Automatic Ni-MH battery charger woes - Page 1
Forum section: electronics / projects

author: VanMarco
author_rank: Contributor
timestamp: January 28, 2020, 04:27:23 am
content: Hello people!I am building this circuit herehttps://www.eleccircuit.com/wp-content/uploads/2013/07/ni-mh-battery-charger-circuit-using-tl072.pngI start in advance by saying that I have added an increased level of difficulty to this build by using mostly used parts, I changed the TL072 with an LM358, and the transistor with a 2N6667. The issue I am having is that the LM358 got hot and likely died. The first error i made was to put D1 in reverse (as you can see from the full page link the 1st image shows it one way, the second the opposite). I since put the diode back as it needed to go, replaced the IC, and now the red led does not turn on nor it charges, also pressing the button "to start" does not seem to do it. I replaced the BC 327 with another one, it was new and the multimeter says it's not faulty.The only difference between 1n6667 and mje listen is that there is a big difference on the base current. It is possible the small elec. capacitor is faulty but I am not sure it would cause such problems based on schematic.I have no idea why such a simple circuit does not work.Full page linkhttps://www.eleccircuit.com/automatic-ni-mh-battery-charger-can-make-by-yourself/Ebay page of a seller that has exactly the same thing i am doinghttps://www.ebay.com/itm/Ni-MH-Charger-Auto-Stop-Circuit-board-1-to-10-cell-800mA-MJE2955T-12-16VDC-/140353600313thanks in advance!

author: BravoV
author_rank: Super Contributor
timestamp: January 28, 2020, 04:33:34 am
content: The circuit's explanation does not mention about how it detects full charge state, which is the "secret recipe" in charging algorithm for Nickel based cell (NiMh or NiCd), without it, it will ruin the battery really fast.Charging LiIon is much-much more simpler.

author: Buriedcode
author_rank: Super Contributor
timestamp: January 28, 2020, 04:57:30 am
content: It looks like a crude delta V charge termination using the opamp as a comparator, with D4 and C4 holding the max voltage of the cells, and R8/C5 the actual voltage of the cells. When the voltage across the cells starts to fall, the voltage on C5 will drop below the voltage of C4, making the opamps output high, turning off charge. R2 and D1 look like they add some hysteresis (the diode to stop C4 discharging through R3 when charging), so charging is held off until the button is pressed again.Kind of a cool circuit, but I would use an actual comparator instead of an opamp. The TL072 has inputs that can sense up to VCC but not to its negative rail, so in this circuit it cannot sense Vbatt below about 2.5V.Using "used" parts is problematic for a couple of reasons, namely that you don't know if they are in any way damaged, and also that their specs may differ enough from the intended parts to negative affect the circuit.

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 28, 2020, 05:00:08 am
content: Looks like a simple voltage slope detector. That said, nowadays it's a lot better to do it in a microcontroller.http://www.ultrasmartcharger.com/phpBB3/viewtopic.php?f=3&t=5

author: VanMarco
author_rank: Contributor
timestamp: January 28, 2020, 07:47:04 am
content: thanks for your reply. So i should buy TL072? all the other parts looks fine, i could put new 1n4148s.

author: MarkF
author_rank: Super Contributor
timestamp: January 28, 2020, 12:24:06 pm
content: Here is an alternate circuit:- Select the sense resistor (RED) for the charging current desired. Some values and currents shown.- Adjust the potentiometer (BLUE) for the desired voltage. Charging current will reduce as the voltage approaches full charge.

author: VanMarco
author_rank: Contributor
timestamp: January 28, 2020, 04:42:00 pm
content: hello markF thanks for the circuit. I was looking for something that tells with an LED when it's done for, but this may be OK as I don't have a TL072 on hand. the resistors shown are all 1/4 w except the current limiting one? thanks!

author: MarkF
author_rank: Super Contributor
timestamp: January 28, 2020, 05:14:42 pm
content: Quote from: VanMarco on January 28, 2020, 04:42:00 pmYes. The current sense resistor is the only one that handles any power.The LED will turn off when it is finished charging. See circuit description:https://www.eleccircuit.com/gel-cell-battery-charger-circuit/

author: MarkF
author_rank: Super Contributor
timestamp: January 28, 2020, 05:37:37 pm
content: Here is a little show-and-tell clip.However, I'm not too thrilled with his construction.He points out at the end that the LM317 gets hot.So, use an adequate sized heatsink.

author: Buriedcode
author_rank: Super Contributor
timestamp: January 28, 2020, 08:56:44 pm
content: That charger circuit is for a gel cell - if charges at a constant current, like an NiMH/NiCd, but then goes into voltage regulation. That circuit is OK for SLA's but will quite possibly kill a NiMH as it requires proper charge termination either by delta T (a sudden increase in temperature), delta V (a slight dip in cell voltage) or a timer. Preferably all of the above.If you set the voltage limit to something reasonable like 1.35 - 1.4V per cell, then it might be "OK", but I wouldn't trust a CC/CV charger on chemistries other than Lithium Ion or SLA.Edit: typos.Edit2: I'm not sure that the charger will ever stop, because the TL072 (and the LM358) have outputs that cannot get within ~2V of VCC - in your case 12V. This means it can't fully turn off Q1/Q1.

author: VanMarco
author_rank: Contributor
timestamp: February 02, 2020, 02:38:23 am
content: Hello. I built this pcb for this.r6 and r7 are just bridges.I have changed the pot to 10K because i dont have a 5K one, and replaced the 2.2k resistor with a 1k one, as this circuit was designed for lead acid, voltage didnt want to go to 5V to charge 4x AAA batteries. One thing happened. I connected the batteries and switched on, the led turned on briefly and then died. voltage at the batteries is 5V. I am not sure it is charging.Did i break something>?

author: VanMarco
author_rank: Contributor
timestamp: February 02, 2020, 03:52:43 am
content: well, batteries are technically charged, now i want to see how long they last.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 02, 2020, 04:18:57 am
content: NiMH batteries really need several redundant systems to determine when to cut the charge. The voltage slope can be hard to detect, so it should be combined with a temperature sensor at a minimum. A timer is good, plus a total charge limit. That's why it's usually better to buy a decent unit. You can also charge at a lower rate for a specified time, so long as you don't let the battery stay on the charger after the time limit.

author: MarkF
author_rank: Super Contributor
timestamp: February 02, 2020, 07:03:29 am
content: Quote from: VanMarco on February 02, 2020, 02:38:23 amNice.Here is the layout I did:Quote from: Conrad Hoffman on February 02, 2020, 04:18:57 amThe NiMH battery chargers are constant current sources that will (if the charging isn't terminated) will chargethe batteries to a voltage higher than their rating. (i.e. You don't set the voltage.)That is why they need to check the temperature and/or voltage drop to know when to stop charging.This charger NEEDS to have it's charging voltage set at the batteries full voltage. This will prevent over charging.It will charge at the set constant current value until it nears completion.As it reaches the set voltage, the charging current will decease and stop when the set voltage is reached.(The LED goes out when charged.)I believe the charging current is somewhat exponential.Whether this is good or bad for a NiMH battery, I don't know.

author: VanMarco
author_rank: Contributor
timestamp: February 02, 2020, 10:48:55 am
content: I think my cells are rubbish. You know those china batteries that come in bright attractive colour like redd, orange and have written on crazy values like 1,800mah and the like, and cost like 5 bucks for 16pcs? those. I think those are 130mAh each, or something along those lines. When i put 3 into a flashlight (led) they last around 15/20 mins or so. MarkF can you show me the underside of your circuit?That's mine.thanks

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 02, 2020, 01:58:38 pm
content: You need to set a higher voltage than "nominal" or the batteries will never be fully charged. The problem is, if you do that, you need those better methods of stopping the higher current charge. Or, you suffer with a longer charge time at lower current and rely on time, usually 16 hours or something. Modern NiMH is better at tolerating a trickle charge, but it still isn't recommended. No idea on the inexpensive batteries. The best seem to be the Eneloops, but they aren't cheap. OTOH, they don't self-discharge much over time, so you can use them in clocks, remotes and flashlights. I'm still using some ten year old "old tech" NiMH AA batteries in my electric tooth brush and they work well because I change them frequently; they don't have time to self-discharge before I run them down. The cells with a high leakage rate would be a bad choice in a flashlight because they'll be dead when you need it most.

author: MarkF
author_rank: Super Contributor
timestamp: February 02, 2020, 03:20:55 pm
content: Quote from: VanMarco on February 02, 2020, 10:48:55 amHere you go:

author: VanMarco
author_rank: Contributor
timestamp: February 02, 2020, 04:40:41 pm
content: it still puzzles me how is it possible that discharged batteries go in there and immediately voltage is the same as the charging voltage.Also is it normal that I get battery voltage on the DC input when the dc input is disconnected?

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 02, 2020, 06:26:30 pm
content: Quote from: VanMarco on February 02, 2020, 04:40:41 pmA lot of batteries have a "hysteresis" effect where the charging voltage is significantly more than the open circuit voltage.QuoteNo, that means you forgot to include a blocking diode.

author: Buriedcode
author_rank: Super Contributor
timestamp: February 02, 2020, 10:37:34 pm
content: Using a CC/CV charger for NiMH is not a very good idea. The peak voltage per cell depends on the cell, and the charging current. Arbitrarily setting it to whatever voltage you feel like will mean either the cells don't get charged much at all, or worse, over charged, reducing their life, possibly damaging the cell.VanMarco: Please post aschematicof the charging circuit you are building/using/testing, along with the parts you used for it. A layout means nothing if we don't know what the circuit is or the values used. Some might be confused because you provided a link to one charger in the original post, but then said you had built a different one later on.

author: VanMarco
author_rank: Contributor
timestamp: February 02, 2020, 11:42:34 pm
content: I did use the schematic of the circuit posted by markF

author: dave_j_fan
author_rank: Regular Contributor
timestamp: February 03, 2020, 02:21:44 am
content: i would differ here as overdesigning nimh circuit is notneededsimple resi and capacitor design works wellcamelion chargers have very simple design

author: IanB
author_rank: Super Contributor
timestamp: February 03, 2020, 04:13:26 am
content: Quote from: MarkF on February 02, 2020, 07:03:29 amThe characteristics of NiMH cells mean that they cannot reasonably be charged using a CC/CV algorithm. An NiMH cell does not have a concept of a "full voltage" that you can program into a charger. Every NiMH battery is different, and for a given battery the voltage can vary a lot with age, temperature and charging current. It this respect NiMH batteries are quite different from lithium ion batteries and cannot be charged that way in any useful sense.

author: BravoV
author_rank: Super Contributor
timestamp: February 03, 2020, 11:00:20 am
content: Curious if the op aware that he is building a Li-Ion chargers, for his NiMh battery ?

author: Zero999
author_rank: Super Contributor
timestamp: February 03, 2020, 12:37:26 pm
content: I'll add my name to the list of those who agree the circuit posted by markF is not suitable for NiMH cells and is probably originally designed for lead acid.The circuit originally posted is much better. U1 should have a resistor to Q1 because relying on the op-amps current limit to restrict the base current is bad design. Yes, a proper comparator IC such as the LM393 or LM311, would be a better idea.

author: VanMarco
author_rank: Contributor
timestamp: February 05, 2020, 03:34:28 am
content: i do have an lm393 on hand. i still cannot figure why the IC gets so hot.

author: mikerj
author_rank: Super Contributor
timestamp: February 05, 2020, 01:44:22 pm
content: Quote from: dave_j_fan on February 03, 2020, 02:21:44 amA simple resistor based trickle charger for NiCDs is ok, provided the rate is low enough but trickle charging NiMH cells is not generally recommended unless performed at very low currents (<0.05C). Certainly you shouldn't be entertaining the idea of full speed charging like this without some method of automatically terminating the charge.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 05, 2020, 03:15:35 pm
content: It's important to understand the battery technology. Battery U. isn't perfect, but they generally give a good overview:https://batteryuniversity.com/learn/article/charging_nickel_metal_hydrideNiMH needs very good charge termination if you want to charge at a reasonable rate. At odds with that, NiMH doesn't charge well at slow rates, and still has to be terminated at about the correct point. I've owned several different chargers that claimed to do NiMH well, but only the most recent one seems to do well without damaging the batteries. IMO, there's no simple circuit that works well for this- you need a battery chargingsystem, where you have several redundant termination methods.

author: Zero999
author_rank: Super Contributor
timestamp: February 05, 2020, 04:50:54 pm
content: Quote from: VanMarco on February 05, 2020, 03:34:28 amAdd a 2k2 resistor in series with the op-amp/comparator output.