Battery Bank - wire for 12v, 24v, or 48v?

[ependydad]

I have four 12v 100Ah Battle Born lithium drop-ins. I'm looking towards the future using Victron components:
- 3,000W inverter w/ hybrid load-support
- solar charging

Currently I have a built-in generator for charging batteries.

If I went with 24v or 48v, I'd need a DC-to-DC converter to step down to 12v to run the existing RV systems (slides, lights, thermostat, fans, etc.).

So, what would you go with? 12v, 24v, or 48v? And why?

[GordonThree]

phemens wrote:
I just did a battery revamp and added 4 lithium batteries, stayed with 12v setup due to pre-existing 12v inverter charger. Very happy with it, They don’t sweat high draw loads, but i’m finding that my Victron 702 is not an accurate indicator of charge. I think it calculates it based on amps out but doesn’t seem to jive with the readings I see on each of the batteries via their BMS panel. Some tweaking to do there. Agree that amps in and out will be the main indicator, but not sure yet how to accurately show that via the 702.

I would trust the Victron, which uses a precision shunt versus the on-board BMS, which *maybe* is using a hall effect sensor but more likely is guesstimating based on cell voltage.

I can't find anyone on youtube that cut one of these batteries open yet to take apart the BMS. All the recommended videos seem like commercials to me.

[phemens]

I just did a battery revamp and added 4 lithium batteries, stayed with 12v setup due to pre-existing 12v inverter charger. Very happy with it, They don’t sweat high draw loads, but i’m finding that my Victron 702 is not an accurate indicator of charge. I think it calculates it based on amps out but doesn’t seem to jive with the readings I see on each of the batteries via their BMS panel. Some tweaking to do there. Agree that amps in and out will be the main indicator, but not sure yet how to accurately show that via the 702.

[GordonThree]

Sounds like a great project!

[ependydad]

GordonThree wrote:
Counting amps out vs amps in is even more important, IMHO.

I have a Victron BMV-702 for that. I think I'll also get the Victron Color Control system once we do this upgrade.

[GordonThree]

ependydad wrote:

GordonThree wrote:
48 volt. The higher the voltage the better. It will give the most margin between "full" and "empty".

With the lithium batteries, does that reasoning still stand? The drop-off for lithium is sharp with very little in the middle of charged vs. not.

Yes, applicable to any technology. On lithium you can't rely on voltage as an accurate state of charge indication, because of that flat discharge line. However having more margin is still a positive.

Counting amps out vs amps in is even more important, IMHO.

---

Higher voltage also lends itself to more efficient conversion inside the inverter. Instead of having to amplify a 12v source with a gain of 10x to get 120v output, a 48v source only needs to be amplified 2.5x (this is over simplified but works as a demonstration). The amplification (boost) causes heat in the inverters's transformer and switches (transistors). The less work they have to do, the less heat they generate.

[ependydad]

GordonThree wrote:
48 volt. The higher the voltage the better. It will give the most margin between "full" and "empty".

With the lithium batteries, does that reasoning still stand? The drop-off for lithium is sharp with very little in the middle of charged vs. not.

[GordonThree]

48 volt. The higher the voltage the better. It will give the most margin between "full" and "empty". It also greatly reduces cable complexity and voltage drop losses.

A DC-DC converter is super easy, get one with a read-out on it so you can dial in a nice stable 13.5 volt output and leave it.

Something like this unit:

DROK

Install it in your distribution panel, rip out the old 12v converter.

[Lwiddis]

Is there any loss in stepping down? Becomes important when you go solar.