charger again... yeah i know

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boelle

Member
i know this has been discussed endless times, but i never found one that suited what i want

i have an charger of the "cooking" model... ie it will continue to push in 15A forever

My idea was to take out the transformer and use that to make a picaxe driven one.

Features that i have in my head:

  • Charge
  • Dis-Charge
  • Messure AH ie figure out how much life is left
  • Desulfation or some way to bring an old battery back to life
  • rapid/slow/trickle charge
  • temp corrected charge

Are there someone willing to help out? remember still a noob when it comes to figure out things

EDIT: maybe a nice display, a small menu.... user must be able to enter what type of battery ie sealed or open type, maybe that can be done with a switch. maybe even extend to not only charge car batterys but also nicad's etc for power tools. often the chargers that come with these tools are not very good. i dont know if the best way is to enter the number of cells and what the nom voltage pr cell is? some commercial chargers can detect this auto. maybe let a pc log or log to a sd card?
 
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techElder

techElder

Well-known member
You have defined your problem, now start with the first item.

Get experience there. You will know what to do after that.

Go to next item. Do the same.

Keep breaking your items down into smaller segments. For example, break the charger down into its basic parts; voltage dropping, rectification, filtering, regulation and filtering.

Work out each piece. Pretty soon you'll have a feel for where the whole project should go.
 
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boelle

Member
Hehe,

I allready have a schematic for a simple charger that just cuts out and in at a given voltage, so could as well use that.... maybe change it so that it uses PWM and a FET of some kind to do the job of slowing down when the limit is near so that it ends with trickle charging...

thinkering on, input welcome
 
J

John West

Senior Member
I'd suggest you start with a search of the site for chargers/dischargers to see who has done what and how.

Detailed displays are often easily done with a common parallel LCD and a serial to parallel decoder bd. They make it surprisingly easy to show what's going on - and they look totally cool.

Then you need a power source capable of generating your charge - and a load device capable of discharging that charge. The size of those devices is something you need to determine based on how much you wish it to do (voltage/current) and how much you want to spend.

Again - search the site here and elsewhere on the web to get an idea of what's been done - what's available - and at what price. Then give it a go and see how far you get - then start asking questions based on a bit more info. The forum will still be here for support.
 
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boelle

Member
Hi again

I allready have a charger that can give me a 17A charge, only bad thing about that one is that it does not turn of when charge is complete. But it can be used to power the whole thing. it even has a rapid / normal charge switch, dont know if i will use that as i plan to run the thing automatic with pwm based on SOC

For discharge a simple headlight bulp would do, maybe make a metal case and put that bulp in the corner and force cool it with a few fans.

thats the basic idea.... since i use a bulp to discharge i guess it makes counting AH etc more easy but could be wrong.

Going on with reading up on what other people did, first time i did not get the brighter picture
 
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John West

Senior Member
Basically - you'll need to make a schematic of your charger as it is now. Then you can see where to put in voltage monitoring and a current sense resistor. Properly conditioned the PICAXE ADC inputs can then read those values so it can make control decisions.

Once you decide how sophisticated you want the control functions to be you can add appropriate circuitry to do the job. It could be something as simple as a relay or something more sophisticated - as you described above.

The key is to be able to gather info on what's happening - make control decisions - then have devices in place to effect those decisions.

The first thing is to gather the wiring info from the charger you have. You need to see how it's wired so you can then manipulate its output.
 
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boelle

Member
hmm... got my brain boiling on that one

at what voltage would a normal 12V car battery be 0% SOC ie total flat?

my idea is to charge it to 100% and then drain them via a bulp and monitor volts and amps and calculate on that.
 
B

boelle

Member
my current charger is the most simple design, very crude and ugly, almost need one of those small paperbag found in planes in front of you ;)

it's just a transformer and Bridge Rectifier plus ampmeter :eek:
 
J

John West

Senior Member
I wouldn't pull a 12V vehicle battery down below about 10.5 volts. That's plenty flat. Do that too often and you'll ruin a good battery.

A full charge on a fairly new battery will be about 13.8 volts when "floating" - meaning not under charge or discharge. That voltage varies somewhat with battery condition, temperature and battery acid concentration.

You'll want, at a minimum, a relay or MOSFET to turn the charger output on and off, and one to initiate and stop the discharge. They must be able to handle the expected current and any momentary excess loads that may occur. If MOSFET's they need to be properly heatsunk as well.

And as I mentioned earlier, a current sense resistor needs to be in place to monitor the current for the PICAXE to read. If you discharge at 5 amps (a typical vehicle headlight) then something around .01 to .1 Ohm could be used in conjunction with an op-amp to bring the voltage drop across it up to easy to read PICAXE levels to measure the current.

A higher resistance could be used to measure current (voltage drop) directly, but the power it would dissipate at 5 amps would mean you would need the resistor to be in the 100W range - pretty expensive. I've actually used coils of copper wire for low value current sensors. They work well.
 
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BeanieBots

Moderator
Maybe... at some time.
It's not complicated and a good read of the text should lead you to a circuit.
However, simple PICAXE -> FET switching would NOT work at the currents you are considering. A good FET driver is all that you would need though.

I'm thinking of an update (higher current) to that project but it won't be for a while yet.
 
B

BeanieBots

Moderator
boelle said:
.... will pop in once in a while and see what happens...
Click to expand...
Alternatively, you could read up on magnetics and determine a suitable core on which to wind your flyback transformer to save me the effort.

Or maybe calculate what current your FET will need to handle based on the PWM frequency you have chosen to run at.
 
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