Battery cuts out

[oldie1982]

I have a small 48v 7ah battery. A KT 15 amp controller.

I normally use assist level 2 and level 5 for a 10% hill that is about 300 metres long.

I had done 16 miles in total and the battery voltage was showing around 44-46v on the lcd8.

As I got on the 10% hill and switched to assist level 5 the battery icon would start flashing on the lcd and the motor would cut out. I dropped down to assist level 3 and the problem went away.

Why would this happen?

Do I have a bad battery or is this because the battery did not have sufficient charge to supply the power the controller/motor demanded?

This never happens when the battery is fully charged. Shows around 54.4 when fully changed on the lcd.

 

[sjpt]

When you draw big current from the battery that will cause the voltage to drop. If the battery is fully charged, the drop may not be that important; e.g. the voltage may drop from 54v to 48v (*). However, as the battery discharges the no-current voltage drops , down to perhaps 42v when 'empty'. The further drop from big uphill level 5 current is sufficient to drop the voltage below the 'safe' level (low voltage cutoff) at which the BMS cuts off power to protect the battery from permanent damage. If it's already at 46v a drop of 6v might drop it to an unsafe (for the battery) 40v. If you reduce current draw by using level 3 then it will perhaps drop 4v to 42v and just keep going.

I fast reacting display would show you the voltage drop as it happens. Your display may be smoothing the displayed battery voltage to make it less jittery and easier to read, so you cut out before seeing the drop. See how it is reading when you take the 10% hill with the 44-46v level of charge on assist level 3.

The better quality the battery cells and the newer the battery is, the less the voltage sag will be.

Of course, it tends to always happen when you and the battery are both tired near the end of a long ride, and you hit that last killer hill.

(*) not precise numbers, just for illustration

 

[Benjahmin]

This happens when a high power demand put on the battery that it is unable to provide. The large current draw causes the voltage to drop suddenly (voltage sag) then the bms (battery management system) goes into low voltage cutoff (lvc) to protect the battery and it's cells.
Causes:
Could be that the battery is reaching the end of it's life. How old is it?
Could be that the controller maximum current (which will be called for in setting 5, uphill) is larger than the battery maximum current rating. In effect a design mismatch.

Either way your battery is not up to what's being asked of it and the system is working as it should to protact it.

Post crossed with sjpt. Obviously he can type faster than me !

 

[oldie1982]

Its a new battery from unit pack power using Sanyo 3500 cells.

Thanks for explaining this, my weight of 95kg plus the 10% hill when the battery is less than half probably does not help matters.

Its a small KT 15amp controller and in the settings of the lcd its set as max current available or something.

 

[Nealh]

Voltage sag and the fact the battery is only 2 cells in parallel, at 44/46v the battery was pretty much depleted and only at a max charge 30/32 % capacity. The cells would have been stressed at max current demand and the two cells in parallel unable to deliver the current asked.
It depends which Sanyo cells and whether they are genuine A cells, or inferior cells or even knock off fake cells.
If depends if the cells are Sanyo BF, BL or GA, even the GA will will be very stressed at 15a esp as only 2 cells in parallel.

One will have to use the battery more discerningly with hills involved and try not to use PAS4 or 5 once voltage is at 48v or less. Try the same hill with a fully charged battery and note how much the battery sags and the lowest volatge the display shows, then you will know how much the cells sag unde rmax load. The volatge shown will be the lowest one will ever want to attempt a similar hill at without lvc coming in to play.

 

[Nealh]

This gives an idication of the stress /voltage sag applied to a cell at any given current load.
From the bottom left the Blue line shows how the voltage sags at a 7a loading the asking current form each cell.
Your BMS lvc will be likely around 3.2v or 3.3v.
Assist 2 level is 20% current so only 3a draw, so each cell will be asked for 1.5a so somewhere in between the dark Blue and lIght Blue line from the top of the chart.

Assist level 3 is 33% current demand is approx. 10a draw so 5a per cell.

With your battery SOC at 44/46v the cells will already be sitting at only 3.5v each so have little or no room to sag very far before hitting lvc at a 7/7.5a loading.




Having only two cells in parallel and only 26 cells altogether, the battery isn't suitable for use on steep hills once the voltgae has reduced significantly.
One may well get 30 odd miles usage form the battery in low power mode but not in high power mode.
As with all batteries one has to balance the usage, either opt for low power and eke out the max capacity and range or opr for higher power and lower range, being mindful that as the voltage decreases the battery won't be able to deliver high power later on in the ride.

 

[Nealh]

Using the graph above if one used max power PAS 5 all the time, the battery would hit lvc at about the half hour mark of use and in Pas 2 one would hit lvc after some 3 hrs.
There is nothing wrong with your battery and it is performing as it should, the only down fall is it is only has two cell in parallel as it is a lighter weight battery and low capacity. Most batteries will have 4 or 5 cells in P and have double the capacity and power delivery and be able to better handle steep hills after some usage.

 

[oldie1982]

I really appreciate the detailed responses. This give me a much better understanding of what is happening.

I did the test last night.

Fully charged battery.

54.3v
Full throttle up the hill 671watts 48.2v.
Just before the end of the hill roughly a minute and a half later display showing 47.4v.
Back on the flat coasting and the battery went back up to 53.2v.

I am led to believe they are genuine Sanyo GA cells. This is what I specified. Battery weight 2KG. 48v 7ah.

 

[oldie1982]

I only ever use the PAS 5 on that hill. Everywhere else level 1 or 2.

I will keep an eye on the battery voltage. My commute is around 8 miles a day. I was seeing If I could get 2 days before charging the battery but I may start charging it everyday. What do you guys suggest?

 

[Nealh]

I would charge daily for an 8 mile commute with such a small battery and PAS 5 use on a hill, if it were flat all the way then one would get away with charging every third day using only PAS 1 or 2. The hill will stress's the battery once the voltage drops so it isn't worth riding for two days.

The sag is heavy at 5.8 volts for a 2p battery, it is ok whilst the SOC is high but not so once it decreases. As the voltage decreases cell temp will increase as well adding extra stress to them.

Although battery specs give a cell it's max current discharge rate, the max rate is usable but any cell will suffer capacity loss with continual max discharge. For cycle life and good longevity 60/70 % of the actual cell discharge rate is recommended.

I have a 6ah 12s/44v 2p battery using LG hg2 3000mah cells, my use is flat terrain and I can get 30 miles without stressing the cells. My use is PAS 1 only as the small hub rides nicely without power so I set the cut off speed to 13mph and ride above this on my own steam,

 

[oldie1982]

Thanks for the help.

I will charge everyday to not over stress the cells.

Is the voltage pretty accurate on the lcd8?

Lastly, to get a freewheeling speed reading I need an external speed sensor correct?

 

[Nealh]

Thanks for the help.

I will charge everyday to not over stress the cells.

Is the voltage pretty accurate on the lcd8?

Lastly, to get a freewheeling speed reading I need an external speed sensor correct?

The displays should be pretty accurate when volatage is at rest one can confirm this with a meter reading as well, the display will be within 0.2v accuracy.
How many wires/pins does the hub motor have ? If nine then it has an internal speed sensor.
Change the P2 value in the lcd8 if it is set for 1 then change it to 6 , if set to 6 try changing to 1.

The P2 doesn't always work though I have a bafang hub and it records road speed but not freewheel speed, I have tried every setting to no avail so the fault may be the display.
I use a lcd4 but have an lcd3 which I should try to see if the lcd is at fault.

 

[oldie1982]

This hub has 9 pins.

The P2 value is set to 0 so I have changed it to 1. I will give this a go. Then I will try P6.

I appreciate all the help.

 

[oldie1982]

The displays should be preety accurate when volatage is at rest one can confirm this with a meter reading as well, the display will be within 0.2v accuracy.
How many wires/pins does the hub motor have ? If nine then it has an internal speed sensor.
Change the P2 value in the lcd8 if it is set for 1 then change it to 6 , if set to 6 try changing to 1.

The P2 doesn't always work though I have a bafang hub and it records road speed but not freewheel speed, I have tried every setting to no avail so the fault may be the display.
I use a lcd4 but have an lcd3 which I should try to see if the lcd is at fault.

Thanks again the P2 setting worked like a charm set to “1”. It now reads the freewheeling speed.

 

[oldie1982]

Sorry I forgot to ask. When pas is at 0 the throttle does not work. The throttle only works when the pas is between 1 and 5. Is there a way to have the throttle on even when pas is 0?

 

[Nealh]

Set P4 to 0 = always active and see if that works.

 

[oldie1982]

Set P4 to 0 = always active and see if that works.

This did not work for me unfortunately.