range

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Tom Thomson
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Joined: Fri Aug 31, 2007 12:48 am

range

Postby Tom Thomson » Mon Nov 17, 2008 11:35 pm

The electric Porsche does not go as far as I think it should. I have an 84V system (14 golf cart batts) and each battery is rated at 192AH. If I assume 80V average this should be slightly over 15KWH. I have a watt-hour meter on the charger and have determined that the car uses (depending on how it's driven) about 300WH per mile. If I have done my sums correctly this is 50 miles of range but after 21 miles the battery pack is down to less than 50V. Any thoughts???
tommyt

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Jeremy
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Joined: Thu Jul 31, 2008 6:35 pm
Location: Salisbury

Postby Jeremy » Tue Nov 18, 2008 7:17 am

Part of the problem is a thing called Peukert effect. Because of this batteries, particularly lead acid ones, tend to give a much reduced capacity when discharged at a rate higher than that at which they are rated.

As a general rule, brand new lead acid batteries used in an EV tend to give about 2/3rds of their rated capacity. This drops with age so that it's not uncommon to find that lead acid batteries that are a couple of years old will only deliver around half their rated capacity. I'd guess that the range you should expect as a maximum would be around 30 to 35 miles with new'ish batteries, less with older ones.

Having said that, the big voltage drop you are getting seems to indicate something more serious, like a faulty battery (or more than one). I'd expect a fully charged 84 volt system to start off at around 96 volts hot off the charger, dropping to around 85 - 87 volts when resting after a short initial run. When fully discharged I'd have thought that the voltage should drop to around 70 to 75 volts at the lowest - 50 volts seems way too low to me.

If you can monitor individual batteries during discharge you may well find that one or two are dropping much lower than the others and limiting the pack overall voltage.

Jeremy

Tom Thomson
Posts: 64
Joined: Fri Aug 31, 2007 12:48 am

range

Postby Tom Thomson » Sun Nov 30, 2008 10:34 pm

Jeremy;
You're right, 50 volts is too low. However, that reading was seen while trying to climb the last hill before home with almost fully discharged batteries. These batteries are almost new (6 mo) and none of the terminal connections have ever been hot or even warm. The difference between open circuit and load is only about 5 - 10 volts. This tells me that my power circuit is ok.
The more I think about it, the more I'm convinced that you are correct in naming the Peukert effect as the culprit. My batteries (Interstate 6V) are rated at a 75A discharge rate and I am nearly always well above that. Can you explain the theory behind this phenomenon?
tommyt

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MaryRCrumpton
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Postby MaryRCrumpton » Mon Dec 01, 2008 12:31 pm

I was reading abot this the other day and found several web-pages that may be of interest, though Jeremy may well explain it more clearly and succinctly than they do ;-)

Anyway, you might want to take a look at:

http://everything2.com/e2node/Peukert%2520Effect - for a basic introduction

http://www.smartgauge.co.uk/peukert2.html - for a bit more detail.

But if you google for 'Peukert effect' you will find lots of informtion out there.

Mary.
BVS North-West Coordinator, based in Manchester.
Contact via Text/Phone on 07751 696 055


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