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12v vs 24v battery bank servicing a 12v/120vac system - which is better?

1holaguy

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Will has repeatedly said in his vlogs that with multiple batteries it is better (more cost effective?)to go 24v or 48v instead of 12v. My understanding is that if I build a battery bank using 8 3.2v 280 AHr cells I can either have 280 AHr at 24v (4S2P) or 560 AHr at 12v (8S1P). The potential out put should be the same and the cost difference will be 2 BMS for the 12v system and 1 BMS for the 24 V system and posably smaller Ga wiring between the battery bank and I/C. Also, in the 4S2P configuration with 2 BMS you could use lower rated units such as 2-50 Amp out put units vs 1-100 amp unit at 24V for the same net out put. If this is going into a 12v environment (12/120 Inverter/Converter and 12 vdc other appliences) I would then, for the 24v BB, require a 24v/12v converter. If this is correct, what is the real world advantage of going 24v?
 
If you have 12 volt dc stuff to integrate with you may want to stay at 12 volts.
If you have big ac loads > ~2200 watts you may want to start thinking 24 or 48 volts.
More info is required to make a good recommendation.

Anyway a power audit is the first step.
 
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24 volt systems use wires that are about half the size, run at half the amperage, and are less than half the cost than a 12 volt system. For example, my space heater uses ~2000 watts @ 115v AC, and pulls ~175 amps @ 12v DC. I use 00 cable, which is expensive (not to mention crimping tools and lugs for large wire), and difficult to work with. It also requires a large, and again, expensive high amperage BMS. Most other components (fuses, busses, lugs, connectors, LVD's and relays) designed to take high amperages are exponentially more expensive than smaller ones.

But I use a 12 volt system for two reasons.
1. I bought my components for a 12 volt setup before I did the research and wasn't willing to buy a new inverter and BMS for 24 volt.
2. I use 12 volt to run lights, motors, and other things designed for 12 volts. I also like the ability to alternator charge, and jump a vehicle. Although if I could do it over, I would have gone with a 24 volt system to handle my large inverter loads, and used a voltage step-down for my smaller 12 volt loads.
 
Since my system is currently already wired with 12 in mind am I correct in assuming I might only have to change the wiring between the BB and the Inverter? Everything is staying the same except the inverter is going from 2500 watt to 3k watt. The wire length from the BB to the inverter location is only about 2.5 to 3 ft (side by side compartments) current wire is 000 stranded copper. Am I over thinking this?
 
Since my system is currently already wired with 12 in mind am I correct in assuming I might only have to change the wiring between the BB and the Inverter? Everything is staying the same except the inverter is going from 2500 watt to 3k watt. The wire length from the BB to the inverter location is only about 2.5 to 3 ft (side by side compartments) current wire is 000 stranded copper. Am I over thinking this?
For that short of a run, 000 should be fine, I also assume you aren't pulling 3000 watts continuously.

To pull 3000 watts out of a 12 volt system continuously, the BMS(s) will need to supply at least 300 amps (you want to over-rate the BMS and not use it near it's full specs. They get hot). In your previous post, you mentioned setups that only are capable of 100 amps. You need a bigger BMS, or low voltage protection of some kind as well.
 
So would you put two 2/0 wires in a single crimp or in separate crimp and stack them on the terminal?
Thanks A. Justice. You cleared up a key issue for me. You are correct, I do not see at anytime a need to to pull 3000 watts continuous. There might be an instance when the coffee maker and the induction plate would be turned on at the same time but not likely intentionally. I look at the 3k watt inverter as having a bit of extra head room so to speak. My plan is to eventually have two 270-280 AHr batteries (most likely at 12v) which, if I follow all this correctly, a 3K watt draw would be handled if each battery has a BMS capable of handling more than 150 amps. If I have any of this incorrect please advise.
 
In reality probably 1000 to 1500. The inverter and batteries are only used when dry camping and although switching to LFP will make extending dry camping to longer periods our life style is such that it would likely not be more than a week. Since the motorhome has an 8K diesel generator I plan to to run the gen for a couple of hours a day or as needed to keep the batteries charged. Mostly 120v will be to run the coffee maker
 
2500 is still doable at 12 volts but on the edge.
If the continuous rating of your inverter is 3000 watts you probably should go 24 volts and then run a buck converter if you have any 12 volt loads.
Is this a mobile application per chance?
If you need to charge from a vehicle alternator that is another vote for staying at 12 volts.
 
How many amps of 12VDC loads are you figuring for?
The most DC draw will be from the inverter so its max is 3000 watts at 120 v would be 250 amps at 12v DC (If my math is correct). This is by no means continuous. This is the theoretical max. The application is in an RV and only during dry camping. Most of the time 120v during dry camping will be electronics with probably the the TV and computer being the largest draw. In the morning maybe add the coffee maker or the microwave for a few minutes. We have an 8k gen so being able to run everything 120v from the batteries is not really necessary. For our use it is mostly for convenience. Afterall, if we find our selves dry camping in warm weather the gen is coming on to run the A/C so at most the inverter would be supplemental to shore or gen (the inverter has that feature).
 
The most DC draw will be from the inverter so its max is 3000 watts at 120 v would be 250 amps at 12v DC (If my math is correct). This is by no means continuous. This is the theoretical max.
As a matter of policy I size for the system for the inverter and ignore the pure dc loads unless they are significant.
I showed you the math.
Its your system and you have to do what you think is right.
 
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Assuming you already have a 12v inverter there's probably no advantage in building a 24v pack, the money you save in smaller wire will be more than lost in the cost of a DC to DC converter.

You also don't need 2 BMS's in a 12V back and 8 cells. You just pair them up 2 by 2 with 4 parallel pairs in series and the BMS treats each pair as a single cell.
 
I am trying to decide whether to replace my current 12v I/C with either a new 12v or a 24v I/C. Your advise is interesting. Are you suggesting to build 4 pairs (8 total) of 3.2V - 280 Ahr cells using - to + and + to - connections with bus bars to yeild four 3.2v - 560 AHr cells and then put them in series ( with bus bars: - to - to - to - and + to + to+ to +) to create 1 560 Ahr 12vdc battery. They connect one say 4S - 250 amp BMS to the 4 cell pairs.? Alternately it would be two 12 v 280AHr , each of 4 cells so it would require 2 BMS? Both would have 560 Ahr of 12 Vdc . The difference would be that in theory the two batteries with say a 150 amp BMS each could yield more amperage (300 combined) vs one battery that could yield up to 250 amps. With a 3000 watt inverter it could be fully services by a 250 amp BMS (250*12=3000). Do I have this correct?
 
I am trying to decide whether to replace my current 12v I/C with either a new 12v or a 24v I/C. Your advise is interesting. Are you suggesting to build 4 pairs (8 total) of 3.2V - 280 Ahr cells using - to + and + to - connections with bus bars to yeild four 3.2v - 560 AHr cells and then put them in series ( with bus bars: - to - to - to - and + to + to+ to +) to create 1 560 Ahr 12vdc battery. They connect one say 4S - 250 amp BMS to the 4 cell pairs.? Alternately it would be two 12 v 280AHr , each of 4 cells so it would require 2 BMS? Both would have 560 Ahr of 12 Vdc . The difference would be that in theory the two batteries with say a 150 amp BMS each could yield more amperage (300 combined) vs one battery that could yield up to 250 amps. With a 3000 watt inverter it could be fully services by a 250 amp BMS (250*12=3000). Do I have this correct?
If you want to turn your eight 3.2V 280Ah battereis into 12V 560Ah battery pack, you will wire them like this: 2P4S

 
If you want to turn your eight 3.2V 280Ah battereis into 12V 560Ah battery pack, you will wire them like this: 2P4S

Bud Martin I tried this link and also to copy and paste it into the address bar of my search engine but it just times out. Is it possible there is a type-0. Will you please re-post it? Never mind, finally caught. Thanks.
 
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I decided to go with 24v since it is a new build and I'm going to use 24v appliances, lights, etc. I will be using Victron MultiPlus 3000/24v inverter for my 110. I decided to go this route also because I want to try to run a 500 watt window AC also.
 
There is an old adage that paraphrased goes - " a chain is only as strong as its weakest link". The same goes for systems. When I ordered my new Aims 3Kw 12v I/C it has a stated max charge rate of 100 amps. So I anticipated that at a charge rate of 100Amps per hour I could run my 8K gen for 3 hours and add 300 amps to my new LFP battery bank. But when wiring up the system I realized that the 120v incoming power is limited to 30 amps (10 ga wire protected by a 30 amp CB) so initially I was thinking "hell, now I will have to change to CB and wiring, except the max would be 50 amp from one leg (120v). But then I wondered " is that 100 amp @120VAC or 100 amp @ 12vdc? I did a quick calculation and 30 amp @120vac is 3600 watts and convert that to 12vdc becomes 300 amp which potentially is 3X the max for charging. I wish manufacturers would be more specific when they quote specs. Now my head hurts.
 

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