Help with battery specs.


Bronze Member
Username: Arnotjie

Post Number: 39
Registered: Apr-10
Hi there

I'm in the market for an extra battery for my system. I'm clueless when it comes to batteries though. I have no idea what most of the specifications mean. haha. I don't want someone to recommend any to me, But I wold appreciate if someone can explain the specs, and what impact it has on car audio, so I can atleast base my decision on something other than just price. haha

Much Appreciated

Platinum Member
Username: Glasswolf

Columbia, South Carolina America

Post Number: 14718
Registered: Dec-03
see "electrical" section and read the alternator, capacitor, and battery pages. also read the battery isolator section

Bronze Member
Username: Arnotjie

Post Number: 40
Registered: Apr-10
Thanks GlassWolf.

I have been on your site hundreds of times, and I've read a few of your papers before.

I have never really been interested in batteries though (hence the lack of knowledge). since I read some of your other papers know that once the engine is turned on, the battery does not really have a use unless the alternator can't provide enough power. However, Upgrading my alternator is not really a worthwhile option for me. To get a high output alternator made, and shipped to Australia would set me back about A$900.

When my 4 channel amp goes in, I will theoretically be running 1100rms, but I will probably never reach that, since I just use it for music. So, my standard 80amp alternator should be able to just handle that with a 0gauge Big 3 upgrade right?

Here's my reasoning behind putting in a battery, and please correct me if what I am saying here is complete bullsh@t.

So, there are two reasons for the extra battery: Firstly, If I do, for whatever reason, use more power than my alt. can provide for a short period, I want to know that there is a battery that is capable of feeding my amps enough power, so I don't damage them.

Secondly, I would sometimes like to listen to my system while the engine is off, And a deepcycle with an isolator in the back would ensure I will at least always be able to start my car after listening for some time.

I know this is a bit of topic from the original question, but maybe I should first determine whether a battery would help or not.


Platinum Member
Username: Glasswolf

Columbia, South Carolina America

Post Number: 14720
Registered: Dec-03
If you plan to use both batteries, then definitely look for a battery isolator from an RV shop or similar such store. That way one battery in back can be dedicated to the audio system, and still be charged by the alternator, but if the battery gets drained, the starter battery is unaffected, and will still start the car to recharge both batteries. This is pretty muth the only way I'd personally set up two batteries in a car for audio.

0 gauge wire is pointless with an 80A alternator though. 4 gauge wire for the lengths used in a car is goot for 125 amperes, which is more than your alternator could supply in total, let alone to the audio system when the car is using about 50+A already.
You could also look into having the alternator you have rewound for higher output. That's mostly just a labor fee and could be done by a local starter/alternator shop. An 80A alt should be rewindable to around 100-110A I'd think.

Bronze Member
Username: Arnotjie

Post Number: 41
Registered: Apr-10
Yep, That's exactly how I planned to install the battery system. Thanks for the tip on the 0 gauge though. I was just about to spend $80 on cable when I have a perfectly good roll of 4 gauge at home.

I have thought about getting the alternator rewound, however, I read on an Australian forum, that you loose some of your low rev amperage. What's your opinion on that?

I never even took in to account that the standard car would be using most of the amps already. A high output alternator might be my only option. How much power would a 1100rms system realistically be using when playing only music. Would it be around 600rms? less? more? I have no idea.

Platinum Member
Username: Glasswolf

Columbia, South Carolina America

Post Number: 14731
Registered: Dec-03
see my pages there. see the "Electrical" section. Those pages have all the information you need.

Regarding low RPM current, the fact is that most alternators are that way because they don't put out full pea current until they are fully spun up. The full output of 99% of alternators is only available above around 900-1100rpm. At idle, you also will see around 12VDC, and above idle, 13.8-14.4 (charging voltage) VDC
This is why you need to drive a car around or rev the engine to charge a depleted battery.

In short on current draw, figure 40-90A for the car itself depending on options, etc.. (luxury models with power everything, heated seats, etc draw more than a stripped Tercel)
Then figure a class D amp is 80^ efficient, and class AB (full range) amps are about 60% efficient on average.
Figure watts = amperes * volts, so take the wattage, divide by your system voltage (12 to 14 volts depending on how you want to do this) then take the result, and multiply it by the efficiency (0.8 for class D, 0.6 for class A/B) to get the current draw for the amplifier at peak output.

NOW, you also need to remember that this is a measure at peak continuous output. momentary peak demands can draw more, while turning the volume down from max will decrease the power used significantly (like half volume is a tenth the power, so a quarter of full volume is divided by another factor of ten)

This is explained as well in the power versus volume page on the link above. has even more info if you want to learn.
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