Why are there no Battery Management Systems that don't stress LiFePO4?

[StadsImker]

Hello,

I am a bit an novice in electronics. Build my first electric bike, a very special one for sure. It a Transport Tricycle for quite a lot of cargo...

I follow the Magic Pie for a longtime, several years... perhaps 7 or 8 and have my own now a little longer than a year. Made my own LiFePO4 battery all by myself... and before learned a lot about what LiFePO4 cells are like.

But I don't understand the filosophy of the companies who develop BMS's at all ! Why?

If I look at a few thing like charge and discharge curves, life cycles and expexted life time of LiFePO4 systems. I think a lot can be changed and also a lot of totally BAD equipment is sold on the market.

In my opinion LiFePO4 cells are charged much to full. Why? Because charging to 3,65 volt gives a LiFePO4 cell stress and stress is NOT good at all for any battery systems. But I see this stress all over the industrie. Even at smartphones, laptop and also the ebike or EV's branches.

In my opinion a Battery pack should be balanced at 3,45 volt and not higher. Because after this voltage the stress get's bigger and bigger.... But manufactors keep building BMS which load battery pack until 3,60 volt per cell. ANd in my opinion it's plain stupid. Even more stupid than a kid you drains his battery pack completly because he loves the ride to last longer. A manufactor of a BMS can symply choose to balance a lower voltages will give his customer a much longer lifetime on it's battery-pack.

Yes, it is true that the pack is full at 3,60 or 3,65 volt. But there is so little more Amps then 3,50 volts. It will drain in a very shorttime. True your e-bike or e-car will have more power for this short time.

I bought all my Battery stuff on various sellers and my two BMS's I bought them on Aliexpress. I pick good ones for LiFePO4, but on Aliexpress you can easely buy the WRONG one... Which charge to much too high voltages, like 3,7v or even 3,9 volts... It's al most criminal and your battery pack will not life a longtime.

I whis I could buy a BMS with lower limmits to keep my battery out of the stress zone of fullest charging. I would like a gentle 3,45 or 3,50 fully charged state. Because my battery will last twice as long. Instead of 12 years (2000 cycles) perhaps 20 years (4000 cycles)

Ofcourse I bought two very large packs of Headway 40152. Both 24 volts 30A (8S2P) and mostly recharge them well before 3,21 volt. Even 3,28 volts is more common... I like it this way, gives me more cycles and no mental stress for me, because I need to come home all the time. My bike weights more than 300 kg with me... And that's not nice to push and I can't leave it on the street.

Hope to hear about what you guys are thinking of my not completly full charged battery filosophy!

Greatings,
Philip  - from the Netherlands (you know the guys who like to be SECOND.. LOL)

[Bikemad]

Hi Philip,

The following table from Powerstream.com shows some interesting results for variations from the typical maximum cell charging voltage:



At a maximum charging voltage of 3.4V per cell there was only a minor loss in capacity of between 1% and 4% compared with the 3.6V per cell for the four different types of 18650 LiFePO₄ cells tested.

3.4V per cell seems to be a reasonable figure to use that can provide near full battery capacity combined with an extended lifetime.

If you regularly use an adjustable balance charger, I would definitely recommend setting the maximum cell charging voltage to 3.4V instead of 3.6V or 3.65V typically used for most LiFePO₄ packs.
Unfortunately, most BMS equipped batteries cannot be changed. 

If you have a vector controller, and don't mind losing a little bit more of your battery's maximum capacity, then I would suggest setting the Undervoltage protection value (V) to 2.8V or even 3V per cell (i.e. 44.8V or 48V for the 16 cell 48V battery) to extend the lifespan of your battery pack even more.

Alan
 

[StadsImker]

Thanks Alan for your reaction!

I think it's a stupid thing of the battery industry. Really a missed chance and even a bit reckless.

But also the electronic designers make these BIG mistakes. Who wants a celphone which quits after 3 to 5 hours heavy use... Nobody! We all buy them because there are no smartphones availeble with larger battery pack. Yes some models show more MAh, but they also use more power.. Bigger and brighter screens, faster processors etc. So they still only hold 3 to 5 hours (if you are lucky) energy. ANd these designers are so stupid because they ignore the fact that a day has 24 hrs and no user will shut of it's phone when the power is 30% or even below that. And that gives battery-cells SO MUCH STRESSssss.... I will kill those nice (stupid) Smartphones.... Don't they care at all? I think a Smartphone with a three day battery life will make others unsaleble. Ofcourse you can use a battery-pack for charging, but than your micro-USB charge-port will be broken in one month... How stupid!

And if you take the time to learn about battery cells and energy. You quickly find out that stress will kill your battery quickly. Stress for batteries is Quick charging (high Qurent), Too full charging, too low discharging and ofcourse small capacity battery, than the user will kill it!... Are these designers total idiots? I question.

Also simple bike led-lights which use AAA batteries... Why? A normal AA penlite pack double the power and means more light time for less... But AA penlite size bike Led-lights are hard to find.

We all live on the same planet, so why not make things last?

You don't need to reply on this... But I think it's stupid. And I get angry and frustrated when LiFePO4 resellers sell BMS with limmits unfit for LiFePO4 to kids and people who make there first Ebike. 3,7volt or 3,9 or even 4,2 volt cell limmits I saw. It's almost criminal. So people watch out for this! It should be max. 3,60, but better less. I you look hard you can find  balancing voltage of 3,5 volt. These are perfect for LiFePO4

But more and more I am thinking of charging without BMS... and bring the charger voltage down to 27,6 volt for an 8 cell (24 volt LiFePO4 pack) because 27,6 /8 = 3,45 a cell if all are the same and in balance and If they are not fully balanced... There is a bit of safety margen.


exmple (27,6 volt) :
                    Balanced                     un-balanced (but still save and no-problem)
Cell 1            3,45v                              3,41v
Cell 2            3,45v                              3,42v
Cell 3            3,45v                              3,43v
Cell 4            3,45v                              3,45v
Cell 5            3,45v                              3,52v
Cell 6            3,45v                              3,44v
Cell 7            3,45v                              3,45v
Cell 8            3,45v                              3,40v

Pack total     27,6 volt                        27,6 volt

Aslong the un-balanced cell don't exceed 3,65 volt notthing will go wrong. In my opinion. But I don't know if this will stay the same for a longtime. Or do I need to check cell voltages every day or charge.... Has anybody any experiance with this?

I think this could be more save than relying on a BMS which can mall function.

Note: For sure, you should not do or try this when you charge a LiFePO4 (24v) pack with 29,2 volts! Any un-balance and one or more cell will get damage!

Greatings,
Philip