Simple Charging

90 Hacking Electronics

commercial NiMH battery charger that will charge them until they are full and then stop. You can then put them back into your project and you are done.

If, on the other hand, you want to leave the batteries in place while you charge them, then you need to understand a little more about the best way to charge your NiMH batteries.

Simple Charging

The easiest way to charge a NiMH battery pack is to trickle charge it, limiting the current with a resistor. Figure 5-2 shows the schematic for charging a battery pack of four NiMH batteries using a 12V DC adaptor like the one we used back in Chapter 1 to make our fume extractor.

To calculate the value of R1, we first have to decide what current we want to charge our battery with. Generally, a NiMH battery can be safely trickle charged with less than 0.1C indefinitely. If the AA batteries we have each hold a C of 2000mAh, then we can charge them at up to 200mA. To be on the safe side, and if we planned to allow the batteries to “trickle” charge most of the time—for, say, a battery backup project—I would probably use a lower current of 0.05C or more conveniently C/20, which is 100mA.

Typically, the charge time for NiMH batteries is about 3C times the charging current, so at 100mA, we could expect our batteries to take 3 × 2000mAh / 100mA = 60 hours.

Back to calculating R1. When the batteries are discharged, each will be at a voltage of about 1.0V, so the voltage across the resistor will be 12V – 4V = 8V.

Using Ohm’s law, R = V / I = 8V / 0.1A = 80Ω.

Let’s be conservative and choose the convenient resistor value of 100Ω. Feeding this back in, the actual current will be I = V / R = 8V / 100Ω = 80mA.

When the batteries are fully charged, their voltage will rise to about 1.3V so the current will reduce to: I = V / R = (12V – 1.3V × 4) / 100Ω = 68mA.

That all sounds just fine, our 100Ω will be great. Now we just need to find out what maximum power rating we need for R1.

P = I V = 0.08A × 8 = 0.64W = 640 mW

So, we should probably use a 1-W resistor.

Figure 5-2 Schematic for trickle charging a NiMH battery pack