Re: Extending the Life of a Rechargable Battery

Keith Williams wrote:
In article <dPd3f.13243$RG1.4466@trndny08>, ehsjr@xxxxxxxxxxxxxxxx says...

Keith Williams wrote:

In article <0%J2f.9286$zQ3.8332@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>, robertbaer@xxxxxxxxxxxxx says...


benn686@xxxxxxxxxxx wrote:


Ive heard its better for the life of a battery to charge a battery to
full capacity, use it until it drains completely and then fully
recharge it and so on.  I heard the battery will go bad faster if  you
partially charge, discharge more frequently.   Is this true?  Does it
depend on the type of rechargable battery?


*NO* battery "likes" to be fully discharged!


Not true. NiCds and NiMH batteries can be fully discharged, but not below 0V on any cell. A multi-cell NiCd battery may reverse charge a weak cell which *will* kill it. NiCds should be stored discharged (but self-discharge is good too).

OTOH, lead-acid batteries must never be fully discharged. Deep discharging them tends to warp the plates causing shorts. "Marine" batteries have thicker plates (thus lower power-density and higher internal resistance) and are less susceptible to plate warping.



That being said, the NiCd cells have a "memory" problem, and the only way to "reset" that is to do an almost complete discharge and then charge; sometimes this is needed moer than once for recovery.
So, this regimen to "reset" their memory theoretically reduces their lifetime - but without the "reset" they are useless anyway, making the theory un-proveable and moot.


The "memory" problem isn't. It's much legend about a totally different issue (one that has been pretty much non-existent for 20 years).



Other uses that can shorten cell lifetime is heavy discharge and heavy, un-controlled charging.


Over-charging and reverse charge during discharge are by far the most common cause of early failure of NiCds.



Roughly, a cell should not be discharged to less than 80 percent of rating, and the discharge rate should not be greater than C/2 or only "bursts" of C *if* the chemistry supports that.


Not less than 80% of rating? Nonsense. Lead-acids shouldn't be discharged below 20%, perhaps. NiCds can be run to down to zero as long as the cells don't reverse polarity. 

Not a pack, just an individual cell. The *proper* discharge
level - the level at which the cells should be considered
completely discharged - is about .9v per cell.  With a single
cell there is no risk of cell reversal, but with a pack,
discharge should be limited to that .9v per cell level.


No, that's the "proper" voltage for a multi-cell application. 

You are agreeing with what I wrote. Why the word "No" at the
start of your sentence?


A single NiCd cell can "properly" be discharged down to 0V. 

That's what I said. "Not a pack, just an individual cell"
which refers to your statement "NiCds can be run down to
zero..."

There
isn't much charge under .8V or so though. Reverse charge is the worry here (which cannot happen with a single cell). 

That's what I said. "With a single cell there is no risk of
reversal, but with a pack, discharge should be limited to
that .9V per cell level."



Lead-acid 12V wet cell batteries should never be discharged
below 10.5 volts.


That depends on the design. 

No - finally we disagree!  Regardless of design, discharging
a wet cell 12V battery below 10.5 is a no-no. That includes
deep-cycle/marine batteries.  The open circuit voltage swing
from 100% charged to 100% discharged is only about 2 volts.
http://www.windsun.com/Batteries/Battery_FAQ.htm
These batteries are 100% discharged when they reach 1.75
volts per cell (10.5 across the battery), per the site above.
Exide specifies a higher voltage: their FAQ says the battery is
100% discharged at 11.7 volts
http://www.exideworld.com/faq/faq_marine.html#stateofcharge

This site lists ~11.8 volts as 100% discharged:
http://www.uuhome.de/william.darden/carfaq4.htm#measure

This site shows 11.5 volts as 100% discharged:
http://www.mpoweruk.com/soc.htm

Even if you do not go below 10.5 volts, removing most
or all of the charge (and by that I mean I mean lowering
the state of charge such that the battery voltage approaches
10.5 volts) reduces the battery life. For exmple, one
of the sites mentions indicates that you'll get more than
double the number of charge-discharge cycles from a deep
cycle battery if you keep the discharge level to no more
than 50% than if you keep it to no more than 80%.

Ed




.

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