Is it possible to hook an alternate battery to the alternator? I am going to run my audio system off a second battery and would not like it to get drained so quickly. Or is there another trick to getting to charge itself again while running?
An additional battery is only going to add an additional load on the alternator which technically will reduce the energy available to everything else (including the front batt you're trying to protect). Adding a second battery only helps engine-off listening. All energy to move a speaker ultimately comes from the alternator. If the alternator is not up to the task, the only thing to do is replace with a bigger one.
The OEM alternator on a good day has 625 Watts to give to an audio system. Amplifiers can't produce more average energy than the alternator can produce. When the amp demands more from the alternator than it has to give, the additional energy comes from the reserve in the battery. Adding additional batteries increases this reserve, but eventually, the system will hit the wall (harder). If you are using amps with non-regulated power supplies, their demand will reduce with voltage sag making them more charging-system friendly. All the amp does is take the energy form the alternator and change it to something useful to move a speaker.
If your amp has a regulated power supply, it will draw more current as voltage sags to maintain it's output. This is harder on the charging system, but you will get the power you paid for over a wider voltage range.
Most people average 20% of the amplifier's power with music. This means the OEM system can handle a system as large as 1125 Watts in the hands of a normal user. The abusive listener is stuck at the 625 Watt mark. If you are an abusive listener, consider adding more speakers to get loud instead of investing in more power (unless you want to upgrade the charging system).
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