# LiFePO4 Pack Voltage vs. Ah



## EVfun (Mar 14, 2010)

The 3 configurations have the same performance potential because you are swapping available volts for available amps (power is volts times amps.) I think the pack size you are looking at will give you a range of about 30 miles. 

The first configuration is a potential problem because the voltage is lower than ideal for most common EV motors and controllers. I've never played down there so hopefully others with experience will chime in. I'm sure suitable parts are available. The battery management system would be cheaper (less cells) and DC to DC converters are available. 

The second configuration puts you in a very traditional voltage range. Newer EVs are increasing going with a higher voltage but 120 to 144 volts is more than enough for most of the common EV motors. It isn't hard to find a suitable DC to DC converter to keep the 12 volt system battery charged (or AC to DC converter that happens to work on DC, like the Iota.) I have a pack of 40, 60 amp hour Thundersky cells in my Datsun and I'm planning the same for my Buggy. 

The third configuration requires that you have a controller with a suitable voltage rating. That is not hard, but may cost more depending on how much power you want. The motor voltage rating is actually not important as the controller job is to buck the voltage down to what the motor can handle [1]. I'm not sure what DC to DC converters are available in that voltage rating. It is not high enough that I would recommend the 240 vac Iota but it is too high for the 120 vac Iota. 

[1] A "buck converter" means that the controller input volts times input amps equals controller output volts times output amps, but the output voltage can never be higher than input voltage.


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## rillip3 (Jun 19, 2009)

The reason you see 120-144 as the top most often is because this is the top voltage supported by most series DC motors (the most common kind used). The numbers are going up slowly though. If you have a motor/controller that will take the higher voltage, it will offer better performance (less resistance, a less-deep draw on the batteries). 

EDIT: the theoretical performance, not counting loses, will be identical for identical watts, just as EV Fun said. But when you take into account losses, the cables, motor and controller will have less resistance losses at higher voltage/lower amperage than vice-versa.

Also, there's the weight/space consideration. Many of the lithium users run with higher voltage packs, because you can get more voltage without significantly increasing weight, and the cells are much smaller. But if I'm using a lead acid battery, every 6, 8 or 12 volts (depending on the battery type) adds another 50-80 lbs and another cubic foot or so of space needed. It's lighter and easier to fit a higher amp-hour battery than to put in another battery. Other considerations trump that at lower voltages (which is why you don't see 48v cars with 400 Ah batteries) but once you get up to the nominal voltage of the motor, it really plays a factor.


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## Yukon_Shane (Jul 15, 2010)

Okay, so it sounds like there's very little difference in performance between these various options but that there may be some benefit to sticking with conventional EV conversion voltages (120-144) because it'll be easier to find parts for those common voltages (controllers, dc-dc converters, etc.)

This has been very helpful.

Thanks


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## dimitri (May 16, 2008)

I know you aren't going for a large pack since your range requirement is low, but you also have to consider max battery C rate for heavy acceleration.

If you go with 100AH cells for example, you want to try and limit your max battery current to be under 300Amps for few seconds bursts, and continuous cruising current under 150-200Amps.

This might be difficult with 128V pack, depending on your car's curb weight. It all depends on your desired acceleration as well. If you don't mind taking it slow, you'd use less current.

Something for you to think about....


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## Yukon_Shane (Jul 15, 2010)

Dimitri: 

My range requirements are pretty small (as is the vehicle) but I do have some pretty steep hills to climb on my commute home and I'd like the vehicle to be able perform fairly well.

So I guess your suggestion is that I might want to make the pack a bit bigger then this to ensure that it's not being discharged. Perhaps 100 Ah but bring it up to 150-160V? Or would it be better to go with the 160 Ah batteries that would allow for higher discharge amperage?


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## dimitri (May 16, 2008)

Yukon_Shane said:


> Dimitri:
> 
> My range requirements are pretty small (as is the vehicle) but I do have some pretty steep hills to climb on my commute home and I'd like the vehicle to be able perform fairly well.
> 
> So I guess your suggestion is that I might want to make the pack a bit bigger then this to ensure that it's not being discharged. Perhaps 100 Ah but bring it up to 150-160V? Or would it be better to go with the 160 Ah batteries that would allow for higher discharge amperage?


Your situation is quite challenging. On one hand you don't need much range, so getting more battery to get more current is not cost effective and with hills you get diminishing returns since bigger battery gets heavier. On the other hand you definitely need more current to climb hills. An obvious solution is to pick high C rate cells, but those are more expensive and pack construction is much more challenging.

If I were you I would look at as many similar sized conversions as possible, contact owners trying to determine energy levels required to climb hills at same speeds that you want from your conversion. Once you have an idea of energy levels involved, then you pick suitable motor/controller to deliver that energy, then design the battery pack to match the motor/controller voltage and whatever amperage you need to match the energy levels involved. Starting with battery design is the opposite approach and IMHO it could lead to disappointing results.

Once thing is clear in your case, try to get higher C rate cells if you can. Even CALB rated 4C limit is better than TS 3C, but remember these rates are for very short bursts, despite what the datasheets claim, if you want your battery to last.

Have you seen Tom's SwiftE thread? I think his car is similar size and he climbs some serious hills in it. But he also wanted range, so he's got 180AH cells. Go thru his thread, he posted most detailed energy analysis, extremely helpful for anyone planning a conversion.

If you are mechanically handy and have tools, it might be within reach to build a pack from Headway cells, which are least expensive and largest cylindrical cells, capable of 5-10C rates. They come in 10AH size, so you put a few in parallel to reach desired AH capacity and max current, then put those groups in series to get voltage. Not as simple as prismatics, but doable. Bike conversions use those often.


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## EVfun (Mar 14, 2010)

Here is the current Thundersky 60 amp hour data sheet. They show test data up to 5C and I currently have been pushing my cells to that point (300 amps on my 60 amp hour cells.) Maximum acceleration current isn't sustained for very long and my 40 cell pack hasn't been dropping below 117 volts. My cells are tightly restrained using the end plates Thundersky provides with the cells. I think these cells have been improving over time, with the spec sheet being updated every so often to reflect them getting lighter, peak amps getting higher and discharge curve getting stiffer. 

If you want (or need) a higher peak power then you can increase either pack voltage or amp hours. The controller will convert it to the amps and volts the motor needs. Your pack choices will provide about 39 HP if you limit the discharge to 3C and about 65 HP at 5C.


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## dimitri (May 16, 2008)

EVfun said:


> Here is the current Thundersky 60 amp hour data sheet. They show test data up to 5C and I currently have been pushing my cells to that point (300 amps on my 60 amp hour cells.) Maximum acceleration current isn't sustained for very long and my 40 cell pack hasn't been dropping below 117 volts. My cells are tightly restrained using the end plates Thundersky provides with the cells. I think these cells have been improving over time, with the spec sheet being updated every so often to reflect them getting lighter, peak amps getting higher and discharge curve getting stiffer.
> 
> If you want (or need) a higher peak power then you can increase either pack voltage or amp hours. The controller will convert it to the amps and volts the motor needs. Your pack choices will provide about 39 HP if you limit the discharge to 3C and about 65 HP at 5C.


Have you been checking temperature of cell terminals immediately after 5C runs? The issue with high C rates is that heat builds up inside the cell where LFP sheets connect to the terminal and it can't dissipate fast enough. You can tell by touching the terminals right after high C run, they get somewhat hot. IMHO doing this on regular basis will shorten their lifespan. Also, the datasheet does not say what the cycle life would be if run at max C rates, cycle life is measured at min C rates, so its a good idea to try and stay away from max rates if you want the cells to last.

Increasing pack voltage and AH means more battery and more $$$, which OP may have tried to avoid due to low range needs. But, in this case a compromise must be reached, use larger prismatics for better max current, and get more range as a bonus, or use high C rate cells, but pay $$$ for them.

Once required energy levels are better defined, it might be a better call to stick with larger prismatics. If you are going to spend more $$$, might as well get more range for it.


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## EVfun (Mar 14, 2010)

Not every time I drive the car but I have casually checked the post temperatures regularly. I stick my right hand in my back pocket and with the first 2 fingers of my left hand I feel all 80 terminals. Nothing has been warm yet. It is easy to do because I have all the cells together in a single block under the hood and they are not covered yet.

I think I may actually have the first batch of LYP TS cells even though they are part marked LFP. The reason I think that is because the cells are also marked LiFeYPO4, the cell posts are 61mm apart, matching the LYP spec but not the LFP spec, and the weight is 2.3 kg each cell, not 2.5 kg like the LFP cells list weight. The cell build date is February 2010.


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## Yukon_Shane (Jul 15, 2010)

Thanks for the advice Dimitri, based on your suggestion this is what I've come up with:

I used the FrankyEV spreedsheet (posted in the wiki section) to estimate the power my vehicle will require to climb an 8% grade hill at 80km/h (a good estimate of the hill I'll have to climb every day on my way home) to be approximately 35kW (seems a bit low but the original ICE only had a peak power of 60 kW and was able to climb this hill at speeds greater then 80km/h with relative ease).

Using 40 X 100 AH batteries would give a pack voltage of 128 and a current at 35 kW of approximately 273 amps. So if I understand correctly this would be a 2.73C discharge rate. I'd have to maintain that rate for approximately 45-60 seconds to make it up the hill at that speed. 

To be safe I'm thinking I might increase the pack to 45 X 100 AH batteries which would give a pack voltage of 144V, and a maximum discharge rate of 2.4C.

Given all of the discussion in the Controller section of 1000A continious controllers my max currnet of 273 amps seems pretty smal but I've checked the numbers several times so I'm pretty sure it's correct.

Any thoughts?


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## dimitri (May 16, 2008)

Yukon_Shane said:


> Any thoughts?


You never mentioned what motor/controller you had in mind. Controllers are rated on the motor side current, so motor current will always be same or more than battery current. 

Sustaining 35-40kW for a minute or more would require controller with decent cooling and good continuous power rating.

Depending on your motor/controller choice I would try to go for 48-50 cells, to get more voltage and hopefully less current. But make sure controller can handle max charge voltage, which will be higher than nominal right after full charge for a short time.


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## Yukon_Shane (Jul 15, 2010)

I knew I forgot to mention something

Yes the motor is a Warp 9" (just came in yesterday) and I haven't choosen a controller yet but was hoping to wait for the Soliton Jr. (500 Amp). I emailed the folks at Evinetics last week to find out when they thought Jr. might be on the market and they responded that it would likely be available for purchase late October or early November. Based on the positive feedback I've found on the Soliton1 (including your own review on this forum) I think it's probably worth waiting a few months for Jr. rather then purchasing a controller that I may or may not be happy with.

If I find I need more power then Jr. can offer then I think I might go with the NetGain controller. I'd prefer the Soliton1 but it's just a bit outside of the budget for this project.


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## dimitri (May 16, 2008)

You won't need more power than Soliton Jr. can offer. During early beta test for Soliton1 I was running a beta unit which had 500Amp limit, so it represented the same power as Jr. will. I was running just fine, I had all the power I needed, except my pack voltage is a little low. That is why I asked the question. Since you are planning on getting a controller which handles up to 300V battery pack, I would recommend you try to up the voltage as much as reasonably possible, while staying with 100AH cells. For example 50 cells, or maybe even a few more. Depends on the budget and available room to put the cells in. You will definitely appreciate higher voltage with this controller.

If I did my conversion all over again, there would only be one thing I would do different, get more cells to get higher pack voltage.


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