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Battery charging at low temperatures

Why doesn’t the design of the Lead-Acid Battery Charge Controller (April 2008) allow charging below 0°C? Even in the fully discharged state, the electrolyte contains about 20% sulphuric acid by weight, and its freezing point is -14°C. (In a fully charged battery, the freezing point is -29°C). My reference for the freezing point data is "CRC Handbook of Chemistry and Physics", 55th Ed. 1974-5, CRC Press.

From my experience in Armidale, cars will start at -10°C and if the battery can deliver a hundred or more amps at this temperature, it will certainly suffer no harm from charging at 10A. (R. S., Armidale, NSW).

  • Most lead-acid battery manufacturers recommend not charging below 0°C. Car batteries have a hard life and are generally not charged correctly due to temperature extremes (both high and low) and insufficient full charging.

    Power supply for Neon Modulator

    I was wondering if it is possible to run the Neon Modulator (SILICON CHIP, November 2001) from a 24V supply. Looking at the schematic, I can see no reason why not but I might be overlooking something. (B. C., via email).

  • The circuit cannot be directly run from 24V. First, the gates of Q2 and Q3 should not exceed 20V and the 100mF 25V electrolytic is not rated for operation from a 24V battery (may be up to ~29V when being charged). Also the neon tube driver itself may not be rated for 24V. Ideally it would be better to run the circuit from 12V using a 7812 3-terminal regulator.

    Headlight reminder trips on all doors

    I’m in the process of installing the wiring in a car for the Headlight Reminder kit (SILICON CHIP, August 2001). One of the sensor inputs is the door switch. However, as these are wired in parallel inside the wiring loom in a 4-door car, it appears the headlight reminder will operate if any door is opened.

    This is a problem when the passengers alight but the driver remains in the car. I checked another car with an original equipment headlight alarm and it only operates off the driver’s door.

    Are there any errata or other advice you could offer to overcome this problem? (P. G., via email).

  • That is a problem with any add-on circuit for a car. The car has the door switches wired in parallel so the headlight alarm will sound with any door opening. You could add your own separate door switch for the driver’s side door only and connect this to the Headlight Reminder instead of the existing switches.

    Alternatively, it might be possible to isolate the driver’s door switch with a suitably rated diode, say a 1N5404.

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