24v vs 48v - pro's/con's

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Stephen Ryan

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Mar 17, 2018
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In the context of a smallpowerwall style project (140 Li-ion cells @ 2000mAh)

If we havebattery pack 'A':
7s 20p --> 25.9v40Ah--> 1036Wh

And battery pack 'B':
14s 10p --> 51.8v 20Ah --> 1036Wh

Why would one choose pack 'B' over 'A'and halve their capacity in favour of voltage, when the inverter type (24v vs 48v)hasn't been chosen yet?

And how does one choose the inverter?

Same power output but vastly different run-times, seems like a waste or have I my basics wrong
 

jdeadman

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Mar 28, 2017
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The Ability to Move Power. Lower the Voltage the Higher the Amps. the Higher the Amps throught a Cable the Bigger that cable needs to be. IE. Bigger cable is more expensive.
 

floydR

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What would be the purpose of this battery?

The total run time would/should be the same. You can get 1036wh out of the battery.


later floyd


Say you have a 250 watt load in theory you would get 4 hrs out of the battery, 25.9v*9.65amps 51.8v*4.82 amps
 

daromer

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Double the voltage equals half the current. Lower current equals thinner wire equals cheaper setup!

In generall the inverter and gear are the same price in 24 vs 48v depending on area you live. In mine thats the case. Actually 48v is cheaper.
 
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I think the 'error' is your statement "Why would one choose pack 'B' over 'A'and halve their capacity in favour of voltage"...

The capacity is not halved, just the current. Capacity is the same at[size=small]1036Wh either way.[/size]

[size=small]There's a reason that power transmission lines are at 110,000 volts... there are less losses when the current is lower. For a fixed amount of power, the only way to reduce current is to increase voltage. P = V.I the losses are lower as most are a function of the current. e.g. heat, magnetic field strength...[/size]

[size=small]When it comes to inverters for our powerwalls, many of them will be rated at, for example, 5kW. In the fine print you'll often see that rating is for 48V, and is only 3kW for 24V. Basically the 'beefiness' of the components within the inverter is to handle a certain current, whereas the doubling of the voltage has minimal effect in that respect.[/size]

[size=small]FWIW cars used to run on a 6V system, but now 12 is standard. Trucks mostly run 24V, and the new Mercedes cars will be running 48V systems. These are gasoline and diesel powered, not EV. The weight savings that MB expect to see by being able to reduce the thickness of all wiring is considerable. I also recall reading that some EV manufacturers are planning to double the charging voltage of their cars in order to speed up charging. They could double the current, but the cables would end up so heavy that they would be difficult to handle.[/size]

[size=small]Hope this helps :)[/size]


I think I should also mention that based on my reasoning, you would expect that a 96V system for a powerwall would be even better than a 48V.

Yes - it would be better! Except that anything over about 60V begins to pose a risk of electrocution. So the "limit" of 48V is for safety only.
 

Korishan

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Grumplestiltskin said:
I think I should also mention that based on my reasoning, you would expect that a 96V system for a powerwall would be even better than a 48V.

Yes - it would be better! Except that anything over about 60V begins to pose a risk of electrocution. So the "limit" of 48V is for safety only.

Well, safety, and that most equipment is built for 48V systems. Getting a 96V inverter the price jumps pretty quickly. And you'd be fortunately to get a full inverter that runs on 120V (not a hacked UPS unit, but actually an inverter designed at that voltage). They are just hard to come by unless you are in the commercial/industrial markets.
 

Crimp Daddy

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Generally speaking, I favor higher voltages...

Id take the 48v configuration over the 24v, but it really all depends on what the application is... most of my smaller projects are 24v (portable power pack, APC UPS conversion)

My next big project is all 15s because most of the 48v inverter I am looking at have a higher efficiency up top and accept a 63VDC input, and my batteries are also lending themselves to that design.

Once you start designing systems that want to pull 5000+ watts, the cable sizing becomes a huge factor.

5000 watts / 24v = 208 amps
5000 watts / 48v - 104 amps

That's the different between 1/0AWG and 4 AWG. With enough parallel strings, you can go even smaller on the branches.
 

Stephen Ryan

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Ah, that was my stupid mistake, confusing capacity for current. Since the Wh rating of the two hypothetical packs above were identical and the Ah rating was halved I instantly jumped to the conclusion that the 48v pack would be capable of pushing double the amount of power but for half as long.

Thanks to all for clearing that up!

This is probably worthy of its own thread but would a 48v inverter incur less losses and be more efficientstepping the voltage up to 230v as opposed to a 24v inverter? (I have no idea how these devices work)
 

Crimp Daddy

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I would say a large part of inverter efficiency has to do with design and overall quality...

Also, once you are in the power game, watt hours is really the only unit one must concern themselves with. I had the habit of taking amp hours because of the 12v automotive DC world. Its assumed everything is 12v and for some reason everyone speaks in amp hours.

I would spend some time looking at inverters I realize that I need to take hte time to choose carefully.... expansion, quality, features, integrations, monitoring, reporting... even the compability with other system like Batrium.

Right now I am leaving hard towards Victron, but there are others that I like too, Magnum, but I have my eye on the more budget options like the MPP Solar gear which others seem to use a lot here too.
 

Korishan

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The higher the voltage, the easier it is to convert to a set voltage point (230V in your case; which, it actually would converter to 240V as it's directly divisible by 48; altho, a few less turns can make the 230V easy enough).
The closer the two voltages are, the smaller the wire can be as well. This is because it's not just voltage that's transforming, but also current. When one goes down, the other goes up. They have to stay in proportion to each other.

So, if you go from 24V to 240V, you need heavy gauge wire on the input, which means either less turns and needing 2 transformers, or a larger transformer. And this equates to a heavier unit overall. Not to mention the copper traces on the PCB's need to be heavier duty to handle the amp loads as well on the 24V setups.
 

not2bme

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I have a 24V setup and I like it. The main advantage is it doesn't take as long to build up the packs if you're looking to expand. I started with a 7S60P 3kw pack 3 months ago and I just added another 3kw pack last month. I did 60P because I ordered 400 cells from alarmhookup in the beginning and I can stack them to go 120P to 180P to 240P in my design. I probably have 2000 cells left to process and I'm slowly working to add 3kw every two months. If I did the 14S I would be looking at 3-4 months per pack which would drive me nuts waiting to complete them. Cables are not really an issue since the battery to the main bus bar isn't that far away, so it's not likely you'll be running more than just a few feet. Plus converting to 48V isn't that difficult since you just rearrange the packs differently.
 

daromer

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It takes equally long time to collect cells for 24V as with 48V! It is EXACTLY the same amount of cells for same total kapacity no matter if you have 12V, 24V or 150V ;)

With that said its a bit more work doing more packs in generall since its more connectors and all. But thats it!
 

CrankyCoder

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Sorry to revive this. It's not terribly old :)

I have been building my battery modules. Initially ill just do 12v for some smaller stuff. But i am looking to move on to a larger system and potentially do some offset for my powerbill. So i have been going back and forth on the voltage of the whole setup. I know alot of powerwall builders have been using the 48v systems, but I have seen most of them outside of the US. I have been looking at inverters like the mppsolar hybrid series and what not and it seems that for US/Canada that they are for 24v battery systems.

Just wondering if that is generally the case. If so, what panels are you guys using with those systems? I do like that the hybrid systems could work in parrallel and give me the 240 for something like my AC. Anyone have a parallel setup? are you using both off the same battery packs? separate?

Thanks!
 

Korishan

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There are a lot of 48V setups in the US, too. There's also a lot of charger controllers, inverters that work with 48V too.

Parallel inverters don't give 240V output, they give higher amps of 120V. Series connected inverters give 240V output, if they are built to handle that type of connection.
 

CrankyCoder

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Korishan said:
There are a lot of 48V setups in the US, too. There's also a lot of charger controllers, inverters that work with 48V too.

Parallel inverters don't give 240V output, they give higher amps of 120V. Series connected inverters give 240V output, if they are built to handle that type of connection.

Here is what I meant. They do split phase using their parallel kit thingyto get the 240.

https://www.mppsolar.com/v3/split-phase-lv-series/

I fully understand the higher voltage allowing for smaller wires and I am a HUGE fan of that. Just wondering what the standards are. Also wondering if anyone knows what the code requirements are for the states.
 

Korishan

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Ahhh, split-phase parallel. Now that makes more sense ;)

Best way to find the codes would be to do a search for with NEC. But, it's probably very close to the same as AC wiring and such.

You could probably get a 'lot' more information and more accurate to your area if you contact your local fire marshal. They usually know that kind of stuff as they do have to inspect that stuff as well.
 

Korishan

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If you go 24V, it'd probably be better to do smaller strings and then parallel the different strings. That would give you more flexibility if you need to work on a string, or add another one into service.
So, you could do 7s80p which would give you 100A capability from each string. Though, you would normally not go over about 50A for each string.
Or, if you already have enough, you could do 7s40p and have 2 strings put into parallel. And then every 280 cells that are ready to go you can put into service. You wouldn't have to wait till you had >500 cells ready to go for each string.

Just an idea
 

higher_wire

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I've been interested in those MPPs, since they're combined cheaper than nearly everything else I've seen and the PIP4048 is fairly popular around here. I'd rather run 48v, but the price is enticing.

I already have a 7s80p (~4.5kWh), but would parallel (rather than series) the second 7s80p I'm building.

Here's a link to the 2017 NEC for reference.
 

CrankyCoder

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Korishan said:
If you go 24V, it'd probably be better to do smaller strings and then parallel the different strings. That would give you more flexibility if you need to work on a string, or add another one into service.
So, you could do 7s80p which would give you 100A capability from each string. Though, you would normally not go over about 50A for each string.
Or, if you already have enough, you could do 7s40p and have 2 strings put into parallel. And then every 280 cells that are ready to go you can put into service. You wouldn't have to wait till you had >500 cells ready to go for each string.

Just an idea

My current packs are 80p. So I was thinking maybe get up to the 7s80p and then start working on another 7s80p. But I definitely see what you're saying. Having everything split would definitely allow for some maintenance without taking out the whole system.


hookemdevils22 said:
I've been interested in those MPPs, since they're combined cheaper than nearly everything else I've seen and the PIP4048 is fairly popular around here. I'd rather run 48v, but the price is enticing.

I already have a 7s80p (~4.5kWh), but would parallel (rather than series) the second 7s80p I'm building.

Here's a link to the 2017 NEC for reference.

I agree. I like the idea of the 48v but that price is definitely a draw. Guess ill have to see once i get my packs ready and i am ready to really step up to that inverter what i can do. I have a couple oftruck inverters now that will take 24v so I will atleast have something to play with lol. Benefits of a pop driving rigs


hookemdevils22 said:
I've been interested in those MPPs, since they're combined cheaper than nearly everything else I've seen and the PIP4048 is fairly popular around here. I'd rather run 48v, but the price is enticing.

I already have a 7s80p (~4.5kWh), but would parallel (rather than series) the second 7s80p I'm building.

Here's a link to the 2017 NEC for reference.

PS. THANKS FOR THE LINK!!! I have some serious reading to do lol
 
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