Need help designing a charger for NiMh batteries - Page 1
Forum section: electronics / beginners

author: engineheat
author_rank: Frequent Contributor
timestamp: July 12, 2017, 11:49:04 pm
content: Hello.I want to design a charging circuit to charge 2 NiMh AA batteries in series, via a 5V USB Micro cable. I have some basic electronics knowledge but not very in depth, so your help/suggestions would be appreciated.OPTION 1:So far, I have looked at TI's BQ2002 chip. Unfortunately, the diagram they have in the data sheet (below) is not very helpful:The diagram above seems too simple and does not provide enough guidance on how to actually build an application. Then I found a spec sheet but the circuit in there seems pretty complex:http://www.ti.com/lit/ug/sluu007b/sluu007b.pdfI wonder if the diagram in page 3 of that spec sheet with the LM317T is the actual circuit I will have to build for my application?OPTION 2:I am also considering LTC4060 IC from Linear Technologies. It seems to require fewer external components than the BQ2002, and there seems to be even a tutorial on using it:http://www.instructables.com/id/How-to-Make-a-NiMH-NiCd-Battery-Charger-Circuit/Here is a diagram for the LTC4060, I wonder if the actual circuit will really be this simple:OPTION 3:Finally, there is this very detailed step by step tutorial that does not use any dedicated charger IC:http://www.stefanv.com/electronics/usb_charger.htmlBut it only uses temperature (thermistor) for charge termination. I feel this is not adequate if one eventually wants to build a charger that's good enough for commercial application.----------------------------------------------------------------------So basically, option 1 seems to be the most difficult route, and option 2 or 3 seems easier. I feel option 3 is too simple and is more of a toy. My goal is to actually build a charger that is safe, robust, and good enough for commercialization. And in the process, I want to learn.Which option do you recommend? (in terms of difficulty level for a novice/quality of the charger, etc...)Thanks

author: amspire
author_rank: Super Contributor
timestamp: July 13, 2017, 02:52:46 am
content: You have probably realized that charging NiMH is fairly difficult. Charging lithium batteries is very straightforward, so it is worth considering using a single lithium cell instead of 2xAA if it is possible.Is this a circuit to charge batteries in a portable device, or are you just building a general purpose AA cell charger?If it is a general purpose charger, then with two AA NiMH cells, you really need to monitor the dV/dT and temp rise of each cell separately - perhaps two separate charging circuits. Lots of cheap chargers just charge a few NiMH cells in series based on dV/dT only and that is a perfect way to damage NiMH cells. I have seen this type of charger often overcharge cells and NiMH is not tolerant to overcharging.If I were building a charger like this, I probably would use a cheap buck converter to reduce the 5V 500mA USB down to 2V at over 1A, and then use this as the charging source with separate circuits for each cell. Both the IC's you have found are probably fine to base a charger on, but you have picked one of the hardest battery types to charge. There will be some head scratching to understand the IC documentation.

author: engineheat
author_rank: Frequent Contributor
timestamp: July 13, 2017, 03:20:16 am
content: Quote from: amspire on July 13, 2017, 02:52:46 amThanks for the reply.I chose NiMh because it is safer than Lithium-ion. You hear stories of Lithium powered laptops or e-cigs exploding. Based on my research, NiMh are not as catastrophic when abused since nothing inside is inherently flammable. It'll vent or leak electrolytes that's about it.I actually plan to use those 2AA packs rather than individual cells. There shouldn't be too big of a difference in cell behavior as opposed to mix and matching cells.The LTC4060 actually is a pretty smart chip (albeit a big costly). It has numerous termination conditions, DV/DT, temperature, and time. I studied the documentation and it's pretty accessible even to a newbie like me. Therefore, I feel the LTC4060 is not a "cheap" charger that only uses DV/DT. In fact, DV/DT is not very reliable if the charging current is low or if cell is damaged, etc... that's why it monitors the temperature as well. I feel temperature is probably the most reliable termination condition. Perhaps that's why OPTION 3 uses temperature termination.Overall, I'm not too worried of cells being damaged. I just can't afford to have things blow up like Lithium batteries do. Usually, AA NiMh has pressure vents so even if overcharged it won't be too bad...I might be wrong though.

author: engineheat
author_rank: Frequent Contributor
timestamp: July 17, 2017, 04:04:08 pm
content: any other comments/suggestions?

author: Benta
author_rank: Super Contributor
timestamp: July 17, 2017, 08:17:17 pm
content: Neither dT/dt nor dV/dt are good cutoff criteria for NiMH or NiCd.Detecting increasing dT/dt means you're already overcharging the cells.Zero or negative dV/dt detection also places your batteries in the overcharge range.The additional timers that your ICs have are just for insurance.A proven criteria that works is d2V/dt2.A forgotten, but extremely good charger IC for NiMH and NiCd batteries was the Temic U2402B, which I've used with great succes. Even with a 3C charge cycle, batteries would only get luke-warm.Unfortunately, it disappeared in the Atmel/Vishay takeovers, and the group designing and supporting charger ICs was disbanded as well.But it's operating principles align completely with the battery manufacturers recommendations, as well as the textbooks on battery charging.Today, the functionality of the U2402B is normally firmware in a microcontroller with A/D, being part of the device where the battery is placed.Links:Textbook excerpt from Google books:https://books.google.de/books?id=6CCOAwAAQBAJ&pg=PA112&lpg=PA112&dq=d2v/dt2+battery&source=bl&ots=p90Gc8Kus6&sig=V7x5kzveBz1b9r2wpYFGLTQidTA&hl=en&sa=X&ved=0ahUKEwifxPDCr5HVAhWGLVAKHTdKBLwQ6AEIRTAH#v=onepage&q=d2v%2Fdt2%20battery&f=falseU2402B datasheet:https://cdn-reichelt.de/documents/datenblatt/A200/U2402B%23TEM.pdfI hope this is an inspiration, at least it gives an idea on how to fast charge NiXX cells.

author: engineheat
author_rank: Frequent Contributor
timestamp: July 18, 2017, 03:00:39 am
content: Quote from: Benta on July 17, 2017, 08:17:17 pmThanks. I'll look into it.You mentioned fast charge. I wonder if speed of charging is not important, can I just make a simple and safe charger by slow charging it at C/10 and just terminate by time? Or perhaps drain the battery first.I've read this is a pretty simple yet safe algorithm if the battery is to be charged overnight (as opposed to fast charging)?

author: amspire
author_rank: Super Contributor
timestamp: July 18, 2017, 04:01:13 am
content: Quote from: engineheat on July 18, 2017, 03:00:39 amYou have to be careful with timed C/10 charging. If the battery is fully charged and you continue, it does increase temperature and pressure in the cells and probably shortens the life. You might find a lot of manufacturers do not actually state anywhere that it is OK.It is not a bad idea using a micro for NiMH charging. It doesn't need a big program. This site contains a suggested protocol.https://www.powerstream.com/NiMH.htmIf you added engineheat's suggested d2V/dt2protocol, it would be even better. With two cells in series, you will have the situation of one cell indicating full charge while the other isn't so it will make the detection twice as hard. Probably want to have the thermistor positioned so it gets the average of the two cells. From my experience, the thermistor is still a great idea.If you are charging the cells while there is load on the cells, you make voltage curve methods much harder. In this case, definitely have a thermistor or other kind of temperature sensor.

author: engineheat
author_rank: Frequent Contributor
timestamp: July 20, 2017, 04:35:32 pm
content: Quote from: amspire on July 18, 2017, 04:01:13 amThanks.I modified my design so that only one NiMh cell is needed. I believe this makes charging easier.I will have a path selector so that when the charger is plugged in, it will not charge the battery if there is a load.Overall, NiMh is "safer" than Li-ion in that it's not as explosive or dangerous when things go wrong?

author: David Hess
author_rank: Super Contributor
timestamp: July 21, 2017, 08:23:25 pm
content: I remember thinking that the U2402B-C was pretty neat but then it became unobtainable. Implementing the same thing with a microcontroller is straightforward if not as easy.Quote from: engineheat on July 20, 2017, 04:35:32 pmNiMH and NiCd batteries cannot fail as catastrophically but most rechargeable lithium failures are do to poor design. If you use a single cell and LiFePO4, then I think the ease of charging will make the lithium rechargeable a better choice *unless* you want to use standard NiMH for ease of user replacement. I would also consider multiple LiFePO4 cells in series with class-B analog charge balancing acceptable also at low power levels.

author: engineheat
author_rank: Frequent Contributor
timestamp: July 24, 2017, 01:24:53 am
content: Quote from: David Hess on July 21, 2017, 08:23:25 pmEven though NIMH can't fail catastrophically, it can still be troublesome if the user puts in an alkaline battery or Lithium battery (there are Li battery the same size as AA).I think I will just leave charging out of the equation now. I will make a battery compartment that takes a single AA battery and the user is free to use whatever battery they want. If it blows up while charging on their own charger, it's not my responsibility.This is a device I eventually want to commercialize, and I feel it's better to keep it simple while starting out. It also brings down the cost. It's a toy device so I don't think people will mind. Some people actually prefer it since they can always swap in fresh batteries in an instant as opposed to waiting for the device to charge via an USB. Any thought?

author: David Hess
author_rank: Super Contributor
timestamp: July 24, 2017, 01:51:48 am
content: Quote from: engineheat on July 24, 2017, 01:24:53 amThat is true. You could design a charger to detect if this happens but for that effort, you might as well design an equally smart rechargeable lithium charger and rechargable lithium batteries are easier to charge anyway except for their extra safety requirements.QuoteI have given this thought in the past. For devices where NiMH energy density is adequate, I would prefer user replaceable cells, probably AA size, with a built in charger that can accept a variety of power sources including 12 volts from a car and now 5 volts from a USB source. For higher energy density and power, I would prefer the same with standard format user replaceable LiFePO4 cells.User replaceable LiFePO4 cells are a major problem however because it is difficult for the charger to distinguish LiFePO4 from other rechargeable lithium chemistries and their charge parameters are not remotely compatible. Offhand I know of no charger which even attempts to distinguish them.