Author Topic: Pulse charger...  (Read 18454 times)

Offline Silvio Klaic

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Pulse charger...
« on: February 21, 2011, 12:34:15 AM »
I decide to start new project, pulse charger...
In my last project with battery capacitance tester I stuck with trying to recover battery by draining it to 8V and recharging in several cycles.
This however didn't produce any result and I have feeling that my old charger is not suitable in recovering process.
This 30 years old charger have transformer with 4 power diodes for rectifier and small light bulb for control...
So I decide to build add-on for this charger to transform it into pulse charger.

After searching web for some useful schematic, I found only these sites:

None of this fits my needs, so I decide to build my own version based on John Bedini principle.
I use 555 timer, set it up for frequency range from 500Hz to 3kHz, then with two out transistors set driver for MOSFET IRF640.
Then I test how this works with different coils and at end I find that 6mH toroid choke coil is optimal for this configuration.
I didn't want to use my test coil so I made one using parts from dead PC ATX power supply.

Here you can see on left desoldered coil, in middle core without wires (inner diameter is 13.5mm, outer 23.5mm and width 9.7mm).
On right picture is my new coil of 6mH, to make it I use wire with diameter of 0.3mm (with insulation) and 8.5m long.
I use instrument to measure inductance, but I think there are 294 turns unless I miscount somewhere. :)

Here is schematic for prototype. I used small light bulb (24V, 4W) as battery for calibrating device. Input voltage from my test transformer is 20V so I need to add resistor R5 of 47 ohm to drop down voltage to about 17V before 12V regulator to avoid heating.
R3 is for adjusting frequency form 500Hz to 3kHz and R8 for length of positive out pulse (duty cycle).

For testing device I decide to use another dead battery, it is 12V sealed lead battery with 7.2 Ah from 1993. It is originally used 5 years in UPS and then collected dust without charging. Using classical charger I fill it up under minute and drain less than 30 seconds to 2V.

Initial test charging with pulse charger is set as 1.4kHz, 60% duty cycle and last about 21 hours. After resting it holds voltage of 2.9V, and connecting 24V/4W light bulb will drop voltage to 0.2-0.3V. During this test charging, MOSFET become hot and there was no progress on battery, so I decide to reduce duty cycle below 50%.
After calibrating again with 24V light bulb I got this:

On left side is signal which produce 555 IC and it reaches 6V with 46% duty cycle. In center image is signal which drive MOSFET, for start it uses +2.4V and after -1.4V to block gate (without MOSFET out is +/- 4V). On right picture you can see out spike, it reaches 200+V and quick drops to 0V (under 46% of duty cycle). With this I solve heating MOSFET, and now only coil become warm.

During calibrating device to 700Hz I found that light bulb becomes bright, even brighter than connected directly to 20V. So I measure current and I detected some interesting phenomena. This 24V light bulb, directly on 20V, drains 3.6A current, but when connected to charger it drains only 230mA. Less than 15 times!
I think I should do another project involving DC pulse currents and regular light bulb of 100W. I'll try to use current directly from mains power to se if I can get same brightness on 100W bulb and to consume only 10W... :)
« Last Edit: February 21, 2011, 11:49:51 AM by Silvio Klaic »

Offline Silvio Klaic

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Re: Pulse charger...
« Reply #1 on: March 01, 2011, 05:06:00 PM »
I made some modification to original schematics.
After testing I decide to remove two driver transistors, because 555 IC have enough pull-up to GND for driving MOSFET alone. However I placed 1uF capacitor to stabilize raise-fall gate voltage.
I was also experimenting with different frequencies and came to conclusion that original setting of about 600Hz is optimal. Higher frequencies have lover voltage peak and I need pulses beyond 100V for battery sulfation problem. So to work with higher frequencies I need high voltage transformer (better solution) or coil with lower inductance - higher consumption (worst solution).

Here is new schematic. I place small incandescent light bulb as control and ballast at output. Without load, coil transfer all HV spikes to MOSFET which can burn out in short time. I also added small ventilator to cool down coil.

Here is prototype board and test casing for it.
Frequency is set up to 600Hz, out voltage on DC scale is 5.8V and 12.7V at AC scale of multimeter. Power consumption of electronics (without coil/MOSFET/ventilator) is about 40mA, and without ventilator it consumes 200mA. Ventilator is from old TNT graphic card and consumes 90mA at 12V.

In theory this charger should never overcharge battery and can be place forever to charge battery (something like trickle charger). But those theories I’m planning to test.
My sealed battery during testing didn’t show any changes. I have recorded internal resistance of battery. In entire time it didn’t change at all, it’s constantly about 257 kohms (yes, kilo ohms).
For measuring (calculating) I use method from Wikipedia:
There are also other methods, like:

Conclusion is; my sealed lead battery is sulfated beyond repair with charge or/and have some structural damage... My plan now is to open it and try with magnesium sulfate to restore it back to life. If someone interested how, here some links:

I’ll test my charger with other batteries, and I’ll report back how things going... :)
« Last Edit: March 01, 2011, 05:10:24 PM by Silvio Klaic »

Offline Silvio Klaic

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Re: Pulse charger...
« Reply #2 on: March 11, 2011, 08:36:10 PM »
Here is update on restoring sealed battery; in short I gave up.
After breaking seal and examining internal plate status, there was no point in repair attempt.
There was crystal structure which deformed and at multiply points shortened opposite plates.
So even if I remove that crystal form, there is question; will I have enough plate material to be able to restore it in normal working condition?
I’m thinking at best to quarter capacity, but amount of work and price of chemicals equals more expensive than new one.

One another observation; while performing tests with measuring internal resistance I found that my method form Wikipedia isn’t good.
So I have to find something else for reliably test internal resistance of battery.

Next, about tests with charger, my initial setting was excellent for small batteries up to 2Ah, in range from 1.2V to 24V.
There is only need to adjust DC output voltage with duty cycle.
My first test was on 70Ah battery and after 2 days of charging voltage raises to 12.4V, so I decide to do testing on smaller battery to see result more quickly.

Next test was done on 4.5Ah battery, and after 4 hours I didn’t notice significant improvements.
Charger consumption with battery was 0.38A and I need to get more power to charge it faster.

This is new setting; on left are out pulses with only light bulb connected.
In middle are pulses for MOSFET gate and on right you can see brightness of light bulb at this setting.
Frequency is 1.2 kHz, with 65% duty cycle on, voltage output 10.6V/23.9V (DC/AC scale at multimeter), consumes 0.29A with only light bulb and 1.12A when charging battery.
With this I was able to fully charge 4.5Ah battery in about 3:30 hours.
Coil was very hot, so I rearrange ventilator from blowing out of casing to in at coil to make it more temperature stable.

Then I calculate that for charging 70Ah battery I’ll need more than two continuous days, which is too long.
So I decide to push charger to limit (and beyond).

This is more powerful setting; 1.6 kHz, 75% duty cycle on, voltage output 12.8V/28.9V (DC/AC scale), it consumes 0.34A with only light and 2.2A with battery.
On bottom left picture are out pulses when battery is charging.
As you can see HV spike is completely absorbed and charging is done only 23% in cycles during coil discharge.
Notice on bottom right how 24V light bulb is now much brighter.
At this point I was not sure if ventilator can keep coil cooled enough. Anyway I decide to push forward and see what happens.

This is end result (I remove pieces of melted plastic casing to get better picture of PCB).
In about 6 hours of charging, coil is gradually heating plastic casing and ventilator to a point of melting.
Ventilator at this point becomes working slower allowing coil to become hotter.
After that, everything gone in runaway effect, ventilator casing was deformed and stops working.
Third of casing was melted away and nearby screw fell on PCB making short connection in which MOSFET blocks in on state and starts heating until breakdown occurs.

So for more powerful version I need to make some heavy duty coil.
However I also need more current at output for faster charging.
There is also problem of HV spikes if there is no load present.
Therefore I think to make new approach, something like this:

This will solve HV spikes problem in primary side by capturing it in capacitor.
Frequency must be tuned in capacitor - coil resonation, therefore each new pulse will go with capacitor discharge and reduce collisions/current waste.
Transformer model will produce more current at output for faster charging and hopefully solve problem if no load is connected (yeah I know this won’t work, let me dream a little bit ;)).
« Last Edit: March 11, 2011, 08:41:48 PM by Silvio Klaic »

Offline Silvio Klaic

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Re: Pulse charger...
« Reply #3 on: March 27, 2011, 03:46:19 PM »
There is error in previous schematic; MOSFET must be P-type to work properly.
I was originally made on test board with P-type and it didn’t work as expected, probably because used MOSFET was too weak.
After that I rearrange parts and made N-type version which worked. Here is modified version:

In here capacitor C8 was placed to reduce load on MOSFET and add better spike output.
When used without C9 it produces 3-4 spikes in short duration and acts like desulfator.
C9 was closed loop with primary coil and significantly increase power output.
Charging in this configuration is much faster, but output spikes are reduced to about 50V.
Running frequency is set to 6.4kHz.

Here is shown coil with only primary (upper left) and prototype device with completed coil.
There is 766 turns in both coils, primary one uses copper wire of 1.4mm diameter, and secondary of 0.6mm.
Inductance in primary is 0.994mH, resistance 0.99 ohm, in secondary is 1.89mH with resistance of 6.22 ohms.
As you can see this coil is big, 500mm in length and 43mm in diameter, but it does job done. :)

However I was not happy with this, because I want bigger pulses and smaller charger, so I started from beginning with new direction.
After searching thru other project with pulsed chargers I find these two most interesting:

And so I create list of what my charger must have:
  • Go on directly at AC line
  • Generating short high voltage spikes (100V+)
  • Pulses must be strong to do actual charging not only desulfation
  • Made in KISS principle
  • Smaller dimensions
So after thinking I decide to try with something like this:

This schematic should fulfill all listed must haves.
CD4060 with potentiometer will provide frequency range from 760Hz to 3.5kHz.
Switch provides selection of duty cycle from 50%, 25%, 12.5%, 6.25%, 3.125% and 1.5625%.
IC3 with two transistors make driver for MOSFET.
Entire device get current from AC line thru C2/D1-2 and power for spikes goes form rectified power AC line.
This should provide powerful and stable 250-300V spikes.

Anyway this is idea, after testing and building prototype I’ll post results.
WARNING: Do not attempt to do this at home, this uses live AC current and it can easily kill you!
Remember, I’m trained amateur and I know what I’m doing. ;)
« Last Edit: March 27, 2011, 03:50:58 PM by Silvio Klaic »

Offline Coplan

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Re: Pulse charger...
« Reply #4 on: November 11, 2017, 01:56:24 PM »
Anyway this is idea, after testing and building prototype I’ll post results.
WARNING: Do not attempt to do this at home, this uses live AC current and it can easily kill you!
Remember, I’m trained amateur and I know what I’m doing. ;)

Sounds like a challenge to me.  :) Seriously though, thanks for the infol. I'd love to make my own pulse charger.