How to calculate safe continuous discharge rate for ebike battery?

[guerney]

The seller of my battery says that the BMS on my LG celled battery is set to 25A (safety cutoff), but for some reason refuses to tell me what it's maximum safe continuous discharge rate is. Is there a handy chart somewhere citing standard cells from Samsung, LG, Sanyo, Panasonic etc. and their safe continuous discharge rates, for various ebike battery package configurations? If not, how can I work it out?

 

[Nealh]

It depends on the BMS rating and the specific cell specs used as each brand has varying cells of mah and current rating.

Can you provide a battery link ?

 

[guerney]

It depends on the BMS rating and the specific cell specs used as each brand has varying cells of mah and current rating.

Can you provide a battery link ?

It's the battery in this kit, from this seller:

Bafang BBS01B 36V 250W Mid Drive Electric Bike Motor Ebike Conversion Kit Mid Motor for Mountain Bike Road Bike : Amazon.co.uk: Sports & Outdoors

Shop Bafang BBS01B 36V 250W Mid Drive Electric Bike Motor Ebike Conversion Kit Mid Motor for Mountain Bike Road Bike. Free delivery and returns on all eligible orders.

www.amazon.co.uk

I've got the LG 19.2ah version. One of the images on Amazon cites these details:

19.2ah: LG 3200mAh(60pcs)

Spitzenstrom: Yes

BMS-Schutz: Yes

Maximaler Schutzstrom: 30A


...further down the page, its says:

19.2 Ah(3200mAh-13S5P)

 

[Nealh]

Cells used are the 10a MH1 18650.

LG MH1 10A 3200mAh 18650 Bench Test Results...safe at 10A but suffers damage | E-Cigarette Forum (e-cigarette-forum.com)

Without knowing the BMS specs one can only speculate that the continuous rating will be 20a with a slightly peak current of 30a.

As per Mooch's testing above the power/current draw should be less to preserve life expectancy.

 

[guerney]

Cells used are the 10a MH1 18650.

LG MH1 10A 3200mAh 18650 Bench Test Results...safe at 10A but suffers damage | E-Cigarette Forum (e-cigarette-forum.com)

Without knowing the BMS specs one can only speculate that the continuous rating will be 20a with a slightly peak current of 30a.

As per Mooch's testing above the power/current draw should be less to preserve life expectancy.

Thanks nealh, that's very interesting. Would 20A be a safe continuous discharge rate for this battery pack, while not reducing life expectancy? Mooch said "I recommend 7A max to ensure decent cycle life." - How do I translate that, to apply to the 13S5P pack?

 

[Andy-Mat]

The seller of my battery says that the BMS on my LG celled battery is set to 25A (safety cutoff), but for some reason refuses to tell me what it's maximum safe continuous discharge rate is. Is there a handy chart somewhere citing standard cells from Samsung, LG, Sanyo, Panasonic etc. and their safe continuous discharge rates, for various ebike battery package configurations? If not, how can I work it out?

You have had some answers, but I believe that nobody mentioned that any battery needs cooling.
More cooling with higher discharge rates of course.
Hopefully there is a temperature sensor installed, that is doing its job correctly....maybe even a small cooling fan for such high discharge rates?
regards
Andy

 

[guerney]

You have had some answers, but I believe that nobody mentioned that any battery needs cooling.
More cooling with higher discharge rates of course.
Hopefully there is a temperature sensor installed, that is doing its job correctly....maybe even a small cooling fan for such high discharge rates?
regards
Andy

There's no fan installed - my controller is set to draw 15A at the moment. The battery case only feels warm to the touch, on the very rare occasions when it's running low on charge.

 

[Andy-Mat]

There's no fan installed - my controller is set to draw 15A at the moment. It only feels warm to the touch when it's running low on charge.

Without looking again, but the OP was talking about 25 amps, and possibly more....That is a huge difference to your battery, the best part of double!
How the heat finds its way within each cell to the outside, can vary with chemistry and physical design too...This needs to be taken into account.
regards
Andy

 

[guerney]

Without looking again, but the OP was talking about 25 amps, and possibly more....That is a huge difference to your battery, the best part of double!
How the heat finds its way within each cell to the outside, can vary with chemistry and physical design too...This needs to be taken into account.
regards
Andy

They might not even be genuine LG cells.

Tesla batteries use Glycol pumped through tubes, to remove heat:

Why do Tesla batteries not overheat? Tesla's battery cooling system. - Easy Electric Cars

When batteries are used, they get hot. Sometimes, safety measures can fail and people can be harmed. Here's how Tesla engineers mitigated that issue:

www.easyelectriccars.com

 

[guerney]

How the heat finds its way within each cell to the outside, can vary with chemistry and physical design too...This needs to be taken into account.
regards
Andy

Good point! To keep the battery case waterproof, and only if it was a serious problem - I'd be more inclined to try drilling copper rods through the casing, connecting to an external heatsink over the battery case, then rely on passive air cooling while in motion.

 

[guerney]

Without looking again, but the OP was talking about 25 amps, and possibly more....That is a huge difference to your battery, the best part of double!
How the heat finds its way within each cell to the outside, can vary with chemistry and physical design too...This needs to be taken into account.
regards
Andy

Or use a metal battery case, to act as a heatsink, and fill that with oil - like they do with some servers:

What Is Immersion Cooling? | Liquid Immersion Cooling | Submer

Discover the science behind immersion cooling, how it differs from water cooling, and the different types that exist.

submer.com

 

[Andy-Mat]

They might not even be genuine LG cells.

Tesla batteries use Glycol pumped through tubes, to remove heat:

Why do Tesla batteries not overheat? Tesla's battery cooling system. - Easy Electric Cars

When batteries are used, they get hot. Sometimes, safety measures can fail and people can be harmed. Here's how Tesla engineers mitigated that issue:

www.easyelectriccars.com

WOW!
I wonder what discharge rate they can achieve without overheating!
Andy

 

[guerney]

WOW!
I wonder what discharge rate they can achieve without overheating!
Andy

"225 amps continuous"

Tesla 5.3 kWh Module (85 kWh Pack) | Batteries | Stealth EV

Shop the Tesla 5.3 kWh Module (85 kWh Pack) and learn more about batteries and other products by Stealth EV. Click here to learn more...

stealthev.com

 

[Andy-Mat]

"225 amps continuous"

Tesla 5.3 kWh Module (85 kWh Pack) | Batteries | Stealth EV

Shop the Tesla 5.3 kWh Module (85 kWh Pack) and learn more about batteries and other products by Stealth EV. Click here to learn more...

stealthev.com

Many thanks.
Andy

 

[Nealh]

Thanks nealh, that's very interesting. Would 20A be a safe continuous discharge rate for this battery pack, while not reducing life expectancy? Mooch said "I recommend 7A max to ensure decent cycle life." - How do I translate that, to apply to the 13S5P pack?

5P means five cells are in each parallel string/group, so even with a reduced max 7a per cell the battery can give a peak 35a.

This graph is for MH1 (single cell) if you intersect 3.3v about the min to go with the 7a discharge line one will see that the cell only can deliver approx. 1400mah out of the 3200 mah rated cell, if one took it to 3.2v then one would get approx. 1750 mah.


For 5p and 3a per cell or a 15a total current draw for the battery then things look much better.
Down to 3.3v one would get approx. 2400mah from the cells or 2600mah down to 3.2v

 

[Nealh]

This danish site give's lot of info on batteries tested independently
Battery test-review 18650 comparator (lygte-info.dk)