Trickle Charging NiMH Batteries

There are lots of fast chargers around for NiCad and NiMH batteries these days but one thing I thought would be useful would be to have a trickle-charger that keeps these batteries 'topped-up' and ready for use. My children are starting to use lots of battery powered equipment and I was finding that so called 'charged' cells were self-discharging if left unused for a few weeks and a trickle-charger similar to those uded on lead-acid batteries would prove very useful.

Battery Characteristics

The key property about NiCads and NiMH cells are they require a constant-current charge to prevent damage or shorting their lifespan. This is quite unlike a lead-acid cell that you apply voltage to and stop charging when the terminal voltage reaches a maximum value. A normal 'slow' charge is often done at 0.1C but this is too high to use as a trickle charge for NiMh batteries although is fine for NiCads. The recommended maximum trickle charge is 0.025C (C/40) which means it would take several days to charge a battery from flat, but thereafter it is kept charged for immediate use. This suits me fine as we need to replace batteries once every two weeks or so. There is always the option of fast charging and then popping onto trickle charge if need be.

Reuse and Recycle

I have plenty of transistors, the odd battery holder and even an unused panel meter I could use for this project, so I set about seeing what I could uncover in my parts box.

I also seem to be awash with old mobile phone chargers which can be reused in many applications so I decided to drive the circuitry from an old Nokia charger which has an output of 6V @ 500mA. On measuring the output of the charger, it was indeed 6.0v and very well stabilized. The batteries I am targeting are AAs and these need replacing in multiples of 4. This makes things a little tricky to use a 6V power source as it does not give much headroom as 4 x1.25 = 5V and in practice, the battery terminal voltage may rise to 1.5 during charging, giving no headroom at all. I decided to use this fact (along with a fixed voltage source) so that the charge current is actually reduced as the battery is nearly charged.

My batteries are 2400mAH which means I should use a charging current of 2400/40 = 60mA. I also have some AAA batteries rated at 1000mAH which would require a 25mA charge current. I decided to make the design capable of charging those too.

My first design is shown below. This would be fine if the power supply was more than 6v, or I was charging less than 4 batteries. Even when using a schottkey diode, the output voltage is insufficient to charge 4 batteries in series. However, I may come back to this for another project as the circuit worked admirably when driven with a suitably high voltage.

The second design I came up with is here. I opted to use PNP transistors as there is one less Vbe drop using this design and I also removed the diode to scavange some precious volts back - and then there should have just about enough voltage to charge 4 batteries in series.

How it works

The transistors I used were some general purpose low power devices I had handy. These have a max Ic of 150mA and an average hFE of 200. If we ignore T1 abd R1 initially, T2 is a simply biased via R2. The base current will be (6-0.6)/10k = 540uA. Given a gain of 200, the Collector current should be 540uA x 200 = 108mA. The actual value depends on the Vbe and hFE of the transistor. When I measured the collector current, it was actually 120mA.

Now add T1 and R1 to the circuit. This is a pure negative feedback loop that is used to limit the current delivered by the circuit. R1 is in series with the output path so the current being fed to the batteries must pass through it. The base-emitter of T1 straddles this resistor so if the current across the resistor aproaches 0.6v, then T1 will switch on. When this happens it will pass current via it's collector to R2. As more current passes through R2, the voltage across it will increase, back-biasing T2 and reducing its collector current. This in turn reduces the current through R1, and the circuit will reach equilibrium. Note there is no resistor in T1's collector path so there is a lot of feedback in the circuit, in fact the feedback current has a greater affect on the biasing of T2 than the default current passing through R2. R1 is 10R. So to get 0.6v across it requires 0.6/10 = 60mA, which is what we want the output current to be. If one, two, three or four flat batteries are put in series across the output, the circuit will provide 60mA

Note the addition of switch SW1 and an extra resistor, R3. R3 is 27R, so if this value is switched in, the current will be limited to 0.6/27 = 22mA. This is the right value for trickle charging AAA batteries.

Finally, due to the limited 6v power supply, the circuit will start to reduce the amount of current it provides once the battery terminal voltage starts to rise. If each cell's terminal voltage rose to 1.35 volts (1.2 is normal, but it does rise slightly as the charge progresses), then the collector of T2 will sit at 5.4v (4 batteries). The emitter of T2 needs to be higher in voltage than its collector for current to flow (say 5.5v). This starts to bias T2 back (only 0.5v Vbe) and the current through R1 must drop to 50mA. As the battery terminal voltage continues to rise, T2 is biased less strongly and the current flowing into the battery with further reduce. Note that with NiMH cells, the battery voltage can fall slight on full charge so there is no guarantee that the current will continue to drop to a minimal value.

Construction

Although there is no need to include a ammeter to show the charge rate going to the batteries, I had one at hand so decided to use it to get an idea of the performance of the circuit. The circuit was built on a small piece of vero board and housed in a plastic box to which had been fitted the meter and a battery holder. The connector on the nokia phone charger was changes to a 2.5mm plug and an appropriate socket was mounted in the box. The finished article is shown below.

With 4 discharged batteries in the battery holder, a 55mA charge current was showing. After 24 hours, the charge started to drop slowly and seemed to settle dowm at 25mA after a few days. Putting 4 fully charged batteries in the charger results in a charge current of 25mA. So it appears that when charging batteries, once the charge current stabilizes at a lower value (in my case 25mA), the batteries are fully charged and just kept gently topped up at 25mA. This means that AAA batteries can be charged more quickly in this mode as the charge rate will drop to a safe value when the batteries are full. However, this is not really the point, this project is a slow and gentle method for keeping batteries in good order!

If the switch is flipped to the 'AAA' position, R3 is put in the current path and flat batteries are seen to charge at just under 25mA and stabilizing at 10mA when charged. I have found in practice that even when AA batteries are fully charged, switching to the AAA position means they are kept at a 10mA charge which seems to be sufficient.

Conclusions

This is a very simple design and works very well. It cost me 89 pence for a battery holder and everything else I had on hand. Batteries are kept charged and ready for use and (so far) I have seen no detrimental effects. If I had had a power lug that output more than 6V, I would have been happy to let the batteries constandly charge at the maximum safe current (60mA/22mA for AA/AAA respectively). However, turning the limited voltage supply to an advantage means that the circuit backs-off the current nicely so that the batteries should be kept charged very gently and give a long and useful life.