I just aquired an 11.1v 800mAh (3 cell i think) li-po and i was wondering if i could put it in my 1:18 rc18t in place of the stock 7.2v 11oomAh ni-mh battery. My motor is a 5800KV brushless and my Esc is rated at 20A, 12v and is li-po ready upto 3 cells so i can't see why it wouldn't work but it is physically smaller than the stock battery. Does that matter?

Also apart from a proper charger what else do i need. For example BEC, balancer, discharger etc. As you can see this is my first li-po and electric car

Thanks, any help would be great

If its already brushless and the esc is 3S rated, it will work. Its going to be faster and have less run time cause the 3S has about 25% less capacity than what its replacing. ESC will handle BEC duties unless you're running a separate one now for some reason.

If you get a lipo capable charger than has balancer already builtin, there's not really anything else you need. Lipos just need to be charged, no discharging needed. If the charger you get doesn't have a balancer, you'll need a separate balancer.

I thought i better just double check it all after asking the first question and after doing so i have found the following information:

INPUT: 4-9 cells ni-mh/ni-cd or 2-3 cell li po

OUTPUT: continuous current 25A, burst current 90A

BEC: 6V/1.5A

I'm not sure what that BEC rating means though I will try and post a link or something to some info on the ESC when i get home from work.

Thanks for your help so far duster

\BEC stands for Battery Eliminater Circuit, so the esc regulates the voltage output to the recievers and servo to 6volts.

OK, now that you've listed the BEC info its clear what they're saying. The BEC specs are just letting you know what your servos and rx will have for voltage and what total current there is available for these devices. As long as you're not running a pr of super current hungry servo, like 333inoz models, you will not have any probs.

The usual prob is to dissipate the heat that comes from stepping the batt voltage down to the voltage output of the BEC. You esc maker has sepcified the range in voltage they can accomodate and you're within this range.

Its going to be fast....

Sweeeeet Thanks for your help guys. I'm thinking of getting the Reedy 9800KV to put with the li-po so i'm hoping it will do just shy of 100k rpm

Now that should be interesting

Will i need to run a regulator? If not when would i need to run one

Not if I understand your question - the BEC will do that for you - it willsupply servos and rx with 6.0v, jut lik ethey are designed to use.

If you're looking to use a separate lipo for just the rx which will output 7.4v, then its a matter of servo specs - what they can tolerate as far as voltage. So do OK, some don't. From setup you've mentioned so far, I don't see any need to run separate lipo for onboard electronics - let the main power lipo supply it all.

Sweet thanks guys for all your help. I'm going to be charging it and dropping it in tomorrow so i will let you's know how it went.

What would be the best amperage to charge this type of cell? (11.1v, 800 mAh, 3cells) No info came with the battery as it's a spare replacement for a helicopter

You need a lipo compatible charger. If you have one, all it will want to know is the number of cells (3) and the batt capacity (800). Lipos are charged at a 1C rate where "C" is the mah divided by 1000 - for your lipo, a 1C rate would be 0.8amps. Be sure you are charging it using a lipo charging protocol. If charged as a NiMH or NiCd, lipo can be dangerous and can cause damage to the battery.

Fully charged the voltage should be somewhere near 12.6, lipo max volts per cell is 4.2v.cell.

Ok so we have had a heap of rain where i live and it's only starting to come good now so i only tested it yesterday and Duster.............you were right, it's bulls#!t fast. With only a few little runs yesterday it was off the charts for speed but it's chewed up my pin tyres, luckily i've ordered foam tyres. I did notice though that the motor was getting pretty hot after about a 5 min run. Am i over/under geared or is this sort of normal for brushless/li-po set ups?

I'm thinking i might not need the 9800KV reedy motor now

Depends on really how hot it was and what the batts and esc were like - it's not normal if it was hot, so if it was "can't hold your finger on for 3 sec hot", its too hot. Handy to have a temp gun so you know what it was. I refer back to this copied out of a post on RCM when ques like this come up, (thanks Batfish) -

"Although not always true, a rule of thumb is:
Hot motor, cool controller, cool batteries = undergeared
Cool motor, hot controller, hot batteries = overgeared
Warm motor, warm controller, warm batteries = happy components

Hot, warm, and cool vary as well, so here's another general rule:
Hot for a motor = generally over 160F (71C)
Hot for a controller = generally over 150F (66C)(or "thermal" range for the controller)
Hot for batteries = generally over 150F (66C) for nimh, 130F (54C) for lipo
Warm for a motor = generally 100F (38C) through 150F (66C)
Warm for a controller = generally 100F (38C) through 130F (54C)
Warm for batteries = generally 100F (38C) through 130F (54C) for nimh, ambient temp through 120F (49C) for lipo
Cool for all components = generally ambient temp and up to 10 degrees F (5.6C) more"

Just be sure there is no binding in your drivetrain. Brushless can be weird sometimes - if you change gearing, run a few min and check temps - if same or worse (hotter), need to go the other way....