# 5.11 Winter storage for batteries, and their state of charge

There is a ritual debate on this topic each year. The concensus seems to
be that (1) It's OK to store a battery on a cement floor, but if you stick it
on an old piece of plywood, any drips or spills will be easier to clean up,
so perhaps the old wives' tale has some value, (2) storing a battery cold in
the winter, provided it is fully charged, is an OK thing to do. The rate of
discharge is reduced by the cold environment, so less frequent recharging
is called for.

Here is an article from Finn Stafsnes, which seems to have some hard data
(fs):

The content is taken from a booklet provided by norwegian battery
manufacturer (Anker-Sonnak).

I have done some linear interpolation between tabulated values. Therefore
minor errors due to non-linear effects may be present. I can only hope
that I have not done big errors in my calculations.

```State............Spec.gravity.......Freezing.......Spec.gravity
of...............@ 25 C, 77 F........point.........@ freez.temp
charge..........kilograms/litre.....deg C, F....kilograms/litre
```

```Full (100%)..........1.280..........-68, -90......not available
.75 %................1.250..........-52, -62......not available
.50 %................1.220..........-36, -33..........1.263
.25 %................1.190..........-24, -11..........1.224
weak.................1.160..........-17, + 1..........1.189
"0 %.................1.130..........-12, +10..........1.156
"0 %.................1.100..........- 7, +19..........1.122
```

If it is impractical to measure the spec. gravity an approximate formula is
given based upon voltage measurment:

Spec.gravity (@ 25 C) = ((Voltage of battery)/(no of cells)) - 0.84
(kilogr./lit.)

The voltage should be measured after the battery has been disconnected
(left to rest) for at least 6 hours.

A discharged battery will gradually be distroyed if stored in a low state of
charge condition due to crystal growth of PbSO4, even if it don't freeze.

Self discharge rate is halved for every 10 deg C (18 F) the storage
temperature is reduced.

Conclusion: Keep the battery well charged all the time. If you don't want
to recharge during the winter, store the battery cold.

And here is a mini-FAQ written by Alan Yelvington:

The efficiency of batteries varies with time, temperature, and state of
charge.

Batteries self-discarge over time. Lead-calcium (die-hard) discharge faster
that straight lead-acid. Their advantage is that they typically do not need
to have the water replaced.

Temperature will kill a battery over time. If a battery gets too hot, its
self-discharge rate goes up. If the battery gets to cold, the reaction that
produces electricity gets slowed down and the full capacity cannot be
"harvested."

The state of charge limits efficiency because of the reactions in the
battery. If a battery is left dead for too long (this means you), the internal
plates will start to accumulate lead-sulphate on them. This insulates that
portion of the plate so that in can no longer contribue to the output of
the battery. It takes extra power in to remove the sulphation that cannot
be recouped. (EDTA will chemically remove the sulphate....)

A typical battery in good condition will return 90 to 95% of the power
put into it under these conditions:

DO NOT recharge at a rate of more that one tenth its capacity. eg. A 220
amp-hour battery should not be recharged at more than 22 amps. The
excess current will generate waste heat and form lead-sulphite. The
lead-sulphite is worse than the sulphate because it cannot be removed.

DO NOT discharge a battery beyond 50% of its capacity.

DO NOT over charge the battery. (Lead Sulphite problem again.)

DO NOT discharge the battery faster than one tenth of its capacity. That
is, don't draw more than 22 amps from a 220 amp-hour battery. You'll
just make waste heat that cannot do work.

DO use the battery and not just leave it dormant all the time. If you
must have a battery for infrequent use, NiCd or gelcells are much better
and are another story altogether. (ay)

Another reader pointed me towards a nice solar panel charge controller
the November, 1993 issue of "73" magazine. It's used by a guy with 200
WATTS of solar panels on his roof.

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