battery discussion [RE-wrenches]

Sun May 6 11:29:03 PDT 2001

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Todd C wrote:

>At the power plant I work at we have a bank of lead calcium cells. I
>don't know much about these guy's chemistry. What experiences do list
>members have with lead calcium batteries? What is they general best case
>life and advantages over lead antimony.

Best advantages of lead calcium are to be experienced when you got 
some in your hot little hands for next to nothing. They were a 
mainstay of my business in late 70's and into 80's until AT&T got 
sued over a public safety issue and had to protect us from their 
"scrap". I know at least one customer still living on his 28-year old 
set that he's had for 18 years.

THE trick to keeping them alive is to respect them as shallow cycle 
batteries. Oversize the set for the application and cycle them no 
more than 50%, except rarely. They have very low self-discharge, much 
less than antimony, so extreme oversizing is not a problem in a 
managed system. Also, they do NOT like high charge rates. When above 
80% SOC, they overcharge at very low rates.

Use a hydrometer until you trust the accuracy of your amp-hour meter, 
because their voltage is so stable, you can't get a SOC impression 
from it.

Specific gravity is about 1100 discharged and only 1215 fully charged 
(may vary according to the specific battery, that was true with 
telephone exchange batteries). The low SG seems to correspond with 
low energy density, so they are not for the space-challenged.

>
>Our bank is 15 years old now and last time we had to really cycle them
>down (2 years ago), we did get their rated capacity, so I imagine they
>are still doing ok. (I know it is bad to take batteries below 50 to 70%
>DOD.) I am curious how other than capacity testing one determines
>current health and remaining life span?

Are they clear-case cells? If so, you can see if sediment is 
substantial. Little sparkles are a good sign on the neg plates, but 
not assurance of good. White powder is usually a sign that they are 
ready for rebirth.

>
>The bank is 10, 12 volt batteries, so ~120 VDC. Their self discharge
>rate is s-l-o-w-l-y increasing. They are being kept at around 70º F. The
>charge voltage set point at 135 volts. Any higher and the inverter kicks
>off from over voltage, so it is hard to do an equalization charge and at
>present they do not consume any water at all. I am thinking about
>removing one battery (6 volts) to lower the pack voltage so I can
>effectively raise the per cell charge voltage (without upsetting the
>inverter) to attempt to keep them a bit happier. Any comments on this
>plan?

I think I remember that EQ overcharging is not advised for calcium 
batteries, unless they have problems showing up.

I would get a V reading AND a S.G. reading of every cell. If they are 
really even (within 5 points SG), you probably have a very good set 
that can last a lot more years.

There are many sources of batteries like this. They are usually 
cycled out per schedule, long before their useful life is over, by 
nature of the applications where they must be trusted for 
emergencies. I used them to run my shop and home in the 80's. The 
ones I used are now part of an old customer's set and still working. 
Don't forget to add water every few years!

Windy

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