Question about Lithium battery vs AGM

[Original wis]

Lithium battery vs AGM

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 watts/12.8 volts)=304 amps. 304 amps/.75 amp efficiency = 406 amps. 200 amp alternator at resting cuts that to 206 amps roughly.

This is what I think though:

One amp has a 150 amp fuse (Smart 3k), one has 50 amps (440 x 4), one has 40 amps (400 x 4). 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

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[hispls 5,000+ posts]

So amp hours are amp hours no matter which medium you choose right?

Not really. A 100AH AGM battery will deliver .1A for 10 hours, might deliver 100A for 1 hour but absolutely wouldn't deliver 1000A for 6 minutes. Anyway, most of those amp hour ratings are taken at a very low current draw and under optimum temperatures and conditions. You should not plan to see anything like those published numbers unless you grossly over-build your battery bank. Honestly I think lead acid batteries are tested to be able to do more like .05A for 20 hours when they figure amp hours. Google is your friend there, it's been a while since I've dug into the topic and seen the graphs.

You should attempt to investigate at what current these published ratings hold true and also consider the maximum and nominal charge and discharge ratings of whatever battery you intend to use.

Keep in mind also that "running an 8000W amp all day" is not a very precise picture of how much current they're drawing over time, in fact, for all we know that "8000W" amp could be a Boss amp that wouldn't make even 2000W if it were hit by lighting and is running at 4 ohm, and the person making the claim is listening to live recordings of bluegrass music.

SOME lithium cells should be able to deliver 800A out of a 40AH bank but you wouldn't have much reserve there and you cannot count on your alternator to load down and deliver current instantaneously or reliably under real world conditions.

What precisely are you intending to do? What will get you by for 3 second tone bursts in an SPL competition is probably a different animal than if you want to just sit in a parking lot with the doors open and be obnoxious for long periods of time.

 

[Original wis]

Not really. A 100AH AGM battery will deliver .1A for 10 hours, might deliver 100A for 1 hour but absolutely wouldn't deliver 1000A for 6 minutes. Anyway, most of those amp hour ratings are taken at a very low current draw and under optimum temperatures and conditions. You should not plan to see anything like those published numbers unless you grossly over-build your battery bank. Honestly I think lead acid batteries are tested to be able to do more like .05A for 20 hours when they figure amp hours. Google is your friend there, it's been a while since I've dug into the topic and seen the graphs.

You should attempt to investigate at what current these published ratings hold true and also consider the maximum and nominal charge and discharge ratings of whatever battery you intend to use.

Keep in mind also that "running an 8000W amp all day" is not a very precise picture of how much current they're drawing over time, in fact, for all we know that "8000W" amp could be a Boss amp that wouldn't make even 2000W if it were hit by lighting and is running at 4 ohm, and the person making the claim is listening to live recordings of bluegrass music.

SOME lithium cells should be able to deliver 800A out of a 40AH bank but you wouldn't have much reserve there and you cannot count on your alternator to load down and deliver current instantaneously or reliably under real world conditions.

What precisely are you intending to do? What will get you by for 3 second tone bursts in an SPL competition is probably a different animal than if you want to just sit in a parking lot with the doors open and be obnoxious for long periods of time.

No demos no burps. Just daily driver. The point is just in case something happens I don't drain the main battery. Don't want to get stranded somewhere at a bad time.

 

[Bobbytwonames 5,000+ posts]

No demos no burps. Just daily driver. The point is just in case something happens I don't drain the main battery. Don't want to get stranded somewhere at a bad time.

Two batteries and an isolator so that you can run your system and not drain the starting battery.