# Battery Calculation Questions



## Zemmo (Sep 13, 2007)

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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## frodus (Apr 12, 2008)

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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## madderscience (Jun 28, 2008)

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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## Chris Sandberg (Sep 13, 2008)

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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## Operation Z (Oct 11, 2008)

Hi all, I’ve 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 PC2150’s/Sears Platinum or do I have to stick with my J150’s? I know the PC2150’s/Sears Platinum are AGM’s, can they be mixed w/lead acid or would I need a BMS?
Any input would be appreciated.


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## madderscience (Jun 28, 2008)

Chris Sandberg said:


> 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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## madderscience (Jun 28, 2008)

Operation Z said:


> Hi all, I’ve 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 PC2150’s/Sears Platinum or do I have to stick with my J150’s? I know the PC2150’s/Sears Platinum are AGM’s, 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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## Operation Z (Oct 11, 2008)

madderscience said:


> 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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## madderscience (Jun 28, 2008)

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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## Operation Z (Oct 11, 2008)

madderscience said:


> Op Z:
> 
> you are running flooded batteries and they are dying off after one year?
> 
> ...


Yes, dying after about a year is correct, since Trojan already warranted 5 of the 12 batteries (within the 1 year warranty period), now 3 of the the older batteries are dying, I check the water levels constantly and frequenly some are very low, I'll check the charger output voltage as you suggest. I know these J150/T1275 12v batteries are fairly new and untested in EV's so I'm sure that is a factor also as compared to the old reliable T105's that your using. We have over 4500 miles on 7 of the 12 in this pack.
Thanks for your help & suggestions
Joe
http://evalbum.austinev.org/preview.php?vid=1436


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## madderscience (Jun 28, 2008)

Op Z-

Ok, one or two more "duh" questions:

you are using distilled water when you top of the water level in the batteries?

If the batteries are using a LOT of water (I have topped off mine twice since the car has been running) then perhaps you are overcharging, not undercharging? They shouldn't use any water during driving, only during the gassing stage of charging.

your battery pack weighs about 1000lbs (12 x 83) so it should be reasonably sized for your vehicle. (looking at your austinev entry for this info) given that weight and the weight of the whole car (3200lbs) a maximum range of 30-40 miles to 80% DOD depending on conditions is to be expected. How far do you typically drive on a charge, and is it high or low speed? Hills? Do you know what your typical discharge depth is, or how many Kwh you are using?

Trojan gives a 75A discharge rating (70 minutes) for the J150 battery (having such a rating is a good sign). 

Hows your rolling resistance? 

trying to help here. replacing batteries is expensive.


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## Operation Z (Oct 11, 2008)

madderscience said:


> Op Z-
> 
> Ok, one or two more "duh" questions:
> 
> ...


Yes I'm using distilled water. The older batteries seem to use the most water so I usually check those first. Normally the car is driven 10-15 miles, so I'd say 50% DOD would be the norm. I'll check the charging rates to see if it is under or over charging. Don't know how many Kwh I'm using or the rolling resistance.
thanks for your response


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## dimitri (May 16, 2008)

I use similar batteries from US Battery, same characteristics as your J150. My EV is very similar to yours in all aspects actually.

Although my batteries are only 4 months old and I only have 2000 EV miles on them, I don't think they will last more than 12-18 months because they cannot supply enough current for EV use. I typically draw 120-150 Amps from the batteries that have 150 Ah 20 hour rate, which is basically 1C rate of discharge. Although I can't find 1C rate in specs, judging by 75 Amp 70 min rating, 150 Amp would drain it in 30 min or less. I think this discharge rate is what kills them prematurely, IMHO. This is primary reason why most successful EVs use 6V or worst case 8V batteries, but it was not possible for me to fit those in Miata and still get decent voltage, so I had no choice.

With good 6V battery you'd get 250-270 Ah 20hr rate, so 150 Amp will be more like 0.6C which is a big difference for lifecycle.

If I was to do it again , I would settle on 8V batteries and 120V if I could cram 15 batteries in the car. Most likely though, I would go for LiFePo4 160Ah cells next time around.

Not sure if this helps, just offering my $.02


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## Operation Z (Oct 11, 2008)

Where do you measure your charger output? Right off the back of my Zivan charger while charging I assume?

Thanks
Joe


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## madderscience (Jun 28, 2008)

measure the voltage right on the output of the charger. don't zap yourself.

voltage goes up during the charge, and will be at its maximum at the end of the charge. ideally you'd find a way to graph the voltage over the whole charge cycle, but if you could measure it at or near the end (but before the charger shuts off) this might give you clue if you are overcharging or undercharging.

divide the peak voltage you find by the number of cells you have (in your case, batteries * 6, or 72) to get the per-cell peak voltage. should probably be about 2.6V for trojans but look at the manufacturer's recommendations to be sure.

theoretically zivans are programmed for the type and voltage of your battery pack. I assume you went through this with zivan when you ordered your charger. If you got the zivan pre-configured from somebody else it is possible it is programmed wrong.


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