[Dan-T]

I'm in the early stages of building an EV and about to buy the battery.
I have a 150kw capable motor/inverter, and main priority is acceleration performance over range.

I'm looking at using Leaf Gen4 modules.

I could buy 24 Gen 4 modules to make a standard 40kwH Leaf pack
That's two sets of 96 series Cells in parallel at 350v

My budget could stretch to 28 Gen 4 modules to make a 46kwH pack
two sets of 112 cells in series at 408v.


As mentioned, I'm not so bothered about range (expect to recharge every night) so the extra 6kWh is not important, but my thoughts were that by having a higher pack voltage I can extract more power at a safe current for the battery.

My question to those with more experience than me is:

Is it worth spending extra money and carrying the extra weight of 4 additional modules for the benefit it will give me? Considering that acceleration, longevity and cost are important to my project.

Thanks
Dan

 

[cricketo]

longevity

Extra modules would allow you to lower per cell charged voltage while keeping the capacity. That would extend the life of the entire pack.

 

[john61ct]

There are so many factors that inter relate for longevity.


But yes reducing the C-rate, peak and average, is a big one.

And bringing top charge per-cell voltage cutoff down by at least half a volt will also help, but not much point going below say 4.125

Increasing the average DoD is of huge benefit, stopping at 3.5Vpc rather than 3.0 could double or triple cycle lifespan

All these factors are helped by greater Ah capacity, but voltage increase not so much.

The biggest killer is a lead foot, and making higher performance available usually exacerbates that.

A light touch, gentle acceleration and minimising use of braking is really the key to longevity.

 

[Dan-T]

Ok that makes sense, so by having more cells in series I have some flexibility to charge to a lower voltage while still achieving over 350v pack voltage, and/or stop discharge at a higher voltage will both keep the battery happier for longer. Is that what you mean?

You mention that it would be better to increase Ah than voltage, so running more cells in parallel.
I did look at that but there was no way that I could divide the cells up into more equal parallel strings with the number of cells I can afford, without reducing the pack voltage too low.
The only thing I could do would be to have uneven strings, so keep 2 96 cell strings, then add the additional 4 modules in parallel (even then they are 2s2p within the modules so wouldn't all be parallel). Would that work though? Will it not cause problems for charging etc. To have uneven strings?

 

[Frank]

Keep the strings even and make sure your controller can handle the highest voltage it will see. Most of us don't drive enough to use even a fraction of the pack life anyway and better cells are always coming if you do ever need to change the pack.

Sent from my SM-T380 using Tapatalk

 

[Dan-T]

Yeah I guess longevity isn't really that important in the long run.

I guess the question is,. For the benefits listed above for having the extra cells in series, is it worth spending an extra £1000 buying more modules?
Does it give £1000 worth of benefit? And does it give any benefit at all in terms of performance?

 

[john61ct]

As already stated, going to a higher voltage increases the potential for higher performance

it does not really reduce stress on the system, and now you're saying you don't care much about longevity

As already stated, increasing Ah capacity - more cells in parallel - is what will give you longer range.

So besides the fact that no one but you can say what is "worth"an extra grand

it does seem to me the answer is no.

 

[brian_]

Increasing total energy capacity - whether that is more cells in parallel or more cells in series - is what will give you both longer range and lower "C rate" (discharge rate relative to battery capacity). Each cell doesn't care whether the current that is produces is combined with the current from other cells in parallel or series, only how much current that cell is producing.

 

[onegreenev]

I'm in the early stages of building an EV and about to buy the battery.
I have a 150kw capable motor/inverter, and main priority is acceleration performance over range.

I'm looking at using Leaf Gen4 modules.

I could buy 24 Gen 4 modules to make a standard 40kwH Leaf pack
That's two sets of 96 series Cells in parallel at 350v

My budget could stretch to 28 Gen 4 modules to make a 46kwH pack
two sets of 112 cells in series at 408v.


As mentioned, I'm not so bothered about range (expect to recharge every night) so the extra 6kWh is not important, but my thoughts were that by having a higher pack voltage I can extract more power at a safe current for the battery.

My question to those with more experience than me is:

Is it worth spending extra money and carrying the extra weight of 4 additional modules for the benefit it will give me? Considering that acceleration, longevity and cost are important to my project.

Thanks
Dan

What are you using for your motor/inverter? What vehicle?

 

[remy_martian]

That extra voltage is next to useless if the car is not geared to exploit the extra HP the pack can deliver, though I suppose it could compensate for "voltage sag" under high acceleration rates.

Since you told us nothing about the car, its use, mission profile, your goals, every post on here, including this one, is a total stab in the dark.

 

[Dan-T]

Double post

 

[Dan-T]

I wanted to understand the theory behind any advantages in running more cells in series etc first without getting sidetracked on the specifics of the car.

The car is a Lexus GS450h
Motor and inverter are both the original units from the GS450h
It will be used for a daily commute of 50 miles and the project goal is to create a RWD EV that handles well and can demonstrate to my petrolhead peers that EV's can be fun, for less than £15k. It must have enough power to break traction and achieve a reasonable 0-60 time (hoping for <7s).

So this gives a criteria of:
Range: >60 miles
Power: 150kW
Cost: as low as possible
Must be safe

 

[remy_martian]

You're in Chevy Bolt territory for power, which has a 0-60 of 6.3 sec at 3700lb. That says your Lexus can't weigh more the about 4000lb to get to 7 sec, assuming comparable gearing and tire sizes (or "effective gearing", since the Lexus has larger tires). Range is 259 miles on 66kWh, so 20kWh pack should work for your 60 miles. 40kWh is excessive (double the weight you need).

I say keep the weight down and minimize the module count.

 

[zepol_wube]

Our conversion, lopified.blogspot.com used a 2013 Leaf pack in a 2003 VW Golf. We had difficulty finding a voltage invertor that would drop 403 to 13.8. Invertors rated for 350 were more abundant. Agreeing with Remy, you are carrying extra weight for your goal, our 2013 pack is only 24kwh and we are getting 80+ miles running 3.1V-4V. The Lexus frame is likey good for the extra weight, we only gained ?207lb? after all the ICE removal. You could add the extra cells... BUT if you are using the GS450h parts, what they expecting for voltages may be your setting factors. Are you using the Leaf BMS? We did and identified two modules failing, if you change the count of modules you will need a matching BMS and that may be your limiting factor. ALWAYS run a BMS!

 

[Dan-T]

The Lexus inverter can accept well over 400v so that's ok, BMS I was going to use Orion which I think should be ok for extra modules. 40kwh of gen4 leaf batteries only weighs 36kg more than 24kwh of gen2 leaf batteries, so I'm happy to stick with at least the 40kwh. Due to the internal configuration of the cells within the modules I can't make 20kwh of gen4 cells while keeping 350v.
I can use the converter in the Lexus inverter to step down the pack voltage to the original lexus hybrid battery voltage in order to run its original DC/DC converter

This was mostly the reason I didn't give the details of the car as it distracts from the original question which was more theoretical about what the advantages were of running more cells or higher voltage in terms of being able to safely pull more power from the battery.

 

[remy_martian]

Are you sure about that added weight? 18kWh of Tesla modules is about 75kg. Seems like you're off by a factor of 2 (2.2?).

Higher voltage means proportionally more HP and taller gearing (if available) for better acceleration. Keeping the mass down directly affects acceleration...you'll lose one or two tenths adding that "36kg" mass. Yes, the car matters. As does every tenth of a second.

 

[zepol_wube]

Well, to your original question, Power = Voltage x Current. Voltage is free, Current requires copper and cooling. We significantly over coppered and require cooling only at highway speeds in 85+F°. Going with higher voltage is much easier. I will say, we are currently stuck as we lost two modules from our pack (0 Volts!) the other Gen 2 modules are still testing out at ~90% SOH. We are not finding any aftermarket modules above 70% and new is $400.ea! By extra and put a few aside.

 

[Dan-T]

Are you sure about that added weight? 18kWh of Tesla modules is about 75kg. Seems like you're off by a factor of 2 (2.2?).

Based purely on the mass I have for Leaf 24kwh and gen4 40kwh cells yes. Tesla modules are irrelevant to the comparison. I have leaf modules available to me

 

[Dan-T]

Well, to your original question, Power = Voltage x Current. Voltage is free, Current requires copper and cooling. We significantly over coppered and require cooling only at highway speeds in 85+F°. Going with higher voltage is much easier. I will say, we are currently stuck as we lost two modules from our pack (0 Volts!) the other Gen 2 modules are still testing out at ~90% SOH. We are not finding any aftermarket modules above 70% and new is $400.ea! By extra and put a few aside.

Thanks for this, it's both useful and very relevant to the original question!

Regarding your comment about adding copper for increased current, is it only the wiring that needs to be improved to draw more current? Are the leaf cells themselves OK?
I saw somewhere a max current draw quoted for a leaf battery, is that just a case of the max current for the internal wiring/busbars? And actually the cells/modules themselves can run a higher current? What's your experience with the 24keh leaf cells so far?

 

[john61ct]

You want to reduce not increase current draw especially for durations more than a second or two.

Besides the obvious crippling of range

increased C-rate stress on the cells means more frequent replacement required.