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I am currently designing a battery system to support some low level electronics. The system will have both 12v and 24v electronics. I am wondering which setup would be better to get the most use out of my batteries, time wise:

  1. Two 12v batteries in parallel and use a step up converter for the single 24v electronic equipment in the system.
  2. Two 12v batteries in series to make a 24v source and use step down converters for all the 12v electronics in the system.

I am not worried about current issues with the wires as the electronics will be located close to the batteries. I just don't want to have to charge the batteries after only an hour of use, so that's why I thought of 2 batteries in series to double the capacity. I was just told by someone I can get a longer energy draw by putting the batteries in series.

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  • \$\begingroup\$ Very similar to this question: electronics.stackexchange.com/questions/83817/… \$\endgroup\$
    – Drew
    Commented Dec 17, 2021 at 5:52
  • \$\begingroup\$ Two 12V 1Ah batteries in parallel is 12V 2Ah or 24Wh. Two 12V 1Ah batteries in series is 24V 1Ah or 24Wh. They have the exact same amount of energy in theory. Which is better depends on how much current the 12V and 24V loads take and what is the efficiency of converting the voltage up or down. Due to internal losses, battery which is discharged at lower current lasts slightly longer. \$\endgroup\$
    – Justme
    Commented Dec 17, 2021 at 6:03
  • \$\begingroup\$ Have all the loads at one voltage then you reduce the conversion losses. \$\endgroup\$
    – Solar Mike
    Commented Dec 17, 2021 at 7:06
  • \$\begingroup\$ I would not use DC converters. Use series batteries for 24V boards, and single batteries for 12V boards. \$\endgroup\$ Commented Dec 17, 2021 at 8:32
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    \$\begingroup\$ Identify the loads. If the high power load is the 24V load, you want a 24V battery, so connect in series, and use a buck converter for the 12V stuff. If the 12V load is the high power load ... well, that'll be less efficient from I^2*R considerations. \$\endgroup\$
    – user16324
    Commented Dec 17, 2021 at 15:01

2 Answers 2

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As the others have stated, capacity is capacity, doesn't matter if the batteries are in series or parallel. There are a few other considerations though.

First off, let's look at your question, 12V batteries (or 24V for that matter) are not a constant 12V output, so you'll most likely need 2 DC-DCs for 2 stable voltage rails. Let's look at which configuration is better. Assuming 1Ah packs for simplicity.

In parallel, with perfect current sharing you'd have 12Vin, and either 12Vout or 24Vout. Power In = Power Out, so 12(Iin12) = 12(Iout12) + 24(Iout24). With a parallel configuration, each pack will see half of Iin drawn from it. If we then go to a series configuration, with the same setup the equation changes slightly: 24(Iin24) = 12(Iout12) + 24(Iout24). In this case, each battery sees the same current through it (Iin24). If the 12V and 24V output currents stay the same in each configuration, we have 12(Iin12) = 24(Iin24) so Iin12 = 2*Iin24. Since each pack sees half the current in the parallel configuration, the current drawn from the packs is the same regardless.

So does it matter? Not in the sense that one configuration will provide more capacity than the other. What you'll have to worry about with either configuration is how you balance the cells. Parallel and series connections each have their own pros and cons for balancing. In parallel you're guaranteed an equivalent voltage on each cell, but it can be tricky when they start to age and sink/source current at different rates. Similarly for series connections they will always sink/source the same current, but their voltages will increase at different rates. I personally find series connections to be easier to balance, and there are a lot of ICs available that do it that way as well.

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  • \$\begingroup\$ Thank you for the response. I think that a series connection will be the way to go then since a decent step down converter will keep the voltage for the 12v rail much more stable as opposed to 2 batteries in parallel. \$\endgroup\$
    – radji
    Commented Dec 17, 2021 at 18:45
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The capcaity in terms of stored energy is the same in both cases. Step-down converters generally tend to be more efficient than step-up ones but for properly optimized designs the difference is not very big. Also your 12V and 24V betteries all contatin multiple cells so if you are going to use 2 similar batteries then there is mot much difference in terms of ballancing or expected wear as well. Also 12V lead-acid batteries can only safely be discharged to around 8V and it is well above what any proper DC-DC convertter can go down to so there is no difference as well.

To be honest I would probably go with a series connection because of slightly less active losses (higher voltage -> less current) and slightly more efficient and more readily available convertors but there is not much to it, you can go with whatever you like or depending on whatever equipment you already have available.

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  • \$\begingroup\$ Thank you for your response \$\endgroup\$
    – radji
    Commented Dec 17, 2021 at 18:49

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