I'm currently repairing a project built by someone else and want to avoid tinkering with it as much as possible.
I am looking to recharge 3 AA NiMH batteries (at 1.2v each), that are in series. I understand that it's difficult to charge in series, and that I should have some sort of regulator in order to not completely destroy the batteries. However, I'm short on time and money right now.
Could I simply use a 3.6v (or higher) DC charger to recharge the batteries?
Thanks!
No, you can't supply 3.6v! You will need a constant current supply.
I found a great, if not in depth, resource for anything battery related is this site: http://www.technick.net/public/code/cp_dpage.php?aiocp_dp=guide_bpw2_00_toc.
As an alternative, if you can use 5v, you could use a single cell lipo battery and charge it with this neat lipo rider.
Edit
As others have mentioned if you DO use a constant current supply you MUST have some way of monitoring the battery charge state either by monitoring change in voltage or temperature! The batteries will likely explode otherwise.
This is not a complete answer, but more a list of thoughts on the subject.
So first, you need to read the datasheet for the batteries! In there you will find the important information such as battery charging currents. I'll take this datasheet as an example. Here you can see that the battery has several charging modes and the specified currents for each of them as well as control parameters. So this type of battery, as well as other NiMH batteries, are charged using constant current, so a "simple" DC charger won't work. You'll need a constant current source for charging the battery and let the battery determine the voltage. There are many constant current sources, but the simplest thing that comes to my mind right now is a simple LM317 regulator in constant current mode.
Now since you have 3 cells in series, this will present some problems. The current through them will be same, so one current source may be enough, but the voltage in each cell may not be same as in the other two cells. You could ignore the problem, but that will be more expensive in the long run, since individual cells in the 3 cell battery may die more rapidly. In my country at least when price of individual NiMH cells is taken into account it turn out that much cheaper option is to use a balancer too. So the idea is that you'll have four additional contacts installed so that voltage of each cell can be measured. Then you can use the leads to discharge individual cell so that it matches the voltage of other two cells. I'm sure that it can be done using discrete components only, but I thing that right now the simplest way would be to use a microcontroller which would control the charging process. You should program it in such way that it can control power access to the LM317 (or whatever you pick) and that it can provide access to each individual cell too. Then you can discharge the cell using a resistor. I thing that it would be easiest to get use FETs for that, since they have low resistance when fully on and don't use current form the microcontroller.
Next you'll need to determine how long the charge process should last. The datasheet I linked has several options and each of them has its own good and bad sides.
The simplest way would be to just slowly charge the battery at (in my example) 70 mA and use the microcontroller as a timer. When the time passes, it should disable the power supply to the charging circuit. Good side of this is that you have a simple way to determine when you should stop charging and the bad side is that charging takes 16 hours.
Next we heave the trickle charge. You simply provide 35 mA to 70 mA and wait. The good side of this is that you shouldn't suffer a catastrophic failure of battery if it gets overcharged and that you don't need a stop circuit at all but the bad side is that it's even slower than slow charge and that, from what I've heard at least, long term trickle charge isn't too good for the battery.
After that we have the voltage change method and temperature change method. So basically when you reach the temperature and voltage change parameters specified, it means that the charging is complete. It is generally the best method for the cells themselves, but it is difficult to implement. You'd need a temperature sensor near the battery and you'll need a way to drive it using the microcontroller. It can be a simple thermistor or a dedicated temperature measurement integrated circuit. It also needs to be close to the battery in order to get good temperature. In addition to that you'll need to track the voltage change. If you plan to implement balancer, than this would only require a minor modification to the system because you'll need to measure the voltage of the whole battery too and then compare them over time. A big downside of this is that the battery voltage dip which indicates end of charge is not very pronounced so it can be difficult to detect. Also newer NiMH cells tend to emit large amounts of heat when fully charged so this makes the temperature change method more precise.
Here is a very rough sketch of what a relatively simple charger would look like:
The "B" blocks on the circuit are there to measure voltage. Almost any microcontroller with an ADC can be used to easily measure voltage and there are numerous tutorials available, so use them to make such a circuit. You can get the whole battery voltage by adding the 3 voltages you get from each cell which could make the measurement a bit easier in cases when battery voltage is higher than microcontroller voltage. Using a 5 V microcontroller could help here.
The "D" block is there to measure the resistance of the thermistor which can be used to detect when the battery is fully charged. I also left it out, but it shouldn't be too hard to get a circuit for that.
I also left out any specifics the microcontroller may need like decoupling capacitors, crystal, its own regulator and so on.
The equation which can be used to get the value of the \$R_1\$ resistor is: \$R_1=\frac{1.25 \mbox { }V} {I}\$ where \$I\$ is needed current. \$C_1\$ is the capacitor from LM317 datasheet.
\$R_B\$ are the resistors used for discharging the overcharged batteries. The value should be determined using several factors like the cell's maximum discharge current, the amount of time each cell should be balanced for, power ratings of available resistors and so on.
Input voltage \$U_1\$ should be at least 3 volts above the maximum voltage expected on the terminals of the LM317. Do note that NiMH cells can in certain (abnormal) situations reach as much as 1.7 V per cell, so input voltage should be determined with that in mind.
Another thing that should be added here would be pull-down resistors on all transistors so that gate voltage is left in known state when the microcontroller outputs low.
NimH are often charged in series.
Factors affecting complexity of implementation include those below.
To make good decisions these factors need to be known when designing the most appropriate solution:
How long you have available for charging.
The longer & slower you charge the easier to do safely while doing it very simply. eg
How long you want the cells to last affects charging decisions.
Lifetime can be measured in cycle of calendar time or both.
Deep discharge daily will produce a short life. 200-500 cycles range usually. Maybe less.
Being cycled 100% every day, 100% once a week, 20% (to 80% capacity) occasionally etc will alter lifetime.
If charging at VERY low rates it is easy enough to do it simply and cheaply.
If you want to charge at high rates then it is better to monitor and control each cell separately, but at lower rates "uncontrolled charging is OK.
C = charge or discharge rate in mA expressed as a proportion of the mAh capacity of the cell.
eg for a 2000 mAh cell C or C/1 = 2000 mA, C/4 = 500 mA, C/10 = 200 mA, C/100 = 20 mA
Charging to an while -charging cell voltage of 1.45V at typical ambient temperatures is fairly safe. 1.4V somewhat safer but may not be quite fully charged (which is not bad). 1.35V getting a bit low. If you lave 1.35 - 1.4V on the cell for some while the charge rate will drop to zero or near zero.
High capacity cells (say over 2000 mAh for AA) should not be trickle charged for any great period. Maybe a few hours at C/10. Maybe longer at C/100. Maybe.
(much) More information available if it seems to be needed.
It is generally bad to charge NIMH in series. Charging in series causes NIMH to go out of balance, because each battery is slightly different and has a different charge-discharge curve.
Because of this when a voltage is placed across all three batteries each one will receive a different voltage and charge differently. If you could match the charge discharge curves it might be possible, but who has time for that.
