Cells in Series for Best Efficiency and Capacity


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How can you calculate how many cells in series for different chemistry? What is the best voltage range? I’ll try to answer these questions in this video. Warning, it’s a bit dry.

48 volts is a nominal voltage. I think of it as the category our equipment falls into. But the batteries, inverters, and charge controllers all work in ranges of voltages. The trick is getting the most efficiency from the equipment. Choosing the right number of cells in series. Understanding your chemistry.

When I built my first powerwall I build a 12s lithium NMC battery. This was simply because the battery was easy to break into 12s modules. Later I started discovering why this configuration had some downsides.

After pouring through manuals and speaking with tech support, I found that inverters love voltages of 48v to 54v. This is the sweet spot. A lot of charge controllers have settings between 54v and 60v. If you want to make life easy, set up your battery to work with these settings.

This video shares my experiences. I make several generalizations. However, every rule has exceptions. The voltages I lay out will not work for every battery.

LTO battery pictured is thanks to Battery Hookup. Use coupon code “davidpoz” for 10% off. Affiliate link: https://www.batteryhookup.com/?rfsn=3333666.ca06e9

Tesla photos are thanks to Carel Hassink
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44 replies
  1. superspeeder
    superspeeder says:

    Hey David, great video! Ego uses 14s in their OPE batteries. They're not as big as retired EV batteries, but for a smaller scale project they might work well with existing inverters and charge controllers. Their biggest battery right now is a 7.5Ah (3P14S) but they are supposed to be coming out with a 10Ah version as well.

    Reply
  2. Shannon Williams
    Shannon Williams says:

    NMC Li-ion: 3.6-3.7 nominal voltage
    3s or 4s isn't optimal for 12v, 7s(24v), 14s(48v)

    LiFePO4: 3.2 nominal voltage
    4s(12v), 8s(24v), 16s(48v)

    LTO: 2.3 nominal voltage
    6s(12v), 11s(24v), 22s(48v)

    Reply
  3. Dustin King
    Dustin King says:

    David have you considered adding a wind turbine? Looks like it's kinda windy at your place it may generate a lot of power. If you don't wanna add it to your existing set up you could run it independently and drop a heater element in your crawlspace water tank. I bet that would help you reach your 180° goal.

    Reply
  4. Scott Henderson
    Scott Henderson says:

    can I use the cc settings for 48v fla battery to charge the 3.2 volt x 16 cell lifepo4 with a programmed BMS? Wondering if I can just switch between the 2 different battery banks (fla are on the way out

    Reply
  5. jo lee
    jo lee says:

    One day I'll make the switch to lithium until then I think I have to stick to sealed lead acid. Once I can move all my equipment from the hundred and twenty degrees Heat into air conditioning I think that's when I will switched. Good video.

    Reply
  6. Chris Deleo
    Chris Deleo says:

    Is there any limit to the number of series connected cells? Is there a factor of diminishing returns do to internal resistance of each cell? Sorry if that doesn't make sense I'm in no way an electrical engineer. Love your videos nice work

    Reply
  7. LithiumSolar
    LithiumSolar says:

    How do you like the Victron 250/100? Is it worth the $900 price tag? I'd love to have an MPPT that can go over 150V but I have a hard time justifying the cost when you can get a PCM60X 150V/60A for less than $200.

    Reply
  8. mitchese1
    mitchese1 says:

    Thanks for this! Do you have any details on mixing battery chemistries? For example if you were to re-do your Volt pack to be 14S (49-56V), could you parallel it with your new LiFePO4 pack in a 16s configuration (48-56), or is there too much of an imbalance in the chemistries when using the pack that you would end up wasting a lot when one pack is charging the other?

    How would you mix packs (like you have, one from Bolt, one from these LiFePO4, and maybe some lead acid)

    Reply
  9. MikeJ F
    MikeJ F says:

    David I’ve got a WZRELB 3000W pure sine wave inverter on its way, I’m running a 7S setup, what should I have as a resettable fuse on the output? I’ve been enjoying your videos.

    Reply
  10. D Dixon
    D Dixon says:

    Awesome video! This is something people hardly ever talk about directly, but indirect in the sense that they assume everyone knows so they don't explain in any detail like you just did. A video on sizing would be good too, for example, using an electric bill to calculate the system size that would match the KwHs used each month. Where to start on that? Battery size, after battery size, one has to think about how many panels are needed at a particular size (watts) to charge the batteries fully in winter on sunny day with only 5 hours of sun. How to size an inverter for said system, how to determine if inverter can be connected in series? A whole detailed series could be done on all aspects of sizing. This is something I don't see covered as a whole, but only in scattered bit and pieces. Also, subjects like charging, how to charge batteries without a charger/bms as in from a bench power supply? Is it possible? A lot of people don't know.

    Edit: Just watched another of your videos, BMS came up …. add size of BMS to the above.

    Thanks for sharing your knowledge.

    Reply
  11. TYP xxi
    TYP xxi says:

    most important point – i have a grid tied inverter and wanted to move to off grid but the grid tied solar inverter has a 70 – 450 Volt range and therefore would need a more likely 84 V – 96 V battery than a 72 V one but mppt chargers for such high voltage are often missing.

    Reply
  12. Jimmy Squires
    Jimmy Squires says:

    David good job. I chose a Victron "48V" Inverter/charger because it had the best voltage range. But I went with 18 LiFePo4 cells because I wanted to reduce the voltage swing to increase the cell life. Also if one cell goes bad, I can remove it and easily reprogram the voltage swing while waiting for a replacement cell. That is the one thing that many forget.

    Reply
  13. dodgeplow
    dodgeplow says:

    I was going to comment on your earlier videos, why not 14s? Is it because you already have the new system batteries? What do you plan to do with the Chevy volt cells now?

    Reply
  14. Kenneth Almond
    Kenneth Almond says:

    Another great video. I might comment that (I believe) the reason you focused on 48v DIY battery bank (rather than 24v or 12v) is that you're talking about house level power. For house level – e.g. 6,000watt or above – is that you need 48v to reduce wire size. To illustrate, running a 6,000watt inverter from a 12v battery requires 6000w/12v = 500amps which is really huge wire thickness; whereas at 48v its 6000/48 = 125amps which is much more mainstream wire, fuses, connectors size. The battery, inverter, amps (wire size) all relate to shape the equipment ranges that are readily available and 48v is one of the sweet spots with lots of stuff (and therefore reasonably priced).

    Reply
  15. Kellyboy
    Kellyboy says:

    Hi David I forgot to say how much I like your videos and your style in front of the camera . Please keep it up, I believe you more than the rest if the guys on YouTube.

    Reply
  16. George Carlin
    George Carlin says:

    question for David.
    David, can you do anything with a battery Type 9MGCD. It's Li-Poly 7.4V 32Wh tablet battery. I have about 50 (or more) of them and can give them to you. I'm in Ontario Canada, 1 hr from Niagara Falls.

    Reply
  17. tee kay
    tee kay says:

    You hit on it ,but explaining that you would be wasting a large percentage (about 1/3) of popular arrangement of LMnCo chemistries if you only charge to max of most systems designed for lead acid.

    Reply
  18. tee kay
    tee kay says:

    Great explanation and examples all in one video especially for beginners. many channels throw around xs and yp without explaining the meaning and specific voltage ranges. I along with others like the spreadsheets. A suggestion for others: just pause video and do a screen shot of the chart. Cant believe you were not familiar with LTO until recently though.. Keep up the good work.

    Reply
  19. bwillan
    bwillan says:

    I think as lithium batteries become more common, the inverter manufacturers are going to make their inverters able to handle the input voltage ranges of the various lithium battery chemistries.

    Reply
  20. TheWanderingStar
    TheWanderingStar says:

    David, very informative video! It answered a lot of questions that I have had regarding chemistry, configurations and inverters. Your explanation and spreadsheet were also well thought out and easy to understand. Very helpful. Thank you!

    Reply
  21. OffGrid Wanabe
    OffGrid Wanabe says:

    Ah yes the sweet spot all components have now putting them all together to get the perfect running system is still left to be determined. Good video as usual. A little higher in voltage is usually better than a little lower and we make more energy at higher voltage when we keep the amperage the same.

    Reply
  22. marthale7
    marthale7 says:

    Yep we build the battery config around both the inverter, and the charge controller. It is good to know that there is an optimal range for the inverter where it is most efficient and it is not being over worked.

    Reply
  23. Brian Davis
    Brian Davis says:

    David what bms did you use for your battery wall? And does it have low temperature cut off? And what did they cost you? Thanks for what your doing its been very helpful trying to figure out what route I'm going to take here.

    Reply

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