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  #1  
Old 11-06-2008, 07:41 PM
Chris Sandberg Chris Sandberg is offline
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Default Battery Calculation Questions

Hello Group, I have a question about the Wiki for battery pack calculations. I was wanting to know if the 1Hr rate can be figured and how important it is compared to the 20Hr,10Hr rates. The second question I have is after figuring the KWH and doing the math from the Wiki for a total AH how do you figure the number of batteries and their AH. My example is a 48VDC controller 250WHM * 32 miles of travel = 8KWH, 8KWH/48VDC = 166.6AH, 166.6AH * 2.25 = 375AH I was wanting to have series 48VDC packs and parallel them for my total power pack voltage of 48VDC. Will the series /parallel idea work for a flat to 200' elevation gain trip 10mls to work and 10mls back plus a little extra on the way home for errands as needed. I will be using a Geo Metro 2 door as the donor car.
A newbie/ beginner trying to figure out my total plan before I start.
TX Chris
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Old 11-07-2008, 08:28 AM
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Default Re: Battery Calculation Questions

Hills take a lot of juice to climb. To go from cruzing speed on a flat stretch to going up a hill, I pull twice as many amps as cruzing speed. So hills will really take a lot of juice. I assume this will be a slower speed vehicle since you are only building it at 48 volts. With lower voltages, you end up pulling more amps for the same amount of work than a higher voltage system. Here is a quick example (not counting any inefficiencies).

48 volts X 300 Amps = 14400 watts
96 volts X 150 Amps = 14400 watts

It is the same amount of work (watts) being accomplished but the batteries only had to work 1/2 as much. With the Peukert's equation (simplified version), the more amps you pull out of your batteries at a time, the less AH is available in a battery as a whole. I had a nice example of this at one time, I can't find it now.
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Old 11-07-2008, 09:20 AM
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Default Re: Battery Calculation Questions

48V is pretty low.... what motor are you going to use? In a car (even that light) its going to suck amps, and if its a small motor, its going to heat up and maybe fail if the hill is very long. Plus, its going to feel like a go-kart. MIGHT get you to 40mph.

I'd consider twice that voltage at least, as stated above. Less work per battery, less peukert effect, higher speed and same amount of motor current (controllers are power converters so to speak).

I'd work backwards. Figure out your needs. What speed, what acceleration/torque... find a motor that matches that requirement, find a controller to match the motor, and then figure what distance you need and take the distance/speed/torque you need and figure out what kind of Wh you need in a battery pack. Also take into consideration the max Amps the batteries can supply. If its too low, it might not perform like you want. I had this problem with my motorcycle.
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  #4  
Old 11-08-2008, 12:11 AM
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Default Re: Battery Calculation Questions

250 WH/Mile (figured battery-to-wheels) might actually be a bit high of an estimate for a geo metro running at sub-freeway speeds, assuming you've got the Rolling Resistance dialed in well.

If you have a 20 mile round trip with 200 feet of climb in either direction (400 feet for the round trip) it should be doable without any extreme measures.

I agree with the comments regarding your planned pack voltage. I assume you are trying to keep costs down. In this case, you can stay in golf-cart territory and still use at least a 72V setup (DONT go below this) and keep with cheaper controller, charger, etc. 72V will give you about 45-50mph tops. A decent 6.7" motor as sold by various EV parts suppliers should be fine but make sure to let the brushes get broken in, and expect a motor upgrade if you ever decide you want sustained freeway speed.

There's a bit of chicken and egg involved in the calculations but a pack twelve 6V golf batteries (720lbs) would give you the range with plenty of margin to do the route you plan for. You could likely get away with a smaller pack of nine 8v batteries as well but for an extra few hundred it would be worth it to get a 33% bigger pack.

The faster the discharge rating you can find, the more accurate it is going to be for estimating an EV's range. Golf cart batteries frequently give a 75-amp discharge rating (minutes at 75 amps) which is likely going to be the closest rating you are going to find. But as a basic rule of thumb, with an otherwise appropriate lead acid battery, dividing the 20-hour rated capacity in half will give you a good feel for the actual useful energy capacity in an EV application.

Cruising at 40mph on the level in a geo metro that weighs around 2500lbs I'd wager you would be using less than 150WH/mile. (my 3400lb MR2 does around 150WH/mile at that speed, as a comparison). The hill would require additional energy according to the equation E=mgh.

E is joules. One kilowatt hour is 3.6 megajoules.
M is mass (2500lbs, or about 1200kg)
g is gravitational constant (9.8m/s^2)
h is altitude change (400ft, or about 120m)

so doing the math and converting from joules to watt hours you get:
E = 1200kg * 9.8m/s^2 * 120m
E = 1.41 million joules or...
E = 0.4 kilowatt hours.

so you only need about 1/2 a kilowatt hour in extra energy to climb the hill, both ways.

so 20 miles at 150 wh/mile plus an extra 1/2 Kwh for the hill gets you 3.5 kilowatt hours used. You might want to add 20% to that number to account for start/stop, headwind, etc. as well. twelve 6v golf batteries have about 6KWH of useful capacity. nine 8V golf batteries have about 4.5Kwh of useful capacity. Both packs should do it, but you'll get more cycles out of the bigger pack due to shallower discharge cycles.

Good luck.
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Last edited by madderscience; 11-08-2008 at 12:26 AM.
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  #5  
Old 11-08-2008, 02:01 PM
Chris Sandberg Chris Sandberg is offline
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Default Re: Battery Calculation Questions

Well Everyone thanks for the replies, I still don't understand the sizing issue AH of batteries and what you need to know or look for after the Wiki calculations are done. Can anyone explane this for a beginner that is a slow learner. It does sound like I will be shopping for a new higher voltage controller the GE EV-1 48VDC controller is just not the trick. The motor I have can do 120VDC so I will be looking at something in this range.
TX Chris
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Old 11-08-2008, 08:25 PM
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Default Re: Battery Calculation Questions

Hi all, Iíve been driving my 280Z-EV for over a year now and noticed I have 3 bad J150 batteries, I was wondering if I could replace the 3 with the Odessey PC2150ís/Sears Platinum or do I have to stick with my J150ís? I know the PC2150ís/Sears Platinum are AGMís, can they be mixed w/lead acid or would I need a BMS?
Any input would be appreciated.
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Old 11-08-2008, 08:46 PM
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Default Re: Battery Calculation Questions

Quote:
Originally Posted by Chris Sandberg View Post
Well Everyone thanks for the replies, I still don't understand the sizing issue AH of batteries and what you need to know or look for after the Wiki calculations are done. Can anyone explane this for a beginner that is a slow learner. It does sound like I will be shopping for a new higher voltage controller the GE EV-1 48VDC controller is just not the trick. The motor I have can do 120VDC so I will be looking at something in this range.
TX Chris
I'll make easy: Use flooded lead acid golf cart batteries. They are the cheapest, most durable (most recharge cycles) and most energy dense (longest range for the weight) lead acid technology. If you are doing 120V now, then ten batteries like the Trojan T-1275 in a geo metro will give you 120V with plenty of range for the 20 mile trip you described in your original post.
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  #8  
Old 11-08-2008, 08:52 PM
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Default Re: Battery Calculation Questions

Quote:
Originally Posted by Operation Z View Post
Hi all, Iíve been driving my 280Z-EV for over a year now and noticed I have 3 bad J150 batteries, I was wondering if I could replace the 3 with the Odessey PC2150ís/Sears Platinum or do I have to stick with my J150ís? I know the PC2150ís/Sears Platinum are AGMís, can they be mixed w/lead acid or would I need a BMS?
Any input would be appreciated.
Note sure what a J-150 is but if you are replacing a few bad batteries I'd stay with the same type. Also, AGM is lead acid. Basically the same chemistry, but the packaging is different, mainly in how the plates are designed and how the electrolyte is contained. And yes, if you are using any kind of sealed or AGM lead acid battery, you should have a BMS of some sort, or plan for a lot of manual battery balancing. You could also use a individual 12v charger per battery, the only drawback there is ensuring all chargers worked and your entire pack is fully charged before driving off.
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  #9  
Old 11-09-2008, 01:30 AM
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Default Re: Battery Calculation Questions

Quote:
Originally Posted by madderscience View Post
Note sure what a J-150 is but if you are replacing a few bad batteries I'd stay with the same type. Also, AGM is lead acid. Basically the same chemistry, but the packaging is different, mainly in how the plates are designed and how the electrolyte is contained. And yes, if you are using any kind of sealed or AGM lead acid battery, you should have a BMS of some sort, or plan for a lot of manual battery balancing. You could also use a individual 12v charger per battery, the only drawback there is ensuring all chargers worked and your entire pack is fully charged before driving off.
A J150 or T1275 is a 12 vollt Trojan battery, thanks for your response, I guess I'll have to stick with what I have or go with all new AGM's and a BMS.
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Old 11-09-2008, 10:38 PM
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Default Re: Battery Calculation Questions

Op Z:

you are running flooded batteries and they are dying off after one year?

what brand is the J150?

Assuming you haven't done this already: You might want to check that you are fully charging the batteries (peak voltage at the top of the charge should probably be 15-16v per battery, but check the manufacturer recommendations) and check the water level on them too. A fellow with a nissan pickup EV here in seattle was having all kinds of battery problems until we figured out that his charger peak voltage wasn't set high enough and the battery pack was never fully charged which meant poor range and the batteries would never balance out (which happens at the top of the charge when the batteries start to gas)

Of course if you are cyclinig the battery pack deeply on every drive I suppose it is conceivable that deterioration would be evident after a year.

For what its worth, my battery pack (21 Trojan T-105s) is about 14 months old now, and so far there hasn't been any noticeable deterioration in range vs. the same tiime period last year, and the batteries are staying balanced. I've put about 3500 miles on this pack so far.

Good Luck.
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