How to charge NiMh batteries?

[Chris Wilson]

The dedicated charger for a dog tracker collar has failed so I need to charge the battery pack external from the collar. It comprise two 1.2V 2/3 AA  600mAH batteries in series. The manufacturer of the cells is Emmerich.

I have current / voltage limited bench supplies, decent multimeters etcetera, will I be able to charge this pack divorced from a coller safely somehow please? Thanks

[FreddieChopin]

You could buy a cheap poundland iMax B6 charger clone. It has LiPo mode normally but it can also do NiMH, lead-acid, car battery etc.

[JustMeHere]

This might work:

[MosherIV]

NiMh can be charged with CV.
2 cells in series gives 2.4V
Set the psu to 3V and limit the current to 60mA. Should take about 10 hours to charge.
If the cells state the max charge current, you can set the current limit to that.
Keep an eye on cell temps, if it gets too hot to touch - stop immediately.

[Andreas]

Hello,

I would use 1.4-1.45V/cell as voltage limit (@20 deg C) and a diode to prevent discharging in case the power supply (mains line) fails.
So total voltage including diode ~3.4-3.5V. Depends on room temperature: with higher room temperature you have to decrease voltage.
(~3mV/deg C and cell)
Current can be increased to ~0.15A (C/4) with voltage limit.

with best regards

Andreas

[Chris Wilson]

OK, great info, thanks. I have made a little discovery, I bought the two collars secondhand and I believe the previous owner may have fitted NiMh batteries in place of what I believe were originally NiCd ones. The dedicated charger that plugs into a port on the side of the collars seemed to over charge the NiMh ones at times especially if left on overnight and part of the next day, and I had to take the battery pack out and put a load on it for a while before the collar would turn on, suggesting it has some form of over voltage protection. Would using a NiCd charger on NiMh cells have this effect? Thanks again, the battery packs are obviously home brew wrapped in insulating tape with rather poorly soldered series tags and leads.

[MosherIV]

Would using a NiCd charger on NiMh cells have this effect?

No, the charge characteristics of NiMh are almost identical to NiCd.
The 2 types if batteries are almost interchangable.
The main difference is the NiCd has higher discharge current capability, NiMh has higher capacity (energy density).

Not sure why discharging the cells brought the circuit back to working. 

[atmfjstc]

Sorry for confusing things a bit by contradicting the other posters, but I've worked with NiMHs a bit and the proper way to charge them is CC, not CV. You'll find most official references saying the same.

To charge a NiMH battery, you basically force current into it (with some limit of course) until it's full, which is when two things happen:

1. The battery voltage stops increasing and even goes down a bit (sometimes this signal is not clear, if charging multiple NiMHs in series)
2. The battery starts heating up at a considerably higher rate than before.

Advanced chargers check for either of these signals so they need some digital logic like a microcontroller, and a temperature sensor if using method 2.

Simple overnight chargers do not actually check for battery fullness at all. They simply pump current into the battery for a fixed amount of time and allow the batteries to overheat. Essentially, what happens is that the energy being pumped into an already full NiMH battery feeds a parasitic chemical reaction that starts turning some of the electrolyte into gas. However, there is another concurrent reaction that reassembles the gas back into the electrolyte, and produces heat instead. As long as you do not pump too much current at one time into the full battery, the repair reaction can keep up and the battery will not be significantly damaged nor will it massively overheat, even if you keep blindly charging it. This is why simple chargers limit the current to a very low and safe value which means they will take around 10 hours to charge a NiMH battery. Advanced chargers know when to stop to they can use higher currents and charge the batteries much faster (even in 1 hour, though the batteries will be less well charged).

To summarize, you can charge them with a lab supply set in constant current mode, but if you want to leave it unattended, you'll have to set the current to at most C/10 and expect the batteries to be a bit toasty at the end. You can also charge them with any standard overnight charger or iMax B6 /clone.

[MosherIV]

Sorry for confusing things a bit by contradicting the other posters, but I've worked with NiMHs a bit and the proper way to charge them is CC, not CV. You'll find most official references saying the same.

Yes, we know. I guess it is good to let op know.

We based our suggestions on the information available, ie the op has a bench psu.
As you said most cheap charger just do a fixed cc and let  the rising battery voltage reduce the current to a nominal trickle current. Typically, they set up a fixed voltage (cv) and use a resistor to convert the cv to cc by using the cell voltage to regulate the current.

The values Andres and myself chose are based on this principle. Hint I chose the quoted 600mAH / 10 as the cc current.

[BravoV]

Ni-based cell can not be charged by CV .. "nor CC" .. without a monitoring circuitry to cut off when it reaches the "peak voltage" of the slope. (google battery university for further details)

Charging just using plain CC , especially for NiMH will roast the battery as it will start to deteriorate the battery quickly, while NiCd is more forgiving for using dumb uncontrolled CC.

Also even charging NiMH with the adequate monitoring circuit as mentioned, the charge rate is also important, most of the cases minimal at 1C rate, as lower than that the monitoring circuit will having a hard time to detect the peak as the slope is really shallow/flat, and will not terminate the charging properly, read overcharged.

Yes, charging Ni-based battery is way more complicated than Li-Ion cell.

[tszaboo]

You cannot charge NiMH with CCCV charger, because the voltage curve will go down at the end of the charge.
Buy a dedicated charger. Best are the ones that have different charge current for AA and AAA. They cost literally 5 EUR/USD.

[macboy]

Hello,

I would use 1.4-1.45V/cell as voltage limit (@20 deg C) and a diode to prevent discharging in case the power supply (mains line) fails.
So total voltage including diode ~3.4-3.5V. Depends on room temperature: with higher room temperature you have to decrease voltage.
(~3mV/deg C and cell)
Current can be increased to ~0.15A (C/4) with voltage limit.

with best regards

Andreas

NiMh can be charged with CV.
2 cells in series gives 2.4V
Set the psu to 3V and limit the current to 60mA. Should take about 10 hours to charge.
If the cells state the max charge current, you can set the current limit to that.
Keep an eye on cell temps, if it gets too hot to touch - stop immediately.

NiMH (and NiCd) are never charged with constant voltage, but always with constant current.

If you charge at a slow rate, then it is safe to overcharge them without damage. So a common easy way to charge cells of either chemistry is at a constant current of 1/10 the capacity (C = 600 mAh so use 60 mA in your case) for 14 hours. This is also used as a conditioning charge for brand new cells or to help refresh old cells.

If you want to fast charge (which I doubt) then you can increase current up to at least 1C (600 mA). Then, you need to monitor BOTH the voltage and temperature of the cells. After reaching full charge, the cell voltage will drop a few 10's of mV, and the cell temperature will rise. Detecting either condition is used to terminate the fast charge. You should also terminate charge after approximately 1.4C worth of charge. So if you are charging the 600 mAH cell at  600 mA, the safety timer should stop the charge after 1.4 hours.

It is very easy to use a good CV/CC lab supply to charge these batteries. Set the voltage to double the nominal pack voltage, set the current to 1/10 C (60 mA), and charge for 14 hours.

[Giuss]

I charge them at 1/10C (a simple LM317 as constant current regulator) without problems