Below is a question to contributing editor Chuck Husick regarding a Sentry battery charger. Chuck response follows. Great stuff.
“Could you go one step further and describe how to check out the circuit when the charger is supposed to have a control circuit that cut the charger off when the control battery voltage reaches full charge? I have an old Sentry C1203NR (20 Amp Neg. Ground, 3 outputs) that is not shutting off (it used to) and I am not quite sure how to troubleshoot it. For example, what should the charger do if I disconnect all batteries from it and then turn it on? What would the symptom be if all grounds are not connected to the ground terminal in the charger? Would that cause it not to cut off when it is supposed to?”
Chuck responds: The charge control used in this device is a relatively simple off / on switch connected to the primary, 120 volt supply. The electronic switch is designed to have hysteresis, it will turn off the AC power when it senses that the batteries are fully charged (by noting a final voltage of about 13.6 volts). Once the switch has removed AC power from the charger the battery voltage will decline. The switch continues to sense the battery voltage, however it will not turn the primary voltage on until the battery voltage has decreased by about a volt (this is adjustable in the circuit) from the level at which it turned off the power. You might be able to check the function of the switch circuit by first assuring that the ground connections to all batteries and the charger are intact. You might then connect the charger and check to see if the automatic switch turns on the charger, applying at least 13 volts. If the charger is working you can then monitor the charger’s output at the battery to see if the charger is turned off when the voltage rises to about 13.6 volts. You might also interpose a standard “D” cell flashlight battery between the positive terminal of the charger and the battery, connecting the output of the charger to the negative terminal of the battery and the positive D battery terminal to the positive terminal of the storage battery. This connection will “fool” the charger by making the battery voltage appear higher than it is.
Unless you are conversant with the circuit used to create the power switch there is not a great deal more that you can do. It may be possible to obtain a replacement switch module from Cruisaire.