icyuod2

battery questions?

i made this little scratch boat a while back. i special ordered the speed control and they ended up sending me a speed control for a 400 series motor instead of a 300 series. instead of sending it back, i ordered a 400 series motor.

to make a long story short, i finally have the proper speed control and motor.

i have this motor (edp 400b-6030 8.4v)
http://www.gwsus.com/english/product...tem/edp400.htm

this speed control
http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXKBF3
and this reciever
http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXMRE6

so i'm ready for my batteries. the speed control says to use 6-8 ni-cd/mh or 2-3 lipo.

i'm planning on making my own packs based on space restrictions. but i'm unsure of a few things. first my battery choices.

ni cd or ni mh? which are best? with a small 13" boat like this, should i use 6, 7 or 8 batteries (if i were to go the ni-cd ni-mh route)
what about milli amps? my local store had 1800- i believe 2300 ma batteries? what should i use and how does this all translate in terms of cost (chargers etc) charge time and run time.

lipo? 2 or 3? ma? costs involved etc etc. i'm eager to get this boat in the water. with the choice of batteries, this boat should be ready to rock, and has a date with the pool.

the more i read on the subject, the more confused i seem to be.
if it were your boat, what would you put in it?
thanks again.

piper_chuck

My Feedback: (12)

Nimh are significantly smaller and lighter for a given capacity, so I'd go with those instead of nicad. As far as those vs lipo, lipos are even smaller and lighter, but they introduce their own complexities, such as needing a special charger and extra care during use. Most chargers can handle a variety of battery sizes, so choose something that fits. Choosing the largest capacity you can reasonably fit will give you longer run times. Choosing more batteries, 8 instead of 7 for instance, will make your boat go faster, but will also cause the motor to get hotter and wear out quicker.

LtDoc

Here are some very 'general' things to remember about batteries, motors, ESCs, and just common electrical stuff.
The most important one is that there are limits to everything. When dealing with model boats, those limits usually can be classified as volume, size, and weight. When one of those limits is exceeded it means something just ain't gonna work. Boat sinks, can't hold the @#$ things, and/or will have to be modified. That particular 'limit' is more on the builder/owner than anything else.
Voltage can be thought of as electrical pressure, just like water pressure, sort of. If there isn't enough, motors usually don't turn very fast. If there's too much, they may spin ~very~ fast, but only for a short period of time before they blow a 'gasket', sort of. The 'best' voltage to use is the one the motor is designed to handle, or close to it anyway.
Current can be thought of as the water in plumbing. An 'ampere'/amp is a particular volume of water, sort of like a gallon. A milliampere is a smaller quantity, 1/1000th of an ampere. Time is a constant, an hour is an hour, or 60 minutes, or 3600 seconds.
The important things to know about any electrical device are it's ratings, it's limitations. If you know those, you can select things that sort of 'fit' together, have the same (or close) ratings.
A battery is a 'fuel tank'. It holds so many gallons of fuel under so much pressure. When a battery holds no more fuel it's said to be dead. OR, if the battery doesn't have the required pressure to force that fuel into the motor, it might as well be dead.
A motor works best at a certain voltage and current delivery. If the voltage isn't what it 'likes' to see, it either goes slower, or beats it's self to death. If the current supply (battery) doesn't hold enough fuel, the motor will spin happily but for a shorter time (Aha! Thats where the hours/minutes/seconds come in!). So, the average current draw of the motor will tell you how long it will spin before the battery goes dead/empty. The biggies are voltage and current draw. Sorry, but the number of turns in a motor tells you absolutely nothing, is a very stupid way of rating a motor. If you know the exact length of the wire used in those 'turns', and what it's resistance is per unit length, you can still figure what the run time is. The 'catch' to that is that the manufacturer never tells you those things. They also never tell you how much heat the motor can dissipate safely before it dies. It does make for a good sales gimmick. (That's a pet peeve of mine, if you couldn't tell. Useless information is just exactly that, useless.)
ESCs. You know what the motor's requirements/limits are and the ESC has to at least meet those requirements/limits. It's also a very good idea if the ESC's limits are greater than the motor and battery's (@#$ things cost more!).
There are ways of sort of extending the limits of motors, ESCs and batteries. The first that comes to mind is 'cooling' them. It does work, but only to some extent. Unless your boat is big enough to hold the required air conditioner cooling system. (I wish I could afford a boat that big!)
It helps to reduce the resistance of the electrical system. That also has practical limit$.
- 'Doc


{There! If that isn't enough to totally confuse you, then you probably didn't need to read it to start with.}

icyuod2

clear as mud! thanks

so what about the ma of the rechargable batteries? what should i select? 1800ma? 2300ma? even higher?

LtDoc

In general, meaning there are always going to be exceptions, the larger the mAh rating the better. You have to balance that against the battery's weight and physical size, as in too heavy, or won't fit.
The capacity of a battery, it's mAh rating doesn't 'do' anything bad to the system, it only tells you about how long the charge is going to last, sort of. Batteries don't provide any more current than is required/acceptable to the system so they certainly won't make stuff blow up just because the mAh rating is very large. It usually works the other way around, the 'system' demands more current than the battery can safely provide, so it sort of looses it's 'cool', gets very hot, pops it's 'cork'! (all puns definitely intended!)
Basically, it's a matter of what will fit without turning a surface craft into a submarine, or slowing it down to a crawl, if it ain't a crawler to start with...
- 'Doc