[evnz]

bms can lithium be used with out them and whats there job
Thanks
Owen

[Ziggythewiz]

Depends on the type of lithium. Some really need it to not go fireball, others, like LiFePo4 do not need it with proper care.

A BMS monitors every cell in the pack to ensure none go under or over safe voltages. It may do this by tripping alarms or cutting the pack to prevent damage, or may shunt cells to prevent overcharge, or remove power from higher cells to bring up lower ones.

[dougingraham]

Originally Posted by evnz

bms can lithium be used with out them and whats there job
Thanks
Owen

LiFePo4 does not strictly speaking need a BMS. And if someone chooses to not use one then they are the BMS and really do need to understand what is going on. Of course you can argue that having a BMS without any understanding of what it is telling you is also dangerous.

The job of a BMS depends on what it was designed to do. There are several different types. Since I am in the camp that believes they are not necessary for LiFePo4 type installations I am not the person to ask about any particular devices. Basically they can do the following:

1) monitor the voltage of every cell during charge and indicate full charge or an error if one cell becomes overcharged. Optionally they can shunt away some current on the higher cells to top balance the pack.

2) monitor the voltage of every cell during discharge and indicate a low voltage condition of a cell to the driver or the controller so as to prevent that weak cell from being ruined.

3) monitor the temperature of each cell and indicate a fault if out of temperature range for charging or discharging.

I hope that answered your question.

[danh818]

Great explanation Doug.

[evnz]

Thank for the replys so main thing i get out of this is as i have less knowlage i will be best to have them so i dont cook them specaily when they cost a 1/3 of my yearly pay
Thank again
Owen

Ps what type bypass the charged cell for the low cells?

[onegreenev]

1) monitor the voltage of every cell during charge and indicate full charge or an error if one cell becomes overcharged. Optionally they can shunt away some current on the higher cells to top balance the pack.

Why top balance when you never drive full. Its safer to have all your cells balanced at the bottom where the danger lies.

2) monitor the voltage of every cell during discharge and indicate a low voltage condition of a cell to the driver or the controller so as to prevent that weak cell from being ruined.

This implies that the driver WILL HEED the warning signal and actually stop. What really happens is that the driver sees a strong voltage but a cell or two may be low. Being a strong overall voltage you keep driving because you KNOW you really have more to use. It will kill a cell. Ask me how I know. Top balanced is just wrong because you always drive less than full. Bottom balance and don't charge so high. That way your rear is covered. You know, cover your ass.

3) monitor the temperature of each cell and indicate a fault if out of temperature range for charging or discharging.

Most BMS systems do not have this function. It is a GOOD function if you have cells that require a temp monitoring because they are not really safe to use. LiPO should use a thermal BMS. But you still need to bottom balance and you need a well matched pack to begin with.

Be safe. How do you decide which of the myriad of BMS systems you should use if any? I still have no clue. I still don't trust them. My cells were damaged by a factory BMS system that failed. I have many that were ruined because of a large failure. Failure of a BMS is a REAL issue.

[onegreenev]

Originally Posted by evnz

Thank for the replys so main thing i get out of this is as i have less knowlage i will be best to have them so i dont cook them specaily when they cost a 1/3 of my yearly pay
Thank again
Owen

Ps what type bypass the charged cell for the low cells?

Before you go out and buy a system you need to do a lot more research. There are plenty of us that do not use a BMS and have done so very successfully. There are a few things you need to know for it to work well and safely but its pretty simple. Really.

How are you going to decide if you do decide to use one? Truly how will you KNOW you have chosen the RIGHT one? Really.

[eva-michael]

Originally Posted by evnz

Thank for the replys so main thing i get out of this is as i have less knowlage i will be best to have them so i dont cook them specaily when they cost a 1/3 of my yearly pay
Thank again
Owen

Ps what type bypass the charged cell for the low cells?

I will recommand you to use a battery system with battery management system. The most important thing is low voltage protection. Battery cell should be cut off at no less than 2.0V/cell and 2.4V/cell in everage for battery pack. Otherwise, it may cause demage to cell if cell voltage comes too low without stop.

[dougingraham]

Originally Posted by onegreenev

Why top balance when you never drive full. Its safer to have all your cells balanced at the bottom where the danger lies.

I agree. But it is not practical for a BMS to bottom balance a pack and since one of the goals is to protect a cell from overcharge this can assist with that. What the BMS maker wont tell you is that you only need to do this once and it will stay that way unless the BMS is badly designed and unbalances the pack.

This implies that the driver WILL HEED the warning signal and actually stop.

Yes. In particular the Wife/Girlfriend/Daughter is just going to ignore a warning indicator as long as the car seems to be driving ok. I think you would need to put the car into some sort of terrible limp mode. Maybe limit the top speed to 15mph. Even at that I suspect they would drive the battery to death.

[dougingraham]

Originally Posted by evnz

Thank for the replys so main thing i get out of this is as i have less knowlage i will be best to have them so i dont cook them specaily when they cost a 1/3 of my yearly pay

The problem with a BMS is all the added complexity. If you dont understand what it is doing I don't see how it can help you. And if you do understand it then you dont need it.

LiFePo4 is pretty simple to deal with.

1) Bottom balance the cells when you install them. This more than anything else this will prevent you from driving a pack to death. What happens is you are driving along and the battery goes dead. Pretty much every cell at the same time. The car stops. You tow the car home or to a charger and charge it up. No damage. What kills these cells is reverse charging them. A weak cell that goes to zero volts will be reverse charged by the rest of the pack if you keep driving. Bottom balance will cause all the cells to dump at the same time. The car stops because there simply isn't enough voltage to make it go.

2) Deliberately undercharge the pack. All you need to accomplish this is a charger that can turn off when a particular voltage is reached. The first time you charge the pack after the bottom balance you find the weakest cell. You can do this when the pack is nearing full by locating the cell with the highest voltage. Once you have located the weakest cell you monitor that cell until it reaches something a little over 3.5 volts and then read the total pack voltage and set the charger to stop at that total pack voltage. Done!

3) Make certain that there are no connections to the pack except at the endpoints. If there are other connections then these connections are a potential source of pack imbalance.

4) Try not to overdischarge the pack. The pack will last longer if you don't do deep discharges very often. Generally no deeper than 80% is considered a good idea for long life and 70% is better.

5) Avoid extreme discharge currents. Most of the cells can tolerate a continuous 3C discharge and a burst discharge of 8C. If you are going to exceed these levels on a regular basis then you probably need a larger pack.