View Full Version : The Truly Versatile Charger Project (tm)

05-16-2003, 12:34 PM
I just completed a spur-of-the-moment project idea, and this time it actually worked the first time -- amazing! :p The idea was to have one do-it-all charger for my various tiny rc's. It should feature adjustable voltage, adjustable charging current, easy plug-and-play operation, low cost to build, and a nice techy look. The results are my "little black box" shown below.

05-16-2003, 12:34 PM
The most interesting part of this mod is being able to adjust the charging current. Why would you want to do that? Well, according to basic NIMH and NiCd charging practice, you should charge at a rate that is related to the capacity of the battery. A 10-hour, overnight charger is usually much gentler on the batteries than a 1-hour rapid charger, right? It charges at a rate of current that is 1/10 of the capacity (1/10C). A 5-hour charger charges at 1/5C, and a 1-hour charger charges at 1C. So a 1800mAh AA battery would probably be charged at anywhere from 180mA to 1800mA in a standard wall charger (we're not talking peak chargers here).

If you relate that to bit batteries, a 150mAh bit battery should theoretically be charged at a rate from 15mA (overnight) to 150mA (1-hour), but since no one likes to wait that long, the chargers that use AA batteries charge at ~1000mA, depending on the battery type. That's 6.67C, way over the recommended max current. If you were doing that to an 1800mA AA battery, you'd be charging at 12 Amps! At the appropriate voltage, 12 Amps will run many power tools and upright vacuum cleaners.

The thing that keeps these bit cells from either (a) exploding, or (b) sustaining heat damage that causes a significant loss in capacity, is the fact that they are charged at such a high rate for a very short amount of time. The tradeoff is that they don't get filled to capacity. It's become widely known that peak-charging can result in runtimes of up to 45 minutes with a stock 50mAh cell and up to 2 hours 14 minutes with a 150mAh cell (thanks, Tiny). But those who use rapid charging generally don't get anywhere near that run-time.

In other words, there is a tradeoff between achieving the maximum capacity and charging quickly. The faster you charge, the less likely it is that you will fill up the battery to its maximum, and the more likely it is that you'll shoot way past that and end up damaging the cell. A common complaint of the Layman's mod is that the runtime is very short, but the reason is partly because of the insanely high charge rate that is being used. The same ~1000mA AA-based chargers that charge a 150mAh battery at 6.67C are like 25C (45 Amps equiv. in the 1800mAh AA example) to the poor little button cells.

Still, sometimes you'd take a big loss in capacity for not having to wait around an hour for your tinyrc to charge completely and safely. It'd be nice to be able to switch the charging current whenever you want. I thought I'd like to have several different rates, ones low enough to charge a 40mAh button cell to max capacity safely, ones high enough to charge a 150mAh in a few minutes, and several in-between rates for when I have 15-30 minutes while I'm doing something else and can leave the car charging.

05-16-2003, 12:35 PM
This question requires a quick revisit to our old friend Ohm's Law. The pyramid diagram below is a useful mnemonic:

/ \ V = Voltage (Volts)
/ V \ I = Current (Amps)
/-----\ R = Resistance (Ohms)
/ I | R \

V = I * R <-- take V off the top, you're left with I and R side-by-side
I = V / R <-- take I off the side, you're left with V over R
R = V / I <-- take R off the side, you're left with V over I

So to change current, you can change voltage, resistance, or current itself. In the AC-to-DC adapter setup, I could already adjust voltage from 3v to 12v with an adjustable wall plug, but the best voltage is one that's close to the voltage of the battery pack being charged. It would be damaging to the car to use high voltages to increase the current, so that rules it out. Current itself would be the ideal thing to change, but the only practical way to do that would be to buy several different wall adapters, each providing a different current. That would be expensive and not very portable. That leaves only resistance, which thankfully is very cheap to adjust thanks to resistors, potentiometers, and Radio Shack. :)

I first thought of potentiometers, but they have several drawbacks -- firstly, that you can never adjust them to the exact position twice, so it's hard to get consistent results, and secondly that most pots this size are linear taper rather than audio/logarithmic taper, which means that the first part of the pot's range will be super-sensitive, and the last part will hardly have any effect at all, since you need consecutively more resistance each time you want to half the current. I decided to use a bank of a few resistors that could be switched on and off individually to provide quite a few different charge currents. Here you can see the switches mounted on the box.

05-16-2003, 12:36 PM
I first estimated the charge currents I would like to have based on 3 battery setups (150mAh, stock 50mAh, and Layman's 40mAh):

(1) 500mA full charge rate from the adapter. This would be the charge rate when all resistors are turned off and I'm in a hurry to race NOW. Still safer than AA-based chargers at ~1000mA.
(2) 300mA, a fast but mostly safe charge for 150mAh batteries. Good balance.
(3) 150mA, the recommended safe charging rate for 150mAh batteries, or a fast stock/Layman's charger.
(4) 50mA, the safest stock/Layman's charge rate.

These rates of current would require the following resistances at 3V:

500mA - none needed

300mA - R = V / I
R = 3 / 0.3 (300 milliAmps = 0.3 Amps)
R = 10 Ohms

150mA - R = V / I
R = 3 / 0.15 (150 milliAmps = 0.15 Amps)
R = 20 Ohms

50 mA - R = V / I
R = 3 / 0.05 (50 milliAmps = 0.05 Amps)
R = 60 Ohms

Those were nice, whole numbers for resistance, but after looking at the same calculations using 4.5V and 6V (for 3 or 4 cell cars) I decided I'd want more max resistance, preferably in a range up to 100 Ohms. I checked Radio Shack's available resistors and found they carried resistors of 10, 15, 22, 33, 47, 68, and 100 Ohms (along with others not applicable to this project). I selected one 10 Ohm resistor (http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F010%5F001%5F001%5F003&product%5Fid=271%2D1101), one 22 Ohm resistor (http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F010%5F001%5F001%5F003&product%5Fid=271%2D1103), and one 68 Ohm resistor (http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F010%5F001%5F001%5F003&product%5Fid=271%2D1106). Using three switches, that gave me eight different possible charge rates for each voltage. Note that individual resistors in series are added together for total resistance. (Ex: a 10 Ohm resistor connected to a 20 Ohm resistor is the same as a single 30 Ohm resistor.)

0 = resistor switched off
1 = resistor switched on

Pos Resist. 3v 4.5v 6v
000 0* Ohms 500mA 500mA 500mA <--maxed at 500mA
100 10 Ohms 300mA 450mA 500mA <--maxed at 500mA
010 22 Ohms 136mA 205mA 273mA
110 32 Ohms 94mA 141mA 188mA
001 68 Ohms 44mA 66mA 88mA
101 78 Ohms 38mA 58mA 78mA
011 90 Ohms 33mA 50mA 67mA
111 100 Ohms 30mA 45mA 60mA

* This table doesn't count internal charger or PCB resistance

As you can see, the change in current is less dramatic towards the bottom, as mentioned in the potentiometer drawbacks, but it still hit pretty close to all the numbers I wanted at each voltage, and it should provide consistent results for each charge level.

05-16-2003, 12:37 PM
I used three SPDT switches (http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F010%5F007%5F010%5F000&product%5Fid=275%2D409), a 4"x2"x1" project enclosure (http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F005%5F009%5F000%5F000&product%5Fid=270%2D1802), an Adaptaplug hobby leads plug (http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F004%5F001%5F003%5F000&product%5Fid=273%2D1742), and of course the resistors. I scavenged a bit-size charging pad from a Cannonball charger, and also added a charging plug for my Micro 4x4 (http://tinyrc.com/forums/showthread.php?threadid=10856). BTW, check out that 4x4 if you haven't seen it already.. it rocks.

Wiring was simple enough. The negative wire from the hobby lead plug (black) went straight to the negative bit charging post and negative wire of the 4x4 charging plug, all soldered together. The positive wire from the hobby lead plug (striped black and white) split into two paths across each switch. If you can visualize a SPDT switch, it has three terminals. The middle one is always being connected to either the top or the bottom terminal. Incoming current will go from the middle (input) terminal to one or the other (output) terminals. The positive hobby plug wire went to the middle (input) terminal of the first switch, the one that would control the 10 Ohm resistor. From there, it would have two choices: run through the 10 Ohm resistor, or bypass it. If it was to run through the resistor, the bottom output terminal was connected to one end of the resistor, and that resistor went on to the 2nd switch's middle (input) terminal. If it was to bypass the resistor, the top output terminal was connected directly to the 2nd switch's middle (input) terminal directly, with no resistor inline. So the 2nd switch would have two wires attached to its middle terminal from the 1st switch, one with resistance and one without, but it is only possible for one of them to be used at a time. The 2nd switch would repeat the process, sending one output with no added resistance (just the resistance from the previous switch, if any) and one output with an added 22 Ohm resistance. Both outputs would connect to the 3rd switch's middle (input) terminal. It would then send both final outputs (one without resistance and one with the 68 Ohm resistor) to the charging base's positive terminal, which is also connected to the 4x4's positive plug wire.

A diagram might help:

START to car
|| [-][+]
|| ^ ^
|| | |
|| [charging base]
\/ neg- pos+
| | \<-<-<-<-<-<-<-<-<-<-<-<-<-<-<-<
hobby lead / \ \
neg- -----------------/ \<-<-<-<-<-<-<-<-<-<-<-<-<-<-<- \
\ \
pos+ ->-> \ |
\ [switch1 top]--\ [switch2 top]--\ [switch3 top]-/ |
\->->[switch1 mid] ->[switch2 mid] ->[switch3 mid] |
[switch1 low]\ | [switch2 low]\ | [switch3 low]\ |
/ \ / \ / \
(10 Ohm) (22 Ohm) (68 Ohm)

If you follow the top path across the switches, you see you'll encounter no resistors. A path across the bottom will hit all three resistors; six other combinations are possible. Attached is a closeup of the wiring, but it's hard to make it out.

Hope this info is useful; I learned a lot doing it. If you already knew all of this, then you're the perfect person to correct any errors I may have made. :) Comments welcome.

Happy modding.

05-16-2003, 12:39 PM
... TRULY an act of god.... hell man, how do you come up with this stuff?!

mad props dude... im scared to make a simply ac/dc adapter-charger...

05-16-2003, 12:54 PM
the second namuna sees this say BITSCIENCE

05-16-2003, 01:22 PM

05-17-2003, 04:11 AM
Yeh I have gone down the same track some months ago.
Designed and built this one from scratch.

It is regulated by 2 x LM317t volt regs to vary the current, and has 2 x 555 timers to take care of charger timing, all selectable with rotary switches. You can see the volt regs hanging off the side on the heat sinks.
It can slam charge or charge more sedately and (obviously) does two bits at once, so you can charge two cars at different voltages at the same time, eg a normal bit and a dual or triple cell.
Havent used a controller to charge my bit for a long time now, and the other good thing is that controller batteries last for ages when you dont use them for charging.
:) uA

05-17-2003, 12:24 PM
Hey uA...

How's about instead of re-inventing the wheel (there's plenty enough of them smart chargers out there)...You get back to work on that Micro-Dyno!!!!??? ;)

Mighty sexy charge box though, nice!

05-17-2003, 09:08 PM
Very nice box, uA! That's not one for beginners.. whew, maybe I can do something like that eventually. This is all new to me. I agree with Namuna.. put those skills to work on something really great! :)

05-17-2003, 11:49 PM
hehehe, thanks guys.........this is not a current project.
The charger was finished ages ago when I needed something to charge various voltage bits, so I came up with that.
Dyno IS progressing. Will probably re-emerge as a completely different design tho as the current design is posing some problems.
I can obtain power production results but with limitations.
As soon as I have a finished product, this will be one of the first places to know about it.
:) uA

05-18-2003, 03:55 PM
Very nice charger! I would make one myself if I didn't already have a high end peak charger from 1/10 th r/c's!

05-18-2003, 05:55 PM
Dudes.. I am in a trouble.. I made a dual bit, but cant chsrg it.. And ive got no skills to do such a charger.. How do i manage?:confused:

05-18-2003, 10:51 PM
Some nice theory there ActofGod. The only problem is that you are limited in your design by the type of plugpack power supply you use and it's current rating.

I'm designing a charger as well, custom for these bits that uses the LM317T voltage regulator as well like Micro's. The beauty of this is that you can configure the output to the Batt's to be of a constant current and/or voltage independant of the plugpack you use. This gives you the ability to be able to choose any plupack as long as it does X volts minimum and is rated to Y amps max.

Here's a pic of my work in progress. It's only a single charger (unlike micro's beast) but I've tested is with a 12V / 1.2 A supply and a 15V / 2A supply and I get constant current along with voltages each time. Leaving the circuit independant of the voltage source. :)


Keep up the good work. (now to get back to my propo emulator, :o )


05-19-2003, 02:48 PM
ActOfGod, thanks for the great description. I know that must have taken you some time to type up that long description and I really appreciate it.

micro_amps and ph2t, it would be great if you guys would also share your designs (i.e. circuit diagrams, parts, etc...).

05-19-2003, 10:40 PM
Thanks for the feedback ph2t, rrvball. I figure if I went to the trouble of learning the stuff and building it, I might as well make it easier for someone else to do the same. My target audience is always the casual tinkerer like myself ;) Now I just need to find some more easy projects to work on.

05-20-2003, 07:51 AM
micro_amps and ph2t, it would be great if you guys would also share your designs (i.e. circuit diagrams, parts, etc...).

Hey rrvball, we are way ahead of you.

:) uA

05-20-2003, 07:59 AM
dude, I think he means with the rest of the scene, lol.......:eek: