# Getting ready to buy batteries soon



## lonestarrpm (Aug 18, 2011)

I'm in the same boat -- I'll take a stab at it. Note I have no experience
but I'm doing the same calculations -- hopefully the grognards here
can infuse their wisdom, yet still keep things at our "novice" level of understanding/terminology.

I'm considering CALBs as well.

Here is what I have been considering.
Take your motor max voltage as your controller output voltage.
Take your max (burst) current of your controller to be your max motor amps.

Let's say your motor is a WARP11 (or 9) and you want to hit the "safe" max of 156V

Multiplied by your 1000A -- is 156KW -- for a brief period of time (10-30 sec?)

Now you have a wattage you are going for.

Figure out what battery aH you are most likely to use. Let's say 130Ah
and they can do ~8C burst, or 1000Ah -- so from a current perspective
you are meeting your requirement.

Now compute your battery pack voltage. I'd like to try to obtain
the motor max V after sag -- if I look at the CALB battery discharge curve lines -- it looks like sag can be approximated at 3V,

(EDITED: I should be clear here -- there are two sags to consider. The battery voltage sagging and the IR drop in the system.
we can consider the IR drop as cell resistance * Amps (~0.5mohm/cell * Amps or 0.5v per cell))

Let's say your motor is a WARP11 (or 9) and you want to hit the "safe" max of 156V

divide by your sag pack voltage -- 156v/3v = 52 cells with IR 156v/2.5v = 63 cells 

Now most folks talk about their pack voltage being V nom of a cell * # of cells.

CALBs vnom is 3.2v so you've built a 201.6V pack.

Now as I understand it -- you can build a larger voltage pack, and trade that off for current draw.

So lets say instead of 8C, you want to limit it to 6C peaks, or from 1000A to 750A. You'll need 208V to your contoller, 156*8/6
but now your cells are delivering 3v-(0.5*0.75) = 2.625v -- NOTE -- you are drawing less current, so your IR drop reduces.

So to get 208v -- 208/2.625v = > 80 batteries

You'll now ship in 208V to your controller under sag (and the cells might not sag as much at 6C vs 8C, but that's just a second order benefit).
But now you'll be running 750A

210*750 = 157.500kW

your not taxing your system quite as hard, you'll have some extra range since your pack is 27% larger -- but you'll have to tote around 
the extra weight and absorb the extra battery costs.

You can also use different cell sizes 180Ah for example and play the same game. I do believe it is harder on the motor controllers to upconvert voltage
(I could be wrong), so my rule of thumb is you have to at least have as much voltage in your batteries as you want to run your motor at.

Anyways -- that is how I understand things.

cheers,

Mike



notailpipe said:


> Hi All,
> 
> A couple months ago I exited "save money" mode and "finish other projects" mode and entered "start building the damn EV!" mode.  So far I have ripped out the engine and mounted my WarP9.
> 
> ...


----------



## notailpipe (May 25, 2010)

lonestarrpm said:


> Here is what I have been considering.
> Take your motor max voltage as your controller output voltage.
> Take your max (burst) current of your controller to be your max motor amps.
> 
> ...


Hi Mike,

Thanks for taking a stab at it. I think your overall method (conserve power in the systems) is correct, but I'm not sure I'd aim at this point (peak volts AND peak amps), because it ends up in a battery pack that's over $10K (which may work for you, that's not wrong in itself, but it's over my budget). This is without BMS on 80 cells as well... But mostly I think it aims at a battery that might be overkill for hard acceleration.

For good acceleration, we're going to be pulling high amps but the voltage will be low in the beginning, starting from 0RPM, because motor voltage is proportional to RPM. Do you know what RPM is max spec for the WarP9, and if that correlates to the 156V case?

For example, just pulling numbers out of my butt here, but say on the way from 0 to 60mph, you never exceeded 4000RPM (because of shift points), and at that voltage you were more like 100V. Then you'd be aiming at 100kW and your battery pack only needs to be 2/3 that size.

I think I need to try to pin down some real numbers first. Unfortunately, finding curves for these things at non-golf-cart voltages is impossible. I have tried those calculators before with varying results, but maybe I'll try one again and see what it calculates my peak motor (controller) amps is to achieve a desired 0-60mph time, and then I'll use that and your concept of power conservation to translate that into peak battery amps.

At that point there is still the question that which range, 5C or 8C, is "better" for pushing the instantaneous current. Obviously 5C will be less harsh on the batteries, but when they list their charge cycles, they only mention DOD%, not how high you ever pushed the discharge current...


----------



## DIYguy (Sep 18, 2008)

A 1000 controller that is maxed, is at 1000 battery amps also.. . ok, minus losses... but close enough.

180 ah can handle 1000 amps for short periods. I have done it with Calbs many times. I did up size the pack a bit...so at full voltage sag..it hits my motor volt limit at about 170 volts. . . about 228 hp. 

Actually, I have pulled a fair bit more than 1000 battery amps also. My Soliton (early version) had the issue with current sensor being affected by externally run conductors close to parts of it. My CA may have been a bit off, but I have seen it read over 1200 amps several times and over 1300 a few times. Of course the duration of these events is pretty short.


----------



## dimitri (May 16, 2008)

notailpipe said:


> I haven't put my desired budget/range in here because, like I said, I'm not looking for an exact Bill of Materials from anyone, but just general advice how to figure out what is the relationship between the two currents.
> 
> Thanks!


Look at it from different angles. Don't think about motor in amps/volts, think in Watts. How much horsepower does a car like yours need to accelerate at acceptable rate? Convert that number to Watts. This is how much power battery has to provide for short bursts. Now, at given power level you want to maximize voltage and minimize current ( within reason ). Voltage gives you idea of cell count and current gives you idea of cell size ( taking C rates into account ). Since cell sizes are limited you pick a combination of cell size and count that approximates to the power figure. This also gives you AH count, which determines range. Now you can see if you can afford this pack and fit into the car, if not, then scale down from there.

This is how I usually approach it and it worked very well so far.


----------



## lonestarrpm (Aug 18, 2011)

dimitri said:


> Look at it from different angles. Don't think about motor in amps/volts, think in Watts. How much horsepower does a car like yours need to accelerate at acceptable rate? Convert that number to Watts. This is how much power battery has to provide for short bursts. Now, at given power level you want to maximize voltage and minimize current ( within reason ). Voltage gives you idea of cell count and current gives you idea of cell size ( taking C rates into account ). Since cell sizes are limited you pick a combination of cell size and count that approximates to the power figure. This also gives you AH count, which determines range. Now you can see if you can afford this pack and fit into the car, if not, then scale down from there.
> 
> This is how I usually approach it and it worked very well so far.


Some quick math.....

1000A * 156V (Warp 11 recommended max) = 209HP
1200A * 156V = 251HP -- A Boxster S (my conversion is 255HP 0-60 5.6ish?)
1400A * 156V (max Amps for a Soliton (sustained I think) = 292.8HP -- close to what new Boxsters have in HP

1400A * 170V (absolute Max Volts for Warp 11) = 319.5HP

Now I believe the Warp 11 is about 87% efficient -- so I have to dial back 
those HP numbers accordingly -- right?

292*0.87 = 254.7HP which is exactly what my Boxster S is rated at 

Boxster S ~255HP 6800 rpm, 229ft/lbs @ 2000 rpm

So if I keep the weight the same (big if!) -- then I should see similar 0-60 times. Maybe a tad better at I'm sure having all my torque at 0 RPMs will make it behave differently if the rubber is up to snuff.

So am I right thinking I need all 156V at time 0 while I pull as many AMPs as I can generate?

thanks,

Mike


----------



## dimitri (May 16, 2008)

See, you are focusing on motor amps/volts again and you take max values for both, which is not realistic for street vehicle, I assume you aren't planning to replace motors after every few races 

My point is to look at power and look at it from battery perspective alone, setting aside motor parameters for a moment, afterall battery will likely be the bottleneck anyway due to cost and weight/volume. Again, I am talking normal car here and normal budgets, not crazy racing world.

So, if you want peak power at 254HP = 190kW, that is what you demand from the battery for short bursts of power. What is the max C rate you want to subject the cells to? This is controversial since it depends on level of risk you willing to take your investment to. Assuming CALB cells, it can be between 4C and 8C. So, you will need a pack of CALB cells between 24kWh and 48kWh to get this much peak power, depending on your max C rate. Now, you want to maximize voltage up to controller's limit, while using standard common cell sizes. Say 100AH cell for example, you need 75 cells for 24kWh or 150 cells for 48kWh. Now you see why few people build Porshe like EV conversions, you need too much battery. Take 80% DOD into account and give some slack to C rate and you are looking at 90 cells of 100AH, which is 288V nominal, perfect for Solitons. At $1.25/AH this is $11k pack. So, why bother talking about motor amps/volts when battery is the limiting factor anyway? Take this 90 cell 100AH pack as rough baseline for high performance car and then scale up or down based on budget and room/weight limits.


----------



## notailpipe (May 25, 2010)

dimitri said:


> Look at it from different angles. Don't think about motor in amps/volts, think in Watts. How much horsepower does a car like yours need to accelerate at acceptable rate? Convert that number to Watts. This is how much power battery has to provide for short bursts.


Thanks, Dimitri, I think the power viewpoint makes sense since we're talking about the equivalent energy (minus conversion losses) to get something up to speed.

Have you found a good calculator that you've seen has real-world correlation? You mention that this method works well for you, so I'm wondering if you've ever gone back and seen how close the 0-60mph times are to what the calculator said it should be. I'd be interested to know if you've found one that matches (close enough) real world data.



dimitri said:


> What is the max C rate you want to subject the cells to? This is controversial since it depends on level of risk you willing to take your investment to. Assuming CALB cells, it can be between 4C and 8C.


What effect to the peak/burst current draws have on the battery lifetime? Any? The battery manufacturers spec the cycles of a battery based on how low you pull it (DOD) but they don't seem to indicate that the RATE you pull it to that level has any effect on lifetime. If there was little effect on lifetime between 4C burst pulls and 8C pulls, then you could buy the smaller cells (giving 8C) and buy more of them (giving higher pack voltage and thus top speed (plus less IR drop at higher voltages)) since you have a fixed budget. To first order, this method will keep the range the same for either case. Any reason I shouldn't use that as my strategy?


----------



## dimitri (May 16, 2008)

notailpipe said:


> If there was little effect on lifetime between 4C burst pulls and 8C pulls, then you could buy the smaller cells (giving 8C) and buy more of them (giving higher pack voltage and thus top speed (plus less IR drop at higher voltages)) since you have a fixed budget.


This is wrong because you again not thinking in power terms, if you decrease cell size and increase count you get same C rate because increased voltage means decreased current at given power level. Manipulation of cell size/count within same budget will give you same power and same range, so there is no point in going too small on cell size, plus you will be limited by controller voltage anyway and get too many connections to manage. I personally would not go less than 100AH for a typical car, so if I locked myself at 100AH cells, then cell count/voltage will be driven by budget and weight/volume limits and also controller's voltage limits.

I don't know of any calculator, the math I use is so basic that you don't need any special formulas. These are basic rules of thumb for typical car with typical budget, we are not talking extremes cases where every part is running at its limits.


----------



## dimitri (May 16, 2008)

As for high C rate effect on cell's life, its all about thermal management, high C rate causes heat buildup inside the cell and it breaks down electrolyte if allowed to build up too much. Prismatics in plastic cases are difficult to actively cool down, so you just need to limit C rate and how long your max bursts are. There is no single answer, some people cook their cells and claim its OK, I personally am conservative and try to keep C rate low. There is also aspect of range vs. power, which is more important to you? I usually work on conversions where range is more important, so low C rate is a natural requirement. High C rate means you won't be driving for too long between charges.


----------



## tomofreno (Mar 3, 2009)

You might want to keep Jim's (rebuilds Warp motors for racers) comments regarding max voltage for Warp9 in mind:
http://www.diyelectriccar.com/forums/showpost.php?p=19647&postcount=2


----------



## lonestarrpm (Aug 18, 2011)

As I understand it you can go with the minimum cell that will get you your peak amps and voltage, you just might not be able to do it very long.

M



dimitri said:


> This is wrong because you again not thinking in power terms, if you decrease cell size and increase count you get same C rate because increased voltage means decreased current at given power level. Manipulation of cell size/count within same budget will give you same power and same range, so there is no point in going too small on cell size, plus you will be limited by controller voltage anyway and get too many connections to manage. I personally would not go less than 100AH for a typical car, so if I locked myself at 100AH cells, then cell count/voltage will be driven by budget and weight/volume limits and also controller's voltage limits.
> 
> I don't know of any calculator, the math I use is so basic that you don't need any special formulas. These are basic rules of thumb for typical car with typical budget, we are not talking extremes cases where every part is running at its limits.


----------



## dimitri (May 16, 2008)

lonestarrpm said:


> As I understand it you can go with the minimum cell that will get you your peak amps and voltage, you just might not be able to do it very long.
> 
> M


That is correct, but those are battery amps and volts ( i.e. power ), not motor. Controller converts power from the battery to power to the motor, so when figuring battery size/power its useless to speak of motor amps/volts, you only end up confusing yourself.


----------



## lonestarrpm (Aug 18, 2011)

dimitri said:


> That is correct, but those are battery amps and volts ( i.e. power ), not motor. Controller converts power from the battery to power to the motor, so when figuring battery size/power its useless to speak of motor amps/volts, you only end up confusing yourself.


Totally get that.

As I understand it
Mist motor controllers allow
Batt v >= Motor v

It will down convert (buck regulator) Voltage
And increase Motor Amps simarily.
But not other way around.

Neglecting controller losses

Batt W = Motor W

Mike
So 

Batt V/Motor V = Motor A/Batt A


----------



## notailpipe (May 25, 2010)

dimitri said:


> Manipulation of cell size/count within same budget will give you same power and same range, so there is no point in going too small on cell size, plus you will be limited by controller voltage anyway and get too many connections to manage.


That makes sense. So the only other variable left is BMS cost. You know I already want your miniBMS system, so I'm sure you can answer this objectively... 

If everything else is the same, what would happen (say) at the other extreme where I buy 300Ah TS batteries until I run out of money... that would save some money on BMS. For 100Ah cells, it's about another 10% cost per cell, but for 300Ah cells, it's about 3.5%. My understanding of the results (please tell me if this is wrong) is that my range and acceleration should be the same, based on what we've discussed, but my overall pack voltage will be lower and this will limit my top speed. If that top speed is sufficient for my needs, any reason not to do this?


----------



## dimitri (May 16, 2008)

notailpipe said:


> That makes sense. So the only other variable left is BMS cost. You know I already want your miniBMS system, so I'm sure you can answer this objectively...
> 
> If everything else is the same, what would happen (say) at the other extreme where I buy 300Ah TS batteries until I run out of money... that would save some money on BMS. For 100Ah cells, it's about another 10% cost per cell, but for 300Ah cells, it's about 3.5%. My understanding of the results (please tell me if this is wrong) is that my range and acceleration should be the same, based on what we've discussed, but my overall pack voltage will be lower and this will limit my top speed. If that top speed is sufficient for my needs, any reason not to do this?


You are correct, you can go the other way and beef up cell size and reduce voltage, while keeping generally the same power, however, lower voltage means higher current, which means more heat loss in battery connections. Also less motor voltage ( since motor voltage is <= battery voltage ) means less power at high RPM = less top speed and poor acceleration at the top.

My example of 288V was for performance conversion, which I thought was your goal. You can find a happy medium by lowering voltage to say 192V ( 60 cells ) and either keeping 100AH size ( less range and less acceleration ) or going for next cell size to balance the power.


----------



## dimitri (May 16, 2008)

Another issue with lower voltage is motor losses since you will end up running motor at lower RPM and use higher gearing to get desired speed. This will result in lots of motor heat and wasting precious electrons.


----------



## DIYguy (Sep 18, 2008)

Just a few thoughts to consider.....

First off, your motor will take more than 156 volts. You can do 170 volts.... especially if you get the Helwig brushes. (Claims of 192 are out there). As long as you have about 12 degrees advance brush timing.

Next, at 1000 amps, your voltage will sag to about 2.6 vpc with the prismatics. So, if I were you, I would try and see if I could fit 66 of the 180 Ah cells in there. It may be too many for your car, but a good goal. 66 * 2.6 = 171 volts. Also, 1000 amps is 5.5 C rate on a 180 Ah cell. Normal driving will be 1C or less most of the time... except on accel or hills. If you can't get the 180's in... try the 160's. . . but still try and shoot for about 65 or 66 of them. You may have to go to 100 ah... but that will be 10C and that's pushing it. Either way, you can't pull those numbers for very long.

Last thing I wanted to mention.... you don't need to shoot for the same peak HP as an ICE. Since the torque curve is so much different, less HP will feel like more (as compared to ICE). Remember, the peak HP #'s on ICE ratings are typically much higher up the rpm scale. They don't develop much at the low end.

Just some food for thought.... based on my experiences....


----------



## lonestarrpm (Aug 18, 2011)

When you are talking sag, do you mean cell voltage reduction AND IR drop
If so I'm calculating that in. Basically sag to 3v due to energy loss and the resistive IR drop of 0.5mohms*1000 amps

I'm figuring 75 cells drops me I to the 156v range at 1000 amps at motor and battery amps
Ironically, same result at 800 amps due to reduced IR drop of pack allows controller to do 1000 a

Where would I find the brushes you are talking about?


----------



## spdas (Nov 28, 2009)

Aloha, I have basically the same setup as you and very happy with my 46 x180ah calbs. They are the same physical size as TS 160 and the new ones are coming in now at 204ah. I get around 90+ miles @ mostly 60mph with them. Top speed 80+mph (it does 75 easily and probably could do 95 on the flat but 95 is a bit scary in this Yaris -- the one time I was going down a hill). But at 95mph (according to the speedo), I was only at 4900rpm.

Francis


----------



## DIYguy (Sep 18, 2008)

lonestarrpm said:


> When you are talking sag, do you mean cell voltage reduction AND IR drop
> If so I'm calculating that in. Basically sag to 3v due to energy loss and the resistive IR drop of 0.5mohms*1000 amps
> 
> I'm figuring 75 cells drops me I to the 156v range at 1000 amps at motor and battery amps
> ...


I haven't tried to measure any changes in resistance...particularly dynamically. I'm chiming in to share real world data... .no need to calculate that. When u pull 1000 amps from a bank of 66 x 180ah Calbs cells, the voltage will sag to 170 volts. That's a fact.

Helwig brushes are now an option on new Netgain motors. If you have a different motor (like I do - a GE), you can order them directly from Helwig. These are split, red top designs typically.

Smaller cells may exhibit slightly different behavior, but 75 x 180 ah cells will not drop to 156 volts at 1000 amps @ 100% duty cycle.


----------



## lonestarrpm (Aug 18, 2011)

DIYguy said:


> I haven't tried to measure any changes in resistance...particularly dynamically. I'm chiming in to share real world data... .no need to calculate that. When u pull 1000 amps from a bank of 66 x 180ah Calbs cells, the voltage will sag to 170 volts. That's a fact.
> 
> Helwig brushes are now an option on new Netgain motors. If you have a different motor (like I do - a GE), you can order them directly from Helwig. These are split, red top designs typically.
> 
> Smaller cells may exhibit slightly different behavior, but 75 x 180 ah cells will not drop to 156 volts at 1000 amps @ 100% duty cycle.


From my original Calculations -- I broke the two components of SAG out 
but the combined effect I modeled was 0.7v. This assumed 0.5mOhm of 
resistance. It is possible that the 180aH, which I assume is being built
out of parallel strings -- will have a lower internal resistance and
therefore a lower IR (or voltage SAG).

I've seen a range of 0.5mOhms as low as 0.3mOhms. I chose to use the upper value -- I'm an engineer -- although for this project I'm the bean counter too 

In the end -- that is why real world advice is the most prudent, 
but it is also wise to understand the reasons behind the results.



> Now compute your battery pack voltage. I'd like to try to obtain
> the motor max V after sag -- if I look at the CALB battery discharge curve lines -- it looks like sag can be approximated at 3V,
> 
> (EDITED: I should be clear here -- there are two sags to consider. The battery voltage sagging and the IR drop in the system.
> ...


----------



## dimitri (May 16, 2008)

You can't assume that IR is constant value under the range of conditions. It will rise under heavy load as cell is unable to supply enough ions fast enough and heat starts to build up as well. So , all this math is an approximation anyway.
IMHO, get as much battery as you can afford and fit into the car and use math as approximate sanity check. Shoot for higher voltage if possible, within reason.


----------



## DIYguy (Sep 18, 2008)

Dimitri is right...
Regarding your comments... my 180ah cells are individual.... not paralleled. The measured resistance is 0.31 to 0.36 mohm... and the nominal voltage would be 211.2 volts.


----------

