I see lithium batteries advertised with different ratings, like supporting a 2000 watt system with a stock alternator or 4000 watts with an aftermarket one. This doesn’t make sense to me.
The power being drawn by the system needs to be replenished. So why would the size of the alternator needed for a lithium battery system be any different from an AGM or flooded battery? The power is still being used, right?
So, why do these batteries claim they don’t require an upgraded alternator when a regular system would need it?
The way lithium batteries are rated has to do with their reserve power. When you’re playing music, it’s not a constant tone, it’s dynamic. So, the battery is usually charging instead of discharging. The battery’s rating is based on how long it can handle that, since it can accept more amperage. A lot of people think you need a 150 amp alternator for a 1500 amp system, but that’s not true. I run two alternators totaling 450 amps at idle and I use 180ah of Cmax. I have amps that pull about 5500 watts each, but my voltage drop never goes more than half a volt, usually staying around 0.2 volts.
@Nori
Okay, let’s say we’re using extreme music, like dubstep with heavy bass. You have a 5000 watt amp running from a 150 amp stock alternator. If you throw in a 7000 watt rated lithium battery, and you’re on a 2-hour road trip blasting music, would the lithium battery be drained, fully charged, or somewhere in between when you get to your destination? With the energy density, I guess we’re no longer worried about undervolting and turning off the ignition system, right?
@Jaden
There are a lot of variables here. Are the subs wired to one ohm and rising to four ohms, making 5k watts, or are they only pushing 1k? Tuning matters because of the rise in impedance. Each setup is different. I have friends who have a 16x rise in impedance, while my truck has almost no impedance rise. I shut off my amps when I get low on ohm protection. If you’re running a 5k watt pure sine wave, it’ll drain fast. But music is dynamic, like dubstep, where the beats pause every second or two. So, I’d say your battery would be about half drained. This is a unique case, though. Someone running 5k watts of bass will also have a secondary battery and some other amps. It’s hard to give a one-size-fits-all answer. It also depends on the loads. My setup uses a Victron 12-12, charges from my Cmax to my stock battery, and keeps things around 14.4 volts. Most of the time, the vehicle only needs about 12 amps to run, 15 with AC, so with 135 amps of charging, it should hold up.
@Nori
Exactly, a lot of people are looking at getting 3000 to 5000 watt amps with just a standard lithium battery, no extra upgrades. That’s what I’m trying to understand, because these lithium batteries claim you don’t need a bigger alternator.
@Jaden
I ran a Sundown SCV 6k with a single 45ah LiPo in my G35, which has small alternators, and never had an issue. Voltage stayed above 13 volts. So, I’d say it’s definitely doable. I even did a battery delete.
@Nori
Hmm, okay, so it sounds like we’re just relying on the time between heavy bass hits to recharge the lithium batteries. I guess it’s about using the battery’s energy density and discharge capacity more effectively now?
@Jaden
That’s the case with any battery, not just lithium. It’s about discharging and recharging. The best part of lithium is that it can recharge much faster than AGM batteries. AGM batteries can only accept a limited amount of current, so even if you have enough amp-hours, the AGM might not be able to take all the power from the alternator. With AGMs, alternators often just prop up the voltage but don’t fully charge the battery. Lithium does better at this. When a big discharge happens and the alternator is working, if you shut off the alternator, the lithium won’t drop much in voltage. An AGM would drop from 14.4V to 13V almost immediately because it can’t handle the full power.
From what I know, Lithium batteries (like LiFePO4) have higher energy density and better charge/discharge efficiency than AGM batteries. For example, you could run a 2kw RMS system with 2 100ah AGM batteries and a 120a stock alternator. You might experience voltage drops because AGMs can’t deliver power as quickly as the amp needs. But lithium batteries can keep up with that.
@Hux
Yes, I get that. But I still don’t understand why the amperage for charging would be any different.
If I’m drawing 20 kilowatt hours from the battery, no matter how big the battery’s capacity, I still need to put those 20 kilowatt hours back, or eventually the battery will be dead, right?
So why do lithium batteries claim they don’t need a bigger alternator?
@Jaden
I think it’s because lithium batteries allow for deeper discharge. With AGM batteries, you only get 50-70% of usable energy. But lithium batteries let you use 85-100% of the energy. I’m not an expert, so it might be good for you to look into how Depth of Discharge (DoD) works.
@Hux
Yeah, I’d like to find some actual research papers on this. It feels like marketing nonsense from lithium manufacturers because I don’t see how we’re getting free energy here. The alternator still has to power everything in the vehicle, no matter how many or what type of batteries are in there.