Bipe Flyer

I put this in the 1/2A group because it is primarily intended for charging 1/2A Receiver packs.

After some discussion in another thread about slow (c/10) charging batteries for the first charge I started thinking that my Accu-Cycle isn't really appropriate for the smallest packs. The lowest setting of 25mA, which actually turns out to be 30mA when I checked it with my meter, is still too high for the initial charge of 120mAH packs. I had been putting them on for 6 hours, but if I forgot to remove the battery then they would get overcharged.

I started looking around on the Internet for charge circuits and found some using an LM317 as a constant current source. This got me thinking and I came up with the following which uses a 7805 5V regulator. The way it works is the regulator will always keep 5V across P1 and R1. I = V/R, so to charge at 20mA P1 + R1 = 250 Ohms. I used a 200 Ohm resistor for R1 to limit the maximum current to 25mA. By adjusting P1 I can set the charge current from about 5mA to 25mA. The neat thing is that this circuit is that it will charge anywhere from 1 to 25 cells as long as the input voltage is a few volts higher than the battery to be charged.

I used a 1K pot that I had, but a smaller value would be more accurate. I used an old 14V DC wall wart that I had lying around and a mechanical light timer set to 14 hours. I put a diode on on the output to prevent the pack from being discharged through the transformer when the power is turned off.

I've attached a schematic and a few pictures. The meter on the left shows the charge voltage while the meter on the right shows the current. One picture shows a 120mAh pack at the end of a charge cycle being charged at 5.69V and 11.03 mA. I swapped in an 8Cell 300mAH pack and the voltage automatically increased to 9.93V while maintaining a charge current of 11.01 mA.

combatpigg

My Feedback: (3)

Thanks for posting that BP! I've been in the same boat with the 150 and 210 packs, and haven't wanted to shell out for a smart charger.

Ragwing

My Feedback: (2)

Bipeflyer
Thanks for posting the shematics, I was looking at wiring something up to step down the current for charging my little 170 pack--but here you've gone and done the planning for me. Thanks-it looks sharp.

Derek

jboy381-RCU

BF,
that's so cool...i want one but,i have no idea what that drawing means...what is a R1,P1 and a D1...i've never worked with this stuff before...should i try to make one or don't bother???i like it because it's like an overnight charger for 100 to 200ma packs...timeing these small packs is a real pain in the @$$...

thanks john

Andrew

A nice application of the 78xx series regulator. These IC's are pretty robust, but tend to heat as the Vin/Vout differential increases. For applications with 4 cells and input voltage near the upper limit, you might want to add a heatsink to the package.

the "other" Andrew

BMatthews

Bipe, something looked wrong with that to me and I went searching. You're battery is coming off the wrong point. It needs to come off the pin 2 ground tab to operate as a current source as in....



A simple transcription error that's easy to make....[8D]

By the way, if any of you want to super charge this circuit to allow for higher charge currents up to 100 ma you can change the fixed resistor to 47 ohms. But either use a 1 watt 47 ohm or two 100 x 1/2 watt's as the heat generated by the 100 ma running through that 50 ohms will be a 1/2 watt. I suggest the higher wattage as a 1/2 watt resistor operating at max doesn't usually live to a ripe old age. I know that in my case I often like to "jam some electrons" into a pack in a hurry or to trickle charge some electric flight packs so I like the higher current capability.

Also, if I may be so bold, a 10 ohm resistor in the battery leg will register .01 volts for every ma of current. With digital VOM's being so cheap these days if you put the 10 ohm in the battery leg and connect up a couple of test jacks to either side of it you have a quick way to measure the charge current with very little if any effect on the constant current. It'll just need a trifle more voltage overhead.

And if you guys are feeling industrious you could build 4 or 5 of these circuits into a single box fed off one 500 ma wall wart and put the 10 ohm sample resistors into the ground side of each charge point and have one common ground test jack and one each for the outputs. Jabbing the meter into each in turn would quickly check all your charge circuits.

All in all a great late Spring project. Thanks for reminding me that I need something like this BF. I currently (ooooo bad pun ) have a 4 station variable charger but I want to get back into more electric flying this summer and a second 4 o 5 output charge bank would be a great project.

Bipe Flyer

John,

R1 is a 200 ohm resistor.
P1 is a 1k potentiometer
D1 is a 1N4001 diode.

As a first electronics project this is about as easy as it gets. Low parts count and very little soldering.


Andrew,
I tried it for a while with the 14V on 4 cells before I put it in the case and it wasn't very warm. Other than the case it's just made up from parts I had lying around.


Bruce,
Thanks for pointing that out, t was a labelling error. I've fixed it and replaced the schematic in the above post with a corrected one.

I have a plug that is normally closed in my circuit that allows me to plug in the meter into the circuit between the output and the battery.

I have a little panel mount LCD meter that I thought about using in a box with 6 charge circuits, but by the time I've bought everything I might as well buy an ACE Super Digipulse.
http://www.acehobby.com/acehobby/pro...lse/index.html

BMatthews

Yeah, it's amazing how some of these "simple" projects can add up. The electronics are often pennies but the controls like that 1K pot can be way more than the other bits put together. Speaking of which.... If you can find trimmer pots with a little thumbwheel on them for adjusting they work quite well and are a small fraction of the cost of the fancy metal pots. I've been using 4 of them on my current (oooo there's those puns again ) charger with great results for over....<cogitate> 20 [X(] years??? ..... I just realized it's been that long but it only seems like a few years back.... I'm getting too old for all this remembering....

Bipe Flyer

I was going to use a trim pot but I wanted a dial instead of sticking a screwdriver into a hole to adjust. I guess I could use the thumbwheel style and cut slots in the case.

Andrew

For those interested in building Bipe Flyer's project (or others), here are two sources of components. I use both regularly and have had good service from each.

www.jameco.com
www.web-tronics.com

the "other" Andrew

nik112233

I implemented recently the mini charger quite successfully. Thank you very much to the author and all those that contrubuted to the thread. Thank you!

nik112233

I followed the schematic very closely. It is so simple it even does not require a board. I only added a LED and a capacitor, see that attached modification of the original schematic. L1 lights up only when there is a load (a battery pack) hooked up and the charger is powered at the same time. C1 is supposed to make the current from the transformer/rectifyer smoother.

In my case the chager current is in approx. 10 - 30 mA range. My non-regulated 9V 400mA transformer supplies around 15V to the charger.

I would only suggest to mods maybe to consider sharing this simple and elegant solution with a DIY electronics forum, so others can benefit too. (I do not do 1/2 or 1/8 models).

Thank you all one more time!

Regards,
Nik

Bipe Flyer

You're welcome. I'm glad you could make use of it.