24v to 12v dc to dc converters, how efficient?

[linuxrunner]

I have a camper with 4 50ah LiFePo4 batteries. I am thinking of changing to a 24v setup from 12v. I have a few high draw items that pull 2000w that we use occasionally. my worry is that I have a 12v appliance that pulls only about .8 amps but runs pretty much non stop.

I understand that a dc to dc converter would be less efficient but I am not sure how much I am going to lose.

Anybody else run into this? We kind of want more wattage capacity and voltage drop is starting to be a real problem which 24v will fix. On the other hand 90% of our usage is that very slow 1amp draw.

 

[Dzl]

I have a camper with 4 50ah LiFePo4 batteries. I am thinking of changing to a 24v setup from 12v. I have a few high draw items that pull 2000w that we use occasionally. my worry is that I have a 12v appliance that pulls only about .8 amps but runs pretty much non stop.

I understand that a dc to dc converter would be less efficient but I am not sure how much I am going to lose.

Anybody else run into this? We kind of want more wattage capacity and voltage drop is starting to be a real problem which 24v will fix. On the other hand 90% of our usage is that very slow 1amp draw.

From what I've seen DC-DC converter efficiency is pretty variable, looking at only 1 company--Victron--models range from ~87% to 97%. I assume this is peak efficiency. I don't know if DC-DC converters are like inverters but if they are peak efficiency will be near full load and they will be less efficient near zero load, but this may be the case.

If 90% of your power consumption comes from that one small load, I think you would be better off sticking with 12V unless it is bottlenecking your system (2000W AC = 2000W / 12V / 85% efficiency = ~200A)

 

[HaldorEE]

What are your DC loads? It is possible to buy dual voltage (12V or 24V) devices. I made a point of buying dual 12V/24V fridge and fans. My LED lights are all 24V. The main 12V load I have is my CPAP. That lets me use a much smaller 24V to 12V DC-DC converter.

Generally, the more heavily loaded a DC-DC converters is, the better the efficiency numbers are. Lightly loaded DC-DC converters can have terrible efficiency numbers (under 50%).

 

[HaldorEE]

24V Examples:

https://www.amazon.com/Caframo-Sirocco-Gimbal-Size-White/dp/B01LDY4TE4/ref=sr_1_2?dchild=1&keywords=Sirocco+II+Fan&qid=1610846775&sr=8-2

https://www.amazon.com/ICECO-Refrigerator-Compressor-Insulated-Cover/dp/B07TXG7G1Y/ref=sr_1_1?dchild=1&keywords=B07TXG7G1Y&qid=1610846942&sr=8-1

Hella Euroled 75 3" Round 24V Warm White Screw Mount White

 

[Factory400]

DC-DC converters are all over the map.
non-isolated have the best efficiency over the isolated variants.

Off the top of my head, I cannot see a problem using a non-isolated converter - I would need more info to make any suggestions.

Isolated units are usually high 80's to low 90's and ensure there is no path back to the source. For a load that you described - 10 watts - the difference is really small. Most converers have the worst efficiency at light loads.

 

[linuxrunner]

Thanks all, it sounds like if I do decide to go 24v it would be best to get a small dc to dc converter that is sized only a bit larger than my load and specifically look for the converters which advertise the highest efficiency numbers. Beyond that if I see a unacceptable drop in run time on the batteries I could start looking at replacing appliances with 24v versions.

That being said I can stick with 12v and save a bunch of money and hassle. . I guess the cheapest option would be new larger wires to solve the voltage drop.

 

[HaldorEE]

Thicker wires might be the cheaper option, although low capacity 24V-12V converters are pretty darn cheap. What equipment do you already have in your system? Do you have an inverter? If so then switching to 24V would require replacing that.

24V does have additional advantages if you are thinking about expanding your solar input (bigger or more panels).

 

[Factory400]

Thanks all, it sounds like if I do decide to go 24v it would be best to get a small dc to dc converter that is sized only a bit larger than my load and specifically look for the converters which advertise the highest efficiency numbers. Beyond that if I see a unacceptable drop in run time on the batteries I could start looking at replacing appliances with 24v versions.

That being said I can stick with 12v and save a bunch of money and hassle. . I guess the cheapest option would be new larger wires to solve the voltage drop.

Balancing all the factors can be challenging for sure.

On my personal system, I chose to go with 24v for the battery and inverter for the high-power stuff and use DC-DC converter for the 12v. The result was rather nice. Modest sized cables, fuses, connectors, etc.
I put the DC-DC converter fairly close to where it is needed most so the voltage drop is not an issue.

Works great and it is not complex.

 

[HaldorEE]

Balancing all the factors can be challenging for sure.

On my personal system, I chose to go with 24v for the battery and inverter for the high-power stuff and use DC-DC converter for the 12v. The result was rather nice. Modest sized cables, fuses, connectors, etc.
I put the DC-DC converter fairly close to where it is needed most so the voltage drop is not an issue.

Works great and it is not complex.

I am doing exactly the same thing. Plus the 12V supply has it's own on/off switch that has to be armed (toggle switch, plus you have to push a momentary button to turn it on). The 12V converter is fed from its own independent low voltage disconnect (LVD) too. I only put non-critical stuff on 12V and the LVD for that is set to a higher voltage (meaning it drops out sooner) than the more critical stuff that is running on 24V.

This means the 12V converter doesn't automatically turn back on when battery voltage rises enough to reset the LVD.

 

[HaldorEE]

This is what I am using for my 12V control panel. The safety toggle switch is used as the On/Off control and you have to press the start button to latch in the power relay to the 24V to 12V DC-DC converter.

I am using a 24V automotive relay instead of the 12V relay that came with the control panel. I know it is silly, but it makes me smile.

https://www.amazon.com/CT-CARID-Ignition-Switch-Button-Carbon/dp/B07PFK194V/ref=sr_1_4?dchild=1&keywords=race%2Bcar%2Bswitch%2Bpanel&qid=1610852097&sr=8-4&th=1

 

[MattiFin]

From what I've seen DC-DC converter efficiency is pretty variable, looking at only 1 company--Victron--models range from ~87% to 97%. I assume this is peak efficiency. I don't know if DC-DC converters are like inverters but if they are peak efficiency will be near full load and they will be less efficient near zero load, but this may be the case.

If 90% of your power consumption comes from that one small load, I think you would be better off sticking with 12V unless it is bottlenecking your system (2000W AC = 2000W / 12V / 85% efficiency = ~200A)

DC-DC converters are lot easier to make efficient across the full operating range than inverters.
For example 70A Victron is specified for <20mA "off load current"

YMMV. Some others might be not as good.

 

[willo]

I'm happy with my victron 24/12. They have a surprising variance between model efficiency.

Honestly, when talking about .8a draw, even 90% only makes it nearly .9a draw. Don't sweat it.

 

[1holaguy]

Balancing all the factors can be challenging for sure.

On my personal system, I chose to go with 24v for the battery and inverter for the high-power stuff and use DC-DC converter for the 12v. The result was rather nice. Modest sized cables, fuses, connectors, etc.
I put the DC-DC converter fairly close to where it is needed most so the voltage drop is not an issue.

Works great and it is not complex.

Factory400, I am looking at the same thing. how did you determine how many amps of 12v you needed so as to size the converter correctly?

 

[Factory400]

Factory400, I am looking at the same thing. how did you determine how many amps of 12v you needed so as to size the converter correctly?

It was an estimate based on turning various things on to see what current they needed and then guessing which ones would be used at the same time.

I figured the awning, water pump, and the slides would not be operating all at once so I chose a Victron 40A that can handle all the lights, water pump, audio, fans, and awning motors simultaneously. The slide, tongue leveler, and stabilizers are operated operated when most everything is still off - they can pull a lot of current. So far, so good.

There is a small isolated battery that is for the emergency break-away brakes too. Being a safety item, I did not want that to rely on complex converter.

 

[willo]

I sort of murdered my victron 40 - my fault, I haven't had a chance to open it up and check the internal fuses.
That said, I put in a 70 - I prefer the bolt terminals over the spades of the 40.
The output voltage of the 70 is also adjustable, which is a helpful if you want it to act as a battery charger output.

After this summer/fall of travel, I added a 24v fuse panel as well so that I can kill off the 12v drains during storage.
This provides a couple of things for me: my monitoring systems run from the 24v side and can stay online, and it reduces load on the converter.
My dometic cooler fridge/freezer can also run on 24/12 so I'll probably convert it over to the native 24v of my pack as well.
I added a 12-24v charger to feed my pack when towing, so my pack should stay topped up on long towing days too.

I added a SLA 12v battery and a small breakaway charger to mine, so I have a similar setup to @Factory400.
When my truck is connected, it charges the breakaway, but I'm thinking of adding in a small maintenance circuit to feed a maintenance charge the battery for 30 minutes a day or something when my camper is down in storage.

Meanwhile, I suggest investing in a good battery monitor - I have a BMV712, but the smartshunt would do fine. Measure your loads and figure out what you really need. You'll want the metering either way, so it's the place to start IMO.

 

[Factory400]

I'm happy with my victron 24/12. They have a surprising variance between model efficiency.

Honestly, when talking about .8a draw, even 90% only makes it nearly .9a draw. Don't sweat it.

Taking a wild guess - I would say that Victron does not have anything to do with designing these other than the enclosure.
They appear to be pseudo standard designs that they probably just buy and put in a box.

If that is true - the SMPS topology, controller, and other details from the 40A to the 70A could be very different. That would easily be noticed in the quiescient current (the draw when it is not doing anything) and the efficiency curve. I am sure they vet the designs rather well though to ensure reliability which is the hallmark of the Victron line.

I sort of murdered my victron 40 - my fault, I haven't had a chance to open it up and check the internal fuses.
That said, I put in a 70 - I prefer the bolt terminals over the spades of the 40.
The output voltage of the 70 is also adjustable, which is a helpful if you want it to act as a battery charger output.

I am very curious to know if the 40A unit is damaged from an overload. If it is anything remotely approaching a decent design - it should just blow the fuse. The fuse may be on the PCB or in some way difficult to get to - but hopefully the core circuits would not be damaged from overload or a dead short.

If it is....I may replace mine before it blows up.

 

[willo]

I abused it by leaving it connected to a 12v FLA that was in place until I installed the new dedicated breakaway battery.
The FLA went completely flat, I'm pretty sure that was just too much on the converter. We also had some cold weeks where my main bank bottomed out and hit my discharge cutoff.

Anyhow, one of these days I'll pop it open. Fuses are visible through the grate on the bottom of the unit.
I wouldn't worry about overloading a 40 in normal use.

 

[HookEm]

Balancing all the factors can be challenging for sure.

On my personal system, I chose to go with 24v for the battery and inverter for the high-power stuff and use DC-DC converter for the 12v. The result was rather nice. Modest sized cables, fuses, connectors, etc.
I put the DC-DC converter fairly close to where it is needed most so the voltage drop is not an issue.

Works great and it is not complex.

How many amps from the 24V/12v converter? (I presume this is for typical 12v loads, right?)

 

[Factory400]

How many amps from the 24V/12v converter? (I presume this is for typical 12v loads, right?)

I have the 40A Victron which is enough to cover most everything. That is on a custom ideal diode in parallel with a small lead-acid battery/charger. That way, if anything goes over the limit of the converter - the battery effectively supplies a boost. This required me to trim the converter to be slightly higher voltage than the battery float voltage.

If/when I have to shutdown the 24v battery bank - the little (backup )battery is still enough to pull in the slides, run the water pump, etc.

 

[willo]

I use a 70A victron unit. I have a biggish 37ft fifth wheel and the slides/leveling system all use 12v.
I was using a 40A but a failing house battery murdered it (blown fuses were easy to replace) But I took the opportunity to install the 70A since I wanted bolt terminals instead of the spades on the 40.