When I let my car idle over the night the battery voltage is around 12.2v and after running it's around 12.5v, and charging voltage is around 14-14.2v, i thought the monoblock amp which i connected was causing the drain over the night and to test this i disconnected the amp and rechecked and this time voltage was around 12.3v, and another thing is when i turn off the engine at stop signal the horn sounds weak and even after starting the car the horn still sounds weak for about good 1-3 seconds and sometimes power steering motor doesn't work about 3-4 seconds soon after i start, i don't know if it's the battery or something else which is causing the issue ? Or is it time to replace the battery for a new one ? But car starts fine and it takes around 1-2 seconds to start the engine.

  • How old is the battery? Commented Nov 5, 2022 at 1:11
  • 1
    At what temperature did you measure those voltages? Battery temperature affects its open-circuit voltage.
    – juhist
    Commented Nov 5, 2022 at 13:51
  • Battery is around 2yrs 8months old and i measured these volatges after an hour of turning the engine off Commented Nov 5, 2022 at 14:15

1 Answer 1


According to these charts: https://footprinthero.com/lead-acid-battery-voltage-charts

...12.2v, the lowest voltage you measured, corresponds to more than 60% of charge left (I'm assuming this measurement was made at rest). These voltages are at room temperature. Now obviously, if there's nothing you can do to increase the 12.2v voltage, not even a long drive with all high power draws like your amplifier turned off, you may have a problematic battery.

Based on these, there could be some other explanation for the phenomena you have observed, but it could be that the battery is nearing the end of its life.

If you turn off the engine at a stoplight, but there's lots of accessory draw, it's possible the battery voltage sags, and therefore the power steering and/or horn can be weak. If you now turn on the engine again, it may take some time until the alternator is capable of pulling the voltage up high enough for power steering and/or horn to work.

It would be interesting to hear what's the system voltage when you have accessories on, but engine off, at the time when the horn is weak and the power steering doesn't work well.

It is possible that the battery is nearing its end of life (which might explain voltage sag), but it's also possible that you simply have too much power draw in your car. You mentioned an amplifier (presumably aftermarket one). Are you sure it isn't drawing too much power when it's on?

I would further diagnose the issue by disconnecting the amplifier, and measuring voltage right at the moment when engine is stopped and thus the horn is weak and power steering doesn't work well. You may also want to buy a clamp ammeter and measure the power consumption of the amplifier by encircling the positive or negative wire going to the amplifier by the ammeter clamp.

If your amplifier is power-hungry and you often use it when the engine is off, you may benefit from a higher quality battery, like a battery that provides more current -- not made of flat plate cells, but made of spiral cells, and that doesn't dislike deep discharge (starter batteries are notoriously bad when used in deep cycle use, but for example AGM batteries aren't generally as easily damaged by sometimes deep cycling).

  • To test this i did check the volatge again after i disconnected the amplifier and it's a mono class D one providing 300w to the sub, i am thinking of testing it with a spare battery from a shop and then i will get a new one, also when the amp is one volatge drops to 12-12.1 if i add headlights to this equation then voltage is around 11.8-11.9v and yes i did check the voltages when engine is idle and after is cooled down. Commented Nov 5, 2022 at 14:20
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    Alternator voltage at idle may not be accurate. It's a reference. Output varies between 13v-15v, 14.5v measured above idle rpm where output ramps up. Idle rpm is the lowest output, barely recharging batteries. Long idling periods may not recharge a battery and any amp simply drains the battery with little recharging occurring at idle. Measure battery/alternator voltage above idle. If voltage doesn't go above 14v, either the battery is on its last legs or the alternator is faulty. Most auto stores like Autozone provide free battery and alternator testing whether in vehicle or bench testing.
    – F Dryer
    Commented Nov 5, 2022 at 19:50

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