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WHY USING A CHEAP GENERIC BMS IS A REALLY BAD IDEA!

MANY of us want to just fit a simple plug and play lithium battery, straight into our powerchairs. But Lithium's are not quite so simple and form a part of a SYSTEM. They are not just a battery in the same way than a lead brick square conventional powerchair battery is. Why?


 BECAUSE, LITHIUM CELLS NEED SOME ACCURATE CARE, THAT A GENERIC BMS WILL NOT GIVE YOU!

LiFePO4 cells promise better range since you can fit more Ah in the same battery space as was there before. If you cannot do that, & increase Ah available when changing to lithium, then there's really very little point in fitting lithium batteries at all. Because you have completely missed out on the main advantage!

They CAN also offer a much better service life. If treated correctly. Or a very short life if not!

This effectively rules out a generic BMS! Its not for nothing these things are called "Battery Murdering Systems" by those that understand this stuff!

Your new Lithium Ion Phosphate (LiFePO4) battery will serve you well, be trouble free, and give a long service life  ONLY if the charge and discharge details in this page is observed very carefully! Lithium batteries die fast when incorrectly treated.

There are many types of lithium battery but only one is suitable for powerchair use. For reasons of service life and safety. That is the LiFePO4 cells. Other lithium's either burn, or have shorter service lives and so are not discussed here. And these LiFe batteries (Lithium Ion Phosphate, LiFePO4 cells) need specialist care in use. And especially so during charging.

Generic BMS setups are also inside the monobloc so called LEAD BRICK replacement batteries, or are fitted to a self assembled lithium pack. This is a cheap and cheerful way of letting the ignorant use lithium batteries without knowing much. It helps sell them to the masses. And is the single biggest cause of breakdowns, failed cells, short service life, balance and charging problems and in extreme cases fires. They are frankly terrible things. The way they work is fundamentally flawed in both charge and discharge modes.

Your new expensive battery pack or cell(s), will soon expire if you do any of the following things to them:

1. charge any cell to above the 3.60 volt maximum. 3.55V is fine. They will live a long life. 3.65v is going to shorten cells life quite drastically. During charge, the charger or charging / cell balance system should NEVER allow an individual cell to exceed 3.60 volts on a LiFe cell for long service life. A typical generic BMS setup does this repeatedly for hours or days at a time.

Heres a chart (Lithium Polymer in this case, so 4.20V charge point is the correct level) showing the drastic affect of too high of a charge voltage on Lithium cells. Check out CYCLES! 

Add an extra .15 of a volt and lose MANY cycles.  This over voltage affect applies to all lithium cell chemistries. Only the correct charge voltage is different with different chemistries. If you take a LiFe (LiFePO4) cell above its max 3.60 volt point you are slowly murdering it. There is no need for discussion here! Its well known by cell manufacturers and experts and even hobbyists that use lots of lithium batteries.  

LiPo cell example:

2. discharge below the 100 percent Ah point. You can do this easily.  (100% is typically safe for 1500 cycles).

3. discharge at too high a rate (too many Amps). Use a LARGE pack, and one with a high C rate rather than a BMS.

4. Store FULLY CHARGED. This kills cells too.

Now avoid the problems above and your CALB, HEADWAY, or A123 or other LiFePO4 cells will live a long and healthy life. With this in mind lets look at the typical BMS specifications below!


 

 
 
 
THE TYPICAL GENERIC BMS SPECIFICATIONS CHART BELOW, FROM WWW.EVASSEMBLE.COM

 

The way they work is fundamentally flawed at least during charging. The CHARGER should be balancing the cells and controlling power proportionally in controlled fashion - not a BMS trying to control it afterwards!

 


  

 

 MY OWN COMMENTS ADDED IN RED

Specification

1. Single Cell Over Charge Protection Voltage

3.75V±50mV  

Cell life drastically reduced any time voltage exceeds 3.60v, and so this is both too high, and ±50mV is nowhere near accurate enough anyway.

In fact they are FULLY CHARGED at around 3.50v if the charge continues until current is very low and you are patient.

The extra voltage used and even suggested by some manufacturers is 'allowed' only because it helps balance the cells faster. Since its way above fully charged resting voltage. It compensates for a weak balance circuit by dragging up low cells and ignoring the overcharged ones. 

When the real solution would be a more powerful balancing circuit than this BMS's minute 100mA setup provides and the ability to "throttle" the charger to a low Amp level that is low enough to prevent ANY overcharge. Exactly as every hobby charger already does. And as I use to charge my own 70Ah 45V lithium battery pack.

As the battery gets to the point where the highest cells are full, right up to this too high voltage, the charger is cut off. Time and time again while the weak balance circuits try to reduce the voltage of the high cells, until balanced. (When the lower cells eventually catch up.)

This shortens the cells service life as it is repeatedly charged from a lower voltage to 3.75 (plus 50mv!) so up to 3.8 volts!

2.Battery Pack Over Charge Protection Voltage

30V±0.3V

TOO HIGH. This should be 3.60 volts, per cell MAX! So that's 28.80v only. Any higher is bad. At this point, the battery charger is totally disconnected...

Too late, too high and on/off/on/off repeat. Regardless of cell balance or individual cell voltage being under or over. It has to do this as the BMS cannot control the charger output level. Only on/off...

3.Battery Pack Over Charge Release Voltage

28.4V±0.3V

TOO LOW... This is the point where it reconnects the charger again... Should hold steady at 28.8v

4.Single Cell Over Discharge Protection Voltage

2.10V±80mV

Under light loads this may be far too low. Under heavy loads it may not be low enough. And without knowing cell count, cell capacity, cell C rate, cell internal resistance, and maximum load in amps it is simply impossible to guess at this figure.

It will allow a small motor to run the batteries too low, and will likely cut off power as you try to accelerate hard or climb a curb in a powerchair. This NEEDS to be logic controlled and programmable, as it already is inside our powerchairs motor controller.

Or it will get in the way and also not do what it was intended. I.e.. to save the cell... And again ±80mV is not accurate enough to be useful anyway.

This will cut in in error as you attempt to say climb a ramp on to a train. And you stop!

5.Battery Pack Over Discharge Protection Voltage

19.2V±60mV

As above, but for complete pack. Again, this is more of a nuisance that will stop you under a heavy load, while allowing you to discharge the battery too deeply on a light load. Should be handled using logic via the controller.

6.Battery Pack Over Charge/Discharge Protection Release Condition

Disconnect Load to Recover into Normal State

It reconnects power when you disconnect the charger... Or stop trying to climb a ramp.

 

7.Maximal Continuous Discharging Current

30A

A typical hi end powerchair can be 2x 120Amps so this needs to reliably do 240Amps.

So needs to be a 300 Amp capable device. This is WAY too low!!! it will cut off power or turn to smoke. 

8.Maximal Pulse Discharging Current

45A

As above. My Roboteq controller can take 500Amps peak. A typical powerchair controller needs 240 Amps.

9.Maximal Charging Current

30A

Adequate for slow charging. Some are less! Beware, they will die or cut off the charger.

10.Battery Pack Over Discharge Current Protection Detection Current

120A±4A

Too low. by about 100%

11.Battery Pack Over Charge Protection Detection Maximal Delay Time

3seconds

How long it takes to figure out you are charging at more than 30 amps, and disconnect your battery!  For what its worth the lithium battery in my own powerchair is safe to charge at 5C or 5x its capacity. So many hundreds of amps.

12.Battery Pack Over Discharge Protection Detection Maximal Delay Time

1second

Approx how long you get trying to climb a ramp before power gets cut off!

13.Battery Pack Over Discharge Current Protection Detection Maximal Delay Time

20milliseconds

Approx how long you get trying to climb a ramp before power gets cut off!

14.Balancing System Working Static State Consume Current

200uA(The voltage of each cells is 3.2V above.)

The amount of current consumed for no purpose all day long!

While it incorrectly tries to balance cells during use. This will cause big problems when you come to recharge.

15.Balancing System Working Static State Consume Current

20uA（The voltage of each cells is 2.0V below.）

After its sent them all dead flat, at well under the safe cell voltage level it then sucks out less power.

16.Impedance

0.1mΩ(The voltage of each cells is 3.2V above.)

This increases the battery total resistance.

17.Dimensions

(Length)95X(Width)60X(Thickness)18mm

18.Weight

100.0g

19.Apply to

8 LiFePO4 Cells in Series

21.Installation method

Double Faced Adhesive Tape or screw

22.The voltage of each cells equalization precision

±5mV---±40mV(The coherence of each cells capacity between±3%0,0.2C discharge testing method)

NOT REMOTELY ACCURATE ENOUGH!
23.The maximal temperature ascent of balancing system module Less than 15℃

Feature

Advanced capacitor balancing technique which is working at charging, discharging and static state all the time. It balances cells by transferring energy from higher cell to lower cell (by nominal 100mA). Save energy and reduce heat emission in balance process.

SEE BELOW! A seriously bad feature...
 

Not shown here directly is that the balance current is TINY at just 100mA That's simply not adequate.

And that this allows the cells to go way over voltage time and time again as the state of charge reaches the end. This can go on for days if the pack is out of balance. Every time, the high cells get pushed to an over voltage life shortening state, and are then released again. On/Off repeat...

And it WILL BE out of balance too, because these damned BMS things keep on trying to balance the cells during the whole usage cycle. The problem with this, is that while all cells may have the same voltage (3.60 volts) when fully charged, at the half way point when every cell has had the same Ah removed during use, this is no longer the case. The voltage drop in different cells not linear and different cells vary markedly. This is correct! On recharge they will all end up full at the same time. But the stupid BMS continually tries to "balance" all day long and actually UNBALANCES the pack for many hours at a time. This then means it need RE BALANCING again on subsequent recharge. So hours of On/Off/On/Off Overcharge the full cells then cut off again and again. Get ready to throw away your valuable battery.

In a correctly implemented system you do NOT NEED OR WANT a BMS setup. You need a BETTER more accurate charge system, and a means of knowing when to stop discharging them with an intelligent system. and maximum current limit should be programmed into the controller. Not the BMS!

And a set of cells, (the battery) should be selected so that its C rate is adequate with plenty of spare headroom, for the maximum Amp requirement of the motor/controller. In this way the BMS is no longer required.

As I do here

The typical BMS is really just intended as a means to sell Lithium batteries to dummies as a kind of one size fits all method of "using" a lithium battery. But its a dumb badly engineered solution!
 

 

 

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