OK ANOTHER DUMB QUESTION....
 

Is there any device that will safely drop the voltage of say a 11.1 lipo to 8.4v while still delivering the maximum current? I know there are adjustable IC voltage regulators but I don't know if there are high current ones.:(

Well, there's not really any efficient way to do it. those IC voltage regulators waste excess current to get to thier voltage, and at most thier rated for 1-2 amps (typically). You could try some intricate voltage dividers, or power supplies, but they would be too big, bulky, and expensive to fit in an RC car. If you're trying to use a 3 cell in a 2 cell ESC, just dont try. Buy a 2 cell pack, or get a 3 cell ESC. John.

Actually I'd like to try to run my e maxx with its 16.8v system on two 8.4v lipos but they're only available in 7.4 or 11.1 volts. That's all. Just figurin' :rolleyes:

A 2S lipo fresh off charger will be 8.4v. Emaxx esc effectively wires the two batts in series so you'll have 16.8v from 2 fully charged 2S lipos. Lipos voltages are talked about in two ways - nominal voltage is 3.7v per cell - 2cells (2S) is 7.4v nominal. Fully charged, lipo cell voltage is 4.2v, so this 2S lipo is 8.4v charged. Minimum voltage is 3v/cell or 6v for a 2S lipo.

If you tried 2 charged 3S lipos, you'd burn your esc out with 25.2volts. 3S charged is 12.6v (3*4.2v/c).

Get the 2 - 7.4v lipos and charge them - they'll do what you want.

I understand that batteries charge up to more than their rated voltage but don't they run down to their rated voltage pretty quickly than mainatain that voltage for a long time?

No,not really. You're kinda splitting hairs anyway - yes, both NiMH and Lipo drop voltage, but at different rates. NiCd and alkalines drop voltage quicker and don't really stop declining. NiMH and Lipo drops voltage the slowest of these chemistries and do show plateaus where voltage drop is real small as they deliver current. Lipo drops less and hold voltage better than NiMH. This is part of the reason NiMH bumped NiCds and Lipo is now bumping NiMHs.

If you want a 16.8v lipo sytem for your stock Emaxx, your only lipo choice is a 2S lipo of some size. It will run like crap on a single 3S (assuming you can figure out how to wire it) cause it will be seriously undervolted. The thing you originally started out asking about does not exist in a form that you could stick in a moving rc vehicle, nothing will handle the current even a stock set of titans will pull and be light enough for an RC to move it.

There is no such thing as a 8.4v lipo being diff than a 7.4v lipo. They are the same battery, just 2 diff way of looking at way batt is rated. Go over to TRX forums and see what those guys are running for lipos. If you're going to run stock esc, it will be a pr of 2S lipos. Your decision is how big should you buy - 5000, 6000, 8000 etc. Check out rclipo.com - you will not find a lipo listed as 8.4v, they're listed as 7.4 - the "nominal" batt voltage. I don't know why they do it this way, but that's the convention, has been since before lipo came into rc useage. Lipos are sold and advertised based on nominal voltage of 3.7v/cell.

You're confusing two voltages that are really the same battery, there's not 2 diff batteries, a 7.4 and a 8.4v. RCLIPO is one of the top lipo internet sellers, but check any of the other lipo sellers/manufacturers CBP, SMC, Orion, Maxamps, Flightpower, Trackpower, Elite, Hyperion etc, you won't see an 8.4v lipo listed.

That's the reason I asked because I see there is no 8.4v lipos listed anywhere.

Thanks for the info. I'll mosey on over.

Just saw this in case you haven't - good FAQ on both Emaxxes

http://monster.traxxas.com/showthread.php?t=423790

WOW!! What an amazing amount of information. Thanks!