Dual Motor Battery Pack- Single or Double

Hi Guys,
Been working on a dual motor project bike over the winter incorporating a front 3k motor and a rear 5k motor. Both being powered by 2 linked Adaptto controllers Mid-Max. The build worked out great and now its battery time.
I intend to run this setup in 32s utilizing Samsung 40T 21700 cells to max both controllers. As this is my first dually Im not really sure which way to go that would be more efficient. A single 150A pack, a 300a pack or 2 smaller 150A packs.
The most obvious being the two separate packs and that would workout fine if both motors were running continuously full time. This is not the case in this application as the front motor is only used to get the bike up to 35mph, then all power is diverted to the rear controller to run the rear motor only.

Which would you believe to be the best coarse of action?
And thanks in advance

One big pack is easier on the cells.

Agree, even when running just the back motor, the cells will be happiest spreading the load among them all. Make it as big as you can possibly fit.

Rule one for all batteries, whatever they spec, the cells aren't happy till you are running half spec, or less. It won't be very long reaching 35, 8) so the cells will get some relief when the load drops 3 or 4k. Chances are by 40 mph, you won't pull the full 5k on the back motor anymore.

The front motor (assuming it's a direct drive hubbie) is being driven 100% of the time and incurring the iron losses 100% of the time, so there's no point in having it cut out at a lower speed than the rear. In fact, the system as a whole will be most efficient only with both motors pulling all the time.

Think of it this way. Losses other than what you give up to friction and the wind are in two forms of heat. One is the iron losses, which increase generally linearly with rpm and occur whether the motor is powered or not. The other is copper losses, which increase with the square of current (resistance X current squared). A given power at the wheel(s) is required to cruise at any given speed, and you can look at both motors together (and controllers too for that matter) as being 2 lengths of copper wire in parallel. Being in parallel that means lower resistance, and lower copper losses, so you want the power from the battery to flow through both motors all the time.

As far as the battery goes, definitely one big battery to the motors. However 32s chargers are much more expensive, and the cheapest way to charge it is probably via 2 16s chargers, so you may want to tap the pack at the mid-point for 2 chargers. Plus it's also always nice to have backup, so if one charger craps out you can still charge each half with the functional charger. I'm a cheapo, so I always opt for dual chargers and charge taps for my high voltage builds.

vipertriplex said:

A single 150A pack, a 300a pack or 2 smaller 150A packs.

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Someone please explain like I'm five.

AFAIC batteries are spec'd in Ah (mAh)

The so-called "max discharge amps" rating seems to largely be fictional, or more diplomatically,

unsuitable for apples to apples comparison between brands.

So, in the context here, does the above translate to

"a pack suitable for my plan to actually draw a peak current of 300A"

or something else?

And why don't people in these circles talk about the Ah storage capacity right off the bat?

"a pack suitable for my plan to actually draw a peak current of 300A"

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That is correct. At least that is what I read.

And why don't people in these circles talk about the Ah storage capacity right off the bat?

Click to expand...

The amp-hour or capacity specs are generally only used when considering the range of a build or to give you a rough idea of how many cells are used in the pack if the cells are known. For instance: lets say you are talking Samsung 30Q cells (a 3000 mah cell rated at 15 amps discharge). If you see someone mentions a 52V 15aH pack, you can safely assume they are talking about a 14S5P pack if you know they are working with the 30Q cell.

In the original question, the main concern is health and performance between "A single 150A pack, a 300a pack or 2 smaller 150A packs." So maximum discharge current is most relevant here. Overall energy capacity or "amp-hours isn't really a concern in this case.

Very strange to me.

Longevity being my primary concern, if I want higher discharge rates I increase Ah capacity.

But in this use case, minimizing weight must prevent that approach.

It can get very confusing. One thing that helps is to start thinking in watts, and watt hours, especially when comparing batteries of different voltages. 36v 20 ah and 48v 15 ah are the same number of cells. Both about 750 watt hours of capacity. So range is similar with either one. When figuring out range, total watt hours you have is the number you need, and the watt hours per mile the vehicle will use, at a given speed. Typical e bikes use 20 watt hours per mile going 18-20 mph, and 35 wh/mi going 25- 30 mph.

But yes, you do have it right, any single cell puts on just so much amps, and to get a pack able to provide more amps, more cells, which means more ah. In the above example, the 36v battery can put out more amps, but since the voltage is higher in the 48v pack, less amps puts out more watts. In the end, both packs are able to put out about the same wattage, as well as the same watt hours. But if you go faster with the 48v, then you use more watt hours per mile.

Gets kinda crazy, with three variables in play. volts, total watt hours, and watt hours per mile. This is why the simulator on the grin technology website is very useful.

Well getting away from the Propulsion use cases.

Say a boat with no big on-demand ICE source, biggest load is the windlass, has to haul the anchor up say 80 yards from the ocean floor. Takes 400 peak then 200A at 24V.

No marine sparkie would talk or even think about a "400A" bank, much less how many Amps each cell might put out. The main factors are Ah consumed per anchor-haulout, and Ah per day, since

there are dozens if not hundreds of other loads on the boat, including line winches, computers, fridge other kitchen appliances, lights & fans, so the overall Ah per day consumption is 99.9% going to be the sizing factor. Note volts are constant, so no need to convert to kWh.

Traditionally lead is the chemistry of choice, but yes the need for fast-discharge may dictate AGM over FLA, especially if

the size / layout of the battery compartment, and the impact of weight on the boat's sailing ability are critical, and an 800Ah bank is impractical, maybe only 600Ah will fit.

The use of an LFP bank will be a game-changer, allow a 1200Ah bank in the same space and weight.

So when I see "200A bank", takes a while to wrap my head around that being a valid way to think,

especially since we need to take published claims about max discharge rate with kg of salt, and

it seems to completely ignore range, which I would think a critical factor?

But since this use case seems to generally use a narrow range of cell sizes and chemistry types, I suppose the better the (actual) discharge rate, the higher the energy storage density anyway, and

just like with solar panels "as much as you can fit" rules, weight constraints giving a pretty absolute limit if talking about a bike / scooter.

I would certainly like to thank all you guys for your input and ideas.
Seems the I's have it on the Dump Truck Pack which is going to require the reading of a few more threads to insure its done properly, along with the help from a few of you volt/watt speed freaks.

John in CR said:

The front motor (assuming it's a direct drive hubbie) is being driven 100% of the time and incurring the iron losses 100% of the time, so there's no point in having it cut out at a lower speed than the rear. In fact, the system as a whole will be most efficient only with both motors pulling all the time.

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I I found your remarks to be interesting John as this is in fact a dual hubbie only they were not paired to mach, they were paired to accent one another. Ill explain. The rear motor is a one off 50mm in 17KV with a whopping top end but seems to lack in bottom end torque converting a good majority of my precious power to heat before it can finally stretch its legs. The front motor is a 40mm in 12KV that has superb bottom end intended to help the rear get to speed. My concern here was the jump in KV. As this is my first dualie running linked Adapttos Im just not sure of its capabilities of making consistent calculations to match rpm fulltime. If it produces drag, whats the point. Please, let me know if I got this wrong...

john61ct said:

vipertriplex said:

A single 150A pack, a 300a pack or 2 smaller 150A packs.

Click to expand...

Someone please explain like I'm five.

AFAIC batteries are spec'd in Ah (mAh)

The so-called "max discharge amps" rating seems to largely be fictional, or more diplomatically,

unsuitable for apples to apples comparison between brands.

So, in the context here, does the above translate to

"a pack suitable for my plan to actually draw a peak current of 300A"

or something else?

And why don't people in these circles talk about the Ah storage capacity right off the bat?

Click to expand...

Sorry if I threw you off but capacity was never really an issue as I new I was going to end up with a high enough capacity in order to get to 300A. 40AH to be exact using 40T cells. This is not an about town two wheeler. Its soely intended for strip racing. 1/8-1/4 mile

pwd said:

In the original question, the main concern is health and performance between "A single 150A pack, a 300a pack or 2 smaller 150A packs." So maximum discharge current is most relevant here. Overall energy capacity or "amp-hours isn't really a concern in this case.

Click to expand...

Shot it rite between the eyes!

Anyone with a big pack design?
Thanks again guys

vipertriplex said:

This is not an about town two wheeler. Its solely intended for strip racing. 1/8-1/4 mile

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Wow, talk about specialized, use case! Didn't even know that was a thing, sounds very cool. To watch

Be careful out there!

Its solely intended for strip racing. 1/8-1/4 mile

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In that case, if you haven't already committed to the Samsung 40T, take a look at the Samsung 30T; they appear to have more punch.

pwd said:

Its solely intended for strip racing. 1/8-1/4 mile

Click to expand...

In that case, if you haven't already committed to the Samsung 40T, take a look at the Samsung 30T; they appear to have more punch.

Click to expand...

This was my first choice as well as I have 2 smaller race packs that are simply outstanding. Seems Im not the only one that thinks so as my supplier always seems to be sold out. Thanks to your reminder I just got off the phone with my supplier who will be receiving a restock in the coming month... Thx again

No time to fully answer now, but you don't want mismatched motors like that unless you either run them at different voltages or the wheel diameter is different by the same ratio as the Kv difference.