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Advantages and real world experience with 2-8s batteries vs a 2p8s 24v solar pack with a single bms.

augiedoggy

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Oct 26, 2022
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I'm expecting my 16 eve 280 cells to arrive from docan tomorrow and I am trying to figure out if there are any real appreciable real world advantages to building 2 separate batteries with 2 bms vs just doing 2 cells in parallel in an 8 cell series pack? this is for a solar system im piecing together along with a 3.6kw AIO (same as sungold but unbranded voltronic) and like 3500w of solar panels for an off grid cabin build. I have the larger 200a jk 2amp active balance bms so far. I have a steel enclosure I might use or I might use a prebuilt enclosure for 16 cells depending.

i'm hoping some folks here have some experience with both configurations.

This system will be for hunting, weekend get aways and will be powering mainly small loads and a fridge part time as well as a microwave and coffee maker.
 
Only positives of running a 2p8s is savings the cost of the second BMS and one large battery enclosure vs two smaller ones.

There are way more positives of running two BMS, two batteries.

For me the second $80 BMS is worth it the extra cost.
 
Any issues with 8s2p disconnecting and ending up at different SoC & voltage, then reconnecting?
Can't fix that with a precharge resistor like inverter input capacitor.

I will be running multiple 48V batteries in parallel, each with JK PB BMS. So I'm wondering if anything might cause disconnect and divergence.

If that is ever an issue, ideally we would want a BMS that monitors voltage across its disconnect (FET or contactor), only closes when they are very close. It could wait until charge or discharge results in them lining up.
 
Any issues with 8s2p disconnecting and ending up at different SoC & voltage, then reconnecting?
Can't fix that with a precharge resistor like inverter input capacitor.

I will be running multiple 48V batteries in parallel, each with JK PB BMS. So I'm wondering if anything might cause disconnect and divergence.

If that is ever an issue, ideally we would want a BMS that monitors voltage across its disconnect (FET or contactor), only closes when they are very close. It could wait until charge or discharge results in them lining up.
I think with their voltage curve it's basically never that harmful to parallel very different SOC Lifepo4 packs. The most they ever are is a couple volts off and they pull each other into line so easily.

At least I have been running a 100Ah server rack battery in parallel with my 300Ah DIY for 9 months now and it's been fine. I've never paid much attention to the 5kWh so I don't know even know if it's ever gone into charge or discharge protection. I doubt it.
 
Thanks everyone.
Im not worried about different packs but more the reasons why it would make more sense to make 2 packs instead of 1 larger capacity pack since all the cells are supposedly grade a and matched in performance. As far as I can tell many of the prebuilt batteries sold as well as ev batteries are setup with multiple cells in parallel as well as series.
 
Only positives of running a 2p8s is savings the cost of the second BMS and one large battery enclosure vs two smaller ones.

There are way more positives of running two BMS, two batteries.

For me the second $80 BMS is worth it the extra cost.
Can you share what those advantages might be? The only ones I can think of are redundancy if a bms fails (but also doubling the chances of having a failed bms) and being able to monitor the attributes of each cell

Right now I have the large single steel enclosure and the $130 jk bms with the heat function and display. as well as a single set of case terminals.
I could build both packs in the single enclosure with 2 bms and displays I suppose
 
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We run a 3 season 24v camp and pull the batteries for winter. With two batteries, we can run one battery on the shoulder season so it’s easier to pack up camp. I couldn’t possibly lift a 2p8s 280ah pack, a single 8s 280ah is about 110lbs.

With two batteries I can see each individual cell voltage. If I had an issue with one battery I can isolate it and still have power.

With two packs each battery has to provide only half the current, less current through the BMS the better. You could run smaller conductors, I still run 2/0 just in case I need to run on one battery.

My cells are also matched and batched. In theory I won’t have any issues but with DIY there’s a much higher chance of a bad bolted connection that needs cleaning redoing.

With commercial packs that run multiple cells in parallel, they typically don’t run bolted connections, the welded connections are proven to be superior so that’s one way they can get away with parallel cells.
 
Thanks everyone.
Im not worried about different packs but more the reasons why it would make more sense to make 2 packs instead of 1 larger capacity pack since all the cells are supposedly grade a and matched in performance.

What some people here have done with parallel first is to mix-n-match Ah capacity so all parallel sets are more close to same capacity than would have been as single cell wide series strings.

Multiple strings with their own BMS increases current handling and balancing current. Given the idea that electronics should be operated well below maximum spec most of the time, a 200A 48V battery is somewhat limiting for larger system.

Also, you can add more capacity in parallel easier than rearranging packs to add cells.

If you are not an EV manufacturer, not sure you will really get matched. Maybe so if they provide test data per cell.

As far as I can tell many of the prebuilt batteries sold as well as ev batteries are setup with multiple cells in parallel as well as series.

That may only be the tiny flashlight style batteries originally found in laptops, then used in Teslas.

Any of the LiFePO4 packs I see inside photos of on eBay are only series connected cells between 100 Ah and 300 Ah.
That would be plenty of current, in fact only smaller cells might reach the desired voltage in automobile sized packs.


I originally planned to use the expensive REC BMS because it is compatible with Sunny Island. One $800 BMS for 4x SI didn't seem outrageous (and contactor handles the high current.)

I ended up buying CATL 4s 228Ah EV packs, without high current access to connections between cells. So I have to have separate BMS per 16s string. Anything from 16s1p up to 16s15p (or is that 16p?) I went with JK PB2A16S20P because other Sunny Island users had success with it. I don't think it has precharge function, so plan to implement a precharge circuit bypassing battery breaker inside each Sunny Island.
 
Thanks, I have been looking and purchased a handful of different types of cells and used ev packs for projects like golf carts and a decent amount of the EV packs are built with multiple cells in parallel.
I dont see myself even using 200amps but the ability to more easily move and carry 2 packs is a very good point. I likely will not be leaving mine in place over the winter either after hunting season ends.

Ive had good luck so far just cleaning the contact points and using an anti oxidating paste on the connections. of the golf cart packs Ive built and those see pretty large amp loads (My 72v pack sees almost 400amps at times though it is shared over 2 packs )

The prebuilt enclosures are all expensive for what they are IMO even on ali
 

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