Re: Battery capacity

Hi Jim,

I saw your post come in last night as I was signing off (MidNight??).
I spent most of the morning reading all the posts. I also took your
advice and took another look at the battery faq and am rereading it.
After reviewing these posts the faq now makes more sense.

Overall I got what I needed at point (and also what I didn't know I
needed!).

Thanks,

Larry

Jim Higgins wrote:
On 31 May 2006 10:48:31 -0700, "larry.greenwood@xxxxxxxxxxxxxxx"
<lgreenwood@xxxxxxx> wrote:


Jim Higgins wrote:
On 31 May 2006 07:21:20 -0700, "larry.greenwood@xxxxxxxxxxxxxxx"
<lgreenwood@xxxxxxx> wrote:

I have two 6 v. 225 ah deep cycle flooded batteries that I just
purchased a couple of weeks ago to use with my solar panels.

So far the most capacity I seem to be able to get so far is a total 50
ah at a 50% dod. I have cycled them several times and have equalized
them twice.

I have been using two 100 watt light bulbs to discharge them at 20 amps
per hour.

Am I missing something here?

Not enough information. What is the exact brand and model of those
batteries... better yet, you do the homework and tell us over what
time period does the manufacturer rate these batteries to deliver
225AH?

Interstate brand. U2200 6 volts @225 ah for 20 hours, according to
their webased spec ***.


That means the specified discharge rate is about 11 amps. (225/20)


Are you saying you are discharging these batteries in series at a rate
of 20 amps? How long do they last at that discharge rate and what is
the voltage just before you terminate the discharge?

From a full charge voltage of 14.1, Battery voltage drops to 12.2 after
a 2 hour 20 amp draw. Thats my reference point for 50%.


That's not the way it works. The manufacturer specifies a cutoff
voltage that signifies the end of useful current delivery - end of
discharge - and the ampere-hours delivered to that point divided by
the rating of the battery at the discharge rate you use represents the
percentage of capacity delivered. The cutoff voltage is more like
10.2 volts, not 12.2 volts.

To test your batteries you need to discharge at as close to 11.25 amps
(225 AH/20 hours = 11.25 amps) as you can and determine the
ampere-hours delivered up to the point that 10.2 volts is reached.
Divide that by the rating of 225 AH to get the percentage of rated
capacity delivered.

At a discharge rate of 20 amps - which is close to double the
specified discharge rate - you can expect the battery to deliver
considerably less than 225 AH. The mfgr may provide tables or graphs
that allow predicting performance at discharge rates other than 11.25
amps.

I very highly recommend to you The Car and Deep Cycle Battery FAQ
located at http://www.batteryfaq.org for a bit more on the
relationship between discharge rate and rated capacity... plus a
wealth of information on other topics related to batteries, including
proper care for long life.

Here is a typical day's test with a discharge of 20 amps.

8:30 13.2 volts @ 20 amps discharge rate
9:00 12.4
9:30 12.3
10:00 12.2
10:30 12.1

Results below 50% discharge at only 40 ahs

Again, 12.2 is not a good measure of 50% capacity. You need to
discharge to 10.2 volts and compare the AH delivered to the AH rating
at the discharge rate you used. If you only have a rating for a 20-hr
discharge, then try to discharge at 11.25 amps instead of 20 amps.


Here is the test with 10 amps.

8:00 13.2 volts @ 10 amps
9:00 12.6
9:30 12.5
10:00 12.4
10:30 12.3
11:00 12.3
11:30 12.2
12:00 12.2

Results below a 50% discharge again only 40 ahs.

If the battery is rated at 225 ahs why wouldn't I get at least 80 ahs
at a 50% discharge?


Because 12.2 volts doesn't reliably represent 50% discharged. Where
did that info come from?


When you recharge, do the cells gas as the end of charge is
approached? What is the voltage and current just before you terminate
the recharge? What is the specific gravity after recharge and after
discharge?

On the times I have checked them, they have bubbled in the bulk and
absorption charges. Not profusely. As they are in final stage of
charging the meter is reading 14.1 volts. After the charge and the
charger is disconnected, the voltage usually drops to 13.8 or 13.9.
When I begin discharging the voltage quickly drops to 13.2 for a minute
or so.

Within each battery the sp is the same. Between batteries there is
slight variation. All cells are measuring between 1.275 and 1.300
after a full charge.

Based on the wide variation in specific gravity and the low end of
charge voltage - 14.1 is only 2.35 volts per cell, barely enough
voltage to produce gas - I'd suggest you perform an equalizing charge.
Fully charge the batteries using the UPS's internal charger, then
continue that charge at 5 - 10 amps for an additional 35 or so
amp-hours. Then record the specific gravity of each cell. If they
differ by more than about +/- 5 points (0.005) I'd continue another
hour or two and read again.

An end of recharge voltage of 14.1 volts is too low for a battery that
you expect to cycle. Is the voltage ever higher than this at any
point in the recharge?

Jim H

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