Using an SMPS to charge a car battery

[eccerr0r]

Re: Using an SMPS to charge a car battery

For those not familiar with car terms, reserve capacity is measured in minutes (i.e. time) for it to drain 25A to dead. So take the minutes of reserve capacity, divide by 60 and multiply by 25A = your Ampere*Hours that most of us are familiar with. Due to Peukert's Law, unfortunately most of these batteries are around the 2 hour mark so the actual C/20 capacities we're familiar with is actually higher than the ReserveCapacity*25.

Fortunately with the hacked ATX SMPS -> car battery charger we don't have to worry about shutoff. But you do have to worry about connecting the battery backwards to the MacGyver SMPS charger... It will cause some damage...

[KeriJane]

Re: Using an SMPS to charge a car battery

No, people kill their car battery stone dead all the time.

Unless it freezes or sits dead for more than a week, they can usually be recharged OK.

Just not with one of these highfalutin newfangled Too-Smart-For-Its-Own-Good Smart Chargers.

And Car Batteries are Expensive nowadays!
A good one*, with high Reserve Capacity (RC) not Cold Cranking Amps, (CCA) is $100 or more and NOBODY, absolutely NOBODY wants to pay for one.
Our Customers don't want to pay.
The Factory doesn't want to pay.
Warranty Companies don't want to pay either! They just laugh.

The company ends up getting stiffed on more than a few batteries, so now we have to do 30 minutes of paperwork, getting authorization and signoffs in Triplicate!
It's so bad, I'm making a contract proposal out of it.
We want straight time for this nonsense!



* "Good" to me means a reputable company as well as "specs". Most modern cars with their efficient starters don't need a lot of Cold Cranking Amps.
Because of all the computerized JUNK** onboard which draws power***, a high Reserve Capacity is far more important.
Your average, cheap, 1,000CCA piece of garbage with little (usually unmarked) RC is lightweight because it contains little lead, leaks acid like crazy and shorts out in a year or two. It can cause all manner of strange electrical problems, and when it shorts out, can fry your alternator in a hurry too.

*** - "Normal" Parasitic Current Draw on a modern Luxury Car can vary between 20-60ma (depending on make, model, year) ALL NIGHT LONG and may stay over 100ma for hours before everything goes to sleep.
For reference, 60ma will kill most high quality car batteries in about 3-4 weeks at most, 200ma will kill it in a day or two.


** Automotive "JUNK" to me includes: Computerized ANYTHING except for the powertrain, fuel pumps, water pumps, oil pumps, radiators, heater cores, automatic or power ANYTHING, Air Conditioners, Camshafts, Valves, Timing Chains or even more than three cylinders!
Remember, I drive a Trabant!

[severach]

Re: Using an SMPS to charge a car battery

Once you're below the trigger voltage for those automatic chargers the battery is usually a goner anyways.

[KeriJane]

Re: Using an SMPS to charge a car battery

There are various "smart" car battery chargers out there that use SMPS.

They are very light for their capacity and produce much cleaner output than our traditional battery chargers.

Most of them perform a check of the battery before charging. Many will refuse to charge a completely dead battery or one that it deems "bad".
They won't even try, and so, chanting the magic spells of Auto Repair, we go trudging off to find a "real" old-fashioned charger.

We've got a pretty good one that can put out maybe 80A, has a built-in battery tester (I'll bet it checks ESR) and can be set for current or voltage limited charging.
It has a "Diagnostic Charge" mode that is pretty good at finding semi-faulty batteries that mysteriously go dead for no apparent reason.

The clean output is great when we re-flash a vehicle ECU. Low voltage during reprogramming will brick an ECU as will bumpy electricity from a traditional charger.

It also has a mandatory timer so the charger will eventually shut off.

Unfortunately, even this gazillion dollar charging station will refuse to even try charging some batteries, and we have to find a "real" charger for the initial charge.

[zaero]

Re: Using an SMPS to charge a car battery

Need to use an op amp circuit to get rid of that efficiency problem of that current limit sense resistor...

You could use an ACS712 module. Not that expensive, low insertion impedance, isolated (no need to bother about level shifters, common mode...) when I did my mod it wasn't available, today I would use it for sure.

[zaero]

Re: Using an SMPS to charge a car battery

Some years ago I modified an old AT power supply to charge a car battery @ 5A.
Modifications done: I put a current limit similar to eccerr0r's suggested schematic, with 0.1Ohm shunt resistor 5W
I moved the voltage sensing point to after that shunt resistor, and set it with a trimpot to 14.3V (bulk charge)
I added an small board that would set the voltage to 13.8V when the current decreases under a certain value (~0.4A if I recall good). It was made with the shunt, an operational amplifier (LM358) and a LM311 which changes the resistors of the feedback/reference divider.

[eccerr0r]

Re: Using an SMPS to charge a car battery

There are a bunch of schematics around that you can look at to see how the two transistors in the tl494 are typically driving two other external transisistors (usually 500mW transistors, or even 2sc945's - not that big) that do the transformer (for isolation) - and the other side of the transformer connects to the bases of the transistor half bridge.

Granted it's a lot of work to get a SMPS to charge a battery, but it's much more power efficient and smaller/lighter You could reduce R2 some more and get more current before it limits current too... Change that resistor to about .1 ohms and get around 7 amps to the battery (though the resistor would dissipate 5W).

Need to use an op amp circuit to get rid of that efficiency problem of that current limit sense resistor...

[budm]

Re: Using an SMPS to charge a car battery

Or just build this http://www.aaroncake.net/circuits/charger2.asp

[budm]

Re: Using an SMPS to charge a car battery

Do you have the diagram how it is being use in the secondary side to drive the transistors in the primary side? How did they interface the C1, C2 output to the primary side. That will be interresting to see.

[eccerr0r]

Re: Using an SMPS to charge a car battery

All the TL494/KA7500's I've seen on ATX PSUs are sitting on the secondary side, completely isolated from the mains/primary side. It uses a transformer which I call a "ticker" that "tickles" the two main transistors on the primary side half bridge. This is different than the UC3842/UC3844 based supplies which sit on primary...

Not to say there are other ways to build this... but I've seen a lot of supplies like the Antec SP400 that did it this way, along with a whole bunch of other PSUs that I've dissected.

[budm]

Re: Using an SMPS to charge a car battery

So the TL494 is already used in the secondary side of this PC power supply, of which power supply section? 12v?
https://cdn.badcaps-static.com/pdfs/...88cd5e8891.pdf
https://cdn.badcaps-static.com/pdfs/...956121acf5.pdf

[eccerr0r]

Re: Using an SMPS to charge a car battery

This circuit is affecting the feedback circuitry. Since the TL494 is on the secondary side, there's no dangerous voltage path to ground, but if you get it wrong it could self destruct like a lot of other things

As for the voltage, toy with the feedback circuitry of course! There's usually a pot somewhere you can change the voltage output. After increasing the voltage to 14 volts or so, add this circuit and you're limited to 2A to charge up to about 13+ volts (which isn't fully charged). Then you're just charging slowly through the resistor which is fine too...

[JonathanAnon]

Re: Using an SMPS to charge a car battery

This hack will basically tell the PWM to stop if it exceeds about 2 amperes into the battery.

Are you talking about the directly influencing the feedback to the primary side PWM chip?

How would you go about hacking the 12 volt rail up to 14/15V... I spose you could rewire the secondary side of the SMPS transformer to join one of the 12V rails and one of the 5V rails... then use a schottky diodes / voltage regulator / filter caps of the appropriate ratings.. Of course the feedback would then cause the SMPS to shut down... Maybe you could toy with the preset on the feedback circuit?

[eccerr0r]

Re: Using an SMPS to charge a car battery

There will always be a drop but if designed properly you can do better than 6V drop. I'm thinking it's possible to do this current regulation in less than 1V...

Here's a quick hack circuit, untested of course, to do the current regulation.

Not sure if it will work but it just might...?

This hack will basically tell the PWM to stop if it exceeds about 2 amperes into the battery.

Advantage of this circuit versus LM350/LM317: Not quite as much loss. Most in the 0R35 resistor which needs to be a 2W unit.
Disadvantage: Well, requires several internal mods to PSU.

And problem with this circuit: The gain is way too high I think. I did not really consider gain and likely this will cause oscillations...


This is wasting about 1.5 watts or so of power. Can do better than this... but this is a fairly easy hack.

[budm]

Re: Using an SMPS to charge a car battery

No matter what you use it will be in series with the batteries so you will always have that voltage drop on the device that charges the batteries, unless you can get -12V that can supply >2A, then you can get 24V out using +/- 12v outputs. Have better idea?

[eccerr0r]

Re: Using an SMPS to charge a car battery

The dropout voltage of those linear regulators will be awful compared to the limited voltages available from a SMPS. I'm sure you can get a volt or two out more but if you need to get upwards 18 volts (few volts to overcome the voltage potential of the chemical reaction to force it to reverse, plus the dropout voltage, plus regulation voltage) to use a LM317/LM350 as a CC regulator... That will surely fry the 16V caps on the output.

[budm]

Re: Using an SMPS to charge a car battery

Cheap, just use LM350, works great.
https://cdn.badcaps-static.com/pdfs/...ba58244431.pdf

[eccerr0r]

Re: Using an SMPS to charge a car battery

Getting a constant current source is not cheap either. The rub is to make a CC source with the guts of the SMPS... I think all one really needs is an op amp, a transistor, and a few resistors to mod a tl494/ka7500 smps to limit current...

Or quite possibly all the stuff is already there, just need to repurpose...

[budm]

Re: Using an SMPS to charge a car battery

You can have the power supply feed the 2A constant current source, the output of the current source will feed the 14.5V Voltage regulator, the output of the regulator will feed the Adnode side of a 3A Diode, the Cathode will be connect to the batteries.
The maximum current will always be limited to 2A by the constant current source. When the batteries voltage reach about 13.5V (14.5 -1V Vf of the Diode), the Diode will not be bias on so the charging will stop.

[amzee94]

Re: Using an SMPS to charge a car battery

Whoo awesome idea