[Lowfer] Lithium packs (Was: A power supply head scratcher)

[C. Turner]

I know that this is still somewhat O.T., but I imagine a lot of us 
LowFERs have these things (LiIon cells) hanging around our shacks...

LiIons are tricky things:

- They don't like to be kept at full-charge forever.  A full 
state-of-charge tends to accelerate their decomposition - particularly 
at elevated temperatures (>90F)

- They don't like to be over-discharged.  Dangerous and irreversible 
chemical change starts to happen when they are <1 volt/cell

- They have a built-in "chemical timer."  From the moment they are 
manufactured, their chemistry begins to deteriorate and within 5 years 
or less, they will probably be hazardous waste if used in any "normal" way.

- Like most types of cells, their lifetime is significantly reduced is 
run through a lot of full charge-discharge cycles.

Having said all of that, there are a few things that you can do to make 
them last longer:

- Keep them cool.  I have read that if you have a laptop that you do not 
use on battery very often, putting the battery pack in the 'fridge when 
you are it on its adapter is better - but that's inconvenient and is 
probably overkill.  Nowadays, a lot of battery packs on laptops are 
situated externally so that they don't get as much heat from the 
computer itself.

- Supposedly, the best storage lifetime occurs when they are kept cold, 
but somewhat discharged.  For most LiIon chemistries, this is probably 
in the 3.3-3.6 volt region.  Supposedly putting semi-discharged LiIons 
in deep freeze will mostly stop that "clock."  I obtained some surplus 
"new" LiIon cells (raw cells - with no attached circuitry) several years 
ago and charged them to about 3.5 volts/cell, wrapped them in plastic 
and kept them in the deep freeze.  When I have pulled them out for a 
project (to "re-cell" a camcorder pack or something like that) and run a 
load test (using my West Mountain Radio CBA-II) to determine their 
health, they seem to be maintaining their capacity - even though they 
are now getting to be quite old, so the "freeze thing" seems to work.  
Occasionally, I'll check the cells' voltages and top-off (after they 
return to room temperature!) those that have dropped below 3 volts or so 
due to self-discharge.

- Avoid either full discharge or constant "full" charge - for the 
reasons mentioned above.  This one is kind of tricky, though as it's 
pretty inconvenient on any device that you are likely to use with 
regularity.  A device that is used relatively infrequently - like a 
camcorder - may be a reasonable candidate for such a regimen, however

***

One problem with keeping LiIons around for a while is that most have 
circuitry in them (battery gauge) that draw a small amount of current - 
plus the self-discharge that, as with most types of chemistries, tends 
to increase with age and, especially, with temperature.  If a LiIon is 
kept around too long with no attention paid to it that voltage can go 
below the "safe" threshold at which point the built-in safety circuitry 
may prevent you from being able to charge them ever again: I've seen 
this very thing happen on several different models of Dell laptops.

I have, for several years, been putting together my own LiIon packs for 
radios - namely FT-817 and my FT-530 HT - and I generally get 3-5 years 
of life out of them before they start to "fade" noticeably - which I 
consider to be a fair trade off for the enhanced capacity when they are 
"newer" and in good shape:  For example, I have a couple of very 
lightweight 3 amp-hour battery packs that will run my old FT-530 HT for 
weeks with casual listening, or for a day or two with very heavy TX use 
- such as during a multi-day public service event.

For charging said packs I use the "Constant-Voltage/Current-limited" 
method.  For my FT-530 packs the charger is just an LM317 on a fairly 
small heat sink with the output voltage adjusted to 8.2 volts (e.g. 4.1 
volts/cell) and after about 6-8 hours on that they are charged to 90% or 
so.  The LM317 limits the charging current to about 2-2.5 amps peak - a 
"safe" level (which would be 2-3C or about 6-9 amps maximum for these 
cells) and the smallness of the heat sink will cause it to throttle back 
to 1-1.5 amps as the regulator heats up.  In that way, charging them can 
be considered to be akin to lead-acid packs (that is, constant-voltage) 
- except that *unlike* with lead-acid cells, you *don't* want to float 
them at "full" voltage any longer than necessary to assure that they 
have charged!

While the specification is 4.2 volts/cell (which translates to 8.4 volts 
for the 2-cell in series pack) for full/maximum voltage on these cells, 
I have read from several sources that dropping the peak voltage by a few 
percent will extend the lifetime of the batteries significantly and that 
the 10% or so maximum capacity that you lose from charging to a lower 
voltage - capacity that rapidly disappears as the pack ages, anyway - is 
traded for longer lifetime. I don't know if this really works, but it's 
easy enough to do.

More recently, with my newer projects I have started using what appears 
to be a more-promising chemistry - LiFePO4 cells.  These do not (yet) 
have the energy density of traditional LiIon packs (they are 2-3 times 
larger for the same number of amp-hours of a comparable LiIon) but they 
are reported to have a charge cycle life of about 2000 charges and a 
lifetime of about 10 years or so - numbers that approach good-quality 
NiCd cells that are treated properly throughout their life.  
Additionally, if you mistreat them (to do so you'd have to defeat the 
"protection circuit" that should be used with them) they are reportedly 
far less-likely to do the sorts of things that LiIons famously tend to 
do - like blow up, catch fire, etc.

Clint
KA7OEI/CT

Peter Barick wrote:
> "Preaching to the choir, no doubt, but it's worth harping about... 
> Clint KA7OEI/CT" 
> --------------------------------
> No, Clint, your harp rings well, and tnx fer your effort in details. Like most I have collection boxes of wall modules, mostly the heavy sort. That is likely what you refer to as "iron" ones. But, yes, I've noticed those light-weights now that come with digicams and small music players, for battery recharging. Not heard mention before of said flaws and will monitor my few for such surprises.
>
> Which brings up this thought -- I have a new music player that has a Li-On battery (my first of this kind). What about recharging? Is it like the Ni-Cds where constant charging can be detrimental, so leave tham run down before an overnight plug-in? The SanDisk instructions made no mention, only how to replace the 3.6V battery.
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