DC-DC Charging Options in Vehicle Mounted Systems

[asot550]

I feel like I'm missing something when deciding how to handle DC-DC charging a house battery while the vehicle is running.

I'll have a 24v house battery (NMC) and a 12v lead acid starter battery in a Chevy Express 3500 based vehicle with a 105amp alternator.

What seems like the most common DC-DC charger is the Victron Orion Tr-smart. There are a number of options at varying price points, but the highest power option is the 12/24-15 which would give me 360w of charging and retails on Amazon for $227.

The other Victron option I'm seeing is their Orion DC-DC converter high power line, which has a 12/24-20amp option (480w) for only $157 on Amazon. As far as I can tell, the only things that you'd be missing out on is bluetooth/app functionality, and multi-stage charging profiles that you get with the Tr-smart. Both have remote shutoff (which I'd tie to a vehicle on/off 12v source, and possibly an additional in-line switch). Both have adjustable output voltages which I'd need for my NMC battery. Is there some other reason why the high power line from victron wouldn't work for house battery charging, or other features in the Tr-smart that would make it preferable?

Finding out about the high power option from Victron also got me thinking though, there are plenty of adjustable buck boost converters available on Amazon. $25 will get you an adjustable CC/CV DC converter that is specced up to 20amps. Grabbing three of these, paralleling them, and limiting them to 15amp each would give 540w of charging power. Paired with a high power relay (or multiple lower power relays) tied to a vehicle on/off 12v source would give you the most redundancy, charging power, and lowest cost. It would just take a bit more man hours to set up.

Anyone have thoughts on any of those options and the pros/cons to each of them?

 

[Supervstech]

If you build it, be sure and show your work here...
Lots of pictures!

 

[justinm001]

They make a 12/24 70a one too. The not smart ones just pump a flat voltage (there's an adjustment screw) the smart has charging profiles iirc. I don't think you can limit the amperage though.

I had a cheap buck boost 12 to 48v adapter fry and take out my starlink so I wouldn't trust it with batteries.

I have the opposite issue with a 24v alt with 12 and 48v house systems. I have gone thru a few of the 70a orions, some my fault and others were just dead. I also have a 24/48 8.5a tr smart but haven't hooked up yet.

 

[featherlite]

To me, a 105 amp alternator sounds fairly small. The rule of thumb is not to draw more than 70% of the alternator’s rated power (in your case 74Ah) to avoid overheating the alternator and burning out the diodes.

I suggest that you use a clamp on DC meter, start the vehicle, and record the Ah being drawn at idle. Then start turning on loads like headlights on high beam, heater fan on high, air conditioner on high, windshield wipers on high, radio on high, etc., and record the incremental increase in Ah draw with each load, until you arrive at the total cumulative Ah draw with all loads.

Compare that total draw to 74Ah and see how much extra Ah capacity you have to charge your house battery. This will allow you to size your charger.

Please report back to us your findings.

 

[Supervstech]

I agree.
105A is a basic alternator with today's fuel injection and electronic everything cars.
If you want to power things with it, look and see what size alternators are available for your vehicle.
See if you can get one with 200A output.
Be sure to upgrade the cables as well.

 

[mikefitz]

The low cost converters have poor current limiting and rely on the load to limit power, connecting to a lithium battery is not a good idea.
The Victron dc to dc will charge to target volts but won't terminate charge, how will you determine charge is Complete?

 

[justinm001]

The low cost converters have poor current limiting and rely on the load to limit power, connecting to a lithium battery is not a good idea.
The Victron dc to dc will charge to target volts but won't terminate charge, how will you determine charge is Complete?

He doesn't have to. The DC to DC converter will limit the amps and put out a stable voltage, say 13.7. The batteries will pull all that power until it can't use any more.

 

[asot550]

They make a 12/24 70a one too. The not smart ones just pump a flat voltage (there's an adjustment screw) the smart has charging profiles iirc. I don't think you can limit the amperage though.

I had a cheap buck boost 12 to 48v adapter fry and take out my starlink so I wouldn't trust it with batteries.

I have the opposite issue with a 24v alt with 12 and 48v house systems. I have gone thru a few of the 70a orions, some my fault and others were just dead. I also have a 24/48 8.5a tr smart but haven't hooked up yet.

The 70a version accepts 24v as the input and outputs 12v so wouldn't work in my case. It'd be a good option for powering 12v devices from the 24v house battery though.

What happened with your 12 to 48v buck boost? I thought those devices were pretty much indestructible. Was it the sealed version with aluminum heatsink?

 

[justinm001]

The 70a version accepts 24v as the input and outputs 12v so wouldn't work in my case. It'd be a good option for powering 12v devices from the 24v house battery though.

What happened with your 12 to 48v buck boost? I thought those devices were pretty much indestructible. Was it the sealed version with aluminum heatsink?

Oh you're right. I saw they made 12/24 just assumed same amperage.

Yes it was the sealed version with aluminum heatsink. It worked for a while and I used it to connect to a dishypowa then to my starlink dish which somehow it fried the starlink dish as the power port was all burnt. Cable and dish both were busted which seems odd it was enough power it melted the cable.

When I tested it, the 48v side was outputting 14v so knew it was dead. I'm assuming it was my fault and my alternator spiked the voltage over 16v and likely it 4x that to over 60v into the starlink.

I found out my alternator was 24v but they put a 12v regulator on it so when I replaced the regulator with a wakespeed so I could charge my lithium directly it couldn't keep the voltage at 12v the entire time apparently. Its a dedicated alternator.

 

[asot550]

To me, a 105 amp alternator sounds fairly small. The rule of thumb is not to draw more than 70% of the alternator’s rated power (in your case 74Ah) to avoid overheating the alternator and burning out the diodes.

I suggest that you use a clamp on DC meter, start the vehicle, and record the Ah being drawn at idle. Then start turning on loads like headlights on high beam, heater fan on high, air conditioner on high, windshield wipers on high, radio on high, etc., and record the incremental increase in Ah draw with each load, until you arrive at the total cumulative Ah draw with all loads.

Compare that total draw to 74Ah and see how much extra Ah capacity you have to charge your house battery. This will allow you to size your charger.

Please report back to us your findings.

This is a good point. The vehicle has a 150w AC receptacle in the cab, so I'm assuming there's at least 200w of headroom. It's also a cutaway model and there's an unused 30a fuse in the auxiliary fuse block in the cab. That should mean there's at least 25a and 300w of headroom otherwise it wouldn't be included. Definitely something that should be tested though.

 

[asot550]

The low cost converters have poor current limiting and rely on the load to limit power, connecting to a lithium battery is not a good idea.
The Victron dc to dc will charge to target volts but won't terminate charge, how will you determine charge is Complete?

He doesn't have to. The DC to DC converter will limit the amps and put out a stable voltage, say 13.7. The batteries will pull all that power until it can't use any more.

@justinm001 is correct. Lithium batteries will pull as much current as they're able at a given voltage. For CC/CV power supplies you start at a lower voltage than the set-point because you're current limited. As the internal resistance resistance rises, the voltage increases until you hit your voltage setpoint on the converter (I'd set mine to 28.5v which is 90% of full charge). Once you're at the voltage setpoint the current starts dropping until the battery can't take any more (full charge).

I think the biggest question is if the Orion high power converters limit current in any way like the AC to DC CC/CV power supplies that I'm used to using to charge batteries. These things: https://www.amazon.com/ALITOVE-Universal-Regulated-Switching-Transformer/dp/B06XJVYDDW/

If the high power converters don't have any means to limit current then the battery could draw more than they're able to provide and could burn them out. There's also the potential issue of burning out the alternator too.

 

[justinm001]

Yes they do limit current and are able to be paralleled but they will pull full amperage the entire time the battery will accept so you need to get a size that isn't too much for your alternator.

With that I wouldn't rely on the fuses and ac converter to determine if you have plenty of headroom for the DC to dc charger. I'm surprised the alternator is so small for a 3500, I'd assume you would have a larger one or a 2nd one as an option. Research your engine and see if there's an easy way to add a 2nd alt then you could get a 24v and a wakespeed then pump a ton of power. Could just need a bracket and new belt. A wakespeed allows you to limit the current and it monitors the alt temp so will derate when it gets too hot... but you're looking at close to a grand but could easily get 24v 100a while keeping your chassis system isolated.

I'd wire the DC to dc converter to the ignition then to a switch. Get a decent size and just be aware of potentially overloading it. You could then add something to turn it on/off every x minutes to give the alt time to cool off. Also ideally wait 5+ minutes after it starts before turning on the charger. These things will prolong the alt, but of course if it dies you can easily swap it out for a larger one

 

[mikefitz]

Once you're at the voltage setpoint the current starts dropping until the battery can't take any more (full charge).

Yes I understand that, the point I was making ( not very well), is that the battery will be subjected to high voltage when fully charged for as long as the vehicle is running. Since NMC has less tolerance to over charge than other lithium batteries there may be unwelcome issues.

 

[asot550]

@justinm001 is correct. Lithium batteries will pull as much current as they're able at a given voltage. For CC/CV power supplies you start at a lower voltage than the set-point because you're current limited. As the internal resistance resistance rises, the voltage increases until you hit your voltage setpoint on the converter (I'd set mine to 28.5v which is 90% of full charge). Once you're at the voltage setpoint the current starts dropping until the battery can't take any more (full charge).

I think the biggest question is if the Orion high power converters limit current in any way like the AC to DC CC/CV power supplies that I'm used to using to charge batteries. These things: https://www.amazon.com/ALITOVE-Universal-Regulated-Switching-Transformer/dp/B06XJVYDDW/

If the high power converters don't have any means to limit current then the battery could draw more than they're able to provide and could burn them out. There's also the potential issue of burning out the alternator too.

Answering my own question, I found this post on the victron forum that states there isn't a user settable current limit on the high power converters (or the tr-smart): https://community.victronenergy.com/questions/170700/orion-dc-dc-converter-current-limiting.html

Max current is 30a (at 24v), which is more than likely too high for the alternator. The 12/24-10 has a max current of 20a, but doesn't have a remote on/off switch so I'd have to add my own relay.

It's shocking how "difficult" this is. It seems like there should be a much simpler and cheaper solution out there...

 

[asot550]

Yes I understand that, the point I was making ( not very well), is that the battery will be subjected to high voltage when fully charged for as long as the vehicle is running. Since NMC has less tolerance to over charge than other lithium batteries there may be unwelcome issues.

Ah, I understand now.

I'm planning to limit all of my charging profiles across all methods to 90% of full capacity in order to hopefully limit over-charge concerns as well as to increase the battery life. I'm planning for a 20% lower discharge cutoff point as well.

I posted on the forum elsewhere, but I ended up with an NMC pack by accident and didn't realize it until after the return window closed. Trying to make the best of a $1k mistake now ?

 

[justinm001]

Answering my own question, I found this post on the victron forum that states there isn't a user settable current limit on the high power converters (or the tr-smart): https://community.victronenergy.com/questions/170700/orion-dc-dc-converter-current-limiting.html

Max current is 30a (at 24v), which is more than likely too high for the alternator. The 12/24-10 has a max current of 20a, but doesn't have a remote on/off switch so I'd have to add my own relay.

It's shocking how "difficult" this is. It seems like there should be a much simpler and cheaper solution out there...

Yupp completely agree. I'm trying to pull 200-250a of 24v to 12v and/or 48v and it seems I need 10 of these which will be a nightmare to manage. You'd think there would be one that you can control the output current digitally and Victron would make one that integrates completely. Not sure why a pwm or mppt controller has so many options but we can't use them

 

[asot550]

With that I wouldn't rely on the fuses and ac converter to determine if you have plenty of headroom for the DC to dc charger. I'm surprised the alternator is so small for a 3500, I'd assume you would have a larger one or a 2nd one as an option. Research your engine and see if there's an easy way to add a 2nd alt then you could get a 24v and a wakespeed then pump a ton of power. Could just need a bracket and new belt. A wakespeed allows you to limit the current and it monitors the alt temp so will derate when it gets too hot... but you're looking at close to a grand but could easily get 24v 100a while keeping your chassis system isolated.

Just looked up the wakespeed and it's a sweet product! Pricey though...

Victron has a very programmable 25a buck boost converter that's a similar price point and has many of the same features.

Neither option gets around the alternator being the biggest limiter for high current applications.

 

[Vigo]

Well, we're talking light duty vehicle alternators here. If you're trying to do 3000-5000w you can look at the generator head on a 3000-5000w portable generator and clearly see that it's 3-5x as large as the typical light duty vehicle alternator. Larger alternators exist but mounting and controlling them is beyond most DIY'ers skill level, and they are pretty expensive as well. At some point you just run into the whole reason why RVs have generators in the first place.. after a certain wattage it ends up being less complicated to turn gas or diesel into electricity with a dedicated small engine and generator head, instead of trying to get that from the 'propulsion' engine.

 

[asot550]

Well, we're talking light duty vehicle alternators here. If you're trying to do 3000-5000w you can look at the generator head on a 3000-5000w portable generator and clearly see that it's 3-5x as large as the typical light duty vehicle alternator. Larger alternators exist but mounting and controlling them is beyond most DIY'ers skill level, and they are pretty expensive as well. At some point you just run into the whole reason why RVs have generators in the first place.. after a certain wattage it ends up being less complicated to turn gas or diesel into electricity with a dedicated small engine and generator head, instead of trying to get that from the 'propulsion' engine.

Yeah, I'm not trying to get the max power out of alternator or really do any ridiculous levels of charging though. 250-500w just to supplement solar. What's grinding my gears is there doesn't seem to be any reasonable cost commercial options on the market that do what I feel should be a relatively simple task of converting 12 to 24 v with a relay built in to only turn on with vehicle power.

 

[Zwy]

Look at the Kisae DMT2430.

$266 at Inverter Supply. https://www.invertersupply.com/index.php?main_page=product_info&products_id=200977

It doubles as an additional charge controller so if you want some portable panels, you can hook them up to the Kisae. I was considering one for my truck camper but instead went with installing dual alternators, a 2400W inverter in the back seat area and use the Growatt AC charger to charge the 24V camper battery off my truck at 60A.

I do believe you can turn the amps down on the Kisae, but contact their support for questions. They respond quickly as I had asked some questions.

What engine in the Express? Usually you can add a second alternator easily as these had dual alternator options when ordering a new van. My GMC truck has the Duramax and I purchased the bracket from GM dealer wholesale for 16 bucks (I watched people on Youtube spent several hours building a bracket), the needed bolts cost about $20 because they make you buy a 5 pack, added another idler pulley and installed the serpentine belt for a dual alternator equipped Duramax. As the Express is commonly used for ambulances and other cutaway chassis, it should have had a dual alternator option.