Capacitive Battery Charger
Published Saturday September 1, 2012: Updated 11/12/2015

Capacitive Battery Charger and De-sulfater
Microprocessor controlled equipment has become the norm these days. Modern day battery chargers are sophisticated and expensive.  
While having lots of whistles and bells is enticing, sometimes going with simplicity is the more elegant and sophisticated solution.  

Back to Basics: There are some amazing, forgotten circuits that were used long before the computer age. One of these grass-roots circuits is the humble capacitive battery charger. 

I am amazed at how simple the capacitive charging circuit really is. 

Here is the parts list.  
  • A capacitor (or several connected in parallel)
  • A bridge rectifier
  • The internal resistance of the battery itself, (without this, the circuit does not work).  
  • Optional amenities: Fuse, power strip with switch, lamp timer, Kill-A-Watt meter, DC volt meter.  

I highly recommend getting your own Kill-A-Watt meter. 
Caution: Do not attempt to connect the charger up to anything except batteries. This circuit can generate upwards of 154 volts DC. If you connect it to other components (in conjunction with a battery), make sure they handle the higher voltage.  

Charge                  Volts/cell     120 volts    96 volts     72 volts       Notes
100% Charged        2.58           154.8          123.8          92.9              Vigorous gassing
80% Charged          2.38           142.8          114.2          85.7              Gassing starts
0% (Equilibrium)      2.10           126             100.8         75.6              Charged battery (not in use)
80% Discharged      1.75           105             84.0           63.0              End of useful usage
100% Discharged    ------           -----             -----            ----                Not recommended

It cost me about $50 for eight large surplus 50uF 440VAC capacitors at NPS and a few bucks for a large 30Amp 400V bridge rectifier on e-bay. The picture below shows the capacitors (wrapped in yellow and silver tape) and the bridge rectifier (top center of the picture).  
The switch allows the charger to be used on 120V or 240 volts. It also selects the charge rate (fast-charge for the 80% bulk charge or slow-charge for topping off the last 20% or for charging overnight). 
A high current capacitive charger/desulfator. 

A capacitive battery charger could theoretically charge up a bank of lead-
acid batteries to 80% charged in only 6-minutes. 
Hypothetically, to restore 12 kWh back into a large battery bank in 6 minutes would require a 500 Amp charger. 
500 x 25µF = 12,500 µF. 500 A at 240 V = 120,000 Watts. Wow, that's a lot of power. While I have never done it that fast before, I regularly charge the pack in my EV truck to 80% in under 3 hours. The last 20% required to get a full charge takes a lot longer due to the chemistry of lead-acid batteries. 

This same capacitive charger circuit will work for any lead-acid battery or string of batteries from 6 volts to 144 volts DC when powered off of 120VAC. 
An occasional over-charge is beneficial to leveling out the voltage of every cell in the pack. While this may also work for other chemistries, the capacitive charger is ideal for flooded lead-acid batteries because they are very forgiving of overcharging. Water lost while overcharging can easily be added back as needed.  
One of the down-sides of the capacitive charger is poor power factor. While power factor alone does not consume any excess energy, it does limit the amount of power available from a given circuit due to its higher amp draw.  

Warning! Dangerous and Free Engineering Advice:  

If you are interested, here is the schematic to my simple charging circuit. I didn't invent this. Mid 2011, I was about to spend $700 on a fancy commercial 120VDC battery charger when Brian at Wilderness EV told me about this circuit. USE AT YOUR OWN RISK! and please, do not use it with lithium batteries! 
Capacitive Charger/De-sulfator Schematic 
Equivalent Circuit

Without any battery load, the output of the bridge rectifier is about 154VDC (or rather 120Hz pulsating DC). The capacitive reactance of the capacitor and the internal resistance of the battery form a voltage divider circuit.  
A home-converted 120VDC lawn mower with an on-board capacitive charger. It can mow nearly a 1/2 acre of lawn per charge.  

Make sure the capacitor(s) you pick are bipolar (don't care what direction they are connect up). Hint: Most electrolytic ones are not bipolar and when connected up to 120VAC will act more like an M-80 firecracker than capacitor. 
Choosing a capacitor
  • Bi-Polar
  • Very low ESR, (effective series resistance)
  • High Quality
  • In the 10-50 μF range (~25 μF per Amp of charge) 
  • Rated for AC voltage RMS * 2 √ 2 *peak voltage
    • 120 v * 2.828 = 340 volts
    • 240 v * 2.828 = 680 volts
The large silver capacitors that accompany motors are perfect for this application. They are typically rated to over 400 volts AC. I have found that the larger the capacitor (physical size) the cooler it will operate and the longer it will last. Try not to use the big blue capacitors from Hong Kong with the wire pigtails. They have too high of an ESR (effective series resistance) and in this application, will over-heat, dry out and quit working in a couple weeks.
A crude rule-of-thumb is to use 25uF of capacitance for each Amp of charging current you want to deliver to the battery or battery pack pack. Higher voltage battery packs require more capacitance for the same Amp of charging current. 
The role of the capacitor is to limit the current going into the battery. Amazingly, it does this without any power loss (like in a resistor). 
Monitor your battery voltage as it is charging and know ahead of time what voltage is required for a full charge. 

Rule of thumb for flooded lead-acid batteries: 
  • 6-volt lead-acid batteries have 3 cells, 12 volt ones have 6 cells.  
  • 80% charged is 2.38 volts per cell (142.8 Volts for a 120 Volt battery pack)
  • Bubbling and gassing starts to occur at 80%. 
  • 100% charged is 2.58 volts per cell (154.8 Volts for a 120 Volt battery pack).  
  • Vigorous bubbling and lots of gassing occurs at 100%.   
At the beginning of the charge cycle the power going into the battery pack is the highest. As the battery pack charges, the power going into it drops until it settles out at some nominal value and the battery reaches a full charge.  
It would be advantageous to get yourself a lamp timer. It will keep your batteries from boiling away if you forget to unplug them after they are charged.  
A Kill-A-Watt meter is also a valuable tool as it will keep track of the energy that it takes to charge up your batteries.  
From that you can also calculate how efficient your battery consumption is. For example, a typical charge in my EV truck is about 13KWH. I drive 40 miles each day (13,000/40) so I end up using 325 watt-hours/mile. As Lord Kelvin once said, "If you can't measure it, you can't improve it." I highly recommend the Kill-A-Watt meter.

Lead-acid batteries are nearly 100% efficient at charging up to about 80 percent SOC (state of charge). The top 20 percent SOC, they are only 50-80% efficient at charging. 

Older batteries are less efficient at charging than newer batteries. My EV truck used to go 40 miles on only 11.5kWh from the wall, now after 10,000 miles and 500+ charge cycles, it takes nearly 15kWh to go the same distance.  
DeSulfator

Amazingly, this simple capacitive charging circuit can also bring damaged or dead batteries back from the dead. In the case of lead-acid batteries, over time, sulfate crystals form inside the battery. Eventually they get large enough and short out across the lead plates, killing the battery for good. No amount of charging on a regular battery charger will ever bring the battery back. The battery has reached the end of its life and you have to replace it. Until Now!  
With the desulfator, the pulsating DC created by the bridge rectifier heats and vibrates the sulfate crystals causing them to break off, opening the short.  
 
It's almost miraculous watching a completely dead battery come back to life and usefulness.  

Resurrection is tricky business. It is best left to deity who has the proper knowledge, power and authority.  
Resurrection of a dead battery however is much, much simpler.  

To prevent the battery from blowing up, I monitor the whole setup with a volt meter and a Kill-A-Watt meter.  
Charger/De-sulfator with power and voltage monitoring. 

I also check the battery temperature with my hand for excess heat. Initially the heavily sulfated battery has a very high internal resistance. This causes the voltage across the battery to be very high, (maybe 109 volts DC or so) but power going into the battery is very low (only 1-2 watts). As lead sulfate crystals start to break up, the internal resistance of the battery goes down, voltage across the pack starts to fall and the power going into the battery starts to go up (10's if not 100's of watts).
 You need to be careful not to over-heat the battery during this phase of the resurrection process. When you hear the sulfate crystals buzzing and crackling inside the battery, you know something amazing (and kind of scary) is happening.  
For a badly sulfated pack, the battery temperature will go way up. If the battery becomes hot to the touch, shut power off to the battery and let it cool down before continuing on.  
After 5-60 minutes of desulfating, (depending on battery size and severity of the sulfate) the voltage drop across the battery will settle to a value consistent with the nominal battery voltage (12V or 6V depending on the number of cells in the battery). The power going into the battery will also settle to a lower value too.  

Measuring just the amp draw of the capacitive charger can be misleading. A capacitive charger that is charging a small 12V battery may draw 220VA but in reality less than 24 watts of real power is being consumed. We must be wise to the tricks of power factor.  

Here is a chart showing measurements that I took when desulfating a dead 7Ah 12V gell cell lead-acid battery. I fed 120VAC through a 30uF bipolar capacitor which is then connected to the input of a bridge rectifier. I attached jumper cables to the output of the bridge rectifier and connected them directly to the battery.  

DC Voltage Across Battery    Watts Going into Battery (Real Power)        Volt-Amps (Real + Imaginary power)
                     109                                                 2                                                                  223
                     90                                                  60                                                                 223
                     85                                                 109                                                                223
                     30                                                  80                                                                 223
                     20                                                  60                                                                 223
                     14                                                  23                                                                 223

It is a good idea to use a power strip with a switch so you can make the battery connections before power is applied to the capacitor. Otherwise you get a surprising, Zap!  
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A 2-Amp capacitive charger, desulfating a small 6V 4Ah gell cell.  

After the desulfating process is complete, the battery (if its plates are not too damaged) will perform like new. It may have slightly reduced capacity (compared to a new battery) but you will have literally brought it back from the dead and given it new life.  

It will be able to be charged up on a regular battery charger too. 
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