Lithium battery vs AGM

[Original wis]

So amp hours are amp hours no matter which medium you choose right? Or is there some conversion factor from AGM to lithium?

I'm hearing people say that they're running an 8k all day with only 45ah battery. But according to basic math, I'm supposed to run 400ah at minimum (3,900/12.8)=304. 304/.75= 406. 200 amp alternator at resting cuts that to 206 amps roughly.

This is what I think:

One amp has a 150 amp fuse, one has 50 amps, one has 40 amps. So they should never need more than 240 amps or a fuse should blow. Which means that if the alternator makes 200 amps at idle I could probably cut that to 100ah and worse case scenario, be able to run the system off battery for 20 minutes more or less being conservative. The math is closer to 25 minutes actually. Which is fine since I'm not building a demo vehicle.

So my plan is to put a group 35 (60ah) under the hood and a 100ah in the trunk which should be easy overkill.

But just in case I'm missing something, can anyone help?

Lewis King

[Justin Zazzi]

Well you could use the Onderdonk multiplier but we'd have to know more about which AGM you're comparing to. The Lithiums they have are are questionable unless you get the right ones. Your basic math checks out but I would rely more on the Pythagorean tbh. Did you get the turbo enabulator, or the retro one from Rockwell? The former is getting harder to find these days but they don't make them like they used to.

(ps: what are you talking about?)

[Original wis]

I'm sorry. I didn't realize how much of the picture that I left out. But thank you for replying.

The issue is that I want to make sure that I have a robust electrical foundation for the system that I want to install. It will have a Taramps Smart 3, a Taramps 440x4, and a Taramps 400x4. The total amount of wattage comes out to 3900. According to the formula for converting watts to amp hours, I'm supposed to run 400ah at minimum (3,900 watts/12.8 volts)=304 amps. 304 amps / 75% amp efficiency= 406 total amp hours.

There's a 320 amp alternator that puts out 160 amps at idle, a 60 amp hour battery under the hood so according to the math above I should add 186 more amp hours to cover the amount of wattage that I want to install (160 alternator+60 amp hour battery = 220 amps. 406 - 220 = 186).

I'm thinking that adding 186 amp hours in rear batteries is way too much. I'm not building a demo vehicle, just a daily driver so I think I will be fine with adding a 100 amp hour battery instead of the 186 that the formula says. The 100 amp hours would allow 25 minutes of playing the system on battery power alone so my thought is that's more than enough to cover any eventualities. But I could be wrong so I'm asking if anyone thinks I'm missing something.

I hope that explains the question in a more cohesive manner.

Thank you,

Lewis King

[Jdunk54nl]

You are making one huge mistake in your math. Unless it is a subwoofer, you can start cutting the wattage output by a lot. A tweeter will be a few watts, a midrange 20ish watts, a midbass 80ish watts. A sub 1000w.
You are also assuming that you will be using this at MAX volume, which would be crazy loud.

[Justin Zazzi]

Thanks for the details Lewis. So what is your goal? It's hard to say if you're overlooking something since I'm not sure what you're trying to accomplish. I assume you want to run the system for a while with the engine off.

If so, the most safe way I know of is to install a 2nd battery somewhere with a battery isolator so that you can power accessories without draining your engine starting battery.

Also like Jdunk mentions, you can be much more conservative with your numbers because music will not use all of the power of your amplifiers all of the time, and it is also doubtful you will run the system at maximum loudness for 30 minutes at a time, and your non-subwoofer speakers will draw a lot less power than you think too.

I'm curious where the equation comes from, the one where you convert from watts to amp-hours. Where can I learn more about that?

In short, you can reasonably reduce all your numbers down to 1/4th for the reasons above which means your 160amp alternator and your engine battery should be plenty.

[Original wis]

Nice thank you for the clarification.

My goal is to build a daily driver. I don't plan to sit anywhere and listen to music with the car off. It'll always be on and idling worst case scenario.

As for the formula, I got it from a YouTube video by Car Audio Fabrication. And I've seen it in other places just can't remember where. And it's built into the ultimate car audio app from Google Play. It has an amperage calculator.



I'll get the 50ah anyway though. It's only $150 or so and when I want to upgrade I'll already have the electrical in place. Can't hurt.

Thank you again

Lewis King

[Justin Zazzi]

Thanks for the app recommendation. The formula seems to be:

alternator current = wattage / voltage / 0.75 * (1/2)

Looks like the formula assumes amplifier has 0.75% efficiency, then assuming 1/2 long-term power draw vs the amplifer's RMS number. Seems reasonable for looking at worst-case maximum output, but a substantial over-estimate for most people listening to music.

The amp-hour number seems to be recommending a certain amount of energy buffer for .... something. It feels like a substantial overestimate though.

Thanks for sharing the app, it looks like someone had a lot of fun making it.

[Original wis]

Yeah the wattage calculator may be way off but the enclosure design is pretty much dead on. I used it along with speakerbox lite and subbox.pro to get a really good build model for my trunk size and checked the numbers in each one against the others. If you use all 3 you'll get a complete box build including cut sheets and be able to see a visual of the completed build.

[ca90ss]

I'm not sure how amp hours are a useful spec in this instance either. It could be marginally relevant with lithium batteries as most are limited by the amount of current they can deliver continuously. For example, a Battleborn 100ah lithium can only put out 100a continuous and 200a surge so in that instance you would potentially need more than one battery for your current requirements but I can't think of any usefulness of AH in regards to AGM batteries unless you planned to listen for long periods with the car off. As has been said, your stock alt and battery are likely enough for your needs and it won't cost anything to try it out first before spending a bunch of money you may not need to spend.

[Original wis]

Gotcha thanks I appreciate it. I'll see how the alternator works and go from there.