Cheap NiMH charger teardown, questions about NiCD, safety?

[edy]

Hi folks,

I recently rediscovered my old 1980's-era Tamiya RC car in the basement, I wanted to get it going again for the kids. After charging the original NiCD battery overnight, I ran it outside but it lasted only a few minutes and died. I hadn't charged that NiCD battery for over a decade at least, so I am sure the battery drained to the point it was damaged.

So I purchased a NiMH + charger pack from eBay and was surprised to find the charger supplied with the new battery was so light-weight (compared to my old NiCD charger) and also it had confusing specifications written on it.

I read up on NiCD vs. NiMH since making the video (linked below) so I understand that NiMH usually needs lower current for charging, which explains the smaller mA rating on the new "cheap" charger. My NiCD charger was a "rapid" charger at 400mA 7.2V and 4-5 hours charge (presumably 400 x 4-5h = 1600-2000 mAh was my NiCD capacity). However, I do not understand why it says output 1.2v - 12v and 100-220mA. Does the charger vary? If so, why and how does it know when? Why does my older charger have a more constant voltage/current rating?

I tore apart the new charger so you can have a look inside. It scared me, the hair-thin wires connected directly to the prongs which were just stuck into the molded plastic (no reinforcement at all). Please have a look and let me know if I can safely charge the NiMH with the older NiCD charger, or whether I can trust this new charger to properly charge and not blow itself up (or the battery) or over-charge or reduce the life of my NiMH.

How am I even supposed to know how long to charge the new battery for? There is no explanation on the time required and I highly doubt the charger has protection circuitry or an auto-shutoff. When I plug it in, the red LED turns on and stays on whether the battery is connected or not. My old charger only turned on the LED when the battery was plugged in.

[edy]

I've added a few snapshots from the video so it is easier to see what I am talking about. You can see the thin wires coming directly from the prongs attached to the PCB. The first thing I see connected is a rectifying diode and a smoothing capacitor.

I haven't analyzed the rest of the circuit but there does not appear to be much there. The charger is rated for 110-220V input, so I assume after transforming it down there would be a voltage regulator to bring this broad input range into a more regulated output. However, why does it specify on the charger 1.2-12V and why the big range in mA? I thought we had to charge at 7.2V constantly?

Because I do not know the exact voltage and mA being output by the charger (or if it is even constant), I am not sure how to calculate the amount of time I can safely charge the battery, or whether I need to feel until it overheats.

[IanB]

Can we please see the (new) battery? Size, writing printed on the outside, specifications given from the purchase link, whatever you have?

It's hard to say anything about how the battery should be charged without some information about it.

[edy]

Here is what the package looks like:



Here is the eBay listing I bought it from:

http://www.ebay.com/itm/7-2V-4600mAh-Ni-Mh-rechargeable-battery-pack-RC-Tamiya-Plug-Charger-/400670353066?pt=LH_DefaultDomain_0&var=&hash=item5d49d066aa

I've attached a larger image as well below.... All it says is NiMH 7.2V 4600 mAh. I'm trying to model the charger circuit in LTSpice IV... going to see if I can reverse engineer it.

[IanB]

OK, I looked at the video. Your new NiMH battery pack appears to consist of six Sub C cells in series (would I be correct in thinking it measures about 46 mm across the width and about 130 mm long?). A capacity of 4600 mAh is believable for such a pack (not necessarily true, but believable).

NiMH cells or batteries are charged at constant current, so the voltage output from the charger matters less than the current output. At a guess the charger is (approximately) a constant current supply.

When charging an NiMH battery at low currents, you can basically just leave it on the charger until it is fully charged (you can tell when this is as the battery will start to get warm once it is charged).

If the charger outputs 200 mA you would divide this into the battery capacity (4600 / 200 = 23 hours) and multiply this by a factor of 1.5x. So if the battery and charger numbers are real, then it will take at least a day to fully charge that battery. Since that is inconvenient for a battery that will be used up in minutes you might want to invest in a better battery charger. Visit somewhere like Hobby King for some choices.

[DanielS]

Good NiMH batteries can handle pretty high charge and discharge currents - some companies make AA batteries rated for 15-minutes charging, which is somewhere in the neighborhood of 8A into 2Ah cells. After my own experiences with those batteries and chargers, I would stick to 1C-rate charging (1h) as the fastest I am comfortable with... the batteries are very hot after charging (likely around 60C - too hot to handle) and it wrecks their lifespan.

The confusing rating is because your cheap charger is just an oscillator that transfers a roughly constant amount of energy per cycle (roughly constant power) so the current it can deliver varies depending on output voltage.

Since your new battery is rated 23X higher than the charger's maximum output, you could probably leave it connected indefinitely but a good NiMH charger would output 1-4A and shutdown when done instead, which would be more practical.

[edy]

Thanks for the help. I read up a little bit more about NiMH here: http://www.powerstream.com/NiMH.htm and here: http://www.talkingelectronics.com/projects/ChargingNiMH/ChargingNiMH.html.

My understanding is that 1.2V cells x 6  = 7.2V. If the battery pack capacity is 4600 mAh, then I have to charge it x1.5 the amount of capacity to fully charge, so 4600 x 1.5 = 6900. If I am pushing in 200mA charge, then I need 6900/200 = 34 hours? So is this a "trickle-charger" that I was provided? Seems like it.... Maybe that is actually safer for the battery, but slow as heck.

My old charger (which outputs 7.2V at 400mA) would probably be better. It would likely cut the time down to half, or around 17 hours. However, the 400mA rating may be too high for the battery and cause it to overheat? I guess I could watch the temperature, hopefully automatically and have a signal warn me so I can time it the first few times to know. I would have to just run the battery dry after each RC run to make sure I have the same charge time from the same 0% discharge level.

For now, at least, since I don't know the exact amount of time, I need to sit around and feel the battery for when it starts to heat up. According to the website I linked above, if I charge at C/10 (or below 460mA for a 4600 mAh cell) then the oxygen recycling catalyst in the cell should be able to keep up with the overcharging state to prevent damaging the battery.

The example provided says to charge a 100mAh cell at 10mA for 15 hours... or 10x15 = 150mAh (which is the 1.5x the cell capacity "C"). So if I multiply that up for a 4600 mAh cell we are looking at 460mA for 15 hours. I have a 400mA 7.2V charger (my old one for NiCD batteries) but I am not sure if this is appropriate for charging NiMH. If it was, I could use it for approximately 17-18 hrs to charge. Otherwise, using the new "cheap" charger rated for 100-220mA I would need at least 30-40 hours.

[Skimask]

I think you're thinking is damn near dead on.  The only thing I throw any exception to is the "1.5x" rule.  I use a "1.25x" factor, which has always worked for me.  The batt pack looks like the typical cheap eBay NiMH batt pack.  4600mah is likely way high, probably more like 3000mah.
You've got a 400ma charger?  If it was me, I'd throw that charger on a spent batt pack for about 8 hours, overnight, whatever, and call it good.

[IanB]

Agreed, there is no problem at all using the 400 mA charger on the NiMH battery pack.

Unless the battery is a quality name brand battery, then I also agree with the 4600 mAh claim being optimistic.

Assume it is 3000 mAh as Skimask says, divide that by 400 mA and you get 7.5 hours. Multiply by 1.25 and that is about 9 hours.

So put the battery on charge with the 400 mA charger and check the temperature after 5-7 hours charging (it might already be partially charged). Keep charging the battery until it starts to feel somewhat warm and call it done.

To maximize the life of the battery in use, do not run it dead flat. Stop and recharge as soon as the car starts to slow down. The easiest way to damage an NiMH pack is to over drain it.

[edy]

Thank you for the advice. If you look closely at the picture of the battery it says SUPER "QUALUTY" BATTERY.  Boy I wonder if my car will end up in a heaping inferno. I will try my 400mA 7.2v charger and start feeling the heat every hour or so. I don't know the current charge state of the battery or its age.

Meanwhile, what do you think of that charger? Am I the only one to question the thin wires, shoddy plug construction molded into the plastic, and gaps between traces on the PCB?

I will try to draw a schematic based on what I see on the PCB as I am learning about power supplies and playing with LTSPICE IV at the moment to simulate rectifier bridges and cap smoothing, transformers and so on.

[FreddyVictor]

If I were you, I would switch to a 2S LiPo (nominally 7.2v, but 8.4v when fully charged)
these are cheaply available and have rapid chargers available (probably ~ 30mins)
waiting >2hours would be way too long 4 me !

I noticed you put the 'rapid' bit of your original NiCd charger in quotes, my original 'charger' for a pack of those NiCd's was I believe a piece of resistance wire which you connected across a 12v Lead-acid battery.
Charging was a nice 'n' speedy 20mins 

[edy]

Yes you are correct... it is not "rapid" as it states 4-5 hours on my original NiCD charger. I am charging the NiMH now. I don't intend on the kids driving the car too long but I may consider buying more batteries if they get into it. I could get a pack for each kid and that way their turn runs out when their battery depletes. Good way to ensure they feel they had their fair share at the wheel. 

[edy]

Update: I have charged it starting from 10am... it is now 7pm... No sign of heating at all from the battery. it has been 9 hours on a 7.2v 400mA charger which is 400 x 9 = 3600. The battery says 4600 mAh (if we believe that) and we need to scale up by 1.25 or 1.5x so I think I will need 4-6 hours more to notice anything. Of course if it is lower mAh I should notice some heat sooner. Even at 3000 mAh we are looking at up scaling to about 4500 mAh which is going to need 11 hours. If it is truly 4600 mAh then up scaling by 1.25-1.5 will get us into the 15-18 hour range. Ouch!

[Skimask]

IF (and that's a big if) your battery is 'actually' 4600mah and you're charging it with a 400ma charger, you may very well NOT be able to feel any appreciable heating when it gets to full charge.  In fact, even if it's 3000mah batt, with the 400ma charger, you still might not be able to feel any heating at full charge.
The delta-Temperature and delta-Voltage "charge termination" methods almost rely on the fact that a guy is trying to fast charge an NiMH battery pack, say maybe around 1/3C to 1C (or more).  Fast charging, quicker temperature rise and/or greater negative delta-V at the end of charge.
"Slow charging" (eg. about 1/10C and below), you probably won't be able to detect any appreciable temperature rise and even more likely, even if you had a good meter hooked up, wouldn't be able to detect an appreciable negative delta-V across the pack when it gets charged up.
I'd say with that 400ma charger, screw the math, quit over-thinking it, and just go with 10-ish hours.  You won't hurt anything.  Might not get an absolute 100% out of the charge, but you won't be hurting anything either.  If anything, the battery pack will last longer (eg. last for more charge/discharge cycles) using that 400ma charger.
Now when you get to the higher end chargers and higher charge currents and such, that's when you gotta start worrying about it.

[IanB]

Can you measure the battery voltage, either while charging or immediately after disconnecting the charger?

A typical NiMH cell voltage will rise to somewhere between 1.45 V and over 1.5 V when fully charged. With a six cell pack you would therefore expect to see a voltage of 1.45 x 6 = 8.7 V or more. If you measure the battery and find this, you may reasonably assume it is fully charged.

[edy]

I am getting 8.12v this morning. I unplugged it yesterday after about 12 hours charge (from 10am to 10pm)... and measured it around 8am now. I assume that is the best I am going to get. I can try to charge a bit more using the trickle cheap charger and measure the voltage. My 400mA charger is 7.2v output but the cheap one I took apart says it will go up to 12v. I'm not sure if that makes a difference.

[IanB]

That's 1.35 V per cell after resting overnight. I'd say it's fully charged.