Insat battery voltage sag

[Dave.ah]

Hi

I've had a frame bottle mount battery built by Jimmy at insat for a year now. It's built with PF cells 13s 5p. Apart from the battery having to be sent back 3 times for Jimmy to repair with various faults from BMS problems to a dead cell it's been great.

The voltage of the pack has always sagged down 5-6v under a 20-25 amp load. This is my first lithium battery so I'm still not sure what an acceptable amount of voltage sag is?

The pack does return 13 amp hours out of the 14.5 and always has done according to my CA, except when it had faults. Jimmy was very good each time and even paid postage costs each way on all occasions.

Just not sure about the sag?

Thanks for any advice.

 

[tommie]

Under a continuous 25A load all ebike batteries will show a voltage sag,

nothing to get concerned about

 

[anotherkiwi]

Try 10P13S or LiPo You need a massive parallel set up or large C rate to stop sag.

 

[Nealh]

PF suffers from higher IR so sag will be noticeable at a higher amp load, although rated as a peak 10a cell 5a continuous appears to be on its limit.
What you are experiencing is about right for PF 1+ voltage sag per cell over 20a load, note the temperature at 20c this will quicken the IR and induce more sag.
https://www.e-cigarette-forum.com/attachments/image-png.613393/

For less sag you need a battery with more cells in parallel as AK has said or use better amp rated cells, HE2, 25R (Green), HG2, VTC5 are all 20amp rated with these you could use 4 or 5 in P and sag should be less.

For less sag with 10a rated cells you will need 6 - 8 cells min paralleled esp with 20-25a load to reduce sag.

 

[Dave.ah]

Thanks for the reply's. Interesting information about PF cells there. The total resistance of the pack is 0.172 ohm's so according to my calculations that would give a 4.2v drop straight from the battery at 25 amps? So it seems normal for my cells?
I'm not worried about the sag as long as it's normal.
Also 25 amps is 5 amps per cell so I'm not over doing it.
Thanks for the advice.

 

[Deleted member 4366]

As a general rule. you divide the maximum continuous current per cell by 2 for ebike applications, so 10A x 5 cells =50A divide by 2 =25 amps max, so you're right on the top of the cells' capability.

5 to 6v sag is a lot. I wouldn't want that much on my bike. Do you have one of those big direct drive motors? They draw a lot of current all the time because of their high speed winding.

 

[flecc]

Do you have one of those big direct drive motors? They draw a lot of current all the time because of their high speed winding.

Got it in one! This is from a post by the OP about his build:

"My bike is a carrera vulcan from halfords with a rear DD hub motor,"
,

 

[Dave.ah]

Yes d8veh as flecc said, it is one of those direct hub motors. I usually run it at 15 amps for my comute and the sag is about 3v. What is an acceptable voltage sag?

 

[Deleted member 4366]

There's no acceptable amount of sag. It'a case of the less the better.

Those DD motors give a battery a hard time because they draw the maximum current so much of the time. Maybe that's why you've had so many problems with yours.

 

[Dave.ah]

There's no acceptable amount of sag. It'a case of the less the better.

Those DD motors give a battery a hard time because they draw the maximum current so much of the time. Maybe that's why you've had so many problems with yours.

Are geared motors lighter on batteries?
The bms had a resistor fault and also a dry solder joint. The dead cell on the other hand I'm not sure what caused that.

What hub motor would you recommend for my setup? It's had a hard life and it's probably time I changed it anyway.

 

[Nealh]

With my older cells that have lost some capacity I see 1 - 1.3v sag @36V or 0.8V - 1.1v sag @48v (13s) by adding a 3s booster pack in series with 20a controller. I always run 2 batteries in parallel to reduce sag and inc range.
My 100a HE2 5P 7s & 3s packs are negligible again I use the 3s booster pack for 36v as the 24v pack is used for the swizzbee which has been documented to draw about 60a peak at times.
On the swizzbee i parallel two batteries for a combined 139a.

 

[Nealh]

For 20/25a controller you need to consider using top cells with 15/20a rating or parallel another battery with your existing.
Most 20a cells are 2500/2600mah, the good ones are HE2, 25R, Sony URNSX, VTC5.
20a HG2, VTC6 for 3000mah or 30Q 15a rated.

 

[Dave.ah]

For 20/25a controller you need to consider using top cells with 15/20a rating or parallel another battery with your existing.
Most 20a cells are 2500/2600mah, the good ones are HE2, 25R, Sony URNSX, VTC5.
20a HG2, VTC6 for 3000mah or 30Q 15a rated.

Yes I understand thank you. I told Jimmy exactly what my setup was before I bought the battery and he recommended pf cells. The battery is still working as well as new a year later. Jimmy told me the pack would be good for 50 amps continuous discharge.

 

[wheeliepete]

Jimmy told me the pack would be good for 50 amps continuous discharge.

I guess it would if the BMS is also rated at 50 amps or more, but it's life expectancy would not be that great and your range would be pretty low.

 

[Deleted member 4366]

Are geared motors lighter on batteries?
The bms had a resistor fault and also a dry solder joint. The dead cell on the other hand I'm not sure what caused that.

What hub motor would you recommend for my setup? It's had a hard life and it's probably time I changed it anyway.

It depends how fast you want to go and how heavy you are. A Q128C is a mighty fine motor, but if your bike is already set up for freewheel gears, a Q128H might be better. You get them from Bmsbattery. They'll probably work quite well with your present controller and battery, though a 20A controller might be better.

 

[Woosh]

the problem with direct drive motors is their very high noload speed because there is no built in reduction gearbox. A geared hub like a BPM code 13 has a noload speed of around 230 RPM, making the power consumption to max out at around 200-210RPM. That will pull around 15A from a 36V battery with some pedaling. Perfect for a 10S5P 36V 15AH with 2C cells like Samsung 29E.
a 1000W DD motor will have a noload speed at over 400 RPM. That will make power consumption to max out at 300-350 RPM - at that sort of speed, the rider won't be able to keep up with pedaling thus all the power has to come from his battery, about 25A-30A at 48V. I can't see a 13S5P battery made with 39E or PF last very long.

 

[Dave.ah]

It depends how fast you want to go and how heavy you are. A Q128C is a mighty fine motor, but if your bike is already set up for freewheel gears, a Q128H might be better. You get them from Bmsbattery. They'll probably work quite well with your present controller and battery, though a 20A controller might be better.

Great thanks d8veh. On my current motor I can hit 30mph on the flat (I'm 13stone by the way). What would my top speed be roughly with the Q128H you mentioned? 26" wheel.

 

[Deleted member 4366]

The cassette motor (Q128C) has a 48v 328 rpm version, which assists to about 28 mph, though 22 to 24 mph is the cruising speed with medium pedalling. I'm not sure about the speed of the freewheel motor (Q128H). I've got a feeling that the 201 rpm 36v one will hit the same speeds as the Q128C when you run it at 48v. The 48v version only comes in 201 rpm version, which maxes out at about 19 mph on the road.

 

[Dave.ah]

T

The cassette motor (Q128C) has a 48v 328 rpm version, which assists to about 28 mph, though 22 to 24 mph is the cruising speed with medium pedalling. I'm not sure about the speed of the freewheel motor (Q128H). I've got a feeling that the 201 rpm 36v one will hit the same speeds as the Q128C when you run it at 48v. The 48v version only comes in 201 rpm version, which maxes out at about 19 mph on the road.

Thanks again. Will the 328rpm version hit 28 mph without peddling? I'm just trying to get an idea of what its like. It looks like a 250w motor which I quite like the idea of.

 

[Woosh]

legality aside, I would not recommend a geared hub for anything over 25mph. The Q128H has for example a gear reduction ratio of 13.2 to 1. When you hit 28mph on a 26" wheel, the RPM is 375RPM. Thus the rotor of the Q128H will spin at 13.2 * 375RPM = 4950 RPM. Your nylon gears won't last very long.
The BPM with a reduction ratio of 5:1 will fare a bit better, 1,875RPM but you are much better off sticking with your direct drive motor. Their ball bearings will last a lot longer.