Battery Chargers

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.

Charger Types

DC Current

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.

Pulse Methods

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.

Burp Charging

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 Charging.

Charge Termination

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 has a 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 uses 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.

-dV/dT

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 for Ni-MH.

Zero Slope

Similar to the -dV/dt method above but instead of looking for the negative slope it terminates at the peak.

Orthogonal Measurements

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.

Temperature

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.

dTemp

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.

dTemp/dt

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%).

Pressure

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.

Others

There are a large number of other patented methods of charge termination. The burp methods for example. Or the battery temp - ambient temp method.

Self Discharge

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.

Smart Batteries

Rechargeable Chemistries

Lead Acid

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 months.

Alkaline

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:
6022639 Zinc anode for an electochemical cell Feb 8, 2000 429/229 (15.6 amp short ckt curr ? batt size)
6589693 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 resistance and load testing.

Ni-Cad

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 used.

Ni-MH

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 heating.

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 around.

Li-Ion

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.

Li-Poly

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.

Chargers

Simple Home Made Charger -
Mike Murphy Charging Kit - for use with the 6140-01-241-2295 0.8 AH Batteries that Mike sells.
Buy 3 of the batteries and get Mike's charger free. See my Squad Radio Battery Snap for a connector.

Commercial

Single Cell Chargers - there are a huge number of these some cheap DC type and maybe others are Burp types.
Rayovac 3in1 - 4 station "D" cell or smaller - will charge their rechargable Alkaline as well as Ni-Cad and Ni-MH

Maha H-C777Plus - 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 charge.
Lab Type Power Supply - can be used for charging all kinds of batteries and will bring back from dead batteries that the Maha will not.
Battery Space Universal Smart Charger -
Maha MH-C401FS - AAA/AA smart charger - supposed to do a good job on Ni-MH cells. Got it for Sanyo Ready To Use (long shelf life) cells
Maha MH-C9000 - 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.
Triton2 Charger Discharger Cycler

Military

706841-801 - 5 Station Magnavox for PRC-68 type Ni-Cad batteries with internal thermistors DC fast & trickle

PP-7286/U - 5 station for a varity of Ni-Cads uses digital circuitry and regulated current for specified time then shutoff

PP-7601 - 6 station for PRC-68 type batteries digital circuitry, not manual or info.

PP-8444A/U - 2 or 4 station suitcase universal charger different versions can charge different chemistries.

Propel BB-590 "Burp" charger very good on the 5590BA

TMQ-34 Weather Station Charger DC low rate (100 ma) charger

Patents

Burp 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

3517293 Rapid Charging of Batteries June 23, 1970 320/129; 320/152
3559025 Rapid Charging of Batteries Jan 26, 1971 320/129; 320/153
3597673 Rapid Charging of Batteries Aug 3, 1971 320/129; 320/153
3609502 High Frequency Battery Charger Employing an Inverter Sep 28, 1971 320/129; 320/139; 320/158; 320/DIG17 - includes Burp

3609503 Termination of Rapid Charging of Batteries Sep 28, 1971 320/129; 320/152; 320/DIG17 - Termination for a Burp charger
3614582 Rapid Charging of Batteries Oct 19, 1971 320/129; 320/139; 320/149; 320/DIG17
3614583 Rapid Charging of Batteries Oct 19, 1971 320/129; 320/139; 320/152; 320/DIG17
3626270 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; 320/31; 320/39

uses the time deritive to determine when charged
References:

Patent Number	Title		Issue date
3597673	RAPID CHARGING OF BATTERIES	Wilford Burkett	Aug 1971
4006396	Universal battery charging apparatus <>Burp	Motorola	Feb 1, 1977
4134056	Apparatus for charging rechargeable battery <>Burp	Sanyo	Jan 9, 1979
4213081	Method and apparatus for charging sealed Ni-Cad batt <>Burp Peak detection (Vnow < Vmax)	no company	Jul 15, 1980
4354148	Apparatus for charging rechargeable battery <>Burp	Sanyo	Oct 12, 1982
4385269	Battery charger BURP	Redifon	May 24, 1983
4388582	Apparatus and method for charging batteries <>Burp delta	Black & Decker	Jun 14, 1983
4503378	Charging system for nickel-zinc batteries <>Burp inflection	GM	Mar 5, 1985
4639655	Method and apparatus for battery charging <>Burp		Jan 27, 1987

4829225 Rapid battery charger, discharger and conditioner May 9, 1989 320/129 - More modern Burp charger
6459243 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.

3258671 Method for Increasing the Capacity of Silver Electrodes (Silver-Cadmium, Silver-Zinc) -U.S. Navy Jun 28, 1966 320/139
4499415 Battery rapid charging circuit Feb 12, 1985 320/139 ; 320/163