What voltage pack to build?

bairleafarm

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Mar 31, 2019
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I want to start building some battery packs. Solar to house power will be in the end result. I have a couple hundred cells that I've separated the good the bad the ugly and am still cleaning up the good ones. Is there a universal pack voltage? Seems to be 24v. I think 40 cell packs is what I will go with. I've also got several trays to do 20 cell packs. I dont have any other solar panels, charger controllers, inverters at this time so I am open to all options.

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bairleafarm said:
I dont have any other solar panels, charger controllers, inverters at this time so I am open to all options.

Given that you are aiming at a house sized supply,your nominal battery voltage should be at least 48v - so 14 cells in series.
 
It really does depend on your requirements. How much power do you need. Both 24v and 48v have there pros an cons.

48v is more efficient than 24v. Less Amps, so smaller cables can be used. One downside, is balancing hardware, you need 14 rather than 7. 24v has the advantage it is easy to add more cells, as you don't need such a large quantity.

I would say that if I were looking to go more than 5kw with a 7kw peak, go 48v.
 
I second the requirements factor. I have a 2400W 24V inverter and it does fine for me. I am careful about which circuits run on what. I avoid on demand items like my toaster, kettle and microwave. Instead I focus on continuously running items like computers, servers, fridge, lights and during this winter even a heat pump unit. So out of this unit I've crossed over 200kwh per month easily. My norm is around 350kwh per month.

The good thing about the 24V was that I only needed 7 packs in series. So I started with a 3.5kwh pack 7s60p, and worked my way up one pack at a time. My only thing was the PIP inverter I used has a parasitic drain of 50W all the time. For a 3.5kwh pack it drained the battery faster than I'd like. So if you were to start with a smaller pack you might want to wait until you get more cells or more efficient inverters.

Now would I have liked a 48V system? The only reason why I would switch is if my requirements is to have a 5000W unit. At that point it's easy to reconfigure since all the cabling is still good, the batteries can be reconfigured easily, so to switch over to 48V is basically a non-event. The only thing I'm out my PIP, and if so, my inverter only costed me $550. Not a whole lot of money lost.
 
After reading and looking I think I will do 24v packs. Thanks for the info guys.
 
If I was building for a home I also agree with 48v no question.

Traditionally the lead acid match would be 14s, but there are plenty of quality inverters out there which will gladly accept a 15s input voltage. I build my 48v setup using a 15s pack.

The reason for doing this was to increase efficiency ofpower conversion... depending on the equipmentefficiency can decrease as input voltage decreases.
 
bairleafarm said:
After reading and looking I think I will do 24v packs. Thanks for the info guys.

Just curious, what are "your" reasons for going 24V? You've read our responses and chose to go with 24V over 48V (I, btw, agree with the others on going 48V).
 
2P7S is a universal voltage. You have a 7S 24V pack in two identical strings (build it like you would be building a 14S pack). If your needs one day exceed what a 24V system can offer, split the two identical strings and run them in series for 14S. That's my plan at least. Going to run 24V for now since that's the BMS and inverter that I have, but I will always have the option to go 48V by just reorganizing my packs.
 
One other advantage I never mentioned. If a 48v system looses a pack, for whatever reason, it will still run OK at 13s. However if a 24v system looses a pack, it will really take a hit. Although that does depend on the inverter used. I thought this would be worth mentioning, as using secondhand cells, depending on condition it is always a possibility that a pack will have to be removed for service. Although I think if I were running 24v, resources permitting,I would build a spare pack.
 
I too started at 24v (7s). But over time realized that for many reasons (shear amps / wire size / inverters / PV array+controller) had to go to 48v system to produce 45Kwh per day (in summer) and I'm only powering 40% of my house!. For example, my home has 240v @ 200amp main panel service. A 12,000watt inverter = 240v @ 50amp - e.g. 25% of grid service.
To supply a 12,000 watt its 48v @ 250amps. At 24v system, that would be 500amps - and you have trouble (and more expense) finding circuit breakers etc at 500amp range. My PV array to supply this 12,000watt inverter is 7Kwh - e.g. I have 2 strings of 48v @ 70amp (and 2 Midnite Classic 150 MPPT controllers). You won't find it convenient to have a 7Kwh PV array at 24v level as you'd need double the controllers (and wiring).

Bottom line - IF you are shooting to power a home, AND that means 240v@50amp or larger... I would research the overall chain of parts starting with PV Array -> Controller -> Circuit Breakers -> Battery Bank -> Inverter and associated things like wiring, transfer switches etc.

I can attest that 48v system WILL get you 240v @ 50amps.
 
OffGridInTheCity said:
I too started at 24v (7s). But over time realized that for many reasons (shear amps / wire size / inverters / PV array+controller) had to go to 48v system to produce 45Kwh per day (in summer) and I'm only powering 40% of my house!. For example, my home has 240v @ 200amp main panel service. A 12,000watt inverter = 240v @ 50amp - e.g. 25% of grid service.
To supply a 12,000 watt its 48v @ 250amps. At 24v system, that would be 500amps - and you have trouble (and more expense) finding circuit breakers etc at 500amp range. My PV array to supply this 12,000watt inverter is 7Kwh - e.g. I have 2 strings of 48v @ 70amp (and 2 Midnite Classic 150 MPPT controllers). You won't find it convenient to have a 7Kwh PV array at 24v level as you'd need double the controllers (and wiring).

Bottom line - IF you are shooting to power a home, AND that means 240v@50amp or larger... I would research the overall chain of parts starting with PV Array -> Controller -> Circuit Breakers -> Battery Bank -> Inverter and associated things like wiring, transfer switches etc.

I can attest that 48v system WILL get you 240v @ 50amps.

If You need that kind of power, You may even aim higher, say 96 Volts. :)

ChrisD
 
But 96V is not something you find for home consumers. Its 48VDC max as of today. That also falls under the max voltage legislation in many countries.

I would love to have a 320VDC system but not doable for a proper cost.
 
A lot higher than 48V you end up choosing between death by internal burns or death by external 3rd degree burns from embeded molten copper. HV DC faults and shocks are not nice, compared to AC. If had been working with DC at the same voltage rather than AC in the past few decades i would not be writing this.

Cheapest way to go 'large' is with a used commercial UPS system, 10kVA 700 (240V battery/20 cells / +/- 120V)
https://www.ebay.co.uk/itm/10-kva-UPS-single-phase/173810606334


In terms of starting off with 24V that is a good strategy if you dont have enough cells and want to build packs first, moving up to 7x2 parallel pack arrangement then 7x4. Once you have enough packs made up you can just reorganise and put 14 in series and you then have your 48V. If you have a modular BMS then gong 24V to 48V is no issue or just keep the balancing at 24V and split.

The only advantage with starting at 48V is your inverter as this can be the most costly part of the build and replacing it later on is an added cost... charge controllers, buy 48V capable ones....
 
daromer said:
But 96V is not something you find for home consumers. Its 48VDC max as of today. That also falls under the max voltage legislation in many countries.

I would love to have a 320VDC system but not doable for a proper cost.

Also the price of circuit breakers starts to really climb once you go over 48v. It's really not practical or safe for home use. 28s is going to be expensive and potentiallydifficult to keep balanced as well. Especially if you are using second hand cells.
 
This thread started with a new user that have gathered a few hundred laptop cells.

Now we are talking 25 KW :huh:

If anyone is serious about building a system capable of 48V @ 500A, I suggest You use something beefier than second hand laptop cells.

ChrisD
 
Chris I use 2nd hand laptop cells and my system is capable of 500A easy...And yes it started with a new guy but most people dont think enough ahead when they plan and that tend to get more expensive in the end. Its important to know where and what limits are out there.

I started with LA some years back and then i started with a LiIon bank that was capable of only 1kW continues. But with that said I started with a base line that allowed me to expand without swapping out to expensive gear along the way. And now I have a system that run my whole house ;)
 
ChrisD5710 said:
This thread started with a new user that have gathered a few hundred laptop cells.

Now we are talking 25 KW :huh:

If anyone is serious about building a system capable of 48V @ 500A, I suggest You use something beefier than second hand laptop cells.

ChrisD

Yep I am serious and am building my system with measly old second hand laptop cells. Almost at the 2800 cell mark and soon it will begin.

At my average mAh results I should have 482.33 Ah which translates to 27.01KWh at 56V. Can't wait. :D :D :D

Oh and while staying on topic yes 48V nominal of course.
Wolf

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Wolf said:
Yep I am serious and am building my system with measly old second hand laptop cells. Almost at the 2800 cell mark and soon it will begin.

At my average mAh results I should have 482.33 Ah which translates to 27.01KWh at 56V. Can't wait. :D :D :D

Oh and while staying on topic yes 48V nominal of course.
Wolf

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I am really impressed by Your spreadsheet. You must have put an immense amount of work into this (hats off).
Now I can not wait to see You get started. Building cell-banks, connecting it all up, and see how it all works out.

I wish You all the best, and hope You keep us posted about how You get along :)

ChrisD
 
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