High Voltage Charging

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In order to charge this car it will need a high voltage charger.
It's either that or we attempt to charge the 60 x 12v batteries in parallel!

Fortunately this is quite easy and cheap.

Mains power in the UK is 240vAC.
The packs in the Mass-EV are going to be arranged as 3 x 20 batteries.
Each battery is, of course 12vDC thus giving a pack voltage of 240vDC.
Total voltage of 3 x 240 = 720vDC.

This is, of course, the design to make charging from domestic mains easy and cheap.
When charging, the packs will be charged in parallel so we only need to charge a 240vDC pack.

If this gets sold in America I guess it will have to have 6 x 120vDC.

Rectified mains peaks at 240vAC x 1.414 (squareroot of 2) = 339.4vDC.
Or about 340vDC so has plenty of voltage difference to charge the battery.

We obviously need to control this as we don't what to overcharge the battery.
We can do this by controlling the AC input to the rectifier using a simple circuit
which normally is incorporated into a dimmer unit for domestic lights.


Original gschem schematic
This circuit was originally published on Electronics Project Design
I recreated the schematic from that image.

It works by removing part of the rising edge (or falling edge on negative sweep) of the phase using a triac circuit.
You can reduce the output by up to 50% linearly.
There's some information about it on epanorama.net.

You can further reduce the output by half wave rectification also, giving a pretty good range.
Certainly enough for the charging of the battery.

The dimmers are sold in DIY stores for under £10 so I purchased one and modified it with a more powerful heatsunk TRIAC.


Original datasheet
Pin Layout of the BTA26

The dimmer is from maplin:

Image of the dimmer from Maplin's website.

Some shots of the board removed from the dimmer:



Original Gimp Image
Better shot of the PCB with some detail.

Once the existing triac was removed it can be identified and the pinout discovered from the datasheet:




Original datasheet
Pin Layout of the BT136

Original Gimp Image

So now we can connect the new high power triac in place, via extending wires.
Of course, we could use this as a 6kW dimmer!

The heatsink and triac were £10 plus a bridge (£2) and a second bridge and switch to use one diode for half wave rectification.
Add a case and wiring and the whole charger comes in around £30.

This gives 13A at around 85v to 340v DC.
The 3 packs are 240vDC 40Ah each giving 240vDC 120Ah in total.
At a 13A charge this would be 120Ah / 13A = 9.23 hours.

So it's quite capable of charging 3 x 240vDC 40Ah lead acid pack to full charge overnight.
In reality you would charge at 12A for a 10 hour charge.

The unit components are actually rated up to 25A so this could charge much faster:
At a 25A charge this would be 120Ah / 25A = 4.8 hours.
For this you would need to have a special supply from your meter.
Also the batteries would probably need cooling, venting and monitoring since they are charging at a much higher rate.

Testing

So work finally began on this as it is now needed to charge the batteries we are going to use in the first prototype.


If you don't like the stereoscopic 3D just switch it off.

The circuit actually tested was:

Original gschem schematic
This seemed to work in that we could vary the average voltage of a 500w/250vAC lamp.
This was, of course, being fed with a variable DC voltage and not AC.

The capacitor was added to measure the RMS DC voltage and verify the meter's reading.
In the final ciruit it will not be needed as the battery does not need a smoothed supply.

There was an issue with the choke once the capacitor was added.
It was being heated.
I guess this was because the current was spiking as it recharged the capacitor during each phase.
This would cause high reactance in the choke.


If you don't like the stereoscopic 3D just switch it off.

As a test the choke was shorted to see if it worked OK, but this blew the 2A fuse quite dramatically.
This would again be because at switch on a high current would pass for the initial charge of the capacitor.


Original gimp image
Apart from the choke there was nothing even remotely warming up.
Not even the TRIAC, although it was only loaded to 4A.
It the real charger it will run around 12A which it still well below the 25A rating.

Even though the ciruit did OK, it would benefit from being designed more for this purpose.


Original gschem schematic

Voltage

Since the lamp was loading the output the RMS voltage was read from the meter, but we really need to see the peak voltage.
This should be around 340v.

As the battery charges the load should decrease and the average voltage should rise.
This would be working as a rugged uniform current charger.

There is an issue with seeing the waveform.
My scope is 100v maximum.

I will need a potential divider to see the waveform using this scope.
I'm guessing 100k high and 10k + 1k low should give me around 10% scale so the voltage should not rise above 34v on the scope.


If you don't like the stereoscopic 3D just switch it off.


If you don't like the stereoscopic 3D just switch it off.


If you don't like the stereoscopic 3D just switch it off.

So we tested this ciruit and experimented with different values for R1, R2 and R3.
Using a value of 0 for R2 and 6.8k (high power resistors !) we didn't really get better than 75% phase.

The low power control was pretty good, but not being able to reach the full power was a limitation.

Inspecting the standard dimmer circuit I understand why it was in this configuration.
The gate was switched from the supply across the TRIAC itself so that when the TRIAC conducted it removed the gate input.
This means you can achieve near 100% phase if the gate is connected to MT2 directly.

Undertanding Triacs

TRIAC or Triode for Alternating Current is the equivalent of a thyristor for AC.


Since AC effectively switches off every half cycle this means it does not lock on in the same was as a thyristor.
The TRIAC does lock on, but only until the end of the current half cycle.

It's like having PWM but without the complex timing circuit.
Since it's AC you get the timing bit for free.

So we tried out the redesigned circuit and it was indeed more stable, but the current in the gate circuit was a bit too high.
The gate circuit in the TRIAC as little resistance so will blow the TRIAC if connected directly to a supply.
This meant it would be better to connect it to the MT2 so the gate supply would be removed when the TRIAC conducted.

A revised circuit was tested:

Original gschem schematic

Charging Times

NiMH cell has an internal resistance of 0.17 ohms

We have 38 of 6 cells so 228 cells.

Total resistance is:
R = 228 * 0.17
R = 38.76 ohms

NiMH cell has a voltage of 1.2v per cell, so
Vbatt = 1.2 * 228
Vbatt = 273.6v

Terminal voltage was actually 285v so the cell voltage is actually 1.25v.
I believe this the no-load voltage.

The battery potential is raised from 285 to about 298 so
Vcharge = 298 - 285
Vcharge = 13v

I = V / R
I = 13 / 38.76
I = 0.335A
I = 335mA

This is about C/20 so sub-trickle charge,
but higher than a maintainence (C/100)

Capacity of a cell is 6.5Ah so charge time is
t = C / Icharge * 1.2
t = 6.5 / 0.335 * 1.2
t = 23.3 hours

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