[SOLVED] Lithium battery 8P1S 3.7v nominal 4.2v Charge high current protection

[benbiles]

I am making multiple 8 x parallel(lithium iron 18650 3.7v 3400mA) battery packs that should be under 100Wh capacity.

The individual cells are not protected for over current.

Each x8 cell pack will need some kind of short circuit protection >100A , 135A ( 500w )

The x8 cell pack has no BMS.

I don't need charge voltage protection as the x8 cell packs will be connected together in series to make one large 36v battery that will have a BMS.

I'm trying to design a modular battery that is designed to come apart.

I need some protection against the small packs shorting out and going into thermal runaway !

can anyone point me to a design that could achieve the low voltage high current protection ?

Should i used an existing IC and a few mosfets ?

Its for an E-bike that can draw up to 500W peek

 

[BradtheRad]

1.
Batteries in parallel invites a problem where merely one weak (under-performing) battery drains the other batteries. In the best case all batteries drop down to an equal voltage. Worst case, it results in discharging them all prematurely.

The usual recommendation is to string batteries in series.

2.
A fuse is the traditional (and probably easiest) method to protect against high current at low voltage. To make 100A rating requires a physically large fuse, physically large wiring and physically large connectors. Just 1/100 ohm resistance causes 100W of heat to be wasted.

 

[benbiles]

yes, that makes sense.

10 x 36v battery packs monitored with a BMS is less parallel than 8x3.7v cell in parallel in every battery pack ?

So maybe since I need 36v min. I should go for slightly less mA hour 18650 cells and put 10 in series for each module.
that way I can get 36v 100Wh packs and fuse each one of those.

Do you know of a self resettable 36v fuse with a high 500W rating ? is there such a thing?

I can use a BMS for the parallel 36v battery packs to whatever size ( common on e bikes )

The down side is that I have to compromise and have less energy density by selecting say 3000mA 18650 3.7v lithium cells to keep under the 100Wh
maximum battery pack size for putting the batteries on a plane.

--- Updated Jan 10, 2021 ---

No, wait , that is wrong.

BMS are designed to manage batteries in series not parallel.

Lots of 18650 cells in parallel will just behave like one large cell ? It's how large lithium cells are made anyway ?
That's a guess and probably incorrect

So I need 8 x 3.7v battery packs with self resetting fuses and large connectors / wires.
The cells will be spot welded with wide nickel strip.

The BMS will manage the 10 x 8 cell battery packs in series to make 36v

How about a 12V 50A PTC Resettable Fuse?

its 600W , so would that trip at 162A at 3.7V ?


this seems to have a low resistance , in the megaohms https://media.digikey.com/pdf/Data Sheets/Tyco Electronics Raychem PDFs/MHP_Series.pdf

it has a hold of 36v 30A , so would allow 1000W discharge , maybe a bit high ? but I suppose it would work for short circuit protection do you think?

 

[BradtheRad]

1.
Your voltage and Ampere levels are in a similar range to a home power system. Devices for circuit protection appear similar. Circuit breakers, etc.

2.

for putting the batteries on a plane

Airlines (some or all) have enacted rules about transporting certain types and sizes of battery packs. It's in response to news reports of fires and explosions (especially with Li-Ion type). No one knows how these rules will change in the future.

3.
Your motor has a particular efficiency curve, depending on what is load, speed, voltage, current, etc. Have you determined what is its optimum power usage?
* At low rpm
* low ground speed
* high rpm
* high ground speed
* uphill (extreme load)
* braking
* with a lightweight rider
* or heavy rider, etc.?

Your spec 500W is reasonable since 1 horsepower is 760W. A healthy human produces 1/2 HP. With gearing (or adjustable belts and pulleys) your motor might operate more efficiently in real-world biking usage.

 

[benbiles]

A home 110v breaker is quite large. I was Looking for a self reseting fuse. I want to keep the contacts as close to the metal strips that parralel the 8 x 18650 battery's as close to the contacts that will connect to the backplane as near as possibe. So large breaker there would be impossible. I wonder if you can have 2 x pptc thermal breakers in parralel. All the pptc self resetting fuses don't allow soldering in the datasheet. I guess excess heat can damage the calibration.. But no mention of pptc socket. Maybe a simple changeable fuse as your surgest is the best way. Would need a high current low resistance fuse holder of some kind. Maybe a screw in type housing? Basically a low resistance 500W fuse in each pattern pack.

Yes I agree the airlines could totally change the resrictions at any time. I've been carrying 98Wh battery's on airlines for 20 years and do it for work. Pro camera battery's. So I'm willing to gambe as long as they are properly manufactured and have some kind of safety rating with proper label I will be OK. The fact is I want them to be safe myself!!! I think 8x parralel 18650's with fused with proper multi point contacts that can be shielded with a plastic slide over cover should be OK. At least as safe as they can be?

Now, a low resistance screw in fuse / fuse holder that can handle 3.7v 150A

--- Updated Jan 11, 2021 ---

This is the battery i use now. its 48v 13 series 3 parallel 13s3p

there isn't enough amps when the battery get 1/3 empty as its only 3 x 18650 in parallel.
I get a little bit of volt drop and BMS kicks it off.

do normal fuses have high internal resistance ?

 

[BradtheRad]

Possibly automotive blade type fuses. Available in 100 A. Cost a few dollars.

Can't get 100A? Then try two 50A in parallel. Because if one fuse goes, the other dies too due to overload.

 

[benbiles]

Possibly automotive blade type fuses. Available in 100 A. Cost a few dollars.

Can't get 100A? Then try two 50A in parallel. Because if one fuse goes, the other dies too due to overload.

View attachment 166891

--- Updated Jan 11, 2021 ---

Great solution, thanks. I should be able to find a blade socket to. I'll work out with ohms law what amps it is at 3.7v

12v 50A is probebly enough but cheap as chips to chnage

 

[KlausST]

Hi,

Do you know of a self resettable 36v fuse with a high 500W rating ? is there such a thing?

No, fuses are not rated for "W", they are rated for "I".

Klaus

 

[benbiles]

Hi,

No, fuses are not rated for "W", they are rated for "I".

Klaus

Yes I know. I assume there aren't many 3.7v 150A or 200A rated fuses out there?So I chose to discribe the fuse in W rating

Can I used a 12v 50A fuse as a 3.7v 187.5A fuse? Both 600W? Or do I have my ohms law wrong?

 

[betwixt]

Yep, your Ohms law is wrong!

The fuse does rupture by overheating but its the fuse link resistance 'R' that causes the (I^2 * R) heat that does it, not the voltage. Think of fuses only operating by current, ignore the voltage, at least for low voltage applications like this.

Brian.

 

[benbiles]

Yep, your Ohms law is wrong!

The fuse does rupture by overheating but its the fuse link resistance 'R' that causes the (I^2 * R) heat that does it, not the voltage. Think of fuses only operating by current, ignore the voltage, at least for low voltage applications like this.

Brian.

That's really good advise ! thank you !

I have a load instrument but I don't think it goes up to enough amps at 3.7v to blow the fuse.

I have to say I'm a bit confused with how I should calculate this.

If I have 36v made up from 10 x 3.7v battery packs.

If I draw 20amps at 36v from the large battery then each 3.7v battery pack in series is using 2amps or 20amps at 3.7v?

I assume the amps are divided up between the batteries in series?

20amps at 36v is not 20amp at 3.7v ( apart from when your rating a fuse ! )

If I know that number I could just say triple the Amp value and add a 12v fuse rated at the same amps?

I understand that it's the power that generates the heat but the Amps vary a lot depending on what voltage your talking about!

My best guess here is I should just use a 50A fuse ( more than double the AMPS at drawn at the 3.7V battery terminals in series )

 

[c_mitra]

If I have 36v made up from 10 x 3.7v battery packs.

These are in series so that the voltage adds up. Total voltage of the set will be 37V. Because they are in series, the same current flows through all the cells in the battery.

If you connect them in parallel, you will get a total voltage of 3.7V but the current capacity will be 10 times. Total current will be divided between the 10 cells. If you are drawing 100A, each cell contributes 10A (assuming they are identical in nature).

In case your cells are in series, you get 37V and you want 10A (total power output will be 370W), but each cell contributes 3.7V but 10A. In series connection, the voltage adds up, not the current.

 

[benbiles]

These are in series so that the voltage adds up. Total voltage of the set will be 37V. Because they are in series, the same current flows through all the cells in the battery.

If you connect them in parallel, you will get a total voltage of 3.7V but the current capacity will be 10 times. Total current will be divided between the 10 cells. If you are drawing 100A, each cell contributes 10A (assuming they are identical in nature).

In case your cells are in series, you get 37V and you want 10A (total power output will be 370W), but each cell contributes 3.7V but 10A. In series connection, the voltage adds up, not the current.

I have 8x cells in parralel to make 10 x 3.7v battery packs. Let's just see them as battery's that need some short circuit protection

Those battery packs will be in series and monitored with a BMS to make 36v nominal.

If I draw 100 amps at 36v each battery pack contributes 10 amps at 3. 6v or 10 amps at 36v?
Ohms law says there is a big difference?

Surely you have to get the Wats value to get to the 3.7v amp value? Each individual cell is providing 3.7v and not 36v so the amp equivilant changes?

--- Updated Jan 11, 2021 ---

I have 8x cells in parralel to make 10 x 3.7v battery packs. Let's just see them as battery's that need some short circuit protection

Those battery packs will be in series and monitored with a BMS to make 36v nominal.

If I draw 100 amps at 36v each battery pack contributes 10 amps at 3. 6v or 10 amps at 36v?
Ohms law says there is a big difference?

Surely you have to get the Wats value to get to the 3.7v amp value? Each individual cell is providing 3.7v and not 36v so the amp equivilant changes?

Oh wait I think your saying the same thing..

I have 8x cells in parralel to make 10 x 3.7v battery packs. Let's just see them as battery's that need some short circuit protection

Those battery packs will be in series and monitored with a BMS to make 36v nominal.

If I draw 100 amps at 36v each battery pack contributes 10 amps at 3. 6v or 10 amps at 36v?
Ohms law says there is a big difference?

Surely you have to get the Wats value to get to the 3.7v amp value? Each individual cell is providing 3.7v and not 36v so the amp equivilant changes?

Ah wait, I think your saying the same thing

 

[c_mitra]

So you are making 8P10S (or 10S8P) set (not 8P1S) battery set.

If done correctly (and that is important), then you can get 100A at 36V. The AH calculation will be 8X3400 mAh (series connection does not increase the current capacity). Not the 100AH you want.8)

When you draw 100A from this set (battery of 10S8P), each individual is giving only (100/8)A and NOT (100/10)A.

Like if you have 10S1P battery set, each cell will deliver (100/1)A.

For the 10S8P configuration, at 100A total load current, each individual cell will take (100/8)A.

You can draw a sketch and see the current values.

 

[benbiles]

sorry c_mitra. yes thanks , I tried to edit message above and it got messed up !

ebike load = 20A @ 37V 740W -> 10 series battery packs x 3.7V 20A 74W


going with betwixt I^2 * R

20A ^ 2 * 0.185ohm = 75

75 A ?

a fuse rating should be 4 x times the maximum load ? does that sound right ?

--- Updated Jan 11, 2021 ---

I'm making 10 3.7V battery packs with 1S8P 18650 lithium 3400mAh that need protection.

Then out of those battery packs I will build a 36V 10S8P that will provide at most 20A 720W

The end result is just a 36V 10S8P battery with a BMS.

The only difference being the 10 battery packs ( made from 1S8P ) can be dismantled for transit.

I will have a backplane with a BMS that the 10 battery packs slot into.

I need to have some protection for the 10 1S8P battery packs in case of short circuit.

sorry please ignore the 100A above , i totally messed up that post and can't delete it.

I think 20A @ 37V 740W -> 10 series battery packs x 3.7V 20A 74W is correct ?

So I will need a fuse rated at around say 3 times the maximum current at each 3.7 battery pack in series , say 60A rating ?