# 75 ah VW batteries?



## terrorr (Jul 17, 2017)

Hi folks. I have the opportunity to buy some VW eGolf batteries. 

My Ranger truck runs on 144 v AC51 motor. Here are the battery specs - 10 of these modules would give me 144 volts.

Each module contain 12 x 25AH cells in a 4S3P configuration

About the module (12 cells)
Nominal Module Voltage: 14.4 V
Maximum Module Charge Voltage: 16.4 V
Minimum Module Discharge Voltage: 12.0 V

Capacity: 75AH

My question is if 75 ah is enough to run the truck? I thought I read that 100ah is the minimum. 

Thanks!


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## john61ct (Feb 25, 2017)

Aroun the block maybe.

My camper's House bank is 800+AH.

Look at KWH instead, you're thinking at higher voltages


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## terrorr (Jul 17, 2017)

I guess I am thinking in terms of discharge rate (C rate?).

This pack will give me 10kWh. I only need 60km range (40 miles) .

Most of the info I have read about ah is in regards to range. More ah more range.

My concern is when I am accelerating or going uphill. Can 75ah batteries handle that? 

Do I assume a 1C rate, 2C rate, 3C rate, 20C rate? How do you calculate the min ah needed in a battery pack (not for range) but for the amps that getting pulled out? 

Hope these questions makes sense. Cheers.


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## john61ct (Feb 25, 2017)

Again, AH are not the unit to use when the voltage varies.

At 14V (likely 12V under a heavy discharge rate) your amps (and AH) are worth a tiny fraction of the usual EV propulsion amps, what 80V and higher?


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## kennybobby (Aug 10, 2012)

How heavy is your truck and how fast do you need to travel for those 40 miles of range?

Here's a general rule of thumb--your average consumption will be no better than (weight x .1) W-Hr/mile.

So if the truck weighs 3000 lbs, then you might expect to get 300 W-Hr/mile with conservative driving habits. For the 10kWh pack (144V x 75Ah), the most range you could expect is

10000 W-Hr / 300 W-Hr/mile = 33 miles.

But that 75Ah number was determined by running the cells from absolutely full to absolutely empty--which you would not want to do if you want to get many charging cycles out of the cells. Assume either 90 or 80% of usage of the cell capacity and that range drops to 26 to 30 miles. If you got to have 40 miles of range in order to commute or run your task, then you need higher capacity or voltage in your pack.

The discharge rate depends upon the specs for the cells, but most li-ion cells can provide 5C for short durations--you would need to find the datasheet for those cells to know for sure. You may be able to determine this from the specs for the egolf also, what is the pack size, the range and max power for which it is rated.


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## brian_ (Feb 7, 2017)

terrorr said:


> I guess I am thinking in terms of discharge rate (C rate?).
> ...
> Do I assume a 1C rate, 2C rate, 3C rate, 20C rate?


That concern is valid. The cells will only withstand a limited rate of discharge, relative to their capacity. But this is a peak demand issue, and to determine what current is sufficient for your vehicle, you need to consider it in combination with the voltage.

I think part of the problem is that discussions between people with a very specific interest often omit some critical assumptions. People with 144 volt (typically lead-acid) battery packs might discuss amp-hours of capacity, just assuming that everyone understands that it is at 144 volts.



terrorr said:


> This pack will give me 10kWh. I only need 60km range (40 miles) .


Now we're looking at range from an energy viewpoint, which is good, but there's the first problem... While each module has a nominal capacity of about 1 kWh and so ten of them will have a nominal capacity of about 10 kWh, I doubt that's enough for 60 km or 40 mi range. That's less capacity than a typical modern plug-in hybrid, and they have trouble going 60 kilometres or 40 miles.

Even if the nominally 10 kWh pack had a full 10 kWh of usable capacity, that would be 167 Wh/km or 250 Wh/mile, which seems unreasonably optimistic, especially for the aerodynamics of a pickup truck and the technology of this low-voltage system.




terrorr said:


> I guess I am thinking in terms of discharge rate (C rate?).
> ...
> My concern is when I am accelerating or going uphill. Can 75ah batteries handle that?
> 
> Do I assume a 1C rate, 2C rate, 3C rate, 20C rate? How do you calculate the min ah needed in a battery pack (not for range) but for the amps that getting pulled out?


If you know how much torque you need, you can look that up on the AC-51 performance chart to see what current is required, then divide that by capacity to get the "C" rate. How much is too much? I don't know; what is the maximum discharge rate in an eGolf?

Just from memory, the eGolf runs around 80 kW maximum, and presumably has 24 of those modules, so at peak it is only discharging the pack at around 3C (a couple hundred amps). A 3C rate would give you 30 kW from the ten modules. The AC-51 performance graphs from HPEVS indicate that 200 amps would only provide peak torque up to about 1800 rpm, and that you would need 500 amps to sustain peak torque up to 4900 rpm, the peak power point; alternatively, 30 kW is less than half of the peak power of this motor. Can the eGolf cells withstand 6C?


kennybobby already posted essentially the same things, but I started composing before reading his response... maybe we have two usefully different ways to express the same information.


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## Sunking (Aug 10, 2009)

Amp Hours do not tell you anything, it is meaningless. You want to work with Watt Hours, and then determine your efficiency expressed as wh/mile.
To determine waat hours is stupid simple 5th grade map unless you live in the USA which would require a Phd to understand the very complicated formula:


*Watt Hours = Nominal Battery Voltage x Amp Hours. 
*

Normally you could use the standard efficiency formula of 300 wh/mile but not in your case because of the weight and size. More like 400 to 500 watt
hours per mile if not higher.



Even more bad news is you cannot use the full capacity, only 80% is as far as you want to push them. So 144 volts x 75 AAH x .8 = 9300 watt hours or 9.2 Kwh usable. Great for a 1000-lb golf cart as that is about what they have. At 500 wh/mile works out 9300 wh / 500 wh-mile = 18.6 miles at best. 



As for discharge rate, only the battery manufacture can tell you that. Commercial EV manufacturers batteries are not required to discharge at high C rates because they run much higher voltages. There is no reason they should spend that kind of money on a high discharge rate battery. All they need is 5C or less for acceleration, and they cruise on Sub C rates of C/2 to C/5.


So to design there are two ways to go. At the very minimum the battery must be fully capable of delivering the maximum Discharge Rate that will be encountered. So if you have a system that draws 450 amps peak, and you have 75 AH cells, they must be capable of 6C discharge rate for about 20 to 30 seconds every 5 or 10 minutes. Otherwise your batteries will not perform, and worse overheat and go into Thermal Runaway. Using that methos range is whatever it is, piss poor because you went with the minimum size battery capacity you could get away with.


Of course no EV manufacture would do that because RANGE is the name of the game and what sells cars. They design on minimum RANGE, and Discharge Rates take care of themselves because they will be much lower than minimum size requirements described above a DIY would use.


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## brian_ (Feb 7, 2017)

Sunking said:


> Commercial EV manufacturers batteries are not required to discharge at high C rates because they run much higher voltages. There is no reason they should spend that kind of money on a high discharge rate battery. All they need is 5C or less for acceleration, and they cruise on Sub C rates of C/2 to C/5.


I agree that they don't need high rates, but this has nothing to do with voltage. Current EVs usually run at about 360 volts (so 80 kW is 222 A @ 360 V, 74 A per cell with 3 cells in parallel), but if they ran at half the voltage they would run at twice the current using the same total pack size with half the number of cells in series and twice the number in parallel (so 444 A @ 180 V, still 74 A per cell with 6 cells in parallel), leading to exactly the same discharge current for the same cells, or the same "C" rate.

I think "C" rate is an annoying term, because it involves time but values are given without using time units. C/1 rate means discharging at the rate which would fully discharge in one hour... and specifically one _hour_, not one second or one day or any other time unit. So since a practical EV needs to run for a few hours to go far enough to use the full range (at sensible road speeds) the rate is C divided by a few (hours). At most, I suppose you could discharge an e-Golf in just under an hour driving at constant speed as fast as it would go, so that's around C/1.

A 5C discharge rate would use the full capacity is 1/5th of an hour, and of course no one will drive the vehicle so hard in street use that the battery runs out in 12 minutes. As I showed in an earlier post, the e-Golf (like the 24 kWh Nissan Leaf) only runs at 3C even at maximum acceleration effort, although an AWD Tesla "Performance" model can run higher than that (because it has four times as much battery but more than four times the maximum motor power).



Sunking said:


> At the very minimum the battery must be fully capable of delivering the maximum Discharge Rate that will be encountered.
> ...
> Of course no EV manufacture would do that because RANGE is the name of the game, and they design on minimum RANGE, and Discharge Rates take care of themselves because they will be much lower.


I agree, in general. Discharge rate is low - and not a problem - for most EVs because their major constraint is total energy capacity, not power capability. You can express this in current, but working it out in terms of power gives the same result.

Although it's not a problem, discharge rate is a design concern for EVs. The need for active cooling depends on the cell and module design, and the discharge rate to be accommodated.

Plug-in hybrids have more of a power issue, as they generally (and in pure series hybrids, always) must deliver full power to the motor entirely from the battery, yet have a relatively small battery (compared to battery-only EVs). Average discharge rate can easily be over 1C, since their range is shorter than the distance they can cover in one hour; discharge rate during acceleration is much higher. This means that discharge rate - not just energy storage - is a design concern. GM has published a maximum (for 10 seconds) discharge rate for the first-gen Volt battery of 110 kW; with only 16 kWh of nominal capacity, that's C/7... so now you see a reason that the Volt battery is popular with DIY builders, even though the Volt is only a hybrid.


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## terrorr (Jul 17, 2017)

First off thanks for all the replies. Looks like my calculations regarding range were too optimistic.

So it looks like that battery pack would give me 9.2kWh usable capacity and maybe 18.6 miles of range. Fair enough. So let's assume that is ok and I only need a range of 15 miles avg/day.

Regarding C rate: this is where things were unclear and I couldn't find any info.

My concern was whether at 144 v the 75 ah battery pack would have a sufficient discharge rate for long enough to power the truck up hill for example.

So assuming 450 amp draw while climbing, the batteries would have to be able to perform at a 6C rate (6x75) = 450 amps.

In the above example if the system was 288 v and using 75ah batteries then the draw would be at 3C. Correct?
If the system was 144 v and using 150ah batteries then the draw would be at 3C. Correct?

I need to know the C-rate specs of those batteries. It sounds unlikely they will have at 6c rate.


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## boekel (Nov 10, 2010)

These cells can do 10C, but need cooling then, that's how they're used in the PHEV versions of the volkswagen GTE / audi e-tron

the e-golf version I'd not use more than 3C (short burst of more power shouldn't be a problem)

The 25Ah cells are from Panasonic.

newer e-golfs (i think from 2017?) have a 50% higher capacity with the same size of battery modules, these are Samsung SDI cells.

also the VW passat GTE uses Samsung SDI cells, same size but only 15% higher capacity than the panasonic 25Ah cells. Presumably due to the higher C-rating of these cells opposed to the e-golf version.

All these battery modules you can monitor with Tom de Bree's 'Simple BMS'

https://www.diyelectriccar.com/forums/showthread.php/fs-tesla-vw-outlander-bms-master-198263.html


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## terrorr (Jul 17, 2017)

Sunking said:


> As for discharge rate, only the battery manufacture can tell you that. Commercial EV manufacturers batteries are not required to discharge at high C rates because they run much higher voltages. There is no reason they should spend that kind of money on a high discharge rate battery. All they need is 5C or less for acceleration, and they cruise on Sub C rates of C/2 to C/5.
> 
> So to design there are two ways to go. At the very minimum the battery must be fully capable of delivering the maximum Discharge Rate that will be encountered. So if you have a system that draws 450 amps peak, and you have 75 AH cells, they must be capable of 6C discharge rate for about 20 to 30 seconds every 5 or 10 minutes.
> 
> Otherwise your batteries will not perform, and worse overheat and go into Thermal Runaway. Using that methos range is whatever it is, piss poor because you went with the minimum size battery capacity you could get away with.


Great. That is the info I was looking for.


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## terrorr (Jul 17, 2017)

boekel said:


> These cells can do 10C, but need cooling then, that's how they're used in the PHEV versions of the volkswagen GTE / audi e-tron
> 
> the e-golf version I'd not use more than 3C (short burst of more power shouldn't be a problem)
> 
> ...


Cool. Thanks.


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## brian_ (Feb 7, 2017)

terrorr said:


> My concern was whether at 144 v the 75 ah battery pack would have a sufficient discharge rate for long enough to power the truck up hill for example.
> 
> So assuming 450 amp draw while climbing, the batteries would have to be able to perform at a 6C rate (6x75) = 450 amps.
> 
> ...


Yes, and both of those alternative configurations require twice as many modules, so the same power means half as much power per module, so half the "C" rate.

Original proposal:

450 amps @ 144 volts = 65 kW; 10 modules in series are required; 450 amps from a single module is 6 times the 75 Ah module capacity

Alternatives:

225 amps @ 288 volts = 65 kW; 20 modules in series are required; 225 amps from a single module is 3 times the 75 Ah module capacity
450 amps @ 144 volts = 65 kW; 2 modules in parallel and 10 module pairs in series (20 modules total) are required; 450 amps from two parallel modules is 3 times the 75 Ah module capacity

If you can fit 20 of the e-Golf modules in (so 20 kWh nominal capacity), that would certainly be more appropriate for both discharge rate and range than making do with only 10 modules.


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## terrorr (Jul 17, 2017)

brian_ said:


> [*]450 amps @ 144 volts = 65 kW; 2 modules in parallel and 10 module pairs in series (20 modules total) are required; 450 amps from two parallel modules is 3 times the 75 Ah module capacity
> [/LIST]
> 
> If you can fit 20 of the e-Golf modules in (so 20 kWh nominal capacity), that would certainly be more appropriate for both discharge rate and range than making do with only 10 modules.


So as it turns out I will be going with 20 modules. Thanks everyone.


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