use either 5v or 3.3v from an ATX power supply. I will keep the transistors and the opamp, as I intend to carry out other functions with them (cut and protection).Quote from: ogden on November 19, 2021, 02:54:39 pmThe circuit is still pretty incomplete. Current limiting will either be built in with a transistor on the output, cutting off Q1 and I will use a simple fuse on the input.

author: floobydust
author_rank: Super Contributor
timestamp: November 19, 2021, 05:32:08 pm
content: I found it did not work. With Eneloops or Ikea Ladda LSD NiMH, they need the "top up" charge or else their capacity drops. This is the peak delta-V phase. Then you must lower charging voltage, then stop charging completely.Steady voltage (after bulk charge) will kill NiMH either from overheating or lack of top-up. I have a cordless phone with constant-current 1.50V (it was NiCd) and NiMH don't do well, they don't last. I just pulled Ladda batteries that had dried out from staying in the charger too much, because they have low capacity without the top-up phase. They do behave differently from Eneloops I have used the same way.You cannot "trickle charge" NiMH, this is not recommended. Max. is 0.033-0.05C which is way less than NiCd, so they dry out if charged too long or in a CMOS memory-backup use.Graph taken fromhttps://lygte-info.dk/info/batteryChargingNiMH UK.html

author: Benta
author_rank: Super Contributor
timestamp: November 19, 2021, 06:25:40 pm
content: The best strategy for charging NiCd or NiMH cells is d2V/dt2detection. This gives the cells a long life, I'm speaking from experience, my cells have until now survived 1000s of charge cycles without significant degradation. All other strategies that I've seen result in overcharging and damage.To understand the idea, p.4 of this data sheet helps:https://pdf1.alldatasheetde.com/datasheet-pdf/view/26117/TEMIC/U2402B.htmlToday, you'd let an MCU with associated ADCs run the algorithm.

author: ogden
author_rank: Super Contributor
timestamp: November 19, 2021, 10:40:44 pm
content: Quote from: Avelino Sampaio on November 19, 2021, 05:01:54 pmFor battery charder it does not make sense to address voltage regulation first. Fuse?? You clearely do not know what you are doing.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: November 19, 2021, 11:00:23 pm
content: Floobydust,thank you very much for the page indicated, it was of great value. Thanks also for sharing your experiences. I will still persist with the tests, using my idea of applying a variable charge to the battery. As you can see in the image, I have some batteries, ranging from new (eneloop) to fully zeroed. My fun will start....

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: November 19, 2021, 11:36:53 pm
content: ogden,have a little patience with me. I certainly couldn't interpret your message correctly. What's the problem with starting with aquick charge(0.5C) and ending with astandard charge(0.1C)? Where is the lack of control in this?

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 20, 2021, 02:43:10 am
content: This is the best open hardware NiMH charger I'm aware of:http://www.ultrasmartcharger.com/

author: ogden
author_rank: Super Contributor
timestamp: November 20, 2021, 09:45:02 am
content: Quote from: Avelino Sampaio on November 19, 2021, 11:36:53 pmIdea of fuse as current regulator made me impatient, sorryQuoteProblem is thatyour circuit is voltage, not current source. If supply is not limiting factor and transistor of your charger does not melt, then it can easily put more than 2C current into poor battery and quickly overheat it. Don't reinvent the wheel. Look for existing circuits, pick one that you like, copy it & test. After that modify and improve - if needed. You can test/simulate your circuits without blowing-up transistors or batteries:https://www.analog.com/en/design-center/design-tools-and-calculators/ltspice-simulator.html

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: November 20, 2021, 10:49:40 am
content: OgdenSorry, it's not yet time for me to give up until I'm fully convinced of my idiocy.Benta and NiHaoMikeas I mentioned, my intention with this project is not to achieve maximum efficiency (100%) in loading. Any value above 80% I will be satisfied. I don't care if this will shorten battery life, as long as it's not terribly significant.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: November 20, 2021, 11:56:01 am
content: The EneloopOn the eneeloop packaging it says that the batteries were ready for use. I unpacked and measured the tensions. All batteries (AA and AAA) measured 1.31v. To find out how much charge they had, I used my electronic charge, set to 500mA, and tested the discharge until I reached the 1v limit. The AA battery (2550mAh) had a load of 1000mAh and the AAA battery (950mAh) was only 300mAh.Project (image 1)I managed to add to the project the part that interrupts the battery charging, when it reaches the limit voltage, which I stipulated at 1.495v.First battery charge (image 2).I did the first charging test with 2550mAh eneloop AA, which I had discharged up to 1v. Charging time was 9 hours, until reaching the voltage limit I stipulated of 1.495vTime to unload and measureThe battery voltage, after 8 hours of rest, is at 1.40v (image 3). I'm going to start the unload test and see how much loading I got.

author: ogden
author_rank: Super Contributor
timestamp: November 20, 2021, 04:39:25 pm
content: Quote from: Avelino Sampaio on November 20, 2021, 11:56:01 amNot even trying to introduce current regulation. Hopefully others do not learn from you. I am out.

author: Benta
author_rank: Super Contributor
timestamp: November 20, 2021, 04:45:36 pm
content: Quote from: ogden on November 20, 2021, 04:39:25 pmMe too.

author: IanB
author_rank: Super Contributor
timestamp: November 20, 2021, 05:46:43 pm
content: Some time ago I did some charging experiments to learn about the characteristics of NiMH cells. Attached is a charging graph for a 1900 mAh Eneloop. You can see that when the terminal voltage reaches 1.48 V in this test, the cell has only received 50% of a full charge. You can also see why charging at constant current and monitoring rate of change of voltage, and/or rate of change of temperature, makes for a sensible charging strategy.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: November 21, 2021, 10:59:00 am
content: IanBThank you so much for bringing this question to me. See inimage 1the graphics that I extracted from the link below (Ansmann).https://datasheet.octopart.com/5030852-Ansmann-datasheet-5400527.pdfNote that forStandard Charger modethe ceiling voltage has decreased while forFast Charge modethe ceiling voltage has increased significantly. You're absolutely right, if I use the cut-off voltage at 1.495v inFast Charger modethe charging will be much lower than expected. See in image 2 the results I got with this project of mine - the AA battery performed well (87.6%) as it spent most of the time instandard charger mode. The AAA battery remained part of the time inquick Charger modeand here I assume that the cutoff voltage would then have to be a little higher than 1.495v, in order to be able to surpass 80% charging.Image 3- Some updates to the project.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: November 24, 2021, 12:39:33 pm
content: Hi!Image 1- The project is advancing and the results so far are very promising. The project is capable of charging batteries ranging from 300mAh to 2700mAh, with an average charge exceeding 85%.Image 2- I added a timing when the voltage reaches 1.495v and with that I managed to overcome the issue of the final cutoff voltage. There was never any change in the temperature of the batteries, as charging always ends inStandard Charger Mode.It's been a lot of fun putting this together with discrete components. At the end of the tests, I will make a version using a PIC12F.. processor.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: November 27, 2021, 10:01:57 pm
content: Hiimage 1: A few more eneloop's arrived.image 2: I will start a new version of the project, using the PIC12F.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: November 30, 2021, 04:41:36 pm
content: Hi!Errors, hits and assembley programming, resulting in another project update.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: December 02, 2021, 08:09:50 am
content: HelloI'm managing to carry out a load between 95% and 100%The programming I am adopting has as its main objective the monitoring of the slope of the voltage curve, and thus getting as close as possible to the top voltage. Monitoring starts from a voltage of 1.48v and every 15 minutes a measurement is taken and then compared with the previous measurement. If the result is less than 3.4mv in the 15 minute interval, it means that we are approaching the top voltage. At this point I decrease the voltage over the battery (from 1.61v to 1.53v) and leave it with a slow charge for another 60 minutes.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: December 03, 2021, 09:08:16 pm
content: Hi!Battery inversion in socketI had already added resistors R2 and R12, in order to protect the PIC and Opamp. Note in the image that the current flowing through the internal diodes is below 1mA. They are well within the safety margin. The problem that I didn't count on is that transistor Q2 (BD139) is fully conducting to the point where Q3 is acting and limiting the current to 0.6A. I need to interrupt the Opamp or transistors so that it blocks this strong conduction. Could someone suggest me how to perform this block?

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: December 04, 2021, 07:52:10 am
content: HiI managed to solve it, adding a mosfet (Q6).I finished the PIC programming and the end result was way above my expectations. I will now plan the assembly of the PCB.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: December 09, 2021, 10:08:16 pm
content: HiFor this project, the 5v power supply will have to show stability under any load conditions, which will vary between 75mA to 2.2A. At first I would use the 5v from an ATX source but I decided to change my mind and use an independent source. I ordered a power supply from banggood (Image1) but the question remains whether I will have the 5v regulated, under any load conditions. While I'm waiting for my order, I found here in my electronics scrap, a 28v+12v source (image2) that has a transformer that seems to guarantee at least 30w of output. So I decided to modify it, taking advantage of the 12v output. See in image 3 the layout of the original output and in image 4 the modifications I made to get the 5v. I've had success with this adaptation, managing to keep the 5v at any required load (images 5 and 6). This source will remain as plan B, if the source I ordered doesn't fit.

author: Benta
author_rank: Super Contributor
timestamp: December 09, 2021, 10:44:46 pm
content: It seems you're speaking with yourself?

author: SilverSolder
author_rank: Super Contributor
timestamp: December 09, 2021, 11:55:16 pm
content: Quote from: Benta on November 19, 2021, 06:25:40 pmThat seems an interesting principle, haven't seen that before. Not sure how the low current phase (after d2V/dt2 detection occurs) is handled, is it just a timer or does it still need -dv/dt to stop charging?

author: vk6zgo
author_rank: Super Contributor
timestamp: December 10, 2021, 01:44:08 am
content: Quote from: ogden on November 19, 2021, 02:54:39 pmThere are "batteries & batteries".Lead/acid "wet" cells have been charged with simple transformer plus rectifier combinations for decades.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: January 12, 2022, 03:54:03 pm
content: HiI finally managed to finish the tests and I'm very satisfied with the results. The font I ordered from banggood met the requirements.I thank the participants for sharing important information and thus contributing to my learning process.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: January 19, 2022, 05:43:42 pm
content: Hi!I took advantage of the box of an old stabilizer and thus concluded my project. Myeneloop, at the end of the charge, reaching around 1.48v.