Depth of Discharge in lead acid batteries

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Depth of Discharge in lead acid batteries

Tom H
I have noticed that much of information about lead acid batteries includes comments that reducing the depth of discharge can significantly increase battery life.  To check this out I did some calculations based on the data I was able to find on the US batteries web page in their pdfs of technical data.  I thought other EV owners might find this useful.

It is true that the number of charge cycles is significantly higher for lower DOD's.  The US batteries chart shows that battery life for a DOD of 20% is 3300 charge cycles and for 50% it is only 1150 cycles.  However, as an EV owner I cannot use these numbers.  What is important to me is the overall amount of energy I can store and discharge over the battery life.  How many miles can I drive before I have to replace my battery pack?  I find that the differences are not so large.  This means I am less worried about always finding a plug to recharge when I am out and about.

Here are the calculations for my system (120 volts using US 2200 batteries rated at 232 AmpHrs - to round off I am using 230 amp-hrs).

At full charge my batteries hold 27.6 Kwatt-hrs of energy.
At 20% discharge I have thus used 5.5 Kwhrs of energy during a charge cycle.
If my batteries last 3300 cycles at 20% discharge rate I have used a total of 18,150 Kwhrs. during the battery life.

At 50% discharge I have used 13.75 Kwatt-hrs of energy during a cycle.
If my batteries last 1150 charge cycles at this rate I have used a total 15,812 Kwhrs during the battery life.

SO the net result is that I have reduced the number of miles I can travel on my battery pack by 13% if I continually discharge  my batteries to 50% rather than 20%.

If I increased my DOD to 60% I reduce my overall mileage by only 17% compared to the case where I recharge every time my DOD reaches 20%.  

Anyone else have similar calculations? or evidence that this is not true and I should go back to worrying about the DOD?

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Re: Depth of Discharge in lead acid batteries

Rush Dougherty
Tom wrote -


 > At full charge my batteries hold 27.6 Kwatt-hrs of energy.

> At 20% discharge I have thus used 5.5 Kwhrs of energy during a charge
> cycle.
> If my batteries last 3300 cycles at 20% discharge rate I have used a total
> of 18,150 Kwhrs. during the battery life.
>
> At 50% discharge I have used 13.75 Kwatt-hrs of energy during a cycle.
> If my batteries last 1150 charge cycles at this rate I have used a total
> 15,812 Kwhrs during the battery life.
>
> SO the net result is that I have reduced the number of miles I can travel
> on
> my battery pack by 13% if I continually discharge  my batteries to 50%
> rather than 20%.

Good point Tom. I've sort of thought about what you've precisely pointed
out. Let me add another factor into my battery pack size/treatment equation.

I will have a daily 54 mile trip for work. Going to work (with a full pack
from an overnight charge, at a lowered time of use cost) is a decrease in
elevation by about 800 ft. And my trip back home is the same route, and an
increase in elevation of about 800 ft, the last couple miles being a steady
increase of about 500 ft. A minor grade, but still work for the motor and
battery pack at the end of a 27 mile or 54 mile trip. I went to
http://veloroutes.org/bikemaps/ and mapped out my route, great site.

So now I am faced with another decison, do I make a battery pack for the
full trip and charge only at home, overnight? Or do I make a pack for just
1/2 the trip and charge at work also? My employer will let me charge there,
I think. And we haven't gotten into the 120v or 240v question... I've also
offered to pay her the KW cost to grease the wheels, so to speak.

If I do a 27 mile pack and still want to go into Tucson on the weekend, or
spend the evening with friends, whatever, I can recharge at a friends, or
even better yet at one of the 200 new Leaf charging stations that are going
to be installed in Tucson (they will have J1772 plugs, which I don't have an
inlet for, but all the Chargepoint charging stations will have a regular 120
v outlet in them also.) But to charge up for the trip back will take x
hours...

So it is not an easy question to answer if I'm going to make my EV take the
place of my Insight that currently gets 60 mpg at a cost of about $.04 a
mile, but I'm starting to have issues with the battery pack.

Rush
www.TEVA2.com
Tucson AZ

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