Need help making Ghetto Battery Charger

[drewjet]

Let me explain what I am wanting to do and then maybe it would clear it up. I race electrathon cars. Which is basically a 3 wheeled go-cart that we have 2 ea 1 hour endurance races with in a day. We are restricted to 2 car sized batteries and 2 hours between races to charge. Many places we race have 110 volt power available, and I have 2 power supplies that can put out 25 amps each and get it recharged in the 2 hours. Some places have no power and we need to use a generator. It SUCKS bringing a generator to an electric event. I have a good supply of 12 volt deep cycle batteries, and figure I could use 2 of them in series to charge 1 of the electrathon's cars batteries if I could somehow turn the 24V of the 2 batteries into 25A/15V CC/CV.

I know I could run an inverter and then use my charger, but that seems really a waste and quite ineffecient.

I looked but found no suitable DC DC Converters. Could something be thrown together with a few FET's and basic electronics?

 

[neptronix]

Here's a ghetto idea as a current limiter...
How about using a brushed motor controller with a suitable current limit at 100% throttle?

Would this not produce steady DC power at a fixed current level?
If this is a bad idea, i'd like to know why.

 

[fechter]

Here's a ghetto idea as a current limiter...
How about using a brushed motor controller with a suitable current limit at 100% throttle?

Would this not produce steady DC power at a fixed current level?
If this is a bad idea, i'd like to know why.

Because there's no inductor.

But if you added an inductor and a freewheel diode to the output, you could make a dc-dc converter out of a brushed motor controller. Many brushed controllers don't have a very good current limiter, but that could be added as well.

It would probably be a lot easier to just swap out the batteries if that's possible.

 

[neptronix]

An inductor would be a magnetic coil of some kind, and works to stabilize the current level somehow.. but draws a bunch of current at startup, which the brushless controller here would expect.. am i right?

And what would the freewheel diode do?

If the current limiting of the brushed controller is fairly useless, can you think of some other current limiting device that can be used? battery swap would be ideal for sure.

 

[TylerDurden]

Maybe a MPPT solar charge controller.

 

[pm_dawn]

iCharger 306b would probably do it for you.

Regards
Per

 

[drewjet]

Thanks for the suggestions. A second set of batteries would work, but they are a bit of a pain to swap out. At $200 a piece, not a real reasonable solution. Same goes for the iCharger 306b or the solar charger. I was hoping there would be some relatively simple circuit that could be made. or if I could tap my external batteries into the DC part of a meanwell charger.

 

[fechter]

In thinking about it, the freewheel diode in the controller should be enough and all you really need to add is an inductor. For really ghetto, you could possibly use a bunch of steel wire (coat hanger wire) chopped up into 6" pieces and bundled with tape for a core, then wrap a bunch of turns of 10ga wire around it for an inductor. A laminated iron core or powdered iron torroid would be better.

Using a pot for a throttle, the you could dial up the output until you reach 25A or whatever you want. With a voltmeter across the load, manually adjust the output to keep it from going over.

Below is a more complex design, that uses a LM431 to regulate the output. The pot would set the output voltage. The current would only be limited by the controller, so may be pretty high. With even more parts, you could have a CC limiter.

 

[NeilP]

Would something like that work at 84 volt and 15 to 20 amps?
I am trig to limit the current on a string of 7 PC PSU's..on initial connection the initial current inrush/surge causes one or more of them to trip out. Need to start with low current and build up


http://endless-sphere.com/forums/viewtopic.php?f=14&t=28632&start=15

 

[pelle242]

I am not a EE so I am just guessing out loud here but...

In a switched power supply the 110 AC is first rectified and then chopped up and passes a transformer to get it to the right voltage. So if you can find a SLA charger that is switched you can feed it 110 V DC (from like 10 batteries in series) after the rectifier bridge.

 

[fechter]

In a switched power supply the 110 AC is first rectified and then chopped up and passes a transformer to get it to the right voltage. So if you can find a SLA charger that is switched you can feed it 110 V DC (from like 10 batteries in series) after the rectifier bridge.

That is correct. Most switching battery chargers could run off a bank of batteries in series. 10 car batteries gets pretty heavy, but it could charge fast.


On the cheapo brushed controller as a buck regulator, yes it might work for that if you can find a controller rated for that voltage. At that power level, you should probably use a 'real' inductor that's made to do the job rather than a ghetto inductor. The other way would be to mod the supplies for soft start. Some have this feature, but I guess yours don't.

 

[NeilP]

The other way would be to mod the supplies for soft start. Some have this feature, but I guess yours don't.

Probably do not have that feature, all just bog standard PC psu's

 

[drewjet]

I don't have the brushed controllers to start with, so this seems a bit of an expensive way.

I did just find these DC DC Converter http://cgi.ebay.com/Isolated-DC-DC-CONVERTER-400W-48-72-84-96-120-144V-30A-/110682709137?pt=LH_DefaultDomain_0&hash=item19c5341c91#ht_2236wt_905

I guess the only question is would it throttle itself back (the cc part)

What do you think?

 

[drewjet]

I email the seller...China... and this was the response

Hello ,

This converter can be used to charge battery, but current is not
30A all the time, when charging it is 30A max, and then the current will
decrease less as the voltage of the battery is upper after charging
.

Regards,

Sharon

What do you think?