Battery recharging/management module?

In one of my project, I will use 4 AA rechargeable NiMh batteries in series. Those usually come in a pack.

I want to create a charger for it so I can charge the device via a USB, without having to take the batteries out.

I know there are battery managment ICs out there, but I still would need a lot of design work. I wonder if there are modules out there so I can just "plug and play"?

Thanks

Yes, but it's rare to charge 4 AA batteries from USB. The nominal voltage is 6V so you need a step-up converter. Have a look at how the ubiquitous USB power-bank type batteries work.

Sparkfun has a charger module intended for LiPo. That may be adaptable to NiMh. I think Adafruit has one as well.

MorganS:
Yes, but it's rare to charge 4 AA batteries from USB. The nominal voltage is 6V so you need a step-up converter. Have a look at how the ubiquitous USB power-bank type batteries work.

Sparkfun has a charger module intended for LiPo. That may be adaptable to NiMh. I think Adafruit has one as well.

What about 2 AA batteries? I would hate to have to use a step up converter that adds complexity.

But 2 AA is only 3V, can the SparkFun or Adafruit modules work? I believe they are designed for Lithium batteries with nominal voltage of 3.7. So if my AA battery pack is only 3V, that should be okay right?

paulwece:
So if my AA battery pack is only 3V, that should be okay right?

No.

There is a simple principle here, only use chargers appropriate for the chemistry of the battery.

A NiMh battery needs a charger designed for NiMh batteries.

A Lithium battery needs a charger designed for Lithium batteries.

There are charge ICs out there for NiMh for DIY designs but I have not seen one in a simple module ready made module like you will find for Lithium batteries.

Whether you can hack on charger for another type of battery, really depends. A Lithium charger uses voltagelevel cutoff to detect charge, a NiMh charger uses the rate of change of voltage to detect charge, two different methods.

srnet:
There are charge ICs out there for NiMh for DIY designs but I have not seen one in a simple module ready made module like you will find for Lithium batteries.

Suppose I get such a IC for NiMh, how much work (additional components required, complexity, etc) does it take to do a DIY design? Are there guidance/tutorials for this?

paulwece:
Suppose I get such a IC for NiMh, how much work (additional components required, complexity, etc) does it take to do a DIY design? Are there guidance/tutorials for this?

The datasheet for such a device will normally contain a typical application circuit.

The reason I want a charging module is to enable convenient charging via a cable without having to take out the batteries from the device.

I wonder if this will also work:

The battery packs that contain multiple cells usually have wires coming out for charging and discharging. Suppose I make a fork in this wire and use one for charging and one for discharging. That way I can just leave the battery pack in the unit and have an opening for the charging wire, and then charge the battery via a standard off the shelf charger for these type of battery packs. As shown below:

bump

Hi, can someone answer my question above?
thanks

The smartest chargers can't charge a battery when there's a load (your Arduino circuit) connected. The dumb chargers don't care. So it is highly dependent on the charger.

One way to do it is to find a DC power input socket which contains a cutoff. It will have three output terminals. One is common to both your circuit and the battery and the other two are for just the battery or just the circuit. When there's nothing plugged in, those two are connected together, so the battery can power your circuit. When you plug in the charger plug, it disconnects the circuit so the charger only sees the battery.

Or put in a switch.

Or make sure your charger is dumb "enough".

MorganS:
The smartest chargers can't charge a battery when there's a load (your Arduino circuit) connected. The dumb chargers don't care. So it is highly dependent on the charger.

So the chargers that charges our cell phone is a "dumb" charger? We can use our phones while charging.

Why would the "smart" chargers refuse to charge a battery when there is a load?

thanks

MorganS:
Yes, but it's rare to charge 4 AA batteries from USB. The nominal voltage is 6V so you need a step-up converter. Have a look at how the ubiquitous USB power-bank type batteries work.

Even though the 4 AAs are in series to power the load, is it possible to charge them in groups of 2 AAs so a step up converter is not needed? So two groups, each group having 2 AAs. The two groups draw power in parallel from the charger.

I have yet to see a balanced NiMh charger, contrary to Lithium, NiMh can take some more abuse, or at least not explode when abused.

NiMh charges in CV (constant voltage), proper termination is dV/dT where as Lithium charges in CC/CV

If you use a low C (charge rate) and trickle charge, it could be simply charged with a timer/dumb charger but will shorten the life and/or not maximize their capacity.

So for a 4xAA pack, you'll need 4.8V which is right in the ball park of USB's 5V output, so it may just work
At 500mA from a PC, each cell will get 125mA, with a typical 2000mAh cell, it will take 16 hours to charge from empty
That's less than 0.06C so it should be fine.
Even with a 1A wall wart, we are talking about 0.13C and 8 hours charge time, you could build Arduino charging circuit :slight_smile: start off as a simple time based then later on, move on to a dV/dT termination by monitoring the voltage drop, may need higher C rate for the proper detection of the voltage drop.

If your Arduino draws lot of current, you may have to take that into consideration as well.

You can read up more on charging Ni-Mh here
http://batteryuniversity.com/learn/article/charging_nickel_metal_hydride

In our cellphone, usually when you plug in an USB cable, the phone is powered directly off the USB connection, it disconnects its load and the battery is only connected to the charging IC.

Or you could simply go for a TP4056 board (the one with protection IC) and a single 18650 3.7V cell with a step up converter if you need 5V out of it, add load sharing with a simple mosfet so it isolates the battery while charging.

You’d then add some kind of switch to connect them in series to generate 6V?

“So the chargers that charges our cell phone is a “dumb” charger? We can use our phones while charging.”
That only charges at 100mA or 500mA levels. A big pack like you showed would take forever to charge at that rate.

One could charge them in pairs but each charger needs it own isolated ground, a isolated DC-DC converter could work but is more complex and expensive than just using 2 separates wall wart of 2.4V, but I see no real benefit to charging in 2S vs 4S?

If you want to tinker, get 4 separate single cell NiMh charger boards on ebay and power them with 4x AC-DC power supply to do balance charging :slight_smile:
That may be overkill though