2wd one battery per motor or 1 battery for both?

I live in a remote hilly rural area, I am building a bike with a long side car, for passenger or cargo, like a few rounds of firewood.
I will have a 36V500W geared motor on the front and rear of the bike. I have ordered 300 Samsung 25R5 20A 2500mAh to build a 36V system with Vruzend V2.
I am going back and forth between building one battery for both motors or one battery for each.
I would like your input as I consider the trade offs
Thank you

[strike][/strike]I have been using 2WD for years and I use 2 complete systems, that is 2 throttles, 2 controllers and 2 battery packs and it has saved me a long walk on several occassions.
But that is not the main reason I use 2 packs(I use LiPoly). To explain that, I need to explain what has been the biggest benefit, something I didn't originally understand when I first built this bike.
It has been the ability to run low power on my main(rear) motor. That means smooth throttle response and being able to use a simple square wave controller w/ a simple 3-speed PAS w/ no big gaps. No need for "throttle taming". But at the time, I have the extra power for that quick squirt across a busy street or that rare long hill.
The way it has worked out, I use the second(frt.)motor about 25% of the time and my frt. pack capacity is about 25% of the main pack and they both dis-charge at about the same rate.
Also, although it doesn't happen often, if on a long trip, I run one battery pack all the way down till the controller's LVC shuts the system down, I can continue on the other pack. No range anxiety w/ this bike

I just re-read your post and it seems to me that you have no choice except to use both batteries.
W/ only a 20 Amp discharge rate, you will have to limit the controller(or other means, like a Cycle Analyst) to prevent the batteries's BMS from kicking in and 20 Amps is not enough , considering what you want to do, for one motor, let alone two.
Frankly, I think you are grossly under-estimating how much power and reserve hills require.
You haven't quantified what your hills look like, but I know that on my 2WD, if I filled my trailer w/ firewood and tried to climb any type of serious hill, I would need at least 1000 Watts X 2 to keep motors and controllers from over-heating and a lot more range than 2500 mAh. Those batteries are tiny and a couple of mile long hills will probably run them flat.
IMO, you need to go w/ some serious motors like the MXUS Q11 or the Leafmotor and batteries that can handle 30 to 40 Amp dis-charges and have more than 10 Ah's capacity each.
Building an ebike to go long distances on the flat is easy, carrying loads up hills is not.

Thank you I am just converting to electric propulsion so I have no experience I am reading as fast as I can.
I was under the assumption that the 300 Samsung would give me a 36V 75Ah S10P30 battery, are you saying that no matter what I am limited to the 20Ah discharge rate of each cell.
In your experience 2 x 500 watt motors would be inadequate for my purpose?

I would recommend a single battery for multiple motor drive. The battery is arguably the most reliable part of the sysem so battery redundancy is IMO not a compelling argument. A single battery gives flexibility in power apportionment, avoids manual charge management, and guarantees equal and balanced voltage/current capability to both systems if that is a design criteria. You always have the ability to run the motors the way the situation requires without consideration for which tank is empty and hooked to what motor. In addition, there are intrinsic simplifications to single battery monitoring while underway and subsequent charging.

The rest of the 2WD issue is fraught with design choices that are affected strongly by the controller and motor selection. Although these can have dramatic effects on the efficiency and drivability of the bike, there really seems little opportunity for them to materially impact the question of one-vs-two battery configuration.

gdinim said:

Thank you I am just converting to electric propulsion so I have no experience I am reading as fast as I can.
I was under the assumption that the 300 Samsung would give me a 36V 75Ah S10P30 battery, are you saying that no matter what I am limited to the 20Ah discharge rate of each cell.
In your experience 2 x 500 watt motors would be inadequate for my purpose?

Click to expand...

are you saying that no matter what I am limited to the 20Ah discharge rate of each cell.

Click to expand...

Yes, the cell discharge value does not change, but as one adds capacity(paralleling), there is less demand on each cell.
And I didn't pay enough attention to your battery spec.s(I'm not a round cell guy). A string of 10 cells in series (10S) would be almost 40 Volts.
And then if you parallel 10 of those strings(10P), you will have a capacity of 25,000 mAh or 25 Ah. More than enough battery, in fact, any more cells than that and I think the bike would be getting really heavy(I sometimes carry 25 Ah's of battery on my 2WD mountain bike and it weighs over 70 lb.s w/out any trailer).
So let's say battery power is not a problem except I don't know how much would be the acceptable safe dis-charge rate for the pack(Maybe teklektik or another expert will chime back in).
So let's say you can pull 40 Amps(2 20 Amp controllers) from your pack and look at motor performance alone.
For that we need to go to the sim. @ Ebike CA. I used the two system feature and guessed that your total rig weighs 300 lb.s(and gave each system a weight of 150 lb.s)
The rest of the spec.s as follows;
mountain bike.
36V/12.5Ah battery(half capacity)
20 Amp controller(for ea. system)
26 " wheels
one MAC 12T motor for each system(these are low-speed 201 rpm rated @ 36 V which would be about the same as a BPM "201")
On a 10% grade(about what a really fit cyclist would be able to do, but he would be on the biggest gears)we get;
An "able to maintain" speed of 14 mph and no "overheat". But interesting enough, that big battery would run "dry" in 9 miles(if there was a hill that long). To get an idea of how much power hills consume, a 25 Ah battery would go at least 40 miles in the flat.
Now if we make that hill an impossible to cycle up 15% grade, we get;
A very low "maintain" speed of 10 mph and overheat in 15 min.s and a crazy low range of less than 7 miles.
So, on paper, it looks doable, but there are some things in the nature of electric motors doing hills that makes it an "iffy" proposition.
The first and most important "rule of thumb" concerning hub motors and hills is;
At approx. half the top rpm/speed of the motor, more energy starts to be converted to heat rather than forward motion.
The motor(s) can never be allowed to start lugging and asolutely never let chug to a stop. Something will start melting.
Last month, here in Mex. where I'm spending the summer, I didn't follow my own advice.
The hill was long(several miles) and not so terribly steep, but the road was so bad, I had to slow down to a crawl several times to cross washed-out gullies. Once the momentum is lost, it's really hard to get going again and on my last leg I melted the wires going to the rear motor.
If you want to stay w/ the 500 Watt motors(and there are several reasons you want to, bigger motor require bigger everything, torque arm, wiring gauge connectors, etc), use the lowest speed rated ones available, which I believe is the "201" versions. The trade off is top speed and the sim shows that on your imaginary bike(12T in a 26: wheel on a 36V Battery), the top speed is only 17.5 mph. But, if the pack is 48 Volts, a more fum top speed bumps up to 22.5 mph.
There really is no advantage to using 36 Volts over 48 V. No bigger wires are needed(you probably want to use 12 AWG silicone) and 48 V controller are as avail. as 36 V ones.
And one last thing, if your total system weight is going to be more than 300 Lb.s, the critical numbers will "tank" real fast as the weight goes up. You will need to "run" the numbers on the sim real carefully.
Hope this helps a bit.
PS-I'm sure you know doing actual work would be much easier if you just bought a gas rig and built the ebike for fun/touring. But I guess there wouldn't be much challenge to that.

gdinim said:

I was under the assumption that the 300 Samsung would give me a 36V 75Ah S10P30 battery, are you saying that no matter what I am limited to the 20Ah discharge rate of each cell.

Click to expand...

No.
A 10s30p battery with a max discharge rate of 20A/cell will deliver 30 x 20A = 600A. Even running this at conservative rates, your available current is enormous.

Just so you can get a charger easily, let's suppose you go with a standard 52V battery (14s) -- that gives you 300/14 ~= 21p or 420A. Still a silly amount of current. Anyhow let's go with a 14s20p just to make things simple. This is 20 x 2.5Ah cells or 50Ah. (huge).

Here's the simulator plot using MAC 10T motors, 40A controllers, and a 450 bike on a 10% incline without pedaling - 20mph, overheats in 16 minutes, and goes 17 miles. The 450lbs is a low value for a couple of 150lb riders and a 150lb bike.

http://www.ebikes.ca/tools/simulator.html?bopen=true&motor=MMAC10T&batt=cust_52_0.2_25&mass=204&hp=0&cont=C40&cont_b=C40&motor_b=MMAC10T&batt_b=cust_52_0.2_25&mass_b=140&hp_b=0&add=true&blue=Lbs&axis=mph&grade=10&grade_b=10&k=1&k_b=1

With only one person on board (310lb) the bike looks like this - 21mph up the 10%, never overheats, and goes for 24miles.

http://www.ebikes.ca/tools/simulator.html?bopen=true&motor=MMAC10T&batt=cust_52_0.2_25&mass=140&hp=0&cont=C40&cont_b=C40&motor_b=MMAC10T&batt_b=cust_52_0.2_25&mass_b=140&hp_b=0&add=true&blue=Lbs&axis=mph&grade=10&grade_b=10&k=1&k_b=1

There's lot's of interesting things going on here. One in particular has to do with the controller rating. If you crank this down to only 20A, the top speed doesn't go down by much but the available torque to the left of the top speed line is much lower for the 20A case. This means that the 40A controller is going to be able to getaway (from speed=0) and accelerate on the incline up to top speed much better than the wimpy 20A controller. In general the controller rating determines/limits hill climbing performance.

Anyhow - this is with that big battery that you proposed. Whether that is still a good plan for your application is for you to decide. I just wanted to emphasize the impact of going with more aggressive controllers in conjunction with the bigger motors motomech recommended. You can evaluate the motors you proposed with the simulator but with a nominal 450lb load, I think you can guess the result....

FWIW: At these higher controller currents and a gear motor with a heavy load I would recommend a CA3 so you get throttle ramping to take the strain off the clutches on getaway if you use these larger controllers. Frankly, the weight of this vehicle is on the outer limits of ebikes and presents a number of unusual design challenges for 2WD and gear motors. In the end you may just wish to go with a single big DD motor for this first build...

Thank you for all the input, I am digesting it all and will come back with questions. It seems immediately obvious from both your experience and the time that you have taken with the simulator that I have to consider a higher voltage and more powerful motors.
I live on Cortes Island where the hills are very steep but short about 1k.
The cell weight is 28lbs the Vruzend 12.5lbs, I can keep the battery at 45lbs, 18bls of motors,the bike and side car will be aluminum, with some steel articulations and I would like to keep its weight at 60lbs. I think that I can keep the weight of the entire rig to under 130 lbs.
I weight 160 I think that a 300lbs single loaded weight is reasonable and 450bls with a passenger or a load of seaweed is reasonable.
it will be a narrow machine 30/32" wide.
The current e bike laws in Canada do not take into considerations the diversity of electric bike design needed to meet the needs of rural or interface living. The 3 wheels and pedal rules are judicious, the number of motors allowed and their wattage has to be increased for load carrying, but not for speed.
I hope this design will make a contribution towards that end.

Your plan is for a battery well large enough to handle 2000w, assuming each motors max is around 1000w. So one battery will work fine.