[amcheekin]

TLDR: Charging house batteries via B2B (DC–DC) vs. Inverter (DC-AC-DC)? Advantages/Disadvantages of each?



Context (my intended build):

Factory: 2023 Transit T-350 Cargo, 148” HR, Ext Length, Single 250 Amp Alternator, Dual 70Ah AGM Batteries

Build (with appropriate busses, circuit breakers and fuses):

• Alternator Charging: CCP2 to B2B (Sterling BB122470 – 70 amp @ 12V in & 35 amp @ 24V out) to 4 each parallel house batteries (Battle Born BB5024 – 50 Ah 24V = 200 Ah @ 24V).
• Shore Power Charging: Victron MultiPlus II 24/3000/70
• Solar Charging: Future addition using MPPT sized for Solar array.
• Backup 12V from CCP1 direct to A/B switch to power 12V fuse panel in a pinch (Thank you GregoryX. I lifted that from your wiring diagram).

Over the last year plus, I have read several posts on this forum and in other places that compare battery charging via both B2B and AC-DC-AC versus using an ACR charge relay. These posts and websites are written by individuals who IMO know what they are talking about. I am convinced that both are better than ACR. But I have not seen (although I am sure it exists and I have missed it) a conversation comparing B2B vs. AC-DC-AC. Advantages and disadvantages.



Things I think I understand:

• LiFePO4 batteries require a specific charge profile. In my case provided either by Sterling B2B or Victron Inverter/Charger or future MPPT.
• Victron 24/3000/70 will charge the house batteries on shore power converting 120V AC to 24V DC when shore power is available.
• Victron 24/3000/70 (when there is no shore power) will convert 24V DC (from the house batteries) to 120V AC to power the house 120V loads.

Things that I need to resolve:

• Sterling BB122470 @ 35-amp output will take about 6 hours of driving to fully charge an empty battery bank. It will take 9 hours if I add the two more batteries that I am considering. These charge times could be cut in half by adding a second Sterling BB122470 costing more and taking up more space.
• A 2000-Watt inverter connected to CCP2 could output 15 Amps @ 120V (assuming 90% efficiency) to feed the Victron inverter/charger.
• Max output of the Victron 24/3000/70 is 70 Amp @ 24V. But I do not know how to calculate the expected output of the Victron at the various potential input amps. If it is as simple as 15 amps X 120V = 1800 Watts / 24V = 75 Amps, then the Victron fed by the 2000-Watt inverter is less expensive than 2 Sterling B2B chargers. Is it that simple?
• CCP2 is fused at 175 Amps, but I don’t want to blow that fuse, so planning on a 150-amp circuit breaker/switch after CCP2 and before either the inverter or the B2B charger.

The questions (finally):

What are the reasons to choose one charging method over the other?

Am I calculating the power output from the inverter correctly?

Thank you all, in advance for your help and expertise.

 

[BadWolf]

When dealing with different voltages, convert everything to watts before comparing.

Don’t so much look at the “24v” on LiPoFO batteries, look at the charge voltage. 24v LiPoFe is usually 28.8v +/- 0.4v
Example:
Sterling 122470
28.8v x 35a = 1,008w

With only a single alternator, you have to deal with the duty cycle (max capacity for continuous load) The van takes ~75a to operate, you don’t want to max it out nonstop, in the thread linked they recommend keeping the load under 100a, which at 12v (13.8v charge rate) its going to limit you to 1,400w

250A 'Heavy Duty' Alternator Charging...

Hey-there everyone. I absolutely love my new 2022 Transit and I have some questions about alternator charging of a large lithium house bank. I'm an EE and battery charging systems (mostly solar) are my professional field so I get how batteries and battery systems work and interact w each other...

www.fordtransitusaforum.com

DC>AC>DC
Cheaper - $300 can get you 2,000w (ccp max is ~2,000w)
Smaller awg wire
Requires selector switch. (Shore or Van)

DC>DC
Expensive - ~$400 get you ~1,000w
(1x Sterling 122470)
(2x Orin 12-24-15 or 12-12-30) 360w each
No switch needed.

One question I have is do you have a 24v device with a heavy load? (ex: A/C)

 

[amcheekin]

One question I have is do you have a 24v device with a heavy load? (ex: A/C)

Thank you for the reply.

24V for the Iceco Fridge and 24V to 24V-12V converter for all the 12V loads

DC>DC
Expensive - ~$1,000 get you ~2,000w
(1x Sterling 122470 or 12120)

I might be missing something, but I believe the Battle Born list the Sterling BB122470 at $380. Or are you assuming needing 2 Sterling B2B's?

I went through the Post by Vanlab. Seems focused on his intention of charging direct from CCP2 without inverter or B2B. This forum and other sources seem to recommend against this method. Sounds like he knows what he's talking about, but its way over my head.

Do you have a recommendation for a 2000 Watt pure sine wave inverter?

 

[BadWolf]

The Iceco fridges list as 12/24v, so do you have a 24v only model? The largest 90L iceco only used 657watts in a 24 hour period per their website. So that is less than 30watts per hour, and at peak they are only 55watts.

Is that the largest load 24v device? Are you installing any sort of air conditioner?

If the Iceco is a 12/24v, have you already bought the power components? If not I would recommend switching to a 12v system. The step down will add ~8% power consumption for all your 12v devices, plus adds points of failure (the stepdown/connections to stepdown). 12v has a lot more devices/support as well

I went and fixed my last post, i thought you were talking about the new sterling B2B
12V to 24V 120A input | 60A output Battery to Battery Charger w/ reverse charging feature

The link for the post was to show they recommend to stay under ~1200w off of CCP2 with a single 250a alternator.

I used the same GIANDEL 2200w as gregoryx, but i'm switching to the Renogy 2000w (rebuilding my 24v system to 12v, had a bad Sigineer Inverter/Charger/Mppt, that blew up the Giandel).

Amazon.com: Pure Sine Wave Power Inverter 2200Watt DC 12volt to AC 120volt with Dual AC Outlets and hardwire Blocks & LED Display Remote for RV Truck Boat Solar Emergency ETL Approved【3 YRS Warranty】 : Automotive

Buy Pure Sine Wave Power Inverter 2200Watt DC 12volt to AC 120volt with Dual AC Outlets and hardwire Blocks & LED Display Remote for RV Truck Boat Solar Emergency ETL Approved【3 YRS Warranty】: Power Inverters - Amazon.com ✓ FREE DELIVERY possible on eligible purchases

www.amazon.com

Amazon.com: Renogy 2000W Pure Sine Wave Inverter 12V DC to 120V AC Converter for Home, RV, Truck, Off-Grid Solar Power Inverter 12V to 110V with Built-in 5V/2.1A USB Port, AC Hardwire Port, Remote Controller : Patio, Lawn & Garden

Amazon.com: Renogy 2000W Pure Sine Wave Inverter 12V DC to 120V AC Converter for Home, RV, Truck, Off-Grid Solar Power Inverter 12V to 110V with Built-in 5V/2.1A USB Port, AC Hardwire Port, Remote Controller : Patio, Lawn & Garden

www.amazon.com

 

[gregoryx]

...

• A 2000-Watt inverter connected to CCP2 could output 15 Amps @ 120V (assuming 90% efficiency) to feed the Victron inverter/charger.
• Max output of the Victron 24/3000/70 is 70 Amp @ 24V. But I do not know how to calculate the expected output of the Victron at the various potential input amps. If it is as simple as 15 amps X 120V = 1800 Watts / 24V = 75 Amps, then the Victron fed by the 2000-Watt inverter is less expensive than 2 Sterling B2B chargers. Is it that simple?
• CCP2 is fused at 175 Amps, but I don’t want to blow that fuse, so planning on a 150-amp circuit breaker/switch after CCP2 and before either the inverter or the B2B charger.

...

Yes, your math is correct. Max charge rate from the Multiplus is 70A @ the DC output side. That is fixed, regardless of the voltage and actual voltage during bulk is probably set to 29V or 29.2V or something like that - dropping a bit with the load - so you really end up with something like 27 * 70 = 1.9kW or so. Minus efficiency losses.

Not sure what the efficiency losses are on the /charging/ side of the Multiplus; but heat and noise (since fans are indexed on heat) are good indicators and it ramps up the fans a bit, but not as much as the higher rates when you're inverting at 3kW, so... I'd guess not 98%... probably in the 90% range? Similar with whatever you'd use as the charging inverter: most inverters are much more efficient at a lower output than their max; though they will quote their ideal max efficiency - even if you'll never actually use that output.

I like to use this chart from the Multiplus 24/3000/70 on the inverter efficiency. I haven't seen such a chart with other vendors; but I'm confident they all have a similar curve of efficiency - regardless of their stated efficiency.

As you can see, optimal efficiency is in the 1kW range - less than half the rated output (3kVA = 2.4kW). Victron states this is "max efficiency" of 94% - most vendors aren't that clear. So take EVERY efficiency claim you see with a grain of salt.

I like to poke at the Sterling efficiency claims of 98% while requiring a LOUD fan and having limitations of space and airflow; actual 98% efficiency would require very, very little cooling since 98% efficiency on a 1kW unit is only 20W... which is not much. Though their new units claim to be much quieter and cooler, so I'm guessing they're doing much better now.

Okay... so back to your questions.

Reversing the logic, since the max charging on the Multiplus is ~1.9kW, anything more than that is available for 120VAC consumption / loads but no more than that will be dedicated to the charging. But that means a lot because if you have any loads going from the Victron inverter, they will not be additionally supported by a DC-DC setup. So... 150A @ 12VDC on the van going into an inverter means ~2kW at idle and as much as 2.2kW @ 14.5VDC. But if you are pushing beyond 2kW, you'll eventually pop a 150A breaker - though you won't ever hit the 175A fuse on CCP2.

IOW, if you go DC-AC-DC and keep the draw on the inverter to ~1.8kW you're quite safe on the 150A breaker (as well as 175A fuse on CCP2).

Qualifying this all with the single-alternator situation... and I can't say if you /should/ pull as much as 150A - and I doubt it will be possible at idle. You can high-idle it with an aftermarket mod.

Final factor that matters for me: I'd rather have all the electrical (and other) stuff managed and configured through the Victron system - especially with the VenusOS / CerboGX setup. That goes away with the Sterling (or other brand) DC-DC setup, so... not my preference.

Let's see... anything else?

On the 24VDC setup, I think it's preferable even if the only devices in 24V are the batteries and the inverter. A 2.4kW inverter that's capable of up to 6kW (as that one is) means pushing over 450A potential @ 12VDC. That is unnecessarily high; cut that in half and you've still got a LOT of amperage. And there's the added benefit of the 24>12 delivering more appropriate 12VDC levels instead of the higher LFP voltage that causes issues in some rare cases.

Back to the DC-DC vs DC-AC-DC, I really like the flexibility to run it at any rate as low as 8A @ 120VAC on up from there to ~15A @ 120VAC. Run it as you see fit - basically delivering the potential of the larger DC-DC units at the price of the lower ones and the flexibility to adjust from your mobile-phone while driving.

 

[orton]

I usually do not need to charge from the vehicle powered inverter. Power from the vehicle powered inverter is my backup if weather conditions prevent charging from the solar panel.

I do use the vehicle powered inverter to power my 750 watt electric baseboard heater in the morning and can use it to heat shower water with a Sous Vide kitchen water heater if house battery is depleted in poor weather conditions when I do not have excess solar power.

I stay warm at night with a 12 volt DC auto rear seat heating pad and let van interior get cold. I set the dash controls for max. heat and the vehicle powered inverter to run the 750-watt electric air heater. In morning I use the remote start to warm van for 15 minutes before getting out of bed.

Vehicle powered inverter provides 120 volt AC power any time the van engine is running. I prefer the added flexibility of the inverter compared to a B2B charger. Cost is similar if you have a shore power charger.

 

[amcheekin]

@gregoryx

Thank you for detailed response.

Since the Victron 24/3000/70 can be set to limit the input amps (seems like 15 amps is the right choice), that seems like the best way to keep the 2200W inverter connected to CCP2 from tripping the 150 amp breaker while optimizing the charging process while driving.

I am closing in on a design that feels right.

 

[gregoryx]

@gregoryx

Thank you for detailed response.

Since the Victron 24/3000/70 can be set to limit the input amps (seems like 15 amps is the right choice), that seems like the best way to keep the 2200W inverter connected to CCP2 from tripping the 150 amp breaker while optimizing the charging process while driving.

I am closing in on a design that feels right.

Yup. Technically, probably need to limit it to 14A @ 120VAC unless you do the high-idle. With ours, the engine will shut off at idle, causing the voltage to drop to 11.5V or something like that for a couple minutes - and if the load is still 15A @ 120VAC output, it's ~1.8kW (plus overhead @ the 12VAC), which pushes the limit of the 150A breaker. It /usually/ won't pop it; but eventually it does.

For the "how high can you go" side of it, I've pushed as far as 18A @ 120VAC and if the voltage is up at alternator levels (~14.5VDC), it'll push over 2kW; but it pops the 150A if / when the Transit alternator shuts off. Sooo.... there's that. You can technically kill the "smart" aspect as well as the engine stop/start and then push 2kW consistently.

 

[Henchman]

Back to the DC-DC vs DC-AC-DC, I really like the flexibility to run it at any rate as low as 8A @ 120VAC on up from there to ~15A @ 120VAC. Run it as you see fit - basically delivering the potential of the larger DC-DC units at the price of the lower ones and the flexibility to adjust from your mobile-phone while driving.

Greg, I'm not clear on how you limit the input into the Multiplus to [email protected]?...Is this programmed through the CerboGX?

 

[gregoryx]

Greg, I'm not clear on how you limit the input into the Multiplus to [email protected]?...Is this programmed through the CerboGX?

Yes. Or through the Victron Connect app IF you have the BT dongle, IIRC. I wouldn't do the dongle. I'd go Cerbo or Raspberry Pi / VenusOS. Multiple ways to config it with Cerbo - either with it online (remote) or local / direct.

This is on the Cerbo touchscreen OR remote control OR local web interface (no internet required from your phone if you're in the van):

This is the status info locally while on "shore power" (kids are out right now but I can still remote to it).

This is from the online web interface:

But the fancy GUI inside Victron Connect doesn't work for the Multiplus remotely - only locally with BT. So... don't bother with the BT dongle; just get the Cerbo GX or build your own with Raspberry Pi and USB dongle.

 

[kenryan]

... I do use the vehicle powered inverter to power my 750 watt electric baseboard heater in the morning ...

... Vehicle powered inverter provides 120 volt AC power any time the van engine is running

All of this can be accomplished with a single inverter + B2B. In my last camper I regularly ran an electric heater off of my single inverter as I traveled down the highway. I would also use the same strategy of firing up the engine when running the microwave or coffee maker, if the batteries were getting low. The charging relay detected the draw and connected to the truck's alternator to replenish the battery ... all without a second inverter, additional wiring and additional switching.

 

[njvagabond]

Not yet discussed is the option of one inverter and a seperate AC-DC charger with switch for selecting CCP as inverter source for charging or house battey for consumption. There is a good wiring diagram somewhere on the forum (maybe @sportcoupe?). When I looked, the cost of decent standalone units plus switches and wiring did not seem to have much if any $$ advantage over other setups.

 

[gregoryx]

Not yet discussed is the option of one inverter and a seperate AC-DC charger with switch for selecting CCP as inverter source for charging or house battey for consumption. There is a good wiring diagram somewhere on the forum (maybe @sportcoupe?). When I looked, the cost of decent standalone units plus switches and wiring did not seem to have much if any $$ advantage over other setups.

Agreed: segregating the inverter and charger in the house system can work out well, price-wise.

I like the premise of a good inverter-charger acting as an automatic-switch and power-conditioner directly from shore-power - even when pushing more power than the non-shore-power might allow; but that's certainly not impossible to set up with the right dedicated inverter and separate AC>DC and DC>DC instead.

Only thing that keeps coming back in the various options are how much alternator-charging and how much shore-charging are possible from each model. It seems the comparisons are often not apples-apples when the power / charge-rates are taken into account. Which seems to be part of @amcheekin initial post questions.

I have no idea on the dedicated AC>DC (lithium) charging options: what does it cost for something 15A @ 120VAC > 70A @ 24VDC lithium charger? The NOCO units don't seem to go that high.

The Sterling 12>24 @ 60A output looks like it's ~$700? That would be the comparable unit to the DC>AC>DC setup if using a Victron 24/3000/70.

Not sure how all that math works out... 🤔

 

[kenryan]

My previous systems have all used an inverter/charger with a built in transfer switch that allowed for charging from shore power, and an automatic charging relay for charging from the vehicle alternator. I was always very happy and never had an equipment failure or any electrical problem, although I did sometimes worry about over taxing my alternator.

But now, with smart alternators, the automatic charging relay seems to no longer be a good option (although there was one thread that disputed this, not sure how it worked out). Orton has said that the reason he came up with the DC/AC/DC charging method was to isolate the house electrical system from the overly sensitive Mercedes electrical system on his Sprinter van.

As long as my Sterling 120 amp B2B works as advertised, I will be happy. Being somewhat simple minded, I usually prefer the simpler approach.

 

[njvagabond]

I have no idea on the dedicated AC>DC (lithium) charging options: what does it cost for something 15A @ 120VAC > 70A @ 24VDC lithium charger? The NOCO units don't seem to go that high

Dunno about 24V but when I looked there were not a lot of options for 12V lithium. IIRC the 12v chargers maxed out around 80A which was in the range of my assessment for a single alternator. Samlux was the best one but $$$ , Aims, Progressive Dynamic and IOTA were ~ $300 but seemed to have some mixed reviews. When looking at decent inverter chargers, the increment over a inverter only was similar or less that the cost of a stand alone charger, but the amperage was greater than a separate charger (an advantage if you happen access to shore power, but for only a short time) So even if you call the cost a wash, then it came down to the cost of the extra switching and wiring vs a good enough vehicle inverter. I thought the design was a neat idea, but tossed the option out pretty quickly based on the available equipment I had seen. Another disadvantage is that house current through the AC distro is not available when driving .... maybe some would be with even more wiring/switching/controls?

 

[orton]

All of this can be accomplished with a single inverter + B2B. In my last camper I regularly ran an electric heater off of my single inverter as I traveled down the highway. I would also use the same strategy of firing up the engine when running the microwave or coffee maker, if the batteries were getting low. The charging relay detected the draw and connected to the truck's alternator to replenish the battery ... all without a second inverter, additional wiring and additional switching.

Correct. B2B instead of a vehicle powered inverter. About the same cost if you already have a shore power charger.

Both B2B and vehicle powered inverter require wiring and installation.

 

[orton]

Not yet discussed is the option of one inverter and a seperate AC-DC charger with switch for selecting CCP as inverter source for charging or house battey for consumption. There is a good wiring diagram somewhere on the forum (maybe @sportcoupe?). When I looked, the cost of decent standalone units plus switches and wiring did not seem to have much if any $$ advantage over other setups.

Another choice is to have one inverter with a switch to select either the house battery or the CCP for 12 volt power.

 

[catfish]

Another choice is to have one inverter with a switch to select either the house battery or the CCP for 12 volt power.

That's what I did and it works great! Did not use any CCPs though.

Look at diagram. Nations 300A drop-in alternator replaced oem 150A. Added 2nd starter bat. I connected my fused loads to alternator side of oem 470A fuse.

Catfish ...

 

[Landcruiser]

Electrical noob here. If you use a victron 30 amp DC to DC charger connected to the CCP 2 stud on the side of the seat to charge the house batteries, and you wanted to add an invertor to occasionally power a microwave or coffee maker while the van is running, can you connect both to the CCP2 stud?

 

[Sams stock van]

There's not much room in the stud wireway under the cover for two stacked terminals and wire. The other issue is two different wire sizes are in play, generally not recommended on the same stud. I have my 50 amp DC charger on ccp 1 with a switched breaker to shut it off. (1 option).

You could go one heavy wire from the ccp 2 thru a breaker to a "1/off /2" switch. Then to each device with proper wire/breaker/fuse from the sector switch.