Car Battery Voltage

[Chris1]

I am using a cheap lead-acid car battery for storage in a solar
application. I know that I'll need to be especially careful with how far I
discharge this battery, since it's not a deep-cycle type. My total
charge-discharge cycle will only be 4-8 AH daily, so I think I should be
OK. But just to be safe, what is the minimum voltage I should allow this
battery to reach?

Chris

 

[st]

On Wed, 06 Aug 2003 17:31:26 GMT, Chris1 <chris54321@hotmail.com> wrote:

I am using a cheap lead-acid car battery for storage in a solar
application. I know that I'll need to be especially careful with how far
I discharge this battery, since it's not a deep-cycle type. My total
charge-discharge cycle will only be 4-8 AH daily, so I think I should be
OK. But just to be safe, what is the minimum voltage I should allow this
battery to reach?

Chris

http://www.tpub.com/doeleadacid/leadacid14.htm
"Avoiding Overdischarge

In order to obtain maximum life from lead-acid batteries, they should be
disconnected from the load once they have discharged their full capacity.
The cutoff voltage of a lead-acid cell is usually around 1.75 V. However,
the cutoff voltage is very sensitive to operating temperature and discharge
rate. Like batteries discharged at a high rate will have a lower cutoff
voltage than those discharged at a low rate. Greater capacities are
obtained at higher temperatures and low discharge rates. The manufacturer
should specify cutoff voltages for various operating temperatures and
discharge rates.

Overdischarge may cause difficulties in recharging the cell by increasing
the battery's internal resistance. Also, overdischarging may cause lead to
be precipitated in the separator and cause a short in the cell or between
cells.
"

please next time use google and "lead acid battery cutoff voltage"


regards
st

 

[Sylvan Butler]

On Wed, 06 Aug 2003 20:00:26 +0200, st <st_news@gmx.at> wrote:

On Wed, 06 Aug 2003 17:31:26 GMT, Chris1 <chris54321@hotmail.com> wrote:
I discharge this battery, since it's not a deep-cycle type. My total
charge-discharge cycle will only be 4-8 AH daily, so I think I should be
OK. But just to be safe, what is the minimum voltage I should allow this

There is no safety issue. So probably you are concerned about
maximum battery lifespan. Right?

If so, 'st's comment is useless. His post would get you 100%
discharged (depth of discharge or DoD), and a car battery has very
few cycles to 100% DoD. You need to stop LONG before 100% to get
maximum life from your battery. I'd recommend 50% and that only if
desperate for power.

Battery will last the longest if you never discharge it. Keep it
floating at full-charge all the time. But that is a battery that
won't do much for you. Battery will last the least number of cycles
if you discharge it all the way every cycle. The less deep you go
each cycle, the more cycles you will get.

So you need to decide the compromise: less available energy by
shallower discharges but more cycles, or fewer cycles each with more
available energy.

The manufacturer of the battery can (or should be able to) give you
a chart showing number of charge/discharge cycles compared to DoD of
each cycle. Then from that you can calculate energy stored over the
life of the battery and determine your most cost effective DoD.

http://www.tpub.com/doeleadacid/leadacid14.htm
In order to obtain maximum life from lead-acid batteries, they should be
disconnected from the load once they have discharged their full capacity.
The cutoff voltage of a lead-acid cell is usually around 1.75 V. However,

please next time use google and "lead acid battery cutoff voltage"

Better to search for "state of charge" information:
http://www.google.com/search?hl=en&q=battery%20voltage%20%22state%20of%20charge%22
The homepower link is probably sufficiently detailed for most.

sdb

--
| Sylvan Butler | Not speaking for Hewlett-Packard | sbutler-boi.hp.com |
| Watch out for my e-mail address. Thank UCE. >>>> change ^ to @ <<<< |
It may be better to live under robber barons than under omnipotent moral
busybodies. The robber baron's cruelty may sometimes sleep, his
cupidity may at some point be satiated; but those who torment us for our
own good will torment us without end, for they do so with the approval
of their consciences. -- C. S. Lewis

 

[Seamus O'Donnell]

I have always worked on that a lead acid is flat at 12v fully charged at 13v



"Chris1" <chris54321@hotmail.com> wrote in message
news:lYacnQAG8P7zpqyiXTWJjQ@dls.net...

I am using a cheap lead-acid car battery for storage in a solar
application. I know that I'll need to be especially careful with how far I
discharge this battery, since it's not a deep-cycle type. My total
charge-discharge cycle will only be 4-8 AH daily, so I think I should be
OK. But just to be safe, what is the minimum voltage I should allow this
battery to reach?

Chris

 

[Sylvan Butler]

On Fri, 08 Aug 2003 17:52:19 GMT, Chris1 <chris54321@hotmail.com> wrote:

right around 50% discharged. Any guess as to the "C" of a generic car
battery?

What group size?

Any idea of the "reserve capacity" (minutes at amps)? There are
some crude conversions on various sites to compute AH given RC.

If you go rough (really, really, really rough) by weight, 60lbs
might be about 100amphours.

sdb

--
| Sylvan Butler | Not speaking for Hewlett-Packard | sbutler-boi.hp.com |
| Watch out for my e-mail address. Thank UCE. >>>> change ^ to @ <<<< |
It may be better to live under robber barons than under omnipotent moral
busybodies. The robber baron's cruelty may sometimes sleep, his
cupidity may at some point be satiated; but those who torment us for our
own good will torment us without end, for they do so with the approval
of their consciences. -- C. S. Lewis