NiCd/NiMH Battery

Fast-Charge Controllers

6

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Detailed Description

The MAX2003/MAX2003A is a fast-charge battery charg-

er that uses several methods of charge termination. The

device constantly monitors your choice of the following

conditions to determine termination of fast-charge:

• Negative Delta Voltage (-

∆V)

• Rate-of-Change of Temperature (

∆T/∆t)

• Maximum Voltage

• Maximum Time

• Maximum Temperature

Figure 2 shows the block diagram for the MAX2003/

MAX2003A.

The first step in creating a fast-charge battery-charger

circuit is to determine what type of battery will be used

and what conditions the battery manufacturer recom-

mends for termination of fast-charge. The type of bat-

tery (NiCd or NiMH) and charge rate determine which

method(s) of termination should be used.

The charging characteristics of NiMH batteries are simi-

lar to those of NiCd batteries, but there are some key dif-

ferences that affect the choice of charge-termination

method. Since the type of charge termination can be dif-

ferent for NiCd and NiMH batteries, it may not always be

possible to use the same circuit for both battery types.

A comparison of the voltage profiles for NiCd and NiMH

batteries (shown in Figure 3) reveals that NiCd batteries

display a larger negative drop in voltage at the end of

charge than do NiMH batteries. Therefore, the negative

delta voltage detection (-

∆V) method of terminating

fast-charge should only be used for NiCd batteries.

This termination method can cause errors in NiMH bat-

teries, since the drop in voltage at full capacity is not as

great, and may lead to an overcharged battery.

Figure 4 shows the temperature profiles of the two

types of batteries. During the first 80% of the charge

cycle, the NiCd battery temperature slowly rises. The

NiMH battery temperature rises more rapidly during this

period. As the cells approach 90% of capacity, the

temperature of the NiCd cells rises more rapidly. When

the cells approach full capacity, the rates-of-rise of

temperature are comparable for both battery types. The

rate of temperature change (

∆T/∆t) can therefore be

used to terminate fast-charge for both NiCd and NiMH

batteries; fast-charge is terminated when the rate of

temperature rise exceeds a preset rate.

Table 1 provides some guidelines to help in the selec-

tion of the proper fast-charge termination method, but

the manufacturer’s recommendations take priority in

case of conflict.

Figure 1 shows a standard application circuit for a

switched-mode battery charger that charges NiMH bat-

teries at a rate of C. Though this circuit is shown for

NiMH batteries, it can be used for NiCd batteries (see

Table 1b). The description below will use this standard

application to explain, in detail, the functionality of the

MAX2003/MAX2003A.

Battery Sense Voltage

The BAT pin measures the per-cell voltage of the bat-

tery pack; this voltage is used to determine fast-charge

initiation and termination. The voltage is determined by

Figure 1, where:

Since BAT has extremely high input impedance (50M

Ω

minimum), reasonable values can be selected for resis-

low enough to drain the battery or high enough to

unduly lengthen the time constant of the signal going to

the BAT pin. The total resistance value from the positive

be between 100k

Ω and 500kΩ to prevent these prob-

lems.

give a more accurate reading by removing any noise

that may be present. Remember that the RC time delay

from the cell to BAT must not exceed 200ms or the bat-

Table 1a. Fast-Charge Termination

Methods for NiMH Batteries

Table 1b. Fast-Charge Termination

Methods for NiCd Batteries

Yes

Max

Temp.

Yes

Max

Time

Yes

Max

Voltage

Charge

Rate

No

Yes

>C/2

Negative

∆V

∆T/∆t

Yes

Max

Temp.

Yes

Max

Time

Yes

Max

Voltage

Charge

Rate

Yes

Yes

>2C

Negative

∆V

∆T/∆t

2C to

C/2

*

*

Yes

Yes

Yes

* Use one or both of these termination methods.