Specifications

2.12.2 Simple Charging Circuit

Even using the previous solution of a separate charger, it would also create the

problem of a limited use device. The user would have make sure to constantly

charge the battery or risk having a non-functioning interlock system, thus

disabling his vehicle completely, regardless of whether he is actually intoxicated

or not.

One possible solution is to use a simple, low cost solution to charge the battery.

The battery would be kept in the housing of the portable unit, but could be

charged by way of a connector on the housing itself from a wall or vehicle

adapter. The advantages of this configuration over the previous are several.

An integrated charging circuit would alleviate many of these concerns. It would

allow the user to simply plug the charging adapter connector into the portable

unit and have the unit charge while driving, or while otherwise sitting idly in the

base unit in the vehicle, or even at home. It would also allow the battery function

to be transparent to the end user – no battery to directly handle. In addition, it will

lower the overall complexity of the system (as viewed by the end user). However,

as mentioned earlier, there are certain considerations to take into account when

charging a nickel metal hydride battery.

Because of the simple electrical circuitry that will be required to regulate the

output from the wall adapter, a contained solution has its own advantages. The

circuitry can be integrated into the portable unit along with the battery. This will

achieve the goal of being able to “plug in” straight from the wall adapter to the

device.

Since the motivation behind this option is as a cost reduction method, it would

also be prudent to consider the most cost effective way of recharging this type of

battery. Long term or “overnight” charging is considered to be the most effective

and cheapest way to charge this type of battery. In battery terminology, this is

stated as charging it at C/10 or below, or charging it 10% below rated capacity or

less. C is expressed in mAh (milliamp-hours), as mentioned previously. The

largest cost and complexity savings from this type of charging method is the lack

of need for any advanced auto-detecting features in the circuit, in order to detect

the end of charge and automatically trail off the charging voltage and current in

order to prevent an overcharge.

This simple current-regulation method can be accomplished by means of the

12Vdc wall adapter with two sets of simple 10R resistors, as illustrated in Figure

2.12.2-2. This will also provide enough voltage in order to charge a 6V battery

(the illustration shows a 12V battery).