Reply To: Battery Alarm

Andrew Sansome

    Hi Peter,
    We have a 45E of the same age and have also had issues with the low voltage alarm sounding despite the voltage measured with a DVM at the panel being around 12.5V. Our VDO panel (2011) does not have any means of either switching off the alarm or changing the alarm threshold. We fortunately no longer have a problem having just changed over to LiFePO4 service batteries which remain above this (incorrect) alarm threshold.
    However before the change we did make one observation: 
    The alarm seemed to be provoked at this panel voltage when a USB charger was plugged into the 12V accessory socket and delivering current. There are two possible explanations the understanding of which requires some knowledge of electronics: 
    A USB charger reduces the 12v supply to a regulated 5v. This is achieved by the implementation of a "buck" converter, a circuit configuration which, while supplying a constant regulated output, draws current in a series of potentially quite large high frequency pulses / spikes. If the design of the charger does not include sufficient input capacitance then these spikes will be drawn from the 12v supply and may (by virtue of resistance / inductance in that supply) transiently drop the supply voltage below the low voltage alarm threshold even though the mean supply voltage (as displayed) remains apparently adequate. The same means of regulation is used within LED lighting.
    So the possibilities are:
    <li>The VDO panel alarm threshold has drifted over the years and is now incorrect (it will be defined by a pair of SMD resistors but I do not have the panel circuit diagram to point you in the correct direction).</li>
    <li>The battery voltage is actually low and the displayed 12.5v is incorrect (check with an accurate DVM at the 12v accessory socket)</li>
    <li>The mean voltage is adequate but a pulsed load is dropping it transiently below the alarm threshold. (disconnect all such possible loads and review). If this is the case then your options are to remove the offending device(s) or try to reduce the supply impedance. The latter requires at least a little knowledge but at its simplest just check for poor connections between the batteries and alarm circuit (So battery terminals , (unlikely) main fuse (unlikely), battery isolation switch (unlikely), one or more fuses on the back of the VDO panel). I would certainly have a good fiddle around with most / all of the fuses / spade terminals, you may be lucky and find that surface oxidation on one or more may be contributing.</li>
    Removing our USB charger was sufficient to enable us to sleep at night!
    Good luck