I think I may have to build one and have it built right into the pit stop of my track, that way all you have to do is pull up and charge, plus I already built in jacks so that you can run your car dead b4 you charge it, then just build the seperate charger in next to it, and BAM! quick racing action

Hello,

I'm new to this forum but totally addicted to micro r/c. I found a cheap bit clone at Wal-mart that comes with a stand alone charger that holds three AAA's. I'm thinking about modding it by taking out the original charging posts and grafting my zipzap charging pad onto the top of it. The Walmart clones were about $13.00 each and come in a bullet shaped container.

Well, again, hello to everybody. I'm sure I'll get to know everybody pretty well.

mA and A are the same thing. Just a different level of measuring current. Like inches and feet. 1000mA is equal to 1A. 500mA is the same as saying .5 A. They are both measured by time. Hence the h at the end of many spec write ups. Why it is called miliAmps rather than kiliAmps as in Ohms is beyond me.
Current flows, just like water. The way you measure it is how much flows past a given point in a certain amount of time. Again, just like water flowing over a dam. Voltage is pressure. Or, how much force is applied to pushing the water over the dam. Which is why when you raise voltage (pressure) more current can flow over the dam faster. And resistance is holding the current back, like the dam itself. If you lower the top of the dam, at a given pressure, more current flows due to less resistance, raise the dam until no current flows and you are said to have maximum resistance or no current flow. Materials with this property are called insulators. This water analogy works well for me as electrons in a wire behave the same way as water molecules in a pipe. Not really though, it's like saying the sun sets and rises rather than the earth going around the sun causing the illusion of the sun moving. When water flows into a pipe, the same water molecules flow out the other end. In a wire, electrons smack into one another like those executive desk toys that bang steel balls together. The first one strikes the next and so on until the last one pops out of the wire to feed your circuit. I'ts not the original electron that you pumped into the wire. This is why copper is such a good conductor. It has more free electrons to supply this chain reaction. It's also why it's so soft. Gold is the best conductor for the same reasons.

I see...
 

Thanks Azimov,

You're analogy makes this so much clearer! :D

sahweeeeeeeeeet......

Someone asked a question on here about whether a higher current charger would charge a ni-cad etc faster - another person's reply suggested it wouldnt.

This is not the case as far as I understand. (The following is based on my understanding, if its wrong, someone please correct me)

Any rechargable battery depending on its capacity has a recommended maximum charging current. A flat rechargeable is greedy, it will try and gobble up as much current as it can get (its like pouring water into an empty container, you could trickle it in, or blast water in really fast) - that doesnt mean its healthy for the battery to accept current at that rate.

Imagine a battery with a 500ma capacity. If you charged it with an adapter rated at 50ma, it would take somewhat over 10 hours to charge (it takes longer than 10 hours, cos some of the power is lost in heat)

That would probably be a healthy charging rate for such a battery.

If you supplied it with 500ma, it would take a little over an hour to charge - the battery would gobble up all the current. However you would shorten the life of the battery because it would be accepting a high current.

I dont know this for sure with Ni-Cads, but this is the case with lead acid car batteries. If you have a 40Ah car battery that was flat, you could charge it real fast with say 20amps of current - but its not healthy, in fact you'll probably boil the electrolyte.

Its the same way in reverse. A ni-cad can deliver quite a sizeable amount of current - it will deliver its full current if shorted, which is usually enough to start melting stuff. That will also likely do permeant damage to the battery.

So to summarise, using a 2Amp adapter with a tiny battery will probably charge it fast, but also shorten the battery's life.

Also, just to backup what others have said, mAh is a measure of current used over the period of an hour. So a battery rated for 500mAh means it can deliver 500ma for an hour, or 250ma for two hours. Thats not strictly true, because in reality the rating is usually calculated based on the battery being discharged over a 20hr period.

So if the battery is 10Ah, that means you could draw 500ma for 20hours. You'd actually get less if you tried to draw the entire 10A over an hour.

Hope I havent confused you all. Im drunk afterall ;)

One more geeky addition, as far as know Lithium Iron batteries are a whole different kettle of fish. For a start they should be pulse charged - short pulses of current. Also, they are pretty dangerous things. If it werent for the safety aspect Li-Ions would have an even higher capacity than they do, but because of their volatile nature they have to be made in such a way that they are safe by giving them pretty thick sturdy casing.

Ive heard a report of one of the early li-ions in a cell phone exploding and causing nasty injury to the users face.

So, what's the easiest way to limit the current going to the battery? Place a resistor in parallel with the charging circuit? Or if you charge more than one car at a time (all in parallel), the current would be split equally between the cars.

Not sure if a resistor would work cos it also has a habit of dropping the voltage too, but you are right in saying that if for example you had a 100ma charger and connected it to four (flat) batteries they should consume 25ma each.

Someone posted a link somewhere on here to a Vellerman kit (the type you just solder together onto a ready made PCB with instructions) for a charging circuit that depending on where you put some jumpers will charge from 1.5-9V at anywhere from 15 to 750ma (if I recall correctly). That looked pretty handy.

In fact if you wanted to make a complete charging station, it probably wouldn't be a big job to modify that board slightly so that you could cram it in a hobby box and use a couple of dials for voltage and current, and have a charging pad coming out of the top. Voilla - you could charge anything from stock to multi-cell jobs at the flick of a switch.

Heres what I did.. http://pic1.picturetrail.com/VOL111/...6/17884689.jpg

If you want to drop the charging current but not the voltage this is how you do it. You must place a resistor in parallel to the charging pads. Every supply will need a different resistor value to cut it down to the proper level. There are ways to figure this out , it's just hard for me to explain to you. I hope this helps
JaredR420

Jared,

Thanks, I'm patient if you want to try to explain it to me. You can send me an email.

Thanks

so how long should you leave your car on the charger if you have 2 50mah batteries wired in series... does this depend on the nubmer of batteris in the charger? what is ideal for this setup, and can this be accomplished by charging through the controller?

hmmm... this stuff is a little over my head.(and i thought all you did with little toys is play with em...)

I just got done building my stand alone charger. It uses 4- AA batteries.
Now I can finally build a 3 cell NimH pack and charge the sucker!:D