Building a car battery charger

[Anonymous No.1747218]

Any electricians/engineers here?

I want to build a car battery charger and i have to limit the huge initial current somehow.

I already have a transformator which gives me 12V output.

But how can I limit the transformators secondary current?

I dont want to put a resistor in series because of the huge power dissipation.

Thanks bros. I hope someone can help me.

btw i know that i have to rectify the voltage from the transformator

[Anonymous No.1747223]

>>1747218
Put a variac on the transformer input so you can charge whatever kind of battery you want.

[Anonymous No.1747224]

>>1747218
Honestly the cheapest / easiest way to do this in a transformer / rectifier style battery charger (as opposed to switchmode) is to select or design your transformer to limit max current.

[Anonymous No.1747225]

>>1747223
This would vary the output voltage, not current

[Anonymous No.1747229]

>>1747225
>what is ohm's law

[Anonymous No.1747230]

>>1747224
Oh shit yeah didn't even think of this; you could even add a mechanically-adjustable shunt for different charging rates.

[Anonymous No.1747383]

>>1747218
I don't know what a "transformator" is.

First question What is the ratings of your transformer? how many amp and volts does it output exactly?

What size batteries will you be charging? Car? Lawn Tractor? A giant ass battery used to start big ol' diesel engine? what?

Yes there are ways to limit the amps. The simplest used in old fashioned car battery chargers was a self resetting 15 amp circuit breaker. It would just sit there and chatter away until the current draw went down.

Now a days a buck converter will do that. But you will need one with a big ol' heat sink and adjustable Constant current.

also you're output voltage needs to be between 14.4 and 15 volts. No higher than 14.4 for sealed lead acid batteries.

Here's one I built

[Anonymous No.1747529]

>>1747224
How can i select/design my transformer so that he limits max current? Just buy a small one with thin wires and therefore higher resistance?

>>1747383
Oh sorry, yeah i mean transformer instead of transformator ofc. Im from germany and transformator is the german word, my bad.
I just measured the output voltage from my transformer and its around 12v, so i cant use that.

I want to charge normal 12v (50-80Ah) car batteries.

Doesnt a buck converter reduces your voltage? I mean, yeah surely if you reduce the voltage the current goes down too, but at the end of the charging cycle the current goes down automatically and then you have to have the full 14.4V at the output

[Anonymous No.1747540]

>>1747224
If you apply a dollar value to your time, it would be hard to beat a mass-produced car battery charger.

[Anonymous No.1747581]

>>1747529
Ok, the rectified output of your 12v transformer (with smooting capacitor) is 12 x 1.414 or 16.968 volts DC.

Most buck converters require the input voltage be at least 1 volt higher than the output so Yes you could use that transformer.

Now about the amperage. what is the rating of the transformer? if you don't know it what is the wire size of the output?

Most car chargers are in the range of 6 to 10 amps and from my experience most car batteries don't pull much more than 8 when they are discharged.

[Anonymous No.1747586]

>>1747529
>Doesn't a buck converter reduces your voltage? I mean, yeah surely if you reduce the voltage the current goes down too, but at the end of the charging cycle the current goes down automatically and then you have to have the full 14.4V at the output

depends on the buck converter. You want a buck converter that will limit current and voltage.
I used this one for my charger I built.

go to ebay and search for
DC-DC Converter 10/12/15/20A 150/250/300/400/1200W Step up Step down Buck Boost
you want the 12amp step down converter like in the picture 4chan would not let me post the link.

You want your battery charger to have a set voltage and maximum amperage. In my case I chose 14.4 vdc and 8 amps.

my transformer put out 17 vdc and 10 amps
I set the buck converter 14.4 vdc and 8 amps.
If the battery draws more current than 8 the buck converter will drop the voltage until the amperage is 8. This is a constant current charge and what you want for Lead acid batteries. Then when the current draw drops to 8 or lower the voltage will go back to 14.4 v and constant voltage charging which at this stage is what is supposed to happen. If you plan on discharging your batteries down to 11v or lower (not a good idea for a car battery) you will want your output voltage to be 15 volts for what is called the maintenance charge this stage is what converts some of the lead sulfate on the plates back to sulfuric acid. But if you ever plan on charging sealed lead acid batteries you should never exceed 14.4 volts dc.

[Anonymous No.1747587]

>>1747540
>If you apply a dollar value to your time, it would be hard to beat a mass-produced car battery charger.

Even if your time is worth nothing it'd still be hard to beat a mass-produced car battery charger. They're cheap and work.

What are you trying to get out of this charger that isn't commercially available for dirt cheap?

Or do you just want to build one to say you did?

I get the diy thing, but if something is available that fits the bill, is dirt cheap, and I don't have to re-invent the wheel then I'm all for it.

[Anonymous No.1747589]

>>1747581
>from my experience most car batteries don't pull much more than 8 when they are discharged.

A healthy discharged car battery is almost a dead short and will draw tons of amps. You see 8 or whatever because the charger either limits it passively by impedance or actively by electronic means.

[Anonymous No.1747591]

>>1747581
It has a rating of 40VA

>>1747586
Thank you very much

>>1747587
I just wanna do it as a project and it looked easy for me. I want to learn a few things about electronics

[Anonymous No.1747593]

>>1747587
Most modern battery chargers are "smart" chargers. They won't try to charge a battery discharged below a certain voltage and they charge based on a hard wired program. This programming does not seem to include maintenance charging which is the process of putting a high voltage on the plates to convert the lead sulfate (which is what kills batteries) back in to sulfuric acid. Old fashioned battery chargers are superior for this. The disadvantage to a non smart charger is that if you leave the charger on too long you can boil the electrolyte off. that's why my charger in the picture above
>>1747383
has a timer. Unfortunately for OP I bought 2 of the last battery charger timers in existence and someone else bought the third otherwise I would have linked to where one can be obtained.

unless you can find one on ebay. A battery charger timer goes up to 16 hours and can switch at least 15 amps.

And also building battery charges is fun.
A maintenance charge can take 6 to 8 hours past the actual charge time.

Many "smart" battery chargers claim to desulfate batteries but my (and other peoples) testing says otherwise.

[Anonymous No.1747595]

>>1747589
well that's not the case. I have tested charging heavily discharged batteries and most of them don't pull more than 5 amps, and WAY over discharged car batteries don't pull hardly any amps because of internal resistance.

[Anonymous No.1747601]

>>1747595
that 5 should read as 15

[Anonymous No.1747606]

>>1747589
this is one of the reason the nonsense of people buying a 50 amp charger is BS. A charger can only charge a battery at the amperage the battery pulls. And there are formulas for how many amps a battery should be charged at. In most cases it's about 10 to 30 percent of rated capacity (amp Hours). so if your battery has a capacity of 110 Amp hours then the maximum charge amps is usually going to be 11 as most manufactures suggest no higher than 10%

this website is very useful for information on batteries. https://batteryuniversity.com/learn/article/charging_with_a_power_supply

[Anonymous No.1747610]

>>1747595
>. I have tested charging heavily discharged batteries

I respectfully suggest that you don't know what you are talking about. If you were correct, it would take an hour or so for a car to recharge the battery if you left the lights on and killed it. Alternators put out way more than 5 amps, and most of it goes into the battery if the battery is down to 8 volts or whatever.

I'm done here, because on 4chan everyone is an electrical engineer. You can have the last word, my friend.

[Anonymous No.1747611]

>>1747589
as counter intuitive as it sounds a battery discharged below 10v will not pull much amps. and below 6 volts will not pull anything more than a few milliamps, as the plates are so sulfated that the battery is now a giant mega ohm resistor.

[Anonymous No.1747612]

The last time my battery was heavily discharged i hooked it up directly to my 12V power supply. I thought "yeah my power supply can easily handle up to 20A the current cant be much higher than that".

After the battery immediately draw way more than 20A and my power supply started smoking i knew better

[Anonymous No.1747613]

>>1747610
and you did not read the post following my last one.

[Anonymous No.1747615]

>>1747612
you probably had shorted plates. which can happen do to a build up of lead sulfate causing the plates to bulge touch. This can be seen with a thermal imager or felt when you charge a battery and one of the cells is warmer or hotter than the others.

[Anonymous No.1747620]

>>1747615
after that incident i checked if i destroyed my power supply, thankfully it survived. just got a little hot. anyway - i then connected a 12V 100W halogen lamp in series to limit the current and charged my battery without problems. Still possible that i had shorted plates?