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NiMH Charger for up to six Cells Schematic Circuit Diagram

Essential Role of Batteries in Modern Society

Batteries have become indispensable in contemporary society. Take a moment to survey the gadgets in your home powered by batteries, and you’ll be amazed by their prevalence. Among these devices, penlight batteries are commonly used, and if you’re environmentally conscious, rechargeable batteries are your choice. In the past, NiCd batteries were prevalent, but their high self-discharge rate and memory-effect issues prompted a shift to NiMH batteries, which offer numerous advantages.

Transition to NiMH Batteries: Overcoming Challenges

NiMH batteries have emerged as the preferred choice due to their immunity to memory-effect and significantly higher capacity. These features result in prolonged usage before requiring recharging. Consequently, practically every household today requires or could benefit from a reliable battery charger. An effective charger must address various aspects to ensure proper charging. Firstly, it must regulate the voltage per cell to prevent overcharging. Additionally, it must analyze the charging curve to ascertain when the battery reaches full charge. If the charging process extends excessively, indicating a problem, the charger must halt charging. Monitoring cell temperature is also valuable, preventing overheating and ensuring safe charging.

NiMH Charger for up to six Cells Schematic Circuit Diagram 1

NiMH Charger for up to six Cells Schematic Circuit Diagram 2

Efficient NiMH Battery Charging Circuit

This circuit design is tailored specifically for charging NiMH batteries, ensuring a controlled and efficient charging process. At its core lies the MAX712 IC, a comprehensive component encompassing all essential functionalities. In Figure 1, the charger schematic is presented, with focus directed towards IC1, the MAX712 from Maxim. This IC, conveniently available in a standard DIP package, simplifies prototyping for hobbyists, fitting directly onto standard through-hole prototyping boards. T1 collaborates with IC1, regulating the current flowing into the battery.

Current Regulation and Adjustable Charging Parameters

R1 plays a pivotal role in current regulation, serving as the resistor used by IC1 to measure the charging current. During the charging process, IC1 strives to maintain a constant voltage, precisely 250 mV, across R1. The charging current can be adjusted by manipulating the value of R1. The calculation for R1 is straightforward: R1 = 250 mV / Icharge. For instance, with a charging current of 1 A, R1’s value must be 250 mV / 1 A = 0.25 Ω. The power dissipated by R1 is given by U × I = 0.25 × 1 = 250 mW, making a 0.5-watt resistor adequate for R1. Depending on the charging current and supply voltage, transistor T1 may require a modest heatsink for effective operation.

User-Friendly Configuration for Charging Parameters

IC1 demands minimal user input regarding the maximum charging duration and the number of cells in the battery being charged. IC1 is equipped with four inputs, namely PGM0 to PGM3, designed for this purpose. These inputs are distinctive, recognizing four different states: V+, Vref, BATT–, or not connected. To enhance user convenience, essential connections have been extended to two connectors, K3 and K4. Various dongles, illustrated in Figure 2, have been developed, plugging into these connectors and enabling users to configure the number of cells and set the maximum charging time effortlessly.

When determining the maximum charging time we have to take into account the charging current and the capacity of the cells that are connected. The charging
time can be calculated with the formula:

Tcharge = Ccell / Icharge × 1.2

where Ccell  is the capacity in Ah (e.g.,1200 mAh = 1.2 Ah).

After the nominal charging time has been calculated, we can use the first dongle that has a value that is equal to or greater than the calculated charging time. For example, if we calculated a maximum charging time of 38 minutes, we have to select the dongle for 45 minutes. When IC1 is replaced by a MAX713, the charger becomes suitable for charging NiCd batteries (but not suitable for NiMH batteries anymore!). The only difference between these two ICs is the value of the detection point at which the cell(s) are considered to be completely charged. The ICs are otherwise identical with regard to pin-out, a method of adjustment, etc. To make it easy to swap between the ICs, we recommend an IC-socket for IC1.


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