Extending the Life of a Rechargable Battery

NiCad are worst for that. Most other types aren't so sedative to discharge depth.

...Jim Thompson

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for some batteries, yes. Just don't drain a lead acid battery. Bad for it.

Hello Jim,

Yes, but AFAIK NiCd are the only ones that can be trickle charged. NiMH supposedly doesn't like that, making stand-by applications rather cumbersome (meaning more $$$) in terms of charger circuitry.

Regards, Joerg

A discharged battery has the potential to grow dendrites (whiskers) between the plates. Some formulas are better than others, Lead acid and NICAD are at the top of the list. Shocking Nicads tend to burn off the dendrites, but they eventually grow back.

Cheers

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

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.

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).

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

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.

You'd better talk to model airplane nuts and electric drill manufacturers about your C/2 or C "rule".

I "resurrected" a NiCd pack in a wireless phone once - it wasn't holding a charge, like I'd get two minutes of talk time, it'd beep, and go dead. Really pissed off one of my clients when the phone went dead on him.

So I took the battery out, poked a 1K resistor into the connector, and let it discharge to approx. 0. Stuck it back in the phone, let it charge for a day, and it worked like new.

I don't recommend this unless you're prepared to spring for a new battery if you actually kill it. :-)

Good Luck! Rich

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No, that's the "proper" voltage for a multi-cell application. A single NiCd cell can "properly" be discharged down to 0V. There isn't much charge under .8V or so though. Reverse charge is the worry here (which cannot happen with a single cell).

That depends on the design. "Deep cycle" or "marine" batteries are better at this than "starter" batteries because of their mechanical construction.

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.

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

Ed

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I read in sci.electronics.design that Keith Williams wrote (in ) about 'Extending the Life of a Rechargable Battery', on Wed, 12 Oct 2005:

The electrolyte concentration is different as well.

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

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

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."

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.

This site lists ~11.8 volts as 100% discharged:

This site shows 11.5 volts as 100% discharged:

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

Ed

"ehsjr"

Eg. A a 6 cell ( nominal 7.2 volt) Ni-Cd pack where all cells are identical.

Freshly charged = 9.0 volts.

On load = 7.5 volts ( at half capacity)

End point = 6.0 volts.

Load cut off = 5.4 volts ( using the 0.9 volt per cell criterion )

However -

In the real world, one cell will always be weakest, reach end point first then go very rapidly to zero.

The pack will still deliver 5 x 1.25 = 6.25 volts with one cell at 0 volts.

So, the weakling can be reversed to -0. 85 volts before the 5.4 volt load cut out operates.

With more than 6 cells, the situation gets worse.

....... Phil

Yup. When the pack ages to the point where one of the batteries is weak enough for the described scenario to occur, it is time to replace or repair it - or live with shorter times between recharge/lower power.

Ed

Clearly you don't want to know the facts.

........... Phil.

Of course it is not what you wrote - it is what I wrote. What you wrote is accurately quoted. And agreed with.

You haven't posted anything to indicate that you have additional facts to offer. You can keep whatever additional facts you have in mind to yourself, or post them. Your choice.

Ed

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Read what you wrote again. You said one should not discharge a *cell* below 80% of its rating (nonsense). You didn't answer the issue about Lead-Acids at all. Ok, you have no comment on lead-acids, but it would be nice if you made that clear.

Then, after I stated that NiCds *cells* could safely be run down to zero, you came back with you stated, in response to my comment that NiCds cells could safely be run right down to *zero*, but multi-cell packs couldn't be. To which you stated:

"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."

Which is *false*. An individual cell can be discharged to *ZERO* and have no damage at all.

Take your foot out of your mouth when you speak.

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Keith, You have confused me with a different poster. I never used the 80% figure. And I most certainly did comment on lead-acids: "Lead-acid 12V wet cell batteries should never be discharged below 10.5 volts. "

No it is not false. It agrees with what you said. It says, with emphasisis added "*the* cell,*S*. Not "*A* cell"

An individual cell can be discharged to *ZERO* and

Jeeez - read it again. the words "not a pack, just an individual cell" immediately follow what you said: "NiCds can be run to down to zero as long as the cells don't reverse polarity. " It agrees with you and emphasises that a pack cannot be safely discharged to 0 volts, while *an individual cell* can..

We are in agreement, and have been.

I can't be responsible for you missing parts of a post or confusing what another poster wrote with what I wrote.

Ed

( Hint - in a group of real cells, one will be the weakest. )

Who wins it mostly ?

......... Phil