Battery understanding of parallel and series

[Vrooom!]

Ok, i know the title sounds like a generic already answered post... however, I'm trying to fully understand the science behind battery packing in series and parallel to increase voltage and capacity.

My confusion is with the amount of batteries required. In a 20s10p most diy and manufacturing seems to utilize 200batteries for this structure. Seems contrary to what the basics I'm finding.

Using 3.6v 3300mAh batteries in the example above you have 20batteries in a series and you clone that series 10 times equaling to the 20s10p 72v 33Ah thus requiring 200batteries.

Since, i am interested in LiFePo4 batteries with the screw thread i will posit another example.

A 3.2v 8000mAh battery. If i purchase 23 and put them in a series pack of 23s1p i am creating a 73.6v 8Ah battery pack, yes?

I purchase 10 batteries and put them in parallel. I now have a 1s10p 3.2v 80Ah battery pack, yes?

NOW, if i run these two battery packs in parallel, will i NOT or WILL I end up with a 1s2p 73.6v 88Ah battery pack utilizing 33 batteries in total as opposed to doing 230 batteries in one pack.

Am i stupid? What am i not getting that everyone else understands and spends $1000 to accomplish what i seem to believe i can do with a few hundred bucks???

 

[E-HP]

A 3.2v 8000mAh battery. If i purchase 23 and put them in a series pack of 23s1p i am creating a 73.6v 8Ah battery pack, yes?

I purchase 10 batteries and put them in parallel. I now have a 1s10p 3.2v 80Ah battery pack, yes?

NOW, if i run these two battery packs in parallel, will i NOT or WILL I end up with a 1s2p 73.6v 88Ah battery pack utilizing 33 batteries in total as opposed to doing 230 batteries in one pack.

Am i stupid? What am i not getting that everyone else understands and spends $1000 to accomplish what i seem to believe i can do with a few hundred bucks???

House fire! Fundamentally, and I’m sure you’ve read, that cells/packs connected in parallel should be at the same voltage before connecting them. If not, the higher voltage cell/pack will force current to flow into the lower voltage cell/pack, virtually instantaneously. For a few hundred dollars you have a decent bomb, but there are probably cheaper ways to go.

 

[Vrooom!]

House fire! Fundamentally, and I’m sure you’ve read, that cells/packs connected in parallel should be at the same voltage before connecting them. If not, the higher voltage cell/pack will force current to flow into the lower voltage cell/pack, virtually instantaneously. For a few hundred dollars you have a decent bomb, but there are probably cheaper ways to go.

That is beautifully refreshing to understand. I was curious how the differing voltage would supersede each other. I ignorantly didn't understand that more advanced part of electrical engineering.

Thank you very much for saving my foolish life!!

 

[E-HP]

That is beautifully refreshing to understand. I was curious how the differing voltage would supersede each other. I ignorantly didn't understand that more advanced part of electrical engineering.

Thank you very much for saving my foolish life!!

Good for you, for asking the question BEFORE connecting them up! Stay safe.

 

[amberwolf]

My confusion is with the amount of batteries required. In a 20s10p most diy and manufacturing seems to utilize 200batteries for this structure. Seems contrary to what the basics I'm finding.

Could you link to those "basics"? Perhaps we could see if they are correct or not.

If you need a 20s 10p battery, then yes, you have to use 200 cells, because 20 x 10 = 200.

The only way to use less cells is to use larger capacity cells, and/or higher voltage cells, so that you use less parallel cells in each parallel group, and/or less series groups.

Using 3.6v 3300mAh batteries in the example above you have 20batteries in a series and you clone that series 10 times equaling to the 20s10p 72v 33Ah thus requiring 200batteries.

In the most common terminology, you're connecting 200 *cells* to get this, not 200 *batteries*. The battery is the entire thing that you build from the cells (also called a (battery)pack).

But yes, that's essentially correct, except: build the parallel cell group sets first, then series those.

(if you don't, then a typical BMS can't monitor the pack as a whole, you would need a separate BMS for each series string set of cells. If no BMS is used, then separately manually managing the cells in each series string will be necessary, unless you are using perfectly matched-characteristics cells. Or you have to add individual parallel connections between each same-s-position cell in the series sets).

A 3.2v 8000mAh battery. If i purchase 23 and put them in a series pack of 23s1p i am creating a 73.6v 8Ah battery pack, yes?

Yes, except I'd call the first item a cell, not a battery.

I purchase 10 batteries and put them in parallel. I now have a 1s10p 3.2v 80Ah battery pack, yes?

Yes, except I'd call the first item a cell, not a battery.

NOW, if i run these two battery packs in parallel, will i NOT or WILL I end up with a 1s2p 73.6v 88Ah battery pack

No. You have a 1s battery that is only 3.2v, and a 23s battery that is 73.6v. It doens't matter what their capacity (Ah) is, two different voltages cannot be directly connected; the lower voltage will short out the higher voltage, and current will flow at as high a rate as the (very low) resistance of the connection will allow. (KFF means Kentucky Fried Finger )

The first immediate effect is the connection between the two different voltage sections will get very hot, very fast. The next effect is the cells at 1p are being rapidly (over)charged by the current flow, heating them; they may just vent but they may burst into flame or actually physically explode, if they are cylindrical cells.

utilizing 33 batteries in total as opposed to doing 230 batteries in one pack.

You can't get away from using the total number of batteries to get the total Wh you need. Wh is just nominal V x Ah. 73.6V x 88Ah = 6476.8Wh.

No matter how you divvy it up, if you need 6476.8Wh you need to get a total number of cells that equal that Wh.

If a cell is 3.2v x 8Ah = 25.6Wh. To get 6746.8Wh with those cells, you have to use 6746.8Wh / 25.6Wh = 253 cells.

 

[amberwolf]

What am i not getting that everyone else understands and spends $1000 to accomplish what i seem to believe i can do with a few hundred bucks???

FWIW, you *can* often build a battery pack much less expensively...by using used EV cells from various second-life companies like BatteryHookup, places that scrap EV car batteries, etc. These cells are almost always better quality, even used, than what you can find for sale "new" (especially the stuff from places like Aliexpress/etc where many buy parts from to try to save $$$).

They even usually come prebuilt into modules with well-made interconnects and enclosures; you just have to do "a bit of reconfiguring" of some of the connections, and add your own BMS in place of whatever electronics it originally had (assuming you want or need one****).


For over a decade I've been using used EV cells (no longer even made) that are at this point probably a decade and a half old, and they're still working--not nearly as well as they did originally, though still good enough to run my SB Cruiser daily commuter heavy cargo trike. No BMS, not needed as they were so well-matched originally by the manufacturer that they all charge and discharge equally, and remain in balance, so I only have to monitor system voltage to know when to stop discharging--all cells are equal so none can get overdischarged (or overcharged). I'm sure it helps that I do not use them for their full capacity range, nor do I use them at their full discharge rate, so they're not pushed to their limits. (this is similar to how typical EVs use their packs, to achieve longer lifespan).

I wouldn't do this with typical cells I could buy "new" anywhere I know of, because they don't provide matched cells, and I'd probably have to buy anywhere from several times to several dozen times as many cells as I actually need, possibly more, just to get enough identical cells to build a perfectly-matched pack from.

I also wouldn't do this with a pack built from many small paralleled cells (like the various cylindricals, etc), as the interconnects can fail and cause groups to become non-identical, among other types of problems.

I'd only do it with large-format (typically pouch) EV-grade cells that came out of a single EV battery pack (because those should be already matched).

 

[offGridDownUnder]

Hello,

Minor point.

200 cells. A battery is a collection of some thing(s). The cells are the thing in this case.

It's only a battery when you assemble more than one cell. Like a battery of hens, or a battery of guns.

The classic AA 'battery' is a cell. The classic rectangular 9V battery is actually a battery - open one and you'll find 6 1.5V cells packaged together.

In many cases, you can get cylindrical AAAA cells from a rectangular 9V battery. I have seen one that was 6 smaller rectangular cells, but most I've opened have been 6 cylindrical AAAA cells.

 

[Vrooom!]

Returning back to this post after some thoughts... What would be required, if possible, to series 2 packs of 48v60aH?

 

[amberwolf]

Returning back to this post after some thoughts... What would be required, if possible, to series 2 packs of 48v60aH?

If you're looking to make a 96v pack, the safest way is to take the BMS out of each one, physically make the two packs into one casing, and replace them both with a single BMS capable of the full voltage and current you're going to use the pack for (but not higher than what the cells are actually capable of (which you may have to test so that you don't damage the cells or worse, since battery sellers are some of the least truthful vendors on the planet)).



It's likely that the individual 48v BMS won't have FETs in them capable of handling the full-charge voltage of both packs in series, and so if one of them turns off for any reason, the full pack voltage will be across those FETs. If they can't take that, they'll fail, and the most common way to fail is shorted, meaning the pack will now be stuck on, unable to turn off no matter what is wrong. Since something was *already* wrong that caused it to turn off in the first place, it'll just get worse, and if it's damaging the cells, possible consequences could be expensive and dangerous. It's a "silent" failure, since the BMS doesn't even know that it's happened, it can't tell you about it, and unless you personally test the FETs every usage to see that the on/off functions of the BMS are working correctly, it's just going leave that whole pack unprotected.


A single BMS capable of protecting the entire set of cells from both packs as a single pack is safer, and easier to test.



Note that if you have an existing 48v system that you're powering from a 48v pack, it's not very likely to survive being run at 96v from two of them in series.

 

[Vrooom!]

In other words a complete overhaul of the wiring system? So to start it would mean taking apart the individual packs just to the point their BMS is exposed, remove both BMS and then join the wires connected to the previous BMSes to a bigger 96v BMS. In that situation though would i be still setting myself up for failure because of the thin gauge wires used in a 48v pack?

 

[amberwolf]

In that situation though would i be still setting myself up for failure because of the thin gauge wires used in a 48v pack?

The voltage of the pack isn't relevant to the wire gauges, and vice-versa.

That only matters for current. (amps).

 

[Vrooom!]

The voltage of the pack isn't relevant to the wire gauges, and vice-versa.

That only matters for current. (amps).

In that case what type of MOSFETs exist in a typical 48v that can handle 96v?

 

[amberwolf]

In that case what type of MOSFETs exist in a typical 48v that can handle 96v?

There's no reason for a BMS maker to use FETs that good, and those would cost more, so they probably wouldn't. They'll probably use the cheapest part they can get that will (barely) handle the current required and (barely) handle the full-charge voltage.

Even one cent per part more is money they could have in their pockets; ten or a hundred thousand BMS with say, average six FETs, 1 cent each...and it's likely to be more difference in cost than that. (can't count how many devices of all types I've seen where parts that cost far less than a penny each were left out, leaving the device "functional" but degraded in reliability, safety, or usability, even for it's intended and advertised purpose. :/ )


Even if you use two separate 48v in series successfully, you now need to keep two *isolated* type 48v chargers, if you want to charge the whole thing at once.

Isolated means that their AC sides are completely disconnected from their DC side. If they're not, plugging both in at the same time effectively shorts across a whole pack (and charger)...

Or you have to put a disconnect between the two packs, so you can isolate *them* during charging. And never forget to disconnect them, or...

 

[Vrooom!]

There's no reason for a BMS maker to use FETs that good, and those would cost more, so they probably wouldn't. They'll probably use the cheapest part they can get that will (barely) handle the current required and (barely) handle the full-charge voltage.

Even one cent per part more is money they could have in their pockets; ten or a hundred thousand BMS with say, average six FETs, 1 cent each...and it's likely to be more difference in cost than that. (can't count how many devices of all types I've seen where parts that cost far less than a penny each were left out, leaving the device "functional" but degraded in reliability, safety, or usability, even for it's intended and advertised purpose. :/ )


Even if you use two separate 48v in series successfully, you now need to keep two *isolated* type 48v chargers, if you want to charge the whole thing at once.

Isolated means that their AC sides are completely disconnected from their DC side. If they're not, plugging both in at the same time effectively shorts across a whole pack (and charger)...

Or you have to put a disconnect between the two packs, so you can isolate *them* during charging. And never forget to disconnect them, or...

How does that coincide with LiFePo4 cylinder or prismatic? How stable are they in that type of application?

 

[neptronix]

FYI. Moved this thread to ebike general because this is not a build thread.

 

[docw009]

In that case what type of MOSFETs exist in a typical 48v that can handle 96v?

They need to handle 110V because the full charge of a 48V battery is 54.6V. And any design engineer would want extra voltage margin, The above scenario described by Amberwolf is not a theoretical scenario. It has happened to at least one poster, where one of the batteries caught fire while he was riding, If it had happened while charging, house fire,

And don't forget 100V DC is now high enough to kill you, especially if you crashed in a puddle of wire and are grabbing bare wires with wet hands.

 

[amberwolf]

How does that coincide with LiFePo4 cylinder or prismatic? How stable are they in that type of application?

I have no idea what your question has to do with the post you quoted (copied below), so I can't answer it.

There's no reason for a BMS maker to use FETs that good, and those would cost more, so they probably wouldn't. They'll probably use the cheapest part they can get that will (barely) handle the current required and (barely) handle the full-charge voltage.

Even one cent per part more is money they could have in their pockets; ten or a hundred thousand BMS with say, average six FETs, 1 cent each...and it's likely to be more difference in cost than that. (can't count how many devices of all types I've seen where parts that cost far less than a penny each were left out, leaving the device "functional" but degraded in reliability, safety, or usability, even for it's intended and advertised purpose. :/ )


Even if you use two separate 48v in series successfully, you now need to keep two *isolated* type 48v chargers, if you want to charge the whole thing at once.

Isolated means that their AC sides are completely disconnected from their DC side. If they're not, plugging both in at the same time effectively shorts across a whole pack (and charger)...

Or you have to put a disconnect between the two packs, so you can isolate *them* during charging. And never forget to disconnect them, or...

 

[Vrooom!]

I have no idea what your question has to do with the post you quoted (copied below), so I can't answer it.

I mean, improperly wired and arranged LiFePo4 cells would also suffer catastrophic failure that can result in damage and human harm or because of their chemistry the battery would just "die"?

 

[amberwolf]

Any battery that's improperly wired (such as shorting across cells, etc) would suffer "catastrophic failure".

But it still has nothing to do with the post you quoted, which doesn't mention any of those things...and I still don't know what question you are asking.

 

[999zip999]

Mr Vroom what do you need this battery for ?
As you must match the battery to your controller,motor and load or needs (hills ect.)

 

[999zip999]

Do yourself a big favor by this book. He is a member here but it is a great book for a person in your situation. Good luck . Be careful some of these batteries using vape pipes are extremely extremely powerful remember you're dealing with a lightning bolt in a small package and like anything like lightning or a snake all it wants to do is get out of his cage.

Amazon.com vlocphy=9031538&hvtargid=pla-323272206900&psc=1&mcid=9e0edc6f3d173de5bf5c64d4ca44bf32&gclid=CjwKCAjwnv-vBhBdEiwABCYQA6NL7Lab1ImA_CUSscduhjGH3AttOUQb7jE3v-E81Vo9qzIYb_1yQBoCpGkQAvD_BwE

 

[Vrooom!]

Do yourself a big favor by this book. He is a member here but it is a great book for a person in your situation. Good luck . Be careful some of these batteries using vape pipes are extremely extremely powerful remember you're dealing with a lightning bolt in a small package and like anything like lightning or a snake all it wants to do is get out of his cage.

Amazon.com vlocphy=9031538&hvtargid=pla-323272206900&psc=1&mcid=9e0edc6f3d173de5bf5c64d4ca44bf32&gclid=CjwKCAjwnv-vBhBdEiwABCYQA6NL7Lab1ImA_CUSscduhjGH3AttOUQb7jE3v-E81Vo9qzIYb_1yQBoCpGkQAvD_BwE

I've decided im not ready to build a battery now or in the near future. I have been trying yo justify purchasing an Eahora M1PS 72v40Ah but my commute is too long at about 45miles to expect consistent success. So i have been trying to figure out how to cheaply or easily increase the battery size/capacity within this frame. It won't work without extensive renovation of the frame setup.

I believe i have realized the answer hidden in plain site. The company included a fast charge cable that connects to a seemingly j1772 charge port. Now i am convinced this is the only viable solution to make this purchase justifiable for my commuting needs. I just have to figure If and how i can utilize that port to charge at a public charging station on the way to work.

I will look for my answer in another topic. Thank you all for your patience and wisdom.

 

[DaLanMan]

Or... As I believe has been said in nearly every range discussion around the forums...

Buy a 2nd battery unit, and when one goes flat... swap.

The dual battery stuff I have seen to run 2 simul units have all be a wee bit hinky for my tastes, but I have an aversion to Lithium fires directly under something I am sitting on. You can have whatever you want under your bum though.