# Battery Comparison

Hi,

My dad and I are converting a 1986 Pontiac Fiero GT to completely electric. We’ve already decided we’re going to go lithium, but in deciding which battery to choose, we’ve run into a problem:

We’ve been using this formula to estimate the number of miles we will be able to drive: (Total Voltage)*(Amp Hours of 1 batt)/(200 Whrs/mile) Our controller (a Zilla Z1K HV: 1000A @ 300v max) can only handle 300 volts which puts the maximum number of Thundersky batteries (max voltage: 4.5 ~ normal voltage: 3.2) at 64. (if you do the math, you’ll actually get 66, but we want to keep the number of batteries divisible by 4 for our BMS.) Anyway, what we found was that with the 100AH batteries, we could use 64 to get about 82 miles out of the car. However, with the same equation, we could use 40 160AH batteries to get the same distance at over \$1500 less. This raised several questions especially about performance. We know two things:
[ul]
[li]more batteries can pull more amps[/li][li]larger batteries have a higher c rating so they can also pull more amps[/li][/ul]
So the question is: Will having fewer but larger batteries affect the performance of the vehicle in any way?

This links to the Excel Spreadsheet I’ve been using to find all my numbers:
xlsx: 2007
xls: 2003 and earlier

Thanks!
~Nick

The key to distance is the total Watt-Hour capacity of the battery system. After looking at your spreadsheets I did not see a correction factor applied to account for the discharge rate.

Battery manufactures us some gimmicks to make their Amp-Hour numbers look better then they really are. For example let’s say you have battery X and they list the amp-hour rating at 100 AH. But that rating is based on the 20 hour discharge rate of 5 amps (100 AH / 20 H = 5 amps). What they do not tell you or bury them in confusing lingo is what the AH rate would be say at the 1, 2, or 5 hour rate. Well for an EV you are going to discharge the battery at very high discharge rate like 1 hour or less. When you do that the battery AH rate is greatly reduced, and at the rates you can encounter that 100 AH rate can be reduced by 75 % so now that 100 AH battery rated at 20 hours is more like 25 AH @ the 1 hour rate.

If you look at the battery charts at the bottom of the following .PDFs, you’ll notice that the AH rating of the battery does not change much as the amount of current pulled increases. Granted, they did only test up to 1CA even though the max continuous discharge rate is 3CA, but judging by the relatively linear decrease in the AH rating to the CA used, there won’t be anywhere near 75% decrease. I’d say about 20% decrease at the most.

In any case, the original question was a comparison of two distinct options. Will more 100AH batteries or fewer 160AH batteries give better performance? Are you suggesting that because of this, it would be safer to go with the lower AH batteries because the voltage is a more reliable number? I’m not sure even that argument works, because if the AH rating is degraded by 20%, that AH rating is a multiplier in the equation. That means the range will be decreased by just as much for each battery as long as each battery is affected by the same percentage.

Thanks!
-Nick

[QUOTE=Roboticmayhem;6748]

In any case, the original question was a comparison of two distinct options. Will more 100AH batteries or fewer 160AH batteries give better performance? -Nick[/QUOTE]Neither one is better than the other. The issue is Kwh capacity. Amp Hour is only half of the equation.

Let me put it this way. Let’s say we had two choices of using a battery pack of 48 volts @ 40 AH, or 96 volts @ 20 AH. Which would I choose? Even though the Kwh is equal I would always go with the higher voltage because it is more efficient requiring smaller, less expensive, and lighter wiring. Weight and space requirements are identical.

So what you have to do is weigh all the elements together to make a good choice.

Ah, I hadn’t even considered wiring. The one thing I did consider though, was the BMS, which is \$250 for every 4 batteries. In the case I described, that would make the 160AH batteries \$1500 cheaper than the 100AH solution. Is having the higher voltage important enough to spend that kind of money?

Thanks!
-Nick

[QUOTE=Roboticmayhem;6752] Is having the higher voltage important enough to spend that kind of money?[/QUOTE]Nick I am not sure where the extra cost comes in. Are you using more than one single string of batteries? Or are you going to install parallel strings to reach the capacity you desire.

Going back to my 48/96 volt example using say 20 AH batteries at 12 volts per cell. To get 48 volt @ 40 AH requires eight cells in a 4 x 2 arrangement or 2 string in parallel.

Or it takes the same 8 cells in series to obtain the same capacity at 96 volts.

The real difference is the 96 volt system will use 1/2 the current, but the benefit of using lower current goes back to the lower discharge rate and the correction factor being closer to 1.

Have you looked at the Telsa specs to get an idea of what they are doing? They run 390 volts and a 3-phase AC induction motor. The two keys Telsa uses to get the 250 mile range is high voltage and 3-phase AC motor. AC motor effeciency runs around 97 to 99% vs a DC motor in the neighborhood of 70 to 80 %

you have probably sorted this out by now… but I am sure you learned that there are considerations for the number of cells for total nominal voltage matched to your motor/controller, and ah per cell to determine range and max output without damage. higher voltage will require lower amp output for same performance. higher ah capacity cells can output more without damage and give greater range.

Yes, we’ve received 52 180AH cells from Sky Energy. The pack has a nominal voltage of 166.4V - perfect for our WarP9 which has been tested and works best arround 160V. This gives us a theoretical range from 100 to 120 miles on a single charge.

ZAP has designed a turnkey lithium battery system that drops into the same space as a G-31 lead-acid battery. New Lithium Battery and Management System Doubles Range of ZAP Electric Cars ZAP Global Blog

An electric car is a plug-in battery powered automobile which is propelled by electric motor(s).Although electric cars often give good acceleration and have generally acceptable top speed, the lower specific energy of production batteries available in 2010 compared with fossil fuels means that electric cars have relatively low range between charges, and recharging can take significant lengths of time. For shorter range commuter type journeys, rather than long journeys, electric cars are practical forms of transportation and can be inexpensively recharged overnight.mustang parts