light dim with sub

I have a 12" Sub powered by a Kenwood 9105 amp at 1ohm. When listening to heavy bass songs my lights dim. I k ow you will probably say that my alternator/battery can't keep up or don't provide enough power. I am not sure how all this works so I need to know what makes this happen exactly. I have used a multimeter on my battery while I have my stereo cranked. The battery tests 14.4 volts down only to 14.20 volts with heavy bass. At the alternator it tests 14.4 volts down to only 14.35 volts with heavy bass. I know other things probably come into play like amps, my alternator is 110 amps. Can someone explain to me how this works and what's going on?2011 Subaru WRX Limited

Your 900W Kenwood draws about 80A at full output. You have other normal loads - maybe 10-30A. Your lights draw over 10A. Your alternator only puts out rated output (110A) above certain engine RPM. If loads or short term burps exceed what the alternator can supply, the system voltage drops, hence dimming lights etc. Depending on the current surge size, the voltage will drop to the battery voltage. (That's typically ~13.6V due to surface charge after charging, dropping to its normal output voltage - typically ~12.7V if fully charged less its internal resistance (times current) voltage drop.) If you merely wish to solve the headlamp dimming, a cap across the headlights should work. Normally I/we would lampoon the use of caps (even small 1.2AH batteries are cheaper and superior), but this is one example where they are suited. Though using a small battery is better, that adds complication do to the need for battery isolation (from the main battery when not being charged except when the lights are on), plus battery maintenance etc. An alternator sized for your load and RPM (with lights & wipers etc) should solve the problem assuming distribution is not an issue (ie, the Big 3). Many do the Big 3 as part of any big amp install and may even add a 2nd battery near the amp for the above and other reasons (eg, an AGM battery with dual fuses and isolation relay). In your case, upgrading the alternator to battery cable alone may help or solve the problem, but that needs to be matched with an equivalent engine (alternator) to amp ground upgrade - ie, engine to chassis or battery, though all 3 are done for the Big 3). The engine to chassis/body GND upgrade - and sometimes the battery- GND - is imperative to prevent damage to other vehicle electrics. HIDs should not experience such dimming issues. Alternator response time should not be a problem with modern alternators (internal electronic regulators). Other threads cover details relating to the above.

Are you sure it would be 80A at full output? I do push it and I only have a single 60A glass fuse going to it from the battery.2011 Subaru WRX Limited

I'm assuming it's 900W is RMS (nothing else counts). Normally one would assume output-power/10V (hence 90A) for design purposes, but I assumed 80% efficiency and used 14.2V - 14.4V as its input. If it's a music or peak rating, then halve those currents. If it were 95% efficient, a 60A fuse will handle its 66A input all day - noting that that is a full 900W output, ie, with appropriate input and gains. Any small drop in "volume" from its full output mans a much larger drop in power, and a lesser drop in current but still more than the volume drop. A fuse will usually handle 10% over-current forever (well, almost as long as at rated current). At 80A, your 60A fuse should last almost 1 hour (depending on its type and Standards etc).