Battery capacity drops and rebounds after "resting"

[batpot]

Have taken my first couple rides on ebikeling kit, and noticed that the battery capacity will drop to ~25%, with about moderate throttle (only going 10mph).
Then when I get home I can watch it rebuild to about 85% after a few seconds.

What does this mean?
The only way the controller knows the battery capacity is the voltage it's putting out, so this means the voltage is dropping, and then slowly rebuilding... Seems like an issue with the battery.

 

[J.R.]

Have taken my first couple rides on ebikeling kit, and noticed that the battery capacity will drop to ~25%, with about moderate throttle (only going 10mph).
Then when I get home I can watch it rebuild to about 85% after a few seconds.

What does this mean?
The only way the controller knows the battery capacity is the voltage it's putting out, so this means the voltage is dropping, and then slowly rebuilding... Seems like an issue with the battery.

Voltage sag will happen. It could also be the battery pack is old, or of lesser quality than others. Or just the kit isn't particularly accurate reading voltage. I wouldn't worry if you're getting the range you expect. I agree 10 mph isn't fast.

What kind of pack is it? What kind of cells in the pack? What chemistry are the cells? Age and specs?

 

[tomjasz]

Voltage sag. From electricbike.com/forum
Voltage sag explained
10-13-2017, 11:44 AM
What is voltage sag?

Voltage sag (or voltage dip) is what happens when you apply a load to a source of electricity. If the source is really big and the load is really small then you won’t notice it that much. But if the load is large enough the voltage will drop.

Now your e-bike battery is powerful but not like a the power lines connected to your home… So it’s limited capacity and limited output power grid. So when you apply a large load to your battery the voltage will drop from the initial resting voltage to to a certain level. Now the amount of sag will depend on many factors but can be put into 2 categories:
The motor : size of the motor and controller and amount of power to the wheel you ask to produce.
The battery : IR (internal resistance of the battery), chemistry, capacity (how many amp hours or watt-hours).

Voltage sag is not all bad, it’s normal, some good high quality battery sag more than some cheaper cells it depends on the chemistry.… But it’s also a good indication on how to use your battery and how to apply power!

For example. If you have a 52v Shark Pack fully charged to 100% (58.xV) while in a resting position and you take your BBSHD capable of 1500W and hit the throttle to accelerate, you would see the voltage “sag” or “drop”a few volts down to 55-54ish Volts.. And will continue to drop until you let go of the throttle, where it will raise back up to slightly less than before obviously.

But when the battery is being used and drained, it generates heat and with less capacity and heat you can get a little more sag near the end. And again it’s normal... the battery has less punch when it's tired!

So let’s say if you’re near 46v (resting voltage) and you ram the throttle wide open....well under load that would put it close to 41V and what is at 41V on your BBSHD? The controller set point to cutoff power to prevent over discharging your battery!

Once you hit about 46V on your 52V battery (or 44V on your 48V) you should consider yourself low on battery and start “limping” home using minimal power to reduce the voltage sag to avoid hitting the low voltage protection….Lower the pedal assist mode, lower your speed, let go the throttle and pedal to get the most milage of whats left…This will also let the battery cool down while you ride back home!

 

[harryS]

Ebikeling's 1000W motor puttering along at 10 mph shouldn't sag a freshly charged lithium battery in good condition. Is it lithium or is it lead acid? How many volts is it?

 

[batpot]

It's a 36v 12ah "Samsung, Lg,and Sanyo 18650 cells", and only 500wl watt motor.
Purchased from eBay vendor with good reputation, and sitting in a waterproof trunk bag.

I bought a watt meter, but haven't soldered the connections for it yet.

Also... I did have to solder a longer wire on, and my solder welds were not good... Could not get the solder to melt without direct heat from the gun.

Did not check the connections, but if they're heating up, that could be the source of the voltage sag under load.

It's a 100w solder gun, and quality Alpha Fry 60-40 rosin core solder with a some supposedly good rosin flux, so not sure what went awry.

 

[tomjasz]

Solder guns kinda suck for electrical stuff. At least I never got decent solder joints until I started using pencil type with quite small tips. Once happy with what I'd learned I grabbed a TS100. Kester solder here, but I still use a bit of Kester liquid flux. A quick review of youtube soldering videos always helped me when learning. https://www.amazon.com/Soldering-El...lectronics+solder&qid=1571069951&s=hi&sr=1-21

 

[tomjasz]

"Samsung, Lg,and Sanyo 18650 cells",

Not to be a harbinger of doom, but the wrong choice in "brand name batteries" can mean a saggy battery. Would you mind terribly sharing who the battery came from? I really like to keep track and do keep notes on various vendors. I helps us all stain top of the good guys and sort the bad!

 

[harryS]

If it's the ebikeling 500W geared motor, I have some experience with them. That motor only takes about 100-150W at 10 mph with throttle only. which would be about 4 amps. I'm using a KT controller and display on mine to get the watts. Shouldn't see much sag with a steady throttle hand.

On the other hand, if you were going uphill, you could pull a battery at 80% down to 30% temporarily by pulling full throttle on a hill, while it would stay at 100% with a fully charged pack. Or maybe you were blipping the throttle during your test ride. So maybe your symptoms are normal.

If this is the first time you've charged the battery and ridden the bike, the pack might need another few charge cycles to get it in balance.

I like the ebikeling 500W geared motors, but didn't like their SW900LCD that came with one of them. It doesn't show speed when coasting, and the PAS was pretty crude compared to a KT controller.

 

[tomjasz]

If it's the ebikeling 500W geared motor, I have some experience with them.

Thanks!

The battery?

 

[batpot]

Not to be a harbinger of doom, but the wrong choice in "brand name batteries" can mean a saggy battery. Would you mind terribly sharing who the battery came from? I really like to keep track and do keep notes on various vendors. I helps us all stain top of the good guys and sort the bad!

Respectfully, I'm not going to throw the battery under the bus until I get some better solder joints.

Also, yes, this is uphill with about 90# including the weight of the Burley hanging off my rear axle, so the mph definitely isn't the whole story.

 

[tomjasz]

Respectfully, I'm not going to throw the battery under the bus until I get some better solder joints.

I'm sorry! That wasn't my intent at all. I could have phrased it better. I have a couple of budget China packs. Both have served me VERY well. I actually pounded one daily all summer. Out in a hot garage, always charging to 100%, and daily. It now sags incredibly. But It still make my 3-5 mile daily run to my pals shop. I got every penny out of it. The batteries were 29E, decent for a 20A 48V motor. But oh my, full throttle and 22MPH and the voltage goes from 53.9 to 43V. It's a crappy build with nickel coated steel rather than pure nickel strips. I think this is the 4th full season of abuse.

Adding the wattmeter is a good plan! I have one but opted for installing a CA3 instead.

 

[batpot]

I like the ebikeling 500W geared motors, but didn't like their SW900LCD that came with one of them. It doesn't show speed when coasting, and the PAS was pretty crude compared to a KT controller.

Yes same issue with the S830 display that comes with the waterproof kit.

I haven't hooked up PAS yet, because I need a magnet disc compatible with a hollowtech spindle, and also hydraulic brake cutouts. Both are on their way.

 

[harryS]

Also, yes, this is uphill with about 90# including the weight of the Burley hanging off my rear axle, so the mph definitely isn't the whole story.

Ah. I think you're OK then. A hard upward pull will deplete a partially discharged battery as you describe.