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What voltage pack to build?
#1
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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#2
(04-03-2019, 01:51 PM)bairleafarm Wrote: 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.
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#3
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.
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#4
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.
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#5
After reading and looking I think I will do 24v packs. Thanks for the info guys.
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#6
(04-04-2019, 05:16 PM)bairleafarm Wrote: After reading and looking I think I will do 24v packs.  Thanks for the info guys.

Keep reading  Wink
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#7
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 of power conversion... depending on the equipment efficiency can decrease as input voltage decreases.
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#8
(04-04-2019, 05:16 PM)bairleafarm Wrote: 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).
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#9
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.
Formerly known as Dallski
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#10
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.
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