Battery cut out at high loads

[Ccount]

I currently have a true 1000w baffang rear hub motor, with 35A controller in rear, and another 1000w front hub motor with separate controller (unknown amps) and throttle. I can nail the rear throttle with no issues. Same with the front throttle, INDIVIDUALLY. These systems run independently from each other, but connected with a simple "Y" connector to the stock Rad 48v battery. The front motor has a much higher speed (30 MPH), but lower acceleration rate, and the rear motor has quicker acceleration, but lower top end (24 MPH+/-). My main intended use is to transverse soft beach sand, and it does so spectacularly. Front wheel does spin a lot in sand if floored, but if feathered along with plenty of rear throttle, it is a great soft sand machine, and the simple revolving front wheel prevents it from "plowing" through the sand.

Obviously I wanted to see what happens on road. Throttles used Individually, and it works great. However, if I "floor" both throttles from a standing start, I imaging I am drawing well over 2000w, and the entire bike dies. No power anywhere. It is not a blown fuse, and it seems to "reset" after about 10 minutes, as if a thermal breaker or a solid state breaker resets. Both battery in and out fuses remain intact. I have not tested for voltage at the battery when this happens, but it MUST be in the battery, because neither controller knows the other exists, and both die completely, and revive simultaneously. At first I thought it may be a low voltage cutoff, but I doubt the front controller even has that function (and it has no programmable functions as far as I know).

What do you suppose is causing things to completely turn off, but eventually return to normal? From there I may be able to modify my dual throttle application to where it continues to stay "battery on and active". Thx!

 

[legsofbeer]

I would assume the battery management system is cutting you off for excess amps, or it may have a temperature cutoff. Seriously, 2000w is a shitload to ask from that battery. I think the output fuse is 40 Amps, so a functioning BMS isn't going to let you do more than that.

You might be better off adding an aftermarket triangle battery or another shark battery using the bidon mounts on the underside of the main tube and driving the motors seperately.

 

[ElevenAD]

its Voltage SAG, that stock Rad battery pack was not made for that type of load! like Legs said, maybe try a triangle pack with more AH or ad an addition rack battery?

 

[Sierratim]

I agree that your problem lies the BMS shutting down the battery to protect it from an over current situation. It recovers, bit the damage to the pack adds up. Assuming you're running the standard 48V 672Wh/14Ah pack at something like 40A, you're at close to a 3C discharge rate. Battery University advises "Some high-performance batteries can be charged and discharged above 1C with moderate stress". 3C will stress even high-performance batteries to an early death.

A quick search didn't bring up any data for battery life at discharge rates of 3C and higher, but this data at 2C is not encouraging;

From Battery University

So if you could hack the BMS to stop cutting power without catching something onfire, you're then facing a very short battery pack life.

I like the suggestion others have made re separate battery packs for each motor.

 

[legsofbeer]

Assuming you're running the standard 48V 672Wh/14Ah pack at something like 40A, you're at close to a 3C discharge rate. Battery University advises "Some high-performance batteries can be charged and discharged above 1C with moderate stress". 3C will stress even high-performance batteries to an early death.

In the spirit of "if I'm asking a stupid question, others have the same question but are afraid to speak up", what is C? I clicked the link, and while useful, the definition of C was not obvious.

 

[Gionnirocket]

In the spirit of "if I'm asking a stupid question, others have the same question but are afraid to speak up", what is C? I clicked the link, and while useful, the definition of C was not obvious.

C is the rate at which a battery is designed to discharge. Going over the C rating causes it to heat up and become damaged, that is why the BMS intervenes.

 

[Sierratim]

In the spirit of "if I'm asking a stupid question, others have the same question but are afraid to speak up", what is C? I clicked the link, and while useful, the definition of C was not obvious.

@Gionnirocket pretty well summarized it. This article from Battery University sheds a bit more light starting with this definition;

"A C-rate of 1C is also known as a one-hour discharge; 0.5C or C/2 is a two-hour discharge and 0.2C or C/5 is a 5-hour discharge."

The capacity of a battery is commonly rated at '1C'. It's basically the discharge rate that will drain the battery in 1 hour. So a 10Ah battery should be able to deliver 10 amps for 1 hour.

To maintain a reasonable service life with a 40A current load the OP would thus need a high quality battery with a min 40Ah rating. That's a big battery!

How big of a battery? Using the LG HG2 battery cells Timpo quoted (a 18650 size cell) you'd need 13 cells in series (13s) to get the nominal 48V pack voltage. The short form spec sheet (attached) notes the nominal C rating as 3000mAh. This is often used in place of the 1C value. So for this cell 1C = 3000mah = 3Ah. You'd need 14 of these cells in parallel (14p) to get a pack that could deliver at least 40A at 1C. So the battery would be a 13s14p configuration to provide a long service life using these cells. Other battery packs might avoid the cut off problem the OP is having, like perhaps the Jumbo Shark that Timpo mentioned, but they will perform over a much shorter service life.

 

[Ccount]

Thanks my friends. Yes, in retrospect it does sound like the BMS is cutting out for protection. Clearly I need another battery! I was surprised that the fuse did not blow as a first line of defense!

 

[Sierratim]

Thanks my friends. Yes, in retrospect it does sound like the BMS is cutting out for protection. Clearly I need another battery! I was surprised that the fuse did not blow as a first line of defense!

Depending on the fuse amp rating and type (standard, time delay, etc), it was probably a race between the fuse and the BMS to see which shut down the power first.

 

[legsofbeer]

I was surprised that the fuse did not blow as a first line of defense!

I think fuses are regarded as the final line of defense.

 

[Duncan]

I currently have a true 1000w baffang rear hub motor, with 35A controller in rear, and another 1000w front hub motor with separate controller (unknown amps) and throttle. I can nail the rear throttle with no issues. Same with the front throttle, INDIVIDUALLY. These systems run independently from each other, but connected with a simple "Y" connector to the stock Rad 48v battery. The front motor has a much higher speed (30 MPH), but lower acceleration rate, and the rear motor has quicker acceleration, but lower top end (24 MPH+/-). My main intended use is to transverse soft beach sand, and it does so spectacularly. Front wheel does spin a lot in sand if floored, but if feathered along with plenty of rear throttle, it is a great soft sand machine, and the simple revolving front wheel prevents it from "plowing" through the sand.

Obviously I wanted to see what happens on road. Throttles used Individually, and it works great. However, if I "floor" both throttles from a standing start, I imaging I am drawing well over 2000w, and the entire bike dies. No power anywhere. It is not a blown fuse, and it seems to "reset" after about 10 minutes, as if a thermal breaker or a solid state breaker resets. Both battery in and out fuses remain intact. I have not tested for voltage at the battery when this happens, but it MUST be in the battery, because neither controller knows the other exists, and both die completely, and revive simultaneously. At first I thought it may be a low voltage cutoff, but I doubt the front controller even has that function (and it has no programmable functions as far as I know).

What do you suppose is causing things to completely turn off, but eventually return to normal? From there I may be able to modify my dual throttle application to where it continues to stay "battery on and active". Thx!

Have you tried to ride it up a set of stairs?

 

[Pay Jota]

After the addition of another battery and mount, what will be your weight? Pushing 90lbs.?