LiFePo4 Battery Management System calibration

[Avantgarder]

I purchased the LiFePo4 48v battery from Golden motor. It works perfect for me.
I read a post on the web recommending to measure and write down calibration values of the BMS so that when calibration values go off it will be easier to reach a proper calibration more easily.
Any thoughts on that ?

[Leslie]

No.  I don't understand calibration.

Maybe to test if BMS is working on all cells, or to find out which chemistry the cells are.  A LiFePo4 BMS should create resistance over the cell at 3.65v, should cut off charger if a cell reaches 3.75 -3.9v 

It depends on the charger and BMS, they should be calibrated together. 

Use a GM charger for GM BMS, or if using a different charger it should be the same volts as the GM charger.


Do not forget to switch bike down and recharge every time after using and make sure charger engages when you plug it in...

Using Lifepo4 and properly charging is better then one left in the garage and forgotten for months.  They are like cars IMO, its good to run them at least once a week and charge back up full.


I would leave it alone and enjoy the riding

[Avantgarder]

I understand from what you say that ideally BMS and charger are a pair.
The question is whether calibration can be done when I want to buy an additional charger from another vendor. One for my home and one for work.
I cannot calibrate the battery to both.
The eBay seller says "It's a 3A 58.4 charger for LiFePo4 battery".
Is that enough to make sure it fits my battery ?
No 2 chargers are the same and while the stock one reached 58.1v in my specific case the non GM one might reach 58.3v...
This is well within tolerance but would it demand the same BMS calibration ?
I'm not saying I will calibrate it since I like the approach of "If it ain't broke don't fix it" which you brought but let's talk about it theory wise.
Also note that I mentioned "calibration" in context of just "measuring calibration values to be written down when calibration values go wrong" (I quote from a post I found).

[Leslie]

It is better to calibrate charger to battery not battery to charger.

Ebay charger, You divide 58.4 into 16 cells if you have Lifepo4 = 3.65v

vs

GM charger 58.1v into 16 cells 3.63v RNDP

3.65v is .02v inny bitty tiny bit difference..

It should work just fine. 

If not, don't blame me.

[Leslie]

It is easy to short the BMS and make mistakes testing you can burn wires or pop a channel or make a loose connection which could lead to battery failure. 

It is good to have a spare BMS waiting before you start to play.

I use no BMS and just charge to 56v and balance every 2 months manually. 

My battery is two years old now and it is starting to become more out each 2 months with no BMS, but has never been damaged because the voltage is 56v. Undercharging by 5-10% is more likey than overcharge.  I never worry about BMS failing, I only worry if I get good charging or over discharging. 

Never discharge below 70% if you can help it and balance will be very good always..

I am thinking of buying a new BMS soon because battery is getting older.

[Avantgarder]

"Never discharge below 70% if you can help it and balance will be very good always.. "

I ride to work a distance of 25km up very steep mountains so I end up getting to work when battery is almost empty.
1. Does this shorten the life of the battery ?
2. How can one keep a battery charged above 70% when even nowadays cutting edge batteries do not have amazing ranges ?
3. Speaking of Battery health, is it true that the slower you charge a battery the healthier it is ?
    In other words, Is a 2A charger healthier than a 4A charger (Assuming BMS can easily handle both) ?

[Leslie]

Thats a pretty hard core ride. 

You can use pedals. 

Yes. All batteries last longer if you are easier on them, both discharge and charge. Buts a charger is not tough on a battery unless its a big charger.  Regen can be like 20 amps max this is agressive but not long charging like a charger.  Up hill discharge down hill regen it all makes heat in the battery and taking away from the electrodes..

Small batteries cost less but if you push them, they last less long.  It is almost a price comparison.

I once made a solar panel on my trailer which was 1.2 amps all the time in the sun.  I use a trailer a lot and I live on flat terrain mostly so this didnt make much difference to my stress on the battery.  I found the solar panels made better watts on the roof then charged into another battery for recharigng when I" got home rather than on a trailer maybe in shade, maybe not faced to the sun, maybe being stolen. :-|

All you can do is live with your purchase and see how it goes and tell us and we all learn from your experiences..  If this pack doesnt make you happy in life span for your uses, get a bigger pack.. 


You can tell how much your battery is flat by how much it charges.  If you have a 10 ah pack and a 2 amp charger it will take more than 5 hours 2*5 = 10 ah to charge from flat. Charging is not efficient 100% so maybe 5 1/2 - 6 hours and maybe a few to balance.

Also remember when you come home it is a lot of down hill, is it not?. Please take this into account this when considering the life span off your pack.  This is not so bad when you think of it.

As much as you ride up hill you will always ride the exact amount down hill.  This can not be cheated unless you walk or transport the bike by any other means than riding.

[Leslie]

I have a 48v 15 ah pack 720watts

I can use 2ah per 10kms if I ride very easily on flats and down hills into my town. 

On the way home I use 4ah, towing and going back up one 500 meter hill 12degs hill towing supplies in trailer..  20km round trip 6ah.

With trailer and 30kg child, riding hard with a few low grade hills,  I have reached 36km using 12ah.   tahts about as far as I would take it.

So you would be close on the mark with a 10ah pack riding up the hills.  It is a good pack.  Your pack is rated at 800 cycles to full discharge @ 1 C.  Take an estimated 100 cycles off  for 2c up hills to work one way. and you have 700 cycles. 

If youre lucky you will get 24 months weekend rests inclusive, before it should start to cut out on the way to work. 

[Avantgarder]

"I have a 48v 15 ah pack 720watts"

The precise way of putting it ... "I have a 48v x 15a x h = 720 watts x h"

V (voltage) ; I (current) ; P (power)
V x I= P
V x I*T = P*T = E (energy)
48v x 15 a*h =  720 watts*h = The energy that can be stored in your battery
=>  your battery can deliver 720w for 3 hours before it runs out.

(A common mistake is that people write   48v x 15 a/h =  720 watts/h   
because they think that     Energy = watts per hour  = watts / hour)     

All the above is Ideally speaking.
Lets say that your maximum current value is 30A but you choose to consume 60A, than your battery will heat up significantly and a lot of Energy will be wasted on heating the mosquitoes flying round the battery instead of driving the pie up hill. In that case the 720 watts*h can decrease to 500 watts*h.
I throw a value of 500 because these numbers are very hard to calculate.

[Bikemad]

"I have a 48v 15 ah pack 720watts"

your battery can deliver 720w for 3 hours before it runs out.

If that were true, Les would be a very lucky guy indeed, but unfortunately the 720Wh (watt-hours) or 259.2kJ (kilojoules) of energy stored in his 48V 15Ah pack would only last for 1 hour if it supplies 720 watts continuously.

However it would last for 3 hours if the continuous current was equivalent to 1/3 of the battery's Ah (ampere-hour) rating .

15Ah / 3h = 5A,  48V x 5A = 240W.

So ideally speaking, the pack can supply 240 watts continuous for 3 hours, or 720 watts continuous for 1 hour, but it cannot supply 720 watts continuous for 3 hours!

How can one keep a battery charged above 70% when even nowadays cutting edge batteries do not have amazing ranges ?

Using a larger capacity battery is the obvious answer. If your 10Ah pack is being drained nearly 100%, then a 15Ah pack would only be 66% drained after the same journey.

Alan
 

[Avantgarder]

Alan,

1. My next battery will be bigger than 10AH, I can tell you for sure...
2. You are 100% correct.  The  "3 hours" thing was a typo. Here is my post again :

The precise way of putting it ... "I have a 48v x 15a x h = 720 watts x h"

V (voltage) ; I (current) ; P (power)
V x I= P
V x I*T = P*T = E (energy)
48v x 15 a*h =  720 watts*h = The energy that can be stored in your battery
=>  your battery can deliver 720w for 1 hour before it runs out.

(A common mistake is that people write   48v x 15 a/h =  720 watts/h   
because they think that     Energy = watts per hour  = watts / hour)     

All the above is Ideally speaking.
Lets say that your maximum current value is 30A but you choose to consume 60A, than your battery will heat up significantly and a lot of Energy will be wasted on heating the mosquitoes flying round the battery instead of driving the pie up hill. In that case the 720 watts*h can decrease to 500 watts*h.
I throw a value of 500 because these numbers are very hard to calculate.

[Leslie]

How does one survive without one of those doovey CA's?

So I measred the current going into my battery.  I get 1.5 amp on a 2 amp charger, lower current helps a more even charge with out the BMS. The lower current is a result of adjusting the voltage my charger down to 56v. 

I assume the charger has more stable reading to estimate how much energy was consumed by measuing how much it need to charge. Cells only go up by .3v to charge usually the way I charge.

If I say get a 3 hours on the charger @ 1.5 amps @ 54v *80% (efficiency losses)

54*1.5*3*80%=194.4 watts

Divid this by charging voltage of 54v, I can estimate that I used .3.59 ah. 

Today my LifePo4 charged for about 3 hours from doing 10km round trip, and hard run home with the trailer.  Works out I could do this 4.1 times on my 15ah pack =41kms.  Which is about spot on to what the CA used to tell me.

I also believe my pack was rated at 15ah @ 1C.