12V or 24V house/aux battery system?

[WilloCo]

I'm building up my cruiser for camping and I've got a pair of 4S LifePO4 packs that are identical. I haven't settled on a proper BMS yet.
The other day I ordered a dual ARB compressor and the next day realized that I'd ordered the 24v version. Bit of an accident.
I also have a Warn 8274 winch that I'm about to rebuild and can easily convert it to 24v since I'm replacing the motor.

This got me seriously thinking about going for a 24V aux/house system.

Advantages of going 24V:
24v winches are faster
Amp loads are cut in half
Wiring can be smaller/stay cooler at same size.
Reduced effect of voltage drop - improved efficiency.
My fridge will run on either 12v or 24v.
24v inverter is trivial to buy
24v solar is trivial to configure
12-24v DC-DC converter cost is similar to the 12-12v converter I was already planning on to support the LifePO4.

Disadvantages of 24v:
Some systems are locked into 24v use only, can't switch over to the main battery if there's a failure.

My battery packs are built from high output 200A rated Headway cells in 4p configuration. Winch load isn't an issue.
Emergency starting could be done from a single battery pack.

Since I haven't collected the various DC-DC bits yet I don't really have any major additional cost here. I will probably spend a bit more on a larger 12-24v DC-DC converter since I'd be running the winch off that system.

Final thoughts - I'm planning on 24v in my camp trailer, so I'd also be able to pull spares from either system if needed.

 

[billiebob]

24V is more efficient. 12V is just the industry standard but that is starting to change. The 4cylinder JL Wrangler comes with a 48V lithium ion battery with a coolant loop to keep it warm and an AC loop to keep it cool.

 

[john61ct]

Sounds like you're all set

 

[dreadlocks]

Higher Voltage is really the way to go with Lithium banks if you are gonna run heavy loads.. compared to FLA's the max current output of Lithium is very small..

Quick google of your winch shows 480A @ 12v at max power.. if you have two 200A batteries.. My math shows chance of triggering BMS protection in 12v, but you'd be fine at 24v.

 

[WilloCo]

@dreadlocks that's 200A per cell in a 4p (parallel) config, so more like 800A max across the pack.
Virtually all of the commercially built packs do have pretty low current.
My pack is made from headway 38120 hp cells. They are 8Ah LifePO4 cells. I have 64 of them total, 32 per pack (originally planned on a pair of 12v packs)
Max discharge rate is 200A per cell and 80A continuous. So to be safe, we'll call it 360A continuous load. Plenty for the winch especially at 24v.

These are pretty impressive cells. There's (an overly long) video of one guy using just 4 of these cells to start his car.

The usable temperature range of these is also pretty good.
Charge -10～45℃，Discharge -20～65℃
AKA
Charge 14F～113F，Discharge -4F～149℃

I'm going to be adding a custom monitoring pack for temperature cutoff and management.

 

[dreadlocks]

what BMS are you using that handles 800A? oh your just raw dogging those cells? Might wanna settle that part of the design to know your actual current limitations.

 

[luthj]

Is your alternator 24V? Do you really want to run the winch from the house bank?

You could use a BMS that doesn't rely on FETs to control the disconnect. Several models support a latching or low power external relay. These are available in the 500A continuous (1500A peak) sizes.

Are you sure the ARB unit doesn't also support 12V? Many of the fridges are dual voltage.

 

[WilloCo]

No the alt system in the land cruiser is 12v. however I will be using a DC to DC converter to charge the lipo bank either way - it's not great to charge lifepo4 off of direct alternator voltage even with a 12v pack.
Instead of a 12v-12v charger, I'll be running a 12v-24v charger. That'll feed into the balancer.
The ARB compressor ships in 12 and 24v flavors. My fridge (dometic) takes 12 or 24. not an issue.
It would be trivial to get a 12v arb compressor, but it just sort of ticked over my thought process.

But yeah, I have decided to go 24v for the aux system.

@luthj has a good point about using a contactor/solenoid to handle the main load. I'll have that, just wasn't going to far into the weeds on it.

I suppose I'll clarify here.

1) I will be using a BMS to balance the packs for charge. A LifePO4 BMS doesn't balance anything during discharge, it only provides cutoff for low/high voltages.

2) I will be running a custom secondary battery system - it'll monitor cell voltages, temps and also cutoff the battery pack - it'll turn off charging, and disconnect loads for temperature state/discharge limits. That'll be my enforcer for battery sanity.

couple more thoughts. I probably wouldn't do this with any of the commercial lifepo4 packs out there. Those are maybe 100-200amp BMS and all power to and from them go through that BMS. Most of those don't play well with making 24v packs out of them either. That's totally fine. In my case, I can safely deal with significant load on these cells and properly support the 8s LifePO4.

My general winching needs are usually a few pulls. Snow days are like that sometimes. If I were planning to winch daily, I'd probably double up my battery pack just to distribute the discharge loads.

 

[john61ct]

compared to FLA's the max current output of Lithium is very small

Wut?

No, going by C-rate, LFP is **much** more power dense than **any** lead.

Not drop-ins of course they are crippled by their cheap BMS which usually can't be bypassed.

But with your own setup, the current has no need to go through the BMS, or you just pay for one with the amps capacity you need.

Of course habitual use over a C or two will shorten lifespan compared to normal House usage rates.

And you need robust infrastructure, fat wires gas-tight crimping great fuses insulation etc to move high currents around safely.

Going to higher voltage helps with that

 

[WilloCo]

A few more details as I develop this.

I've been taking a serious look at which DC-DC converter I'll run. It'll be extra important when I crank up the winch.
I have three contenders, all made by victron. There are some offbrand options but victron has two features that I require:
1) Adjustable voltage - so I can tune the charge output.
2) on-off control built in - so I can set it up to only run when the car is on, and extra cut-off controls from my battery monitor.

The contenders:
Victron Orion 12/24-20A DC-DC converter - non-isolated (aka no transformer), can peak at 30 Amps, 20 Amps continous. Also I can parallel up to 5 of them for more amperage.

Victron Energy Orion 12/24-20A DC-DC Converter

Part Number: ORI122420020

www.solar-electric.com

Victron Orion-Tr Smart 12/24-15A Isolated DC-DC charger w/bluetooth

Victron Energy Orion-Tr Smart 12/24-15A Isolated DC-DC Charger with built-in Bluetooth

Part Number: ORI122436120

www.solar-electric.com

5 amps less continuous and peak, bluetooth interface for control/monitoring. Parallel any number for more current. $100 more per unit.

I'm not sure how I feel about the bluetooth - it's really a bit of a security hole in a vehicle setup and I work near lots of nerds.
The smart unit can actually detect the engine state automatically, so that's actually pretty nice for bootstrapping while I work on my custom BMS bits.
I can still use the on/off input as well.
Of note, it's also a bit more water resistant.

Of note, the non smart(bluetooth) version of the above for about $50 less:

Victron Energy Orion-Tr 12/24-15A DC-DC Isolated Converter

Part Number: ORI122441110

www.solar-electric.com

All charge and loads will go through the LiFEPO4 BMS, with the exception of the winch, which will bypass the BMS but also have a LV/batt temp cutoff.

 

[shade]

If you're concerned about Bluetooth security, Victron may provide a way to completely disable it, as they do with their batteries.

The Orion-Tr and Orion-Tr Smart lines are different enough that you should compare their manuals. Also, the Orion-Tr Smart is a new product, due to be released in NA real soon now.

DC-DC chargers (well, converters) work best when located close to the battery they're charging to minimize voltage drop.

 

[john61ct]

Note that a DC-DC **converter** usually does not include the required functionalities of a **charger**

Victron makes great stuff, but personally I'd stick to Sterling BB series for DCDC charging until the new Victron has gotten user feedback for a couple years, or approval after very thorough testing by trusted techie members.

Of course if you don't need charging output, you should not be paying these crazy prices, there are dozens of quality DC converters on the market.

But yes Victron makes quality gear, as a rule.

 

[WilloCo]

The price on the sterling is a little high, but I do like the power throughput - 50Amps is nice.
However, reviewing the specs says they are designed for GEL, AGM or Flooded Lead Acid batteries. I'm using LifePO4 so those don't seem to be a good choice.

 

[john61ct]

I have yet to see a single charge source with LFP labeled presets I would apply to a bank of mine.

Most designed for lead are fine, so long as they support **user custom** adjustments of setpoints.

Which very few DC-DC chargers do.

Sterling's do. The BB series without the optional remote.

 

[DRP]

Note that a DC-DC **converter** usually does not include the required functionalities of a **charger**

Victron makes great stuff, but personally I'd stick to Sterling BB series for DCDC charging until the new Victron has gotten user feedback for a couple years, or approval after very thorough testing by trusted techie members.

Of course if you don't need charging output, you should not be paying these crazy prices, there are dozens of quality DC converters on the market.

But yes Victron makes quality gear, as a rule.

I’m not the techies member but I am running three other VictronConnect units in parallel to my three Battle Born so I will be an early adopter of the 12/12/30 Orion Smart.
I have two 75/15 and one 100/30 charge controllers and the Orion unit appears to be built on the same 30A chassis as the 30A SC.
I’ report back when it’s up and running