WOODS ELECTRIC’S
LITTLE BLACK
BOOK OF
BATTERY
CHARGING
The writing’s black, anyway.
LEAD-ACID BATTERY CHARGING
HOW DOES CHARGING WORK?
Current forced into the battery causes a chemical reaction, converting lead sulphate to
lead on the negative plates and lead dioxide on the positive plates.
BATTERY OVERCHARGING
When most of the lead sulphate has been converted, the battery is fully charged. The
charging current then begins to electrolyse the battery’s water into hydrogen gas and
oxygen gas.
In “Sealed Lead-Acid” (SLA) batteries, the gasses produced by moderate charge rates
can be re-absorbed internally. In unsealed batteries, and at high charge rates,
dehydration (drying out) can occur.
SPARKING DANGERS
Batteries may produce hydrogen and oxygen gas during charging (see “Battery
Overcharging”).
Also, batteries in engine bays and under bonnets may be near fuel vapours.
The batteries may be knee-deep in explosive gas mixtures.
Only one small spark is needed…. Boom, flying shards & acid spray.
Chargers are designed to be connected and disconnected in a specific sequence to avoid
sparking:
• Connect battery clips with charger power OFF.
• Apply charger power, and charge the batteries.
• Turn the charger power OFF
• Disconnect batteries.
See, no sparks! Nice and safe!
BATTERY CAPACITY
Battery capacity is measured in “AMP-HOURS” (AH). For example, a 70AH battery will
deliver one amp for seventy hours, or seventy amps for an hour, or 35 amps for 2
hours…. in theory.
In practice, a battery’s exact amp-hour capacity will depend on the current drain,
temperature and battery age. Battery manufacturers make this information freely
available, if you ask them.
A battery’s “COLD CRANKING AMPS” (CCA) is a measure of its ability to deliver brief
bursts of current to starter motors, and is not related to the battery’s overall storage
capacity.
BOOST AND FLOAT CHARGING
FLOAT CHARGING
Forces current into the battery until it reaches 13.7V (27.4V), then holds the battery at
this voltage. Holding the battery at this voltage gives an excellent compromise between
maintaining a fully-charged battery condition, versus unnecessary gas production.
BOOST CHARGING
Forces current into the battery until it reaches 14.4V (28.8V), then holds the battery at
this voltage until the current demanded by the battery falls to a low level (typically 10% to
50% of charger capacity). Generally, the charger then reverts to FLOAT charging for
long-term maintainance.
Boost charging gives the fastest charging time for a given charger capacity.
Deep-cycle batteries require boost charging for best fully-charged battery condition.
SLA’s will accept boost charging, but do not require it.
BATTERY TYPES
WET LEAD-ACID BATTERIES
These are general-purpose batteries, and will accept high charge and discharge rates.
They need maintainance (ie. check the electrolyte level regularly, and specific gravity
when you think of it).
SEALED LEAD-ACID (SLA) BATTERIES
These require less maintainance than wet types, but they prefer longer charging times
and don’t like heavy loads (due to their higher internal electrical resistance).
DEEP-CYCLE LEAD-ACID BATTERIES
These are better suited to survive repeated deep depletion and recharging than wet or
SLA types, but need boost charging. They need a similar degree of maintainance to wet
types.
“CALCIUM” BATTERIES
These batteries are very good at cranking engine-starter motors. They will deliver high
peak currents (“Cold cranking amps”) due to their low impedance. They require very
specific charging routines.
HAWKER “GENESIS” BATTERIES:
These have good cold-cranking ability due to their low impedance, despite being a form
of SLA battery. They are also good in traction applications. They require very specific
charging routines, with relatively high charging currents.
DEFINING OUTPUT CURRENT
AMPS AIN’T AMPS!
The current delivered by most chargers to the battery is NOT nice, pure DC. The current
waveform usually looks like a series of pulses (100 per second).
The current can be measured as “RMS” amps or “Average” amps. The chemical
reactions in the battery during charging respond to “Average” amps, so you’d think that all
manufacturers would quote their chargers’ performance in average amps.
Unfortunately, the RMS amps always measure higher than the average amps… so this is
the figure which some manufacturers quote.
Just to add to the confusion, everyone in the audio profession has been taught (rightly)
that RMS measurements are the only valid type.
Look at the specifications carefully. If the chargers’ RMS current is specified, its average
current will only be about 60% of this figure.
HEAVY MATHS IN THIS BIT
The waveform below is for a typical 10 amp (average) charging current.
Xa: 60.00m Xb: 0.000
Yc: 10.07 Yd: 15.27
A
a-b: 60.00m freq: 16.67
c-d:-5.200
b
30
a
24
18
d
12
c
6
0
-6
0
10m
20m
30m
Ref=Ground X=10m/Div Y=current
40m
50m
Although the AVERAGE is only 10 amps, the RMS current is 15.3 amps – and the peaks
reach nearly 30 amps.
60m
The current waveform is equivalent to 10 amps of pure DC, with 11.5 amps of AC (not
sinusoidal) superimposed on top. Despite the large amount of AC, the charging current
never actually reverses; it simply pulses between zero and thirty amps.
The total RMS current (15.3A) consists the root-mean-square sum of 10 amps DC, with
11.5 amps of superimposed AC:
15.3A(rms) = √(102 + 11.52)
You can verify this at any time with a charger, a battery and an AC/DC RMS amp-clamp
meter.
THE MATHS IS FINISHED – BUT…
The average current gives a measure of how much current is charging the battery. The
RMS current merely gives an indication of how much heat is generated in the batteries’
internal electrical resistance by the leftover AC current.
The amount of heating in the battery is proportional to its internal electrical resistance,
and proportional to the SQUARE of the RMS current.
Since the RMS current from an unfiltered charger is around 53% higher than from a
filtered charger, its heating effect will be around 2.3 times higher.
FILTERED CHARGERS: WHY BOTHER?
“Filtered” chargers give near-pure DC output. There is no additional battery heating
caused by AC ripple from the charger.
The AC current ripple is wasted as heat. This extra heat generation is relatively small in
wet batteries, since these generally have fairly low internal electrical resistance, and
relatively large surface area.
However, internal heating becomes a problem for SLA’s due to their compact size and
higher internal resistance.
SLA batteries should be charged with filtered chargers to avoid overheating and
premature battery failure.
Filtered chargers cost more (and weigh more) than normal ones due to the extra
components required for filtering, but are quite necessary when charging SLA’s quickly.
Filtered chargers should also be used in applications where the DC power quality must
be of the highest purity – for example, battery systems supplying high quality audio
installations.
BATTERY BOILING
IS THE BATTERY REALLY BOILING?
There are two forms of activity which can be described as “boiling” in a battery –
ELECTROLYSIS and THERMAL BOILING.
•
•
ELECTROLYSIS, or electrolytic bubbling, is caused by overcharging, leading to
bubbling of hydrogen and oxygen.
THERMAL BOILING is caused by allowing battery temperatures to become high
enough to boil the acid mixture.
Electrolytic bubbling is accompanied by a mild acid smell. The battery may be warm, but
not too hot to touch.
A brief period of electrolytic bubbling in wet-cell batteries is not harmful, and signifies that
the battery is fully charged. The bubbling may even help in avoiding stratification, by
mixing denser and lighter layers of electrolyte which may form in stationary batteries.
Thermal boiling is accompanied by an extremely strong acid smell. Some surfaces of the
battery will be too hot to touch. The battery case may distort, or even melt, due to high
temperatures.
WHAT CAUSES OVERCHARGING AND ELECTROLYTIC BUBBLING?
Gross overcharging, and prolonged bubbling, can occur for a number of reasons – all of
them avoidable!
“Basic” chargers ($29.95 from Woolworths’): These generally have a low charge rate
(2.5A); there are few ill effects if overcharging (and resulting mild outgassing) continues
for only a few hours. Small batteries (eg motorcycle batteries) may be at risk of
dehydration.
“Float” or “Constant Voltage” chargers (eg Woods Neptune chargers): It is difficult to
force these chargers to overcharge. They will overcharge batteries if:
• a cell within the battery is shorted (Working cells must share the charger voltage)
• the charger voltage is set too high (more than 13.8V for a 12V battery)
“Manual” chargers (eg Woods Dialomatic chargers): The charge rate and battery
condition are in the users’ hands. Don’t leave batteries unattended. Disconnect the
charger soon after bubbling is noticed..
“Smart” or “Automatic” chargers (eg Woods Automatic types): Boost charge is terminated
when the current taken by the battery tapers to a low value. Overcharge can occur if the
charger stays in “Boost” mode, because the current has not tapered off. This can be
caused by:
• A shorted cell in the battery (the charger keeps force-feeding the battery to bring its
voltage up to the “Boost” level)
• A load on the battery during charging (even when the battery accepts low currents,
the charger supplies and senses the load’s current)
• The charger is too small for the batteries (eg: a 15 amp charger for 200AH batteries;
the charge current takes too long to taper to a low value, so the charger stays in
“Boost” too long)
• The batteries are too large for the charger (See above!)
WHAT CAUSES THERMAL BOILING?
Thermal boiling occurs when the charger current to the battery heats the battery’s internal
resistances above boiling point.
This may be due to high internal battery resistance, or to excessive charging currents.
Battery heating is proportional to its internal resistance. If its resistance triples, so does
the heating effect.
Battery heating is proportional to the SQUARE of the charging current. If the charging
current triples, the heating effect increases NINE TIMES.
NOTE: The heating effect from unfiltered charger current is about 2.3 times more than for
filtered charger current. Always use filtered chargers for SLA batteries!
Thermal boiling may occur because :
• Bad battery condition (high internal battery resistance in one cell or several!)
• The charger is too large for the battery (eg: a 60 amp charger on a 40AH battery)
• Batteries need filtered charger to reduce superimposed AC (eg gell-cells or SLA’s).
CHOOSING A WOODS ELECTRIC CHARGER
CHARGER TYPE:
You want…
If you are dealing with “wet” batteries with a variety of voltages
and capacities, or want manual charge control, and you’re
willing to attend the batteries during charging:
We make…
WOODS DIALOMATIC
If you’re charging wet batteries, and want to walk away and leave
the charger on auto-pilot:
If you’re charging Deep-Cycle batteries:
WOODS AUTOMATIC
If you’re charging “Calcium” batteries:
WOODS AUTOMATIC with
“Calcium” software
If you’re charging SLA’s or gel-cells:
WOODS FILTERED
AUTOMATIC
If you have two or three battery systems, and the budget for only
one charger (ATTENTION ALL BOAT OWNERS!)
If you’re charging “wet” batteries which may have heavy loading
during charge:
WOODS NEPTUNE
WOODS AUTOMATIC
WOODS NEPTUNE
QUICK PRODUCT DESCRIPTION:
DIALOMATIC chargers are unfiltered. They have manual control of charge current. They
will charge any battery voltage, from single cells up to the chargers’ nameplate. A good
workshop charger.
NEPTUNE chargers are a “Float” or “Constant Voltage” charger (unfiltered) with three
diode-isolated and individually regulated outputs. Choose this one for boats with multiple
battery banks.
AUTOMATIC chargers are “Boost-Float” chargers (unfiltered). They pamper your
batteries. A “must” for deep-cycle batteries. Turn them on, walk away and leave them to
it.
FILTERED AUTOMATIC chargers: Just like the “vanilla” Automatics, but the filtered
output has very low AC ripple. Choose these for gel-cells and Sealed Lead-Acid
batteries, and for top-quality audio installations.
CHARGER CAPACITY:
This is EASY! No rocket science is required…
1/ Figure out your battery’s amp-hour capacity.
Note: Batteries in parallel – capacity adds!
Eg: Two 12V/200AH in parallel = 12V / 400AH
Batteries in series – capacity unchanged!
Eg: Two 12V/200AH in series = 24V / 200AH
2/ How many hours will you allow for recharging?
Note: We suggest more than 2 hours, and less than 10 hours.
3/ Charger capacity equals battery amp-hours divided by recharge time:
And remember to add the GST! (the charger must return about 10% extra to the battery)
eg: for 200AH to recharge in 6 hours, you need:
33 amps
(plus 3 amps GST)
36 amps.
A 30amp or 60 amp AUTOMATIC charger would be a good choice so far…
4/ Add any load during charging:
eg: 200AH battery, 6-hour recharge
PLUS 15 amps of fridge consumption:
Total
36 amps
15 amps
51 amps
5/ Choose the nearest WOODS charger!
Since you need 51 amps, a 60 amp AUTOMATIC charger would be a good choice.
Recharge time would be less than five hours. A 30 amp NEPTUNE might suffice, but
recharge time would exceed 13 hours.
FINALLY:
We hope that you find battery charging to be an incredibly BORING activity.
We hope you never experience sparks, explosions, smoke, boiling, acid smells and
sprays, or dud batteries.
Charge on, regardless!