dc-dc converter as an aux. pack charger?

[joe tomten]

Today I added an auxiliary pack in series with my 16s 15ahr ping. It is 4s ~16ahr, and brings me up to a 20s pack.

Big fun.

I was wondering if i could use one of the lyen dc-dc converters (http://endless-sphere.com/forums/viewtopic.php?f=31&t=21459) to let me do an opportunity bulk charge of the 4s pack at work, while I'm charging the ping.

I'll balance charge the 4s pack at home at night.

The voltage input and output should work.

Pretty sure i can get the wiring right, I just want to know if there are any issues about the amperage draw through the converter that would trash the converter the first time i tried it.

thanks,
Joe

 

[amberwolf]

You'd probably need to put some sort of current limiter on it, otherwise if the pack was way down in voltage compared to the DC-DC, it'd try to draw huge current from the DC-DC. IF the DC-DC has a protection built in it'd just shutdown, but A) if it doesn't have protection it'll probably fry, and B) if it has protection that crowbars the output stage during shutdown it'd short out your pack.

A diode on the output will fix B), but you'll need a current limiting circuit to fix the problem's source.

 

[joe tomten]

Amberwolf,

thanks for the response. Might be easier just to buy a 20s charger.
Couldn't find one for cheap though.

Would something like this work?

http://www.opencircuits.com/Basic_Circuits_and_Circuit_Building_Blocks#Current_Limit_or_Constant_Current_.28_Transistor_Based_.29

How does on figure out the values of the resistors?

Joe

 

[amberwolf]

Looks like it should work. You might need different transistors depending on the actual current capabilities of those specified, and how much you want to be able to draw thru this.

Based on the description at the site, R2 would be calculated so that near the limit you wish to impose, it would have about 0.7V or more across it, to turn on Q2 so it shuts off Q1.

R1 is there to limit current into the base of Q1, AFAICS, so it would depend on which transistor you use and it's max input current needed to get a specified output, or amount of transistor turn-on.

Now, R2 is gonna pass all the current to the load, AFAICS, so it has to be a high-wattage resistor, probably a really low ohms.