Mid drive with big battery sag

[Evelosimo]

Hello,
This is the first time I post after months of always finding without having to ask, what a great resource this is.
I recently purchased a used mid drive motor and battery for what appeared a decent price. The motor is a 48V Xofo 15A controller, the 14Ah battery appears to be made of low powered Chinese cells. I finally hooked up the motor for the first time a couple days ago and went for a ride, everything appeared fine at first, with lots of power, however after a few minutes I noticed on the display that every time the motor went on, the voltage went very low, eventually the little battery indicator even disappeared and after about 20 minutes the motor would not even go. That was with a new battery I had purchased, a 8Ah with a rather low powered cell but good quality. I switched battery and then tried it with the battery the motor came with, however after about 5 minutes the whole thing shut down and I have not been able to get that battery to work it again, that is no power. The other battery, which is new and better quality seems to still work but sags a lot until it stops. Both battery were fully charged, they both still had substantial charge after the motor went out or at least my voltmeter said so.
I am wondering what is causing this, could it be a faulty controller on the motor? I imagine since both batteries seem to react adversely. Maybe the older battery that was previously hooked up to the motor has had some damage because the previous owner burned out the controller ( he did not mention any of this but that is a different story)
Any ideas is greatly appreciated as this is all new to me.
Thank you!

 

[amberwolf]

however after a few minutes I noticed on the display that every time the motor went on, the voltage went very low, eventually the little battery indicator even disappeared and after about 20 minutes the motor would not even go. That was with a new battery I had purchased, a 8Ah with a rather low powered cell but good quality. I switched battery and then tried it with the battery the motor came with, however after about 5 minutes the whole thing shut down and I have not been able to get that battery to work it again, that is no power. The other battery, which is new and better quality seems to still work but sags a lot until it stops. Both battery were fully charged, they both still had substantial charge after the motor went out or at least my voltmeter said so.

There are two possibilities.

The first is that the batteries are simply not designed for the amount of power the system is pulling from them, so they either drop below the system's LVC (low voltage cutoff) and the system shuts off (display will stay on, but motor won't go), or their internal BMS drops below it's LVC, and shuts off the output (display will turn off and motor won't go).

The solution for this is to replace the batteries with ones that are capable of the power that system needs under the conditions you're using it.

Or, possibly, to parallel both batteries on teh bike at the same time, so each one sees less load, and they sag less, and perhaps do well enough for you right now, until you can get a new better battery.



The second is that the controller is designed for a higher voltage battery than you have, so that the batteries you've got appear to it to be empty before they actually are, and so it shuts down the motor thinking it is protecting them.

The solution to that depends on the system.

If it has settings in the display you can access, for battery voltage/type, then you can change that to match your battery.

If it does not, then you would have to replace the batteries with one(s) that are the right voltage for that system.

 

[Evelosimo]

Thank you for your answer, very much appreciated, I did check the motor specs and it it is rated pretty low, 250W to 350W, I have an older version but similar to the one on the link. Both batteries and motor are 48V but I will check in the motor display setting and make sure it is set on 48V.

http://www.xofomotor.com/index.php/products_15/26.html

I also noticed that the sag happens with very little power, even on the flat at a lower setting. Also on the first ride the motor seemed a bit warm considering how little I used it, I climbed a couple of steep hills for 3 minutes at a time in low gear but nothing too challenging, I cycle a fair amount on a non electrified mountain bike and am in pretty good shape (or at least I like to think so) so I still did most of the work.
I do have a stronger new battery I have access to but I am a little hesitant of hooking it up to the motor after what happened to the other 2 batteries, is there concerns over damaging the bms if there is something faulty with the motor?

 

[goatman]

im wondering if they wired the hall/phase combo wrong in the factory, actually you bought this used? do you know what the no load amp draw is? maybe someone was in messing with the wiring

 

[Evelosimo]

There seem to be no voltage drop when there is no draw on the motor but I should check how much amp it is drawing. I suspect that the person I bought it may have been careless, the price was just too good to be true and many things the previous owner said turned out to be misleading. He also had it mounted on a downhill bike with a very small rear cassette for the size of the motor, I suspect he may have worked the motor too hard going uphill and burned some parts in the controller which in turn had a negative effect on the battery, would that make sense?

 

[amberwolf]

Thank you for your answer, very much appreciated, I did check the motor specs and it it is rated pretty low, 250W to 350W

Then you're going to push it a lot harder than it is meant to with the "48V Xofo 15A controller" you have, as that is around 720W.

Basically, you have a group of parts that are not the right ones to work together.

The controller is more powerful than what the motor can take, so it's heating it up too much when the motor asks for more power than it can handle. You'd need a controller that is more like 7A or 8A to limit the power to the motor to it's rating, which will greatly reduce the assist it gives.

Alternately you can replace the motor with one able to handle 750W or higher, so that it doesn't overheat with that controller.

(note that motor ratings are a bit ambiguous anyway, but a really small motor can't handle a lot of power, it has nowhere to put the heat).


The controller is more powerful than what the batteries can safely output, so they are sagging in voltage more than one that was properly made to do so. (many may claim to, but if it's cheap, it's cheap for a reason, usually becuase it's made of poor quality components, and/or it's "specs" are outright lies, and it isn't even remotely capable of what is claimed. A very common problem.).

To fix this, you need better batteries, or a much less powerful controller that doesn't stress the batteries. How much less, depends on what the batteries are *actually* capable of (not what someone "rated" them for).

 

[Evelosimo]

Thank you for the advices, I am going to put this one on hold, I do have another ebike, it seems weird that XOFO would make a motor with too powerful of a controller. Maybe when I get a moment I will open the motor up and check if somebody tried to change the factory settings on the controller. I did find that there was not much of a difference between pedal assist settings, it seemed to have similar power wether it was on 1 or 9, so maybe somebody messed with the controller. Lesson learned on buying a used ebike kit, hopefully I won’t make that mistake too often.

 

[Evelosimo]

I finally got around to open the motor and look at the controller, interestingly all the contacts inside the controller compartment are buried in silicone, which is a good thing considering its location inside the mid drive motor with no gasket between the controller compartment and the motor. This also means that no one has played with it, however as I was putting it back together I noticed that the Anderson connector between the battery and the motor on the negative side was not clicked in properly and had to replace it with a new one that clicked properly. This turned out was the problem, now there is no more battery sag with any of the batteries I have, the only problem is that every once in a while the motor cuts out for a fraction of a second but this seems to no affect the performance. Anyway I though I would update in case someone runs into a similar problem. As for the motor cutting out briefly, if anyone has any suggestion as to where to look, please let me know.