© Brooke Clarke 2005
DC Voltage with Current Limit
Trickle or Float
This web page is a direct result of
testing many battery chargers for use with the 5590BA Battery Adapter
that uses 20 or 22 AA cells and the need for a consolidated
A battery has some total Amp hour
life. For example if the battery is rated for 500 charge
discharge cycles and 2 Ah capacity then it's total life is 1000
Ah. This is related to both how it's charged and to how it's
discharged. If it's abused by discharging to deeply or by
over charging then the life is shortened.
The spec sheet has information on the minimum premissable
discharge voltage and recomended charging methods.
The problem with charging is the battery can have it's life
shortened by over charging. Severe over charging can kill
the battery just as severe discharging can kill some batteries.
If a discharged battery is put on a charger the problem is
straight forward and almost any of the charge termination (see below
) methods will work.
The problem is when a partially or fully charged battery is put on
the charger. For example if you use the C/10 for 16 hours
charge on a full battery at the end of the 16 hours the battery
will not have any more charge than it had at the start. It
will be warm to hot and it's life will have been shortened by one
charge and some electricity will have been wasted. If a full
battery is put on a fast charger that uses -dV/dt charge
termination and the charger ignores -dV/dt for the first few
minutes to eliminate false stops on batteries that have not been
used for some time it may never terminate the charge. In
this case if there is no thermal limit charge termination the
battery will be killed by excessive temperature.
The most common charger is just a DC current source. Most
any battery can be charged by feeding it a current that's it's
specified discharge Capacity divided by 10 (C/10). For
example a Ni-MH AA cell may be rated at 2.3 Amp Hours so it's C/10
charge current would be 230 ma. If the world was perfect you
would only need to put this current into the battery for 10 hours
and you would have put in 2.3 Amp Hours, but the battery and the
charging circuit have resistance that converts power into
heat. Also the battery may generate heat as part of the
chemical reaction related to charging, like a Ni-MH that nearing
full charge which also waste power. So instead of 10 hours
is common to see 12 to 16 hours for a slow charge.
For rates between C/10 and 1 C it may be possible to charge a
little faster, but it's not a good idea because you might be
getting into the area where the battery is going to have a
problem. This type of charger typically adds a timer to
control the amount of charge that's put into the battery.
Another way to prevent overheating to to monitor the battery
temperature. For example the charger for the Ni-Cad BB-590
battery does not check the battery chemistry because when a sealed
Ni-Cad is charging the chemical reaction causes a temperature rise
that's not present with Ni-Cad batteries.
If DC current is used in an attempt to fast charge a battery, say
at a rates way above C/10 the battery will not charge and might
vent, explode, catch fire or some combination of these. NOT
a good idea.
DC Voltage with a Current Limit
This is a common way to charge a Lead Acid battery. When the
battery is near dead it will draw current at the limit and as it
charges the terminal voltage goes up and so the current
decreases. At full charge only a trickle of current is
flowing into the battery. One problem is that the value of
the voltage depends on the battery temperature.
There are a number of methods where the charging current or
voltage is applied as a pulse or as pulses of different
amplitudes. By measuring the voltage with no load or with
one or more different loads some idea of the battery State Of
Charge (SOC) can be determined and this can be used to terminate
the fast charge and switch to a trickle to topping charge.
A Burp Charger uses high current pulses for charging (maybe 10 C)
and also has high current (maybe 10 C) pulses discharging the
battery intermixed with the charging pulses. In order to
work the energy in the discharge pulses needs to be much much less
than the energy in the charging pulses. Wilford Burkett has
a number of patents on this method. As is common with
patented ideas they don't show up in reference text
books. For example the 2"+ thick "Handbook of
Batteries" Third Edition, ISBN 0-07-135978-8 has no mention of
Burp or Pulse charging methods, yet the patent office has a large
number of patents on these topics.
My experience comparing the Maha C777+ charger that uses a pulse
method to measure the battery condition compared to the Proper model BB-590
that uses the
Galaxy 1702 smart Burp Charging controller chips indicates that
burp charging is a much better way to go. It does a fast
charge while only warming the battery, whereas the C777+ gets the
batteries very hot, and if the external temperature sensor is not
in good contact with a battery being charged these a good
possibility of damaging a battery by overheating it.
6 Dec 2007 - I think a Burp Trickle charger could be made that
would maintain Ni-MH or Ni-Cad battery packs, i.e. put back the
power that would otherwise be lost to self discharge. If
this is of interest let me know
See the Patents
section below for more on Burp
The standard method of charging a
battery when it's known to be discharged is to charge at C/10 for
16 hours. The Maha C9000
break-in mode (aka IEC capacity analysis) that: (1) 16 hr charge @
C/10, (2) rest 1 hour, (3) discharge at C/5, (4) rest 1 hour, 16
hr charge @ C/10. This is recommended for new batteries
(which have been sitting for many months and so are close to fully
discharged) or batteries that are known to be discharged. It
gives the battery a full capacity charge discharge cycle.
Note that a battery that was 100% efficient on the charge cycle
would only need 10 hours at C/10 rate to charge. At 16 hours
an extra 60% of it's capacity is being fed to the battery.
C/10 and Time
When slow charging (C/10 is considered a slow charge) the battery
will not overheat and so there's no time limit. For most
batteries 16 hours is enough to be sure you have fully charged the
battery. The PP-7286/U
this method. There is a potential problem with this method
when the ambient temperature is hot in that a battery with a
partial to full charge at the start might over heat.
As a Ni-Cad battery nears end of charge the voltage rises faster
that it did for most of the charge then peaks (the slope is zero)
then falls off with increasing negative slope. By looking
for the negative slope the charge can be terminated very close to
the 100% of charge point. This does not work for a C/10
charge but need some higher rate.
A similar effect is present on Ni-MH cells but the size of the
peak is much smaller. So a charger made for Ni-Cads that
uses -dV/dt termination will not properly detect end of charge on
a Ni-MH cell.
As of Dec 2007 both Sanyo and Duracell (the only ones I've checked
recently) do not recomend -dV/dt as a charge termination method
Similar to the -dV/dt method above but instead of looking for the
negative slope it terminates at the peak.
When making a measurement it's good to have Orthogonal
parameters. For example when measuring the
capacity of a battery the cell voltage is used to
determine the end of discharge. A plot of battery
cell voltage will have a steep slope (be nearly at right
angles to the time axis).
But if zero slope is used the slope of the curve is
parallel to the time axis so it's very difficult to say
when it's exactly zero.
This method can have problems with false termination due to
noise or temperatue changes and so is not recomended.
This is an absolute max temperature charge termination. When
a Ni-MH cell receives over charge at rates above C/10 it will get
very hot so a temperature sensor can be used to terminate the
charging. Tends to over charge when it's cold and undere
charge when it hot. This is not recommended as a normal
charge termination method, but is recommended as a safety back
termination method. the reason is that when any battery gets
very hot the life is shortened. For some applications where
battery life is not a concern like racing there is an advantage is
getting the battery hot because it produces more power.
In delta Temperature the change in temperate is sensed. This
has an advantage in that the ambient temperature has less
impact. If absolute temperature is used and it's very hot
then the charging may be terminated too soon. In delta Temp
the battery starts out at the ambient temperature and charge is
stopped when the battery temp increases some specified
amount. This can work for fast charges where the charge time
is fairly short, but has problems it the charge time is many hours
because the ambient temperature changes during the night and day.
Duracell recommends this method when the parameters are matched to
the specific battery. For their "D" cell Ni-MH charged at 1C
terminate at delta T of 15 deg C (27F). No topping charge
needed. And use a 60C (140F) safety termination.
This is an improvement on the dTemp method since it's looking at
the slope and so will not be as influenced by the daily
temperature ambient temperature changes. Duracell has a plot
of battery capacity vs. the number of charges (1 to pver 300)
comparing -dV/dt to dTemp/dt. For the first 250 charge
cycles the -dV/dt method provides about 5% more capacity but after
250 cycles dTemp/dt provides more capacity and the -dV/dt capacity
starts falling off rapidly and the battery is dead by cycle
350 but the dTemp/dt battery goes to about 430 cycles.
Duracell recomends this method with a 1C charge rate and
termination at 1 deg C/min (1.8 deg F/min) with a 60 deg C
temperatue backup. To make up for the 5% lower capacity this
method prvides they also recomend a top off charge of C/10 for a
half hour (that equals C/20 or 5%).
When a battery is charged not only does the temperature increase
but also the pressure inside increases. There are no
commercial chargers that I know of using the pressure
increase. Probably because it's not easy to measure.
There are a large number of other patented methods of charge
termination. The burp methods for example. Or the
battery temp - ambient temp method.
When any primary or secondary
battery is sitting on a shelf with no load attached it will
discharge. For the common Ni-Cad or Ni-MH chemistries this
may amount to one or more percent per day. For some data see
my Ready to
(low self discharge) web page.
In addition to discharging the battery also looses the ability to
be charged. This is a temporary effect, but all the chargers
I've seen don't know how to handle it. If you see that the
capacity of a battery is much lower than expected it's good to put
in on a few cycles of discharge and charge and note how the
capacity is changing cycle to cycle. If it does not improve
the battery is dead. But if it does improve keep cycling
until it flattens out.
Trickle or Float
A lead acid battery can have a
constant voltage applied that's near the fully charged battery
voltage and typically can only supply small currents. If the
cell voltage decreases due to self discharge then current flows
and keeps the battery fully charged. This is very good for
backup batteries and for maintaining a battery in a parked car.
Ni-Cad and Ni-MH batteries have a self discharge rate of about
C/300 to C/500 (that's 0.3 to 0.2 %/hour). But these
chemistries do not accept a DC charge at this low a rate according
to Sanyo. Duracell does recommend a C/300 trickle charge to
maintain it's Ni-MH cells.
Note if the hourly self discharge rate is C/400 that's 0.0025C per
hour. Since the discharge depends on the current capacity it
decreases and the capacity decreases. To get the daily rate
use (1-C/400)^24 so for C/400 the capacity after 24 hours is 0.94
or the battery has lost 5.8% of it's capacity. This is an
exponential type discharge not linear. Also it's very
temperature dependent, the hotter the faster the self discharge.
The self discharge is improved when a battery is stored in a cold
location. But the capacity of a battery like Ni-MH or Ni-Cad
decreases at cold temperatures and decreases rapidly for
temperatures near water freezing.
Li-Ion and Li-polymer batteries have
a venting & catching fire type problem if their terminal
voltage gets above or below specified limits or if the current
exceeds a specified limit (either charge or discharge). To
prevent this each cell has a safety circuit and the pack has a
current monitor. If any of the prohibited conditions exist a
FET switch opens the terminals. These protection circuits
are digital in nature and can be connected to a smart chip that
can communicate using a one wire serial protocol with the host
load or charger. The chips also have coulomb counting and
can predict the battery discharge time (gas gauge).
Some of these use stock chips and public data protocols and can be
used in other applications. Others use a micro controller
with a proprietary interface protocol and can not be used by
anyone else. So that great eBay deal for a laptop battery
that's for a computer you don't have may be next to useless as far
as the gas gauge is concerned.
A primary battery has a chemistry
that only goes in one direction (discharge). If you try and
put charge into this type of battery it's not going to charge and
may very well do something nasty. A lot of thought and
effort goes into preventing attempts at charging primary
batteries. For example an early BA-5800
(all military batteries with a name starting BA- are primary
types) used in a PLGR that was designed to charge a battery when
was connected to a vehicle
tended to explode the primary battery. The sockets on the
BA-5590 family of batteries are divided into two types primary
with 5 keying slots and secondary with 4 slots to mechanically
prevent a properly designed charging device from connecting to a
primary battery. The BA-5590
primary battery has no connection to pin 3, but all the secondary
batteries in the family connect negative pin 1 to pin 3.
This way if a charger uses pin 3 instead of pin 1 it can not
charge a primary battery.
This is one of the oldest types of
rechargeable battery and comes in a number of different
forms. I'm not going to spend much time on them except to
say that you need to keep them away from electronics. I
got a very good deal on a Gibbs precision oscillator because it
was designed with the gel cell batteries in the same box as the
electronics. When during normal charging the batteries
vented with a track of sulfuric acid over time the traces were
etched from the PCBs in the warmer parts of the package.
Typically charged with constant voltage and current limit.
The Cyclon cylindrical Lead Acid deserves special mention.
I learned about this battery when working with a military O-1814
Rubidium Frequency and Time
standard that uses them for a back up battery. Virtually
all the other batteries I've looked at prohibit discharging to
zero volts. For example if you discharge a car battery to
zero volts it's dead and will no longer take a charge.
BUT, the Cyclon can be discharged to zero with the proviso that
it will be charged within a reasonable short time, like what
would happen when there's a power failure.
When other types of batteries are used in backup systems the
controller needs to have a way of disconnecting the battery when
the cell voltage gets to the minimal allowed to prevent pulling
the cell to zero volts.
It turns out that the designers of the O-1814 should have also
done that, since it was used in a vehicle mounted system , not
an line powered system where the battery would automatically get
charged. In the vehicle system if a soldier turns of the
power to the whole system at the central power panel then the
O-1814 backup battery runs to zero volts and does not get
charged until the next time the system is used which may be many
The Alkaline chemistry goes way
back to the Leclanché Battery
which could be recharged by changing the electrolyte. The
more modern Alkaline battery has a chemistry that can go both
ways, but has not been optimized to do it efficiently.
Rayovac does make a rechargeable Alkaline battery but it has
limited charge cycles when compared to other rechargeable.
For many years Alkaline batteries used a small amount of Mercury
to prevent the zinc from oxidizing. When Mercury was
removed from Alkaline batteries there was a lot of development
work (and many many patents) on ways to replace it. If you
look at a package of Energizer or Wal-Mart Alkaline batteries
you will see a couple dozen patent numbers, and almost all of
them are related to ways of getting around the Mercury
problem. But a couple of them are related to getting more
energy out of the battery, see:
Zinc anode for an electochemical cell Feb 8, 2000 429/229 (15.6
amp short ckt curr ? batt size)
High discharge electrolytic manganese dioxide and an electrode
and alkaline cell incorporating the same July 8, 2003
429/224; 429/218.1; 429/229; 429/231.8
These patents are aimed at providing high discharge currents and
are a big improvement over conventional Alkaline cells.
For more see my 5590BA web page on internal
and load testing.
This was the rechargeable battery
that enabled a very large number of battery powered devices like
drills, tooth brushes, electric shavers, hand held radios, etc.
that needed a battery that could supply high currents and was
small and light weight. There was a "memory" problem with
early Ni-Cads, but it's not clear to me if this is still true of
modern Ni-Cads. Know for their good performance under
heavy discharge and heavy charge. A workhorse battery.
Typically charged using a constant current. If above C/10
then a charge termination methods is needed such as a timer,
terminal voltage, it's slope vs. time, or some pulse method is
These provide more Amp Hours than
a Ni-Cad of the same size. They may not have the Ni-Cad
memory problem by may have a decrease in capacity after some
amount of use. This effect may also be caused by improper
charging (i.e. overheating during charge). For example a
battery that should have a run time of 6.9 hours only has a run
time of 4.2 hours after a single over charge incident.
These batteries get very hot when fast charged and they are near
full (maybe at 80% of capacity). If an over temperature
method is used to terminate charge either some capacity is
sacrificed or there is a danger of damaging the battery by over
The peak voltage and voltage slope methods that work for a
Ni-Cad will not work for a Ni-MH unless they are much improved
versions since the voltage increase with the Ni-MH is smaller
and harder to detect. A charger that can charge a Ni-MH
will typically also work with a Ni-Cad, but not the other way
These cylindrical cells offer the
highest Watt Hour capacity for the volume they occupy.
This is the current standard battery for laptop computers, cell
phones and other high tech products.
They require protection circuits and specialized chargers.
These pouch cells offer the
highest Watt Hour capacity for the weight and are popular with
RC model airplane hobbyists. They have the same need for
protection circuits and special chargers as the Li-Ion batteries
although the RC airplane folks typically remove the protection
so that they will not crash the plane due to a low battery, but
they may burn up the family car as a result.
Single Cell Chargers - there are a
huge number of these some cheap DC type and maybe others are
- 4 station "D" cell or smaller - will charge their rechargable
Alkaline as well as Ni-Cad and Ni-MH
Desulfating Battery Charger, Maintainer,
- single station
combined charger discharger with LCD readout of mAH put in to
removed, uses mag mount thermoter. No information on how
it works, but appears to be a DC type with pulse for state of
charge measurement. Gets Ni-MH cells very hot on every
Lab Type Power Supply - can be used for charging all kinds of
batteries and will bring back from dead batteries that the Maha
Battery Space Universal Smart
- AAA/AA smart
charger - supposed to do a good job on Ni-MH cells. Got it
for Sanyo Ready To Use
- this is a charger
analyzer for AA or AAA cells, but I don't see why it could not
also work with C or D cells, i.e. any single cell rechargable
battery that was Ni-Cad or Ni-MH.
Charger Discharger Cycler
Samya AQ-K800i Cell Phone External Battery Charger
||The modern energy saving Switch Mode
Power Supply is rated for inputs of 100 to 240 VAC 50/60
Hz (i.e. it's a worldwide wall wart) with an output of 6
VDC @ <= 400 ma.
Works best when two batteries are used, one in the cell
phone and one that's been charged ready to be swapped.
Radio Shack 23-335
This charger has a clear plastic cover. The first
generation Maha MH-C401FS
also came with a clear plastic cover, but
they said to remove it and didn't ship later units with
the cover because it can overheat the batteries while
they are charging.
This Radio Shack unit can both discharge and charge AA,
AAA or 9V batteries, either Ni-Cad or Ni-MH. It
gets within 100 mAh of the MH-C401FS on a 2600 mAh AA
Switches for selecting battery chemistry and Standard or
Hi Capacity and a button to discharge.
The table on the bottom has AA and AAA battery capacity
ranges for the various switch combinations but not for
- 5 Station Magnavox for PRC-68 type Ni-Cad batteries with
internal thermistors DC fast & trickle
Power Supply is
designed to charge mil vehicle batteries or power radios
- 5 station for a varity of Ni-Cads uses digital circuitry and
regulated current for specified time then shutoff
- 6 station for PRC-68 type batteries digital circuitry, not
manual or info.
- 2 or 4 station suitcase universal charger different versions
can charge different chemistries.
Weather Station Charger DC low rate (100 ma) charger
Electricity: Battery or Capacitor Charging or
127 Battery or Cell Discharging
Pulsed discharge (Burp)
137 Battery or Cell Charging
current or voltage amplitude
current or voltage differential (e.g., slope, etc.)
charging (e.g., plural charge rates before a maintenance charge,
There are those on the internet that think Burp Charge is snake
oil and does not work. Burp Charging does NOT cause the
cells to get hot. The Maha C777+ charger stops charging
Ni-MH cells based on an over temperature error (140 F). Burp
Charge is a pulsed discharge during the charging process.
Wilford Burkett's Burp Charge Patents
Rapid Charging of Batteries June 23, 1970 320/129
Rapid Charging of Batteries Jan 26, 1971 320/129
Rapid Charging of Batteries Aug 3, 1971 320/129
High Frequency Battery Charger Employing an Inverter Sep 28,
320/139; 320/158; 320/DIG17 - includes Burp
Termination of Rapid Charging of Batteries Sep 28, 1971 320/129
320/152; 320/DIG17 - Termination for a Burp charger
Rapid Charging of Batteries Oct 19, 1971 320/129
320/139; 320/149; 320/DIG17
Rapid Charging of Batteries Oct 19, 1971 320/129
320/139; 320/152; 320/DIG17
Battery Charger for Single Cells Dec 7, 1971 - also Burp
4413221 Method and circuit for determining battery capacity, Fred
Benjamin, Robert H. Heil (Christie Electric Corp), Nov 1,
1983, 320/48; 320/14; 324/427; 324/435
Uses discharge pulses during
charging. analog circuitry
4746852 Controller for battery charger, Ray J. Martin
(Christie Electric Corp), May 24, 1988, 320/20; 320/14; 320/21;
uses the time deritive to
determine when charged
CHARGING OF BATTERIES
battery charging apparatus <>Burp
|Feb 1, 1977
for charging rechargeable battery <>Burp
||Jan 9, 1979
and apparatus for charging sealed Ni-Cad
batt <>Burp Peak detection (Vnow <
|Jul 15, 1980
for charging rechargeable battery <>Burp
||Oct 12, 1982
||May 24, 1983
and method for charging batteries
|Black & Decker
||Jun 14, 1983
system for nickel-zinc batteries <>Burp
|Mar 5, 1985
and apparatus for battery charging
|Jan 27, 1987
Rapid battery charger, discharger and conditioner May 9,
- More modern Burp charger
Multiple plateau battery charging method and system
to fully charge the first plateau Oct 1, 2002 320/155;
320/160 - has an overview of other types of charger patents.
Method for Increasing the Capacity of Silver Electrodes
(Silver-Cadmium, Silver-Zinc) -U.S. Navy Jun 28, 1966 320/139
Battery rapid charging circuit Feb 12, 1985 320/139
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