Balance charger for Chevy volt module

There's another thread going on about 48V batteries that I've been adding thoughts to. I think it may be helpful to suggest a dead-man high voltage cutoff (HVC) switch that I've been working on. I believe that a crude failsafe is better than nothing at all.

The heart of the circuit is a lowly 555 timer:



Fundamentally the timer is a high powered set/reset flip flop. It's set then the trigger (pin 2) falls below 1/3 VCC (pin 8), which drives the output (pin 3) high. The FF is reset when the threshold (pin 6) reaches 2/3 VCC. Timing circuits are produced by combinations of RC circuits connected among the timer's pins. However we can use it as a voltage controlled cutoff switch.

The plan is simple: the 555 triggered on turns on the charger. The battery reaching it's target voltage turns off the charger. To keep the numbers simple let's drive the 555 with a regulated 12 volts.

The trigger is trivial. Tie pin 2 to a pullup resistor to Vcc, with also a normally open switch to ground. So as a default the trigger is deselected. Pressing the button triggers the part and turns on the charger.

The high voltage cutoff is equally as simple. Measure the battery voltage and set it to trigger the threashold at 8 volts (2/3s of VCC). Done with a simple voltage divider. Say you want to be real safe and cut off the charger a 4.0 volts/cell. So the battery would cut off at 48 volts. So you need a voltage divider that will translate 48 volts to 8 volts. Fortunately there are a million voltage divider calculators out there. I use this one:

http://www.ohmslawcalculator.com/voltage-divider-calculator

Since the upper resistor is going to have the higher resistance, let's fix R2 at 1K ohm and see what R2 needs to be to translate 48V to 8V. Result is a 5 to 1 ratio. So 1K for R2 means 5K for R1. Finally we need to quickly compute the power. The current is 48V/6000 ohms = 8 mA. The power is 48V*8mA = 384 mW. So 1/2 watt resistors will do the job. If we wanted to use 1/4 watt resistors, we'd have to get the amperage below 250 mW. 250 mW/48V = 5.2 mA. So let's reduce the current to 5 mA. 48V/5mA = 9600 ohms. Doesn't really give good round resistor numbers. 12K would give us R1=10K and R2=2K. 48V/12K is 4 mA. Looks like a winner.

So tie a 10K resistor to the battery + terminal, a 2K resistor to ground, and tie them both together and feed the divider to pin 6 of the 555. According to this tutorial:

https://www.jameco.com/Jameco/workshop/TechTip/555-timer-tutorial.html

other connections should be reset (pin 4) to Vcc, ground (pin 1) to ground of course, and control Voltage (pin 5) to ground via a 0.01 mF capacitor. Pin 7 discharge is an output (open collector) and is unused in this application.

So there's the dead man switch. Press the button, and the output of the 555 goes high. When the battery reaches 48V (in this example), pin 6 resets the FF, the output goes off, and most importantly stays off. Also note this particular circuit needs to be driven with a regulated 12V power supply to ensure that the threashold voltage is a steady 8 volts.

The most trivial way to drive the charger with the 555 is using an SSR. I bought a handful of 40 A overkills from Marlon P. Jones for $5 a piece on sale. Drive the input with any DC voltage from 3-32V and it'll switch up to 240V @ 40A supposedly. Way overkill for anything that plugs into a wall.

None of this precludes using other monitoring or control. It's literally a dead mans switch that protects the battery even if everything else blows up. See the other thread for suggestions on CC/CV charging, switching power supplies, and using Arduinos as control circuitry.

BTW it's always possible to just watch it. I just recharged my Volt module today using a Meanwell 48V 12A supply. Biggest problem I had was trying to figure out how to keep the battery from sparking when connecting it to the supply. It seems that a precharge resistor and a bypass switch are necessary to charge the output caps of the supply so that it won't spark.

Hope this helps,

ga2500ev

I have done some research into Chevy Volt batteries.

check it out
http://www.diyelectriccar.com/forum...-battery-bms-charging-questions-190594p4.html

If your looking for an affordable, professional "smart" charger

check out

http://evcomponents.com/elcon-tc-hk-h-1800w-charger.html

~$300.00 delivered, it's even from one of the DIYElectricCar sponsors

merkurmaniac said:

Be aware that serious limits on throughput exist. no more than 15 amps in or out, and limited to around 600watts, maybe 900 if you brought in high voltage and put out a little higher.

That being said, I have NOT used it to charge my Volt battery module yet, I plan to this weekend. I'll be charging at 150watts or less, as my demand schedule can allow it. I'll be mowing for a half hour, then literally have the rest of a week to charge. This works for me, may not for you.

Click to expand...

Been waiting for your report - but it hasn't appeared yet.

Anyway, I bought one of the DROK units and tried it out this weekend. My first run was very conservative, starting with it set to the battery voltage, then raising it in steps to my target voltage. I used a 12 Volt 20 Amp regulated supply to power it (because it was at hand).

In summary, it works like a charm, and did exactly what I would have wanted. It wasn't fast (took 8 -9 hours), but I am not in a hurry. An overnight trickle charge would be fine for my situation.

The highest charge current I saw was just under 2 Amps. That may have been due to my own inexperience (learning curve encountered), or perhaps my supply should have been more capable. I do have a boat anchor supply that will put out 25-30 Volta at 50 Amps - and will try it set to a higher voltage next time. I did note that my 12 Volt supply got pretty warm during
the test. Don't know what the current draw was. The boat anchor has an ammeter, so I will get better data next time.

I will say that the instructions leave something to be desired (written by a non-native speaker of English). It takes a few passes through them to decipher the procedures. At least there IS some info. I also bought a Lithium Battery Capacity Meter that came with nothing. It reads Voltage OK, but shows every battery I connect it to to be at 100% Capacity. I have no idea how to adjust or calibrate it.

Bottom Line: The DROK unit appears to be just what is needed to properly charge a Chevrolet VOLT Li-Ion battery - without using a BMS (provided you cal live with the low charge rate) . I have on order a cell tester - with which I will explore what is going on in the individual cells. Will report on that when I have had a chance to do so.

Dave

Here is a pic of my ratproof outdoor cabinet. The drok is running fine. I am charging right now at 1 amp, so thats barely 50 watts. The Alienware Laptop charger is rated at 240 watts, so I am sure it would be fine living at half that rate. I just got my electric mower running Saturday, so this was my first real charge operation. I have mine controlled by my sprinkler system as a zone called "charger" , and this way, I'll not depend entirely on the DROK to keep things between the ditches if I leave it powered all week. My plan is:

1. settle on a charging rate, maybe 1.5 amps.
2. run my battery down next weekend mowing.
3. set the unit to charge for 24 hours.
4. take photos of the voltage each time I walk past my tool shed as it charges.
5. graph it and make a chart.
6. then, when I run the mower later on, I'll bring it into the shed, consult my chart, and see that if its at 44.1 volts, then about 18 hours.. or whatever. Set my sprinkler via phone to 18 hours, and walk away.

btw, I had it set at 48V and 2 amps, at some point, it forgot the amps. Reverted back to 1 amp and 48volts. A little bit of aggro.