24V to 12V buck converter with ON/Off

[FilterGuy]

Folks,
I have bought into the idea that it is a good idea to use signals from your BMS to turn loads and chargers on and off (as apposed to using relays or FETs to interrupt the current.) For stationary systems where the only load is the inverter, this turns out to be relatively easy to do. However, for mobile systems where you have 12V DC loads, it gets a bit more complicated.

For a 12 Volt system, you can usually use a Victron battery protect. (Yes.... this is using FETS, but it is typically much smaller current and you don't have large banks of capacitors like you do on the inverter).

For a 24 Volt system with a 12Volt buck converter, the ideal situation would be to use the BMS Discharge Enable signal from a Chargery or ElectroDaucus to turn the buck converter on and off. Do any of you have a recommendation for such a beast? I did a quick look and a lot of them just have DC-in and DC-out. No controls.

 

[John Frum]

Victron battery protect also works at 24 volts

 

[FilterGuy]

Yes, but it is another component.

 

[Dzl]

I believe the high current version (the ones that look a bit like inverters) of the Victron Orion series might fit the bill:

https://www.victronenergy.com/upload/documents/Datasheet-Orion-DC-DC-converters-high-power,-non-isolated-EN.pdf

 

[cass3825]

24V to 12V 60A 720W DC DC Step Down Converter Voltage Regulator w ACC Switch 3 Years Warranty Newest

Wide input voltage from 18 to 35V Work for appliances from 0AMP to 60AMP automatically 100% full rated power 100% full stable output current 100% burn-in test High-efficiency up to 97% Waterproof sealed Cooling by free air convection Surface mountable 3 years warranty ACC Enable Over-current...

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[RickP]

I have a Victron Orion 24/12-70 that has an exterior connector for on off. It’s a simple make/break sort of thing, and comes looped through (a short wire connecting the two terminals) from the factory. Connect whatever is opening/closing the circuit and you‘re done. I just use a SPST switch as an on/off, but you might be able to wire your system in, though it sounds like you might still need a relay to break the circuit.

 

[DonPhillipe]

If there's a choice, I'd say stay away from 24V systems in vehicles. In most mid-size RV's and campers, a 12V source is needed at a higher amperage load for the generator starter at minimum. This is a factor as well as the emergency solenoid switch most RVs have which allows starting the coach motor with the house bank as an auxiliary power source when the main chassis battery goes flat. I don't think a 70A buck converter will fill that bill. The single point of failure exposure of running the entire coach off a 24v to 12v buck converter is the third reason to give this idea a second thought.
There's no reason a working 12V system at a higher amperage cannot be delivered. If one is basing the design of a 24V system just to save on the cost of copper, there are more headaches created than few dollars saved by going with a 24V system.

On the home front, I'd say in contrast to go as high a voltage as one pleases and to the best economy of scale.

 

[FilterGuy]

If there's a choice, I'd say stay away from 24V systems in vehicles.

I used to completely agree, but now I have mixed feelings on it. I agree that the 12V Loads in a camper makes 12 volt systems more attractive. Where I get hung up is on the BMS for the battery build. I can't find a high amp 12V BMS that I like. However there are several 24V BMSs that I do like.

In most mid-size RV's and campers, a 12V source is needed at a higher amperage load for the generator starter at minimum. This is a factor as well as the emergency solenoid switch most RVs have which allows starting the coach motor with the house bank as an auxiliary power source when the main chassis battery goes flat. I don't think a 70A buck converter will fill that bill.

I have not built a system with powered generator starter, so at least so far, that has not been an issue for me. What is the current of a generator starter?

As far as emergency start goes, you don't need a huge current. You only need to be able to top-up the starter battery. That is what the little starter packs that you can buy do.

The single point of failure exposure of running the entire coach off a 24v to 12v buck converter is the third reason to give this idea a second thought.

I would be replacing the battery protect (Single point of failure) with the Buck Converter (Single point of failure). Granted, the buck converter is more complicated so it is not a 1-1 trade off.

The truth is, there is no single 'correct answer' to this. Each situation has it's own unique requirements....not the least of which are the values and preferences of the builder and/or owner.

 

[DonPhillipe]

If you look at the commercially sold LION's which go for about $750 at Costco, they claim to be a drop-in lead/acid replacements and they also claim you can strap 4 of them in parallel. So a parallel setup and of course the individual BMS's in each battery case will cause the amp capacities of each BMS inside to add up and if this works as claimed, this would solve a lot of the current limitations of the BMS's for the 12V systems, or at least all the ones I have seen in this particular "cluster" of design suggestions. Most I have seen seem in the video series appear to be designed around a single BMS system which actually wouldn't be a problem if there were BMSs which carried more current at a reasonable price.

Otherwise one can't really speculate what level of charge the engine battery will have when an issue is encountered but one can speculate that a 70a buck converter wouldn't support the jump start via the solenoid that has been a feature in RVs and at least in the Winnegego line since the early 1980's. Being a RV owner I do know from my own personal experience that every time I have needed the jump start button I have been ready to move then and not wait for e.g. a 70a converter, or any kind of charger actually, to boost the battery. Actually any type of recovery mechanism other than a direct jump is normally not going to cut it because of the long time it takes to charge a battery. Also if the chassis battery is down, it could also be completely depleted to the point a charger might no longer be able to recover it and therefore a direct bridge to put the entire capacity of the house battery across the starter would be needed. And even if charging is desired, how much charging is that buck converter going to provide at its lower output voltage? Assuming even that the chassis battery was still in good condition and something like headlights being left on had depleted it, it would still take a reasonably high static output voltage from that buck converter to make any headway in the charge cycle. Likely even a level of 14.2 seen from a conventional alternator will not get the battery up in a significantly short time, at least to a starting level. I don't know the ambient voltage of the buck converters people are using, but I would assume a output quite a bit lower than e.g. the standard 14.2V of an alternator. And again even if the buck converter approached 14.2V,, no one would want to wait, particularly when they were accustomed to the old lead-acid system that offered the push button "boost solenoid" instant-starting capability.

Examining the rightness or wrongness of the design was not exactly my intention. I am simply saying that there is a price to pay going either way and for the person who is either building out a coach in the image of a modern day RV or wishing to place large 12V loads on the system, then a 12V lithium configuration is likely a better choice unless one is willing to make a lot of sacrifices and give up function to save a little money with the lower cost of cables sized for 24V. I'll personallyleave switching to 24V to the future as auto mfgs seek to save even more on their copper costs and thus add to our pain with more incompatibilities in appliances.

I will guess the reason to go 24V just comes down to how much is too much when trying to reduce the size of the cables and save money, and wondering if that is the sole objective as compared to (perhaps) an idealistic uniqueness of being able to say "I have a 24V system". After all hasn't the retail industry been successful with selling all new rechargeable tools by simply upping the voltage to a new number? (As anyone who is convinced to buy a whole new set of power tools will likely say - "hey look how cool this is - I replaced my old 18V system with a 40V system - WHAT A VAST IMPROVEMENT" (wink, wink). ) Anyway I can't buy-in totally to the cost of cables being a driving factor for going with a higher voltage because what's the difference in a foot or two of 4/0 cable vs 2/0 cable and especially since all the instruction videos seen from this resource show the entire system laid out on a single board and with components in extreme close proximity. I would think the cost savings is not enough to go the route of exposing one's self to dealing with a 24V system while still living and driving in a 12V world.

So in closing if one would ask, would I like autos, solar setups and the entire automotive industry and accessory pool to be on a 48V aircraft-like system? Of course, but only if 48v had been deployed long enough that all items in the coach were operating at that same voltage and if it had been around long enough that all accessories, starters, etc ran on 48V.

 

[FilterGuy]

First.... a little math:

Lets say your starter draws 400 amps and it takes 10 seconds to start the engine. That means you need 4000 amp-seconds of current.

Lets assume a 35 amp buck converter. Assuming you have to put all 4000 amp seconds into the battery that is 4000/35=115 seconds.

The reality is, you *don't* have to put all 4000 amp seconds into the battery. You just need to top it off with probably less than half of the current needed to start the car. Furthermore it takes less than 5 seconds to start an engine. Consequently, the typical would be closer to 20-30 seconds (and that matches my experience) I don't find 30-120 seconds unreasonably long for what should be a very rare occasion, but others might. (Like I said, owner preference is a big driving factor in design decisions)

Yes, you can parallel a lot of 12volt batteries, each with their own BMS. However, this is not my preferred way of building out high amp banks. I prefer parallel-first configurations with a single BMS, and this requires a high amp BMS. Since I can't find a good High-amp BMS with low temp disconnect, that means using BMS signals to directly control the load and charge devices... and the choices for that type of BMS drives me toward 24Volt. (Please see the premise in my original post).

The smaller wire sizes are a big plus in tight spaces (like a vehicle). The lower cost is also a plus. However, those are not the driving factors in my liking 24Volt systems.

BTW: I am not against 12 volt systems.... I installed one a couple months ago. Every situation is different.

 

[Dzl]

I will guess the reason to go 24V just comes down to how much is too much when trying to reduce the size of the cables and save money, and wondering if that is the sole objective as compared to (perhaps) an idealistic uniqueness of being able to say "I have a 24V system". After all hasn't the retail industry been successful with selling all new rechargeable tools by simply upping the voltage to a new number? (As anyone who is convinced to buy a whole new set of power tools will likely say - "hey look how cool this is - I replaced my old 18V system with a 40V system - WHAT A VAST IMPROVEMENT" (wink, wink). ) Anyway I can't buy-in totally to the cost of cables being a driving factor for going with a higher voltage because what's the difference in a foot or two of 4/0 cable vs 2/0 cable and especially since all the instruction videos seen from this resource show the entire system laid out on a single board and with components in extreme close proximity. I would think the cost savings is not enough to go the route of exposing one's self to dealing with a 24V system while still living and driving in a 12V world.

I think you are overlooking a couple of the primary reasons people move to 24V systems.
Like you I was intitially skeptical, weighed the pros and cons, and decided that any savings in wiring and slight efficiency gains did not justify the added complexity and comparative lack of availability of components. However as I learned more, I came to see there are reasons to choose 24V beyond savings on copper. In my opinion what makes more sense largely comes down to system size, and the particulars of your situation.

1. Wiring cost and efficiency (this is partially offset by the cost and efficiency of a converter)
2. BMS availability and cost
3. Charge controller size and cost
4. Inverter size

It has been my observation that there is a tipping point, with points 2-4, where higher voltage starts making more sense, and at some point there is no real choice. I feel that tipping point is roughly around:

• 120-200A for a BMS
• 50-60A for a charge controller
• 2000-3000W for an Inverter

All that said, I do think you make some good points about the advantages of 12v, and its a useful perspective, a lot of people (@FilterGuy is not one of them) erroneously assume higher voltage is better always better without thinking it through, particularly those whose experience is more off-grid than mobile or marine. One thing that I think is very much overstated is efficiency, sure wiring efficiency are cut to 1/4 of what they were (say from 4% voltage drop to 1%) but then if they must pass through a 90-95% efficient converter, that efficiency gain is lost.

I like 24v because its a balance between 12 and 48 in many respects. But honestly I'm still torn, my current plans are to build around a 24v system, but every now and then I reassess and consider 12v. My situation is probably different then yours because I'm building out a mobile system from scratch (the house side) not modifying an existing RV.

 

[Dzl]

and the choices for that type of BMS drives me toward 24Volt. (Please see the premise in my original post).

Have you checked out the Victron converter I linked to above, I know you are pretty familiar with Victron stuff, so it might suit your needs pretty well:

https://www.victronenergy.com/upload/documents/Datasheet-Orion-DC-DC-converters-high-power,-non-isolated-EN.pdf

 

[BiduleOhm]

[...] I prefer parallel-first configurations with a single BMS [...]

Why?

 

[Dzl]

Why?

I like the simplicity of P first. Of course there are trade-offs and disadvantages to either option. I do like the redundancy/compartmentalization of S first though, but I think in many cases its added cost and complexity.

edit: FWIW, Dacian (Electrodacus) advocates 'parallel first' and notes that that is the standard for professional/commercial applications.

 

[DonPhillipe]

Still there's something to say for anyone who can jump a diesel motor starter with 35A source, even if it is only on paper. If I'd only known this while battling with the 200A roll-aways and long AC cords I had to battle during my youth on the farm.

 

[FilterGuy]

Have you checked out the Victron converter I linked to above, I know you are pretty familiar with Victron stuff, so it might suit your needs pretty well:

https://www.victronenergy.com/upload/documents/Datasheet-Orion-DC-DC-converters-high-power,-non-isolated-EN.pdf

Yes.... I did and you are correct.... it looks like a great peice of kit. Sorry I did not respond sooner. the thread went off in other directions.

 

[FilterGuy]

I like the simplicity of P first. Of course there are trade-offs and disadvantages to either option. I do like the redundancy/compartmentalization of S first though, but I think in many cases its added cost and complexity.

edit: FWIW, Dacian (Electrodacus) advocates 'parallel first' and notes that that is the standard for professional/commercial applications.

I don't know if I have ever heard him explicitly state a preference, but I have noticed that @Will Prowse also seems to steer toward parallel first.

My primary reason for parallel first is simplicity.

The two arguments I hear the most for series first is

1) redundancy: For most of the users of the systems I have built, they would be lost trying to shut down half the system. Furthermore, adding BMSs adds points of failure.

2) Monitoring individual cells. If you start with reasonably matched cells, it is unlikely they will diverge as they age. Furthermore, my users are not going to be watching the cells anyway. They would have no idea what they are looking at.

So, for me, I prefer parallel first but will occasionally build serial first if the situation demands it. Others have different points of view... and that is fine. What works for me may not be the right thing for the next guy.

 

[Dzl]

I don't know if I have ever heard him explicitly state a preference, but I have noticed that @Will Prowse also seems to steer toward parallel first.

He addresses it with some regularity on his google group. Here is one instance.

My primary reason for parallel first is simplicity.

The two arguments I hear the most for series first is

1) redundancy: For most of the users of the systems I have built, they would be lost trying to shut down half the system. Furthermore, adding BMSs adds points of failure.

2) Monitoring individual cells. If you start with reasonably matched cells, it is unlikely they will diverge as they age. Furthermore, my users are not going to be watching the cells anyway. They would have no idea what they are looking at.

So, for me, I prefer parallel first but will occasionally build serial first if the situation demands it. Others have different points of view... and that is fine. What works for me may not be the right thing for the next guy.

Good points. I'm of the opinion (currently) that both have pros and cons, and context, budget, and sophistication/involvement of the end user and to some degree the builder will determine what makes more sense.

 

[FilterGuy]

Good points. I'm of the opinion (currently) that both have pros and cons, and context, budget, and sophistication/involvement of the end user and to some degree the builder will determine what makes more sense.

Yup. All designs are trade-offs among many different choices and one size does *not* fit all. I have preferences but often go a different rout based on the particulars of the situation. I can be stuborne, but I like to believe

 

[BiduleOhm]

Furthermore, adding BMSs adds points of failure.

But with only one pack you have a single point of failure so if it fails you have nothing left. With multiple packs you may have more chances one will fail but if it is then you have the other packs still running. Same as one hard drive vs multiple ones in RAID.

Anyway, everyone do what it prefers, no problem with that. I was just curious to see why you prefered this design as I couldn't see the benefits (besides it's simpler of course)