Ni-mh Charging
Martta Borromeo
Then I realized that TP4056 is for lithium batteries and not nichel ones.
Here you can see a video describing exactly what I was trying to do, this guy answered to my doubts about why my circuit was not working properly with those batteries.
This is the first project I take out of the "development box", to be fed with batteries instead of direct supply by cables, so I'm pretty new about battery world.
I took NiMh ones because of their great reliability with respect to Lithium ones.
Thank you to both of you.
I've already seen both those sources, here I wanted some more practical hints, because I saw the theory behind those batteries but I didn't get how to practically realize all the theoretical tips in the sources.
All you need to charge a NiMh battery is a constant current source, which can be as simple as a power supply in series with a resistor. Either look at the manufacturer's specification or if you are not sure charge at no more than C/10. So for 1Ah battery charge at no more than 100mA. What is your application? Does the power supply have to supply the load as well as charge the batteries at the same time? Are the batteries charged in one place then connected to the load? Or what? If they are for standby use, as in, charging for months and months then used for maybe an hour or 2 I would charge at a much lower rate, maybe C/100.
To clear up some confusion that I occasionally read, although you've not mentioned it; the predecessors to NiMh, nickel cadmium batteries, had a memory problem (as do I, but that's something different...). If you only partially discharged them before charging then they remembered that they were only partially discharged and would lose the remaining capacity. The answer was to always make sure they were completely discharged before charging. This is NOT necessary with NiMh (or any other kind of battery).
Thank you for the answer.
My application is an Epaper typewriter, consisting in a lolin D32 pro (ESP32) connected to an Epaper screen and a USB host driving a mechanical keyboard.
The power consumption is around 180 mA as a peak at the start-up and 110 mA during normal use.
@PerryBebbington
Well written. If I remember, NiCad battories were charged using constant current, monitoring the voltage. When the voltage, during charging, decreased charging should be cut off.
What is the cut off criteria for NiMh as well as LiPo?
@OP
Is the load connected 24:7? In that case charging current needs to be the load current + charging current, some 200 mA. Is there a way You can know if the load is disconnected and then reduse the charging current?
Railroader:
@OP
Is the load connected 24:7? In that case charging current needs to be the load current + charging current, some 200 mA. Is there a way You can know if the load is disconnected and then reduse the charging current?
The load is connected only in use, I switch it off when not in use, so yes I can see when it's loading or not.
The fact is that if I need a voltage monitoring during charge I can't switch off the load because it must be active to check the voltage.
I think that both You and I ask for charge the cut off criteria. I can't tell, for Your battory, whether constant C10 charge is okey or if it will shorten battory life.
Step in, You helpers knowing more!
There are NiMH battery charger chips, for example: BQ24401 data sheet, product information and support TI.com
Since C/10 constant-current charging is so trivial, and unlikely to result in fire or explosions if misapplied, most designs don't get that complex. Bit I have heard that the C/10 CC is less-than-deal for modern NiMH cells.
westfw:
There are NiMH battery charger chips, for example: BQ24401 data sheet, product information and support TI.com
Since C/10 constant-current charging is so trivial, and unlikely to result in fire or explosions if misapplied, most designs don't get that complex. Bit I have heard that the C/10 CC is less-than-deal for modern NiMH cells.
So without getting complex with SMD such as chips like that (that I could consider in future developments), I'm wondering if I can keep it simple.
If I have a USB source (5V), can I drop it down to 4.2V (that is the max charging voltage of three 1.2 V cells) and charge at 110 mA that is C/10?
Not worrying about when the charge is finished?
I read on the link above that at C/10 the battery doesn't care about overcharging but I don't know. (Section "Overnight charge").
But as already said you want something approximating to a constant current source NOT a fixed voltage. A voltage source and a resistor works but 5V is a bit low to start from. Your battery voltage may vary from about 3V (empty) to over 4V (full). If you use a 20 Ohm resistor that will give you about 100mA when the battery is empty but that will reduce to less than 50mA as the battery fills up.
Lots of constant current circuits out here (that's an actual Google link ). Easily done with two transistors and a sense resistor, for example. Or have a look at one of the many NiMH charger circuits.
Most of the circuits that are suggested in those Google searches can be built from through hole parts. But if you want to go fancy with full fledged integrated BMS... well, those I can imagine are SMD only.
Of course you can have the Arduino do basic battery management as well. It's easy enough to add a switch so the Arduino can switch on and off the charging current, likewise it's easy enough to have the Arduino measure the battery voltage. When the voltage goes over a certain point, switch off the power. You can even keep the Arduino in deep sleep during the process, just waking up every second to check the voltage while charging.
wvmarle:
Most of the circuits that are suggested in those Google searches can be built from through hole parts. But if you want to go fancy with full fledged integrated BMS... well, those I can imagine are SMD only.
The first reason is that voltage is not a reliable indicator of charge for NiMH since they are different from Lithium ones and the second reason is that Arduino (or in my case esp32) voltage measurement through voltage dividers is not so accurate to be used as watchdog for a safe battery management.
The charge algorithm for NiMH is similar to NiCd with the exception that NiMH is more complex. Negative Delta V to detect full charge is faint, especially when charging at less than 0.5C. A mismatched or hot pack reduces the symptoms further.
NiMH dislikes overcharge, and the trickle charge is set to around 0.05C. NiCd is better at absorbing overcharge and the original NiCd chargers had a trickle charge of 0.1C. The differences in trickle charge current and the need for more sensitive full-charge detection render the original NiCd charger unsuitable for NiMH batteries. A NiMH in a NiCd charger would overheat, but a NiCd in a NiMH charger functions well. Modern chargers accommodate both battery systems.
It is difficult, if not impossible, to slow charge a NiMH battery. At a C rate of 0.1C to 0.3C, the voltage and temperature profiles do not exhibit defined characteristics to trigger full-charge detection, and the charger must depend on a timer. Harmful overcharge can occur when charging partially or fully charged batteries, even if the battery remains cold.
The same scenario occurs if the battery has lost capacity and can only hold half the charge. In essence, this battery has shrunk to half the size while the fixed timer is programmed to apply a 100 percent charge without regard for battery condition.
Many battery users complain about shorter than expected service life and the fault might lie in the charger. Low-priced consumer chargers are prone to incorrect charging. If you want to improve battery performance with a low-cost charger, estimate the battery state-of-charge and set the charge time accordingly. Remove the batteries when presumed full.
If your charger charges at a high charge rate, do a temperature check. Lukewarm indicates that the batteries may be full. It is better to remove the batteries early and recharge before each use than to leave them in the charger for eventual use.
The material on Battery University is based on the indispensable new 4th edition of "Batteries in a Portable World - A Handbook on Rechargeable Batteries for Non-Engineers" which is available for order through Amazon.com.
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