BIG FAT WARNING!
These MAINS and HIGH VOLTAGE experiments can kill you!
In order to charge this car's battery it will need a high voltage charger.
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.
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.
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.
So work finally began on this as it is now needed to charge the batteries we are going to use in the first prototype.
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.
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.
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.
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.
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.
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 way 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.
I have some resistors which appear to be 2w (Resistors),
so using 5 parallel capacitors connected directly to one terminal for testing:
I filled in the values on the above circuit.
Testing on Real Batteries
Started with a relatively safe circuit which was extremely low current just to get a feel for the batteries reaction
This is a test which I expanded to charge 3 complete full batteries in parallel which were all very low.
excessively discharged or deep-discharged
batteries, first allow a trickle current to flow,
and then proceed with the rapid charge current once
the battery voltage has risen.
Rapid charge start voltage: Approx. 0.8V/cell
Rapid charge transition voltage restoration current:
0.2 ~ 0.3 CmA"
In this document it refers to "CmA" which is assuming the capacity (C) is in mAh.
This is indicating we should use a charge of C/5 to C/3.33.
Pretty much all these cells were well below the 0.8v, in fact alot would be 0v.
MCU Controlled PWM
Since, in a different project here MCU control of mains is actually quite easy and cheap,
This can be extended using AC-DC then using an ADC to measure and control voltage.
So a first basic AC circuit driving a lamp was constructed:
BIGGER FATTER WARNING!
These 3-phase MAINS, HIGH VOLTAGE experiments can kill you!
The battery packs in the Mass-EV v0.1 are 3 x ~400vDC, so this will not charge from 240vAC even rectified to approx 330vDC.
In order to charge these a 3-phase charger using the 415vAC would be needed, unless we use a step-up transformer.
This is a very basic charger, not really that cheap since it uses a £16 resistor.
But it's reliable would work at C/100.
Top left is the pack voltage (418vDC) bottom left is the input voltage (583vDC) and you can see the current is 820mA.
The resistor is dissipating about 130W, well within the rated 200W.
This was charging 32 x 12v 100Ah batteries (384vDC total), which of course was no problem.
Extending this to all 3 packs (99 batteries).
So to provide some degree of isolation the single 200Ω resistor was replaced with a pair of 100Ω resistors,
one to the positive terminal and one to the negative.
Also a pair of 100W/100Ω resistors are cheaper (~£5 each) than a single 200W/200Ω one.
Charging from Domestic
In order to charge a 415vDC battery from domestic UK mains the voltage first needs to be stepped up above the battery or it won't charge it.
This can be done quite easily and cheaply using a voltage doubler circuit called a Delon circuit to give 680vDC
The diodes D3 and D4 (half the fullwave bridge) were left connected in parallel.
This was tested and gave a charge current of 1A, so equal to the 3 phase equivalent.
The 2700μF capacitors were swapped for 100μF and the current dropped to 0.75A, but was still charging so it was fine.
This should provide up to 580vDC which means it would charge a 400vDC battery.
The battery in a G-Wiz is a simple 48v Lead Acid wet cell, so this doesn't need a sophisticated charger.
This is just a 500w transformer, bridge and smoother and using a 10R/60W resistor for current limit.
Charge rate is about 1A into 200Ah battery so C/200.
The open circuit voltage is about 80v which is 3.33v per cell.
This can't be used for trickle charge as the gassing voltage is less than 2.5v per cell.
It would need to be limited to 60v for the pack for that which can be done with a LM317.
48v EV Motorbike charger
Using a purpose build charger from a 400va 48v transformer we get better results:
There is need of measuring the voltage of each 12v battery, but there are 99 of these so it must be something that scales cheaply.
Also it does not have to be extremely accurate as it's really just measuring the relative voltage against the pack voltage.
The most obvious way is to use an optocoupled circuit, so we need to check the accuracy of the optocoupler.
Optocouplers of the require quality cost 10p - 50p, a 1k resistor is less than a penny, and the Arduino Mega is from £13 - £35 which should manage the whole pack.
This means we can monitor the whole pack with less than £100 of components.
Wiring would be a considerable cost, but again should be within that budget.
The downside is that this will draw a small current constantly.
Up to 15vDC in the circuit, minus the voltage drop across the optocoupler LED (1v) would be across the 1k resistor.
I = V / R
I = 14v / 1k
I = 14mA
Also need to know the power is withing spec of the resistor:
P = V2 / R
P = (14v)2 / 1k
P = 0.196w
So a 1/4w cheap resistor would be fine
This is across an 100Ah battery so if this was the only thing draining it the time for full discharge would be:
t = Q / I
t = 100Ah / 14mA
t = 7142.86 hours
t = 297.62 days
A0 and A1 were 12v (2 batteries in series), and A2 and A3 were on 6v singles.
The check was run over a few hours with a very rough recified mains charger attached.
This is the raw ADC input 0<=ADC<=1023 which needs the mapping graph above to get the voltages where 5v output = 1023.
The values are roughly 200 and 340, so 0.97v and 1.66v which map to about 14v and 10v.
These are shifted because the collector resistor is different: 470R vs 300R,
but it very obviously measures the batteries independently and seems quite stable.
All current electric cars have an auxiliary 12v battery which is prone to going flat.
Even a Tesla has this auxiliary 12v battery system.
In our car the 12v comes directly from the traction battery, so no "going flat" problems.
You can leave the lights on for like a year and the car will still run.
So here we have a 360w DC-DC converter which uses a flyback converter to convert 420vDC to 12vDC.
The output of this is an isolated 12vDC at 30amps, for an input current of less than 1 amp from the traction battery at full power.
These are very efficient (approx 80%) and, given that these form the basis of most modern mains power packs, are very reliable.
The car will have 3 of these (one for each traction battery) so the combined power will be over 1kW at 12vDC.
Obviously triply redundant as well, and will provide a good solid 12vDC even when the traction battery is far too flat to power the car.
In the circuit are also a local 5vDC buck converter to provide 5vDC at 3A.
So no need for an auxiliary battery which is prone to going flat in less than a day.
Also there are some 5vDC systems in the car (Raspberry Pi and Arduino).
These get their power by dropping the power from 12vDC to 5vDC using a buck converter circuit.
Again very efficient >85% and high output (3A).
So there will be plenty of high power USB charging points in the car.