Overheating uphill?

[Kfraser]

Hey everyone, I'm new to owning a used GM.  I think it's a Magic Pie 3 with a 48v lithium battery.  I did not do much research before and just jumped on what I thought was a good deal.  I realize now people always sell things for a reason!  The initial driveway test seemed great.  I live on a hill and rode it up an down a bunch of times with no pedaling.  I figured everything was going to be smooth sailing on the way to work the next day.

My commute home is 10km 310 vertical metres.  Pedalling a normal bike was getting old, especially on hot days.  The first time I took it to work, of course I was pinning it down hill and goofing around.  It died suddenly on a climb about 1 km from work.  The grade was maybe 6%, nothing crazy and I was pedaling.  I figured the battery was a dud and that the guy lied to me about only having 70 charge cycles on it.  Going to work is mostly downhill with only 83 vm and a distance of 10km.  I would have expected the unit to get me to work under it's own power easily.  I started pedalling and then it started working right as I got to work.  I thought maybe the regen had charged it a bit.  Good thing I had the charger with me!  It took about 45 minutes of charging. 

On the way home I decided to use it sparingly after doing a little research about how to get better range.  I got home about 10 minutes faster than pedaling alone and did not have to kill myself to get there.  I used about half throttle on the hills and pedaled the flats with only my leg power.

As I got home, the meter reads half full and I decided to keep going up the hill I live on.  I got 100 feet or so past my house and it died.  I believe the lights stayed on.  SO I pointed it down the hill and got to my driveway.  I rode in circles for about 3 minutes and then got power back with a green light on the meter even.  I went up the hill and it died at the same spot.  I did this 3 times and then decided to take a side street that was pretty flat.  I west up and down the side street about 5 times with just battery power and it was fine. 

So, I guess my question is - is this normal behavior for the Magic Pie and steep hills when the battery is on the low side?  Or is the motor shutting off do to a temperature thing?  Could I have over torqued the nuts at the drop out and I'm squeezing the bearings too much causing drag and overheating?  Is my battery maybe just a little older than the seller told me?

Thank you!

[Bikemad]

The LED indicators are pretty useless as a battery gauge when using a lithium battery:

My experience with the throttle is, that all LED's are ON during the whole time and the last 500 meteres of driving with batterie power one after the other is going OFF.
So the functionality of this voltage-watcher is not very useful.

This basic LED battery gauge was originally designed for lead acid type batteries which have a more pronounced voltage drop throughout their discharge, but the LiFePO₄ batteries in use today have a higher voltage which drops very little in comparison as they discharge, and the LEDs would probably remain fully lit for 90-95% of the battery's use.



Take a look at this post to find out how a resistor can be used to adjust the gauge operation to better suit the battery type being used. As your cable has already been chopped it should be relatively easy to solder a variable resistor into the battery gauge power supply wire.

Alan

I have just tested a 48V throttle on a variable power supply and here are the results:

• All three LEDs remain ON above 48.4V
• When the voltage falls below 48.4V the Green LED goes OFF
• When the voltage drops below 42.7V the Amber LED also goes OFF
• Below 42.7V, the Red LED will remain ON until the battery's BMS cuts the power completely

By the time the Green LED is permanently OFF (with the throttle fully released) the resting voltage voltage of the battery will be below 48.4V. If you look carefully at the above chart, you should be able to figure out that a LiFePO₄ Lithium battery would have less than 4% of its total capacity remaining at this point, a LiMn Lithium battery would have less than 7% remaining, whereas a lead acid battery would have around 30% remaining.

A lead acid battery also has a much greater voltage sag under load, therefore the LEDs would start to go OFF even sooner while using the motor with a lead acid battery.

So, I guess my question is - is this normal behavior for the Magic Pie and steep hills when the battery is on the low side?  Or is the motor shutting off do to a temperature thing?  Could I have over torqued the nuts at the drop out and I'm squeezing the bearings too much causing drag and overheating?  Is my battery maybe just a little older than the seller told me?

I would say the behaviour is perfectly normal for a low battery.
If it was cutting out because the controller was getting too hot, the battery lights would be unaffected as it would only disable the controller itself, and not the battery supply to the LED gauge.
Over torquing the axle nuts has no effect on the bearings as it simply clamps the dropouts and washers etc. between the nuts and the machined clamping face of the axle:



if it doesn't have a date of manufacture, it is impossible to say how old the battery might be, but its condition would also be affected by the type of use it had been subjected to since it was originally purchased.

It it had been left in a completely discharged condition for a long length of time (e.g. not used or recharged over the winter period) this could cause the cells to deteriorate, resulting in a noticeable drop in their capacity.



If you installed a cheap wattmeter, and used the battery from completely full to completely empty, you would be able to find out the actual available capacity of your battery.

Alan
 

[Kfraser]

Thanks so much for the detailed help, I really appreciate the time it took you to put that together.   I'm an ebike newb but a mountain bike mechanic. 

Sounds like you are right and the battery is just more used than the seller told me.  I will just try to use it up now the best I can.  So when the motor cuts out, it's because it can't produce the wattage to maintain the climb?  Then after it comes back to life, there is enough power to travel on the flats for an extended period?

Just a few followups:
1.  Should I be trying to use the full charge before charging the battery?  Or does that even matter with this type of battery?  I'd like to not have to charge at work but I worry I'll be stuck on that hill on the way home and this bike weighs like 80 pounds now!
2.  What is I connect an mtb hydraulic rear disk brake and just keep the ebike disc brake connected to the controller so I can engage the regen without slowing down, could that work?
3.  Can these batteries be serviced?
4.  Does the cycle analyst LCD thing have a battery level gauge and estimate your run time?

Thanks!

[Bikemad]

When the motor cuts out under load, it is usually because the overall battery voltage (or a single cell's voltage) has fallen too low and the battery's BMS simply disconnects the battery to prevent further discharging of the cells.

If you turn the battery OFF and ON again using the keyswitch, the "no load" battery voltage will be above the minimum voltage setting and the motor should continue to run until the voltage drops too low again, which is why there is usually enough power to travel on the flats for an extended period with a reduced load on the battery.

1. There is no memory effect with lithium batteries, and running them completely flat each time is not the best way to look after it. The battery will have a longer lifespan if it only partially discharged before being recharged, instead of being completely discharged every time.

2. Engaging regen will slow the bike down very quickly, therefore you cannot really use regen without slowing down.
With most cable operated brakes, the regen is activated by a very small amount of brake lever movement, usually before any mechanical pressure is actually applied to the brake pads/blocks.

3. It should be possible to replace cells in the battery, but as all the cells are the same age and will have been subjected to the same use, it is probably best to change all of the cells together if some are already starting to deteriorate.
The earlier LiMn 48V batteries had 39 separate cylindrical 18650 cells, whereas the later LiFePO₄ 48V batteries have 16 of the large rectangular cells:



4. The Cycle Analyst is a lot more expensive than the wattmeter shown in my previous post, but it does do a lot more.
Full details on the Cycle Analyst can be found here.

Alan
 

[StadsImker]

On hot days when I ride my VERY heavy Transport bike. Sometimes my GM MP5 also stops suddenly. I is in my case overheating protection that kicks in.

You need to know that accelerrating takes a LOT of Amps... When you drive on 48volts. The GM can use 30A or even more, because you ask it to go faster. More Amps more heat... When you are cruising, the GM doesn't use 30 Amps but only a thirt of that. (on a flat road) Hills and dirtroads and accelerating makes your GM go into the high Amps. And It will get hot... So it is very normal that your GM get into it's max limits for temprature. Also you don't drive fast and cooling get's much more difficult.

If you drive in traffic or decellerate and accelerate a lot it will use MUCH more from your battery pack.

So I think that's your problem you use to often or to long and High current... And your GM starts to heat up to much. Advise peddle more... Or buy and extra Magic Pie... and make an all wheel drive Ebike. 

Greetings,
Philip - from The Netherlands.

[Hastings]

Did you persue your original thought ie overheating ?   Why go up the hill at your home with a full battery and see if it cuts out ?

[Kneedeep]

If you bought this used I will add the Lifep04 could be out of balance also.

[Kfraser]

Thanks to everyone who responded.  You are all right, it seems to cut out once the charge is low and I force it to output too much up a hill.  It's not an overheating thing.  I was passed by 2 people with hub motors yesterday both uphill.  My battery needs servicing or replacement I guess.  It gets me home in half the time and half the rider input of pedaling myself.  I may look at skinnier tires.  I'm running 2.35 Schwalbe big apples.  If I could find something lighter and cheap that could carry the heavy load and have less drag and rotational weight, that may help me out. 

Thanks to everyone that responded!

[Pwd]

To help prevent the battery from cutting out on you, you could lower the maximum amps on the controller if you have a programming cable. That should reduce the strain on your battery. If its at 25 amps continuous now, try lowering it to around 18. I'm not sure if there is a max burst amps parameter you can change too.

If that doesn't work out, you can probably try replacing some bad cells as Bikemad has pointed out.

EDIT: I also highly recommend a wattmeter.