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Old April 21st, 2010, 08:58   #19
m102404's Avatar
Join Date: Apr 2006
Location: Toronto
Originally Posted by Shooting Addict View Post
Whats a PCB I've heard thunder cactus say it's necessary for lipos setups?
That's not entirely correct (as per his last/latest post). He's said that a PCB is not absolutely necessary...but a good thing to consider.
Personally, I do not use one.

A PCB (protection circuit board) is a set of chips/circuits that cuts off the draw of power when the voltage drops below a given threshold.

The primary reason for doing so is to help ('cause everything can fail) ensure that the voltage of your lipo cells do not drop below their safe lower threshold.

A typical LiPo pack is made up of 1 or more cells. Each cell has a nominal voltage of a 7.4v pack has 2 cells...a 11.1v pack has 3 cells...etc...

Each cell has a lower threshold of 3v. Once it drops below that there is a high risk that the cell is permanently drained and will not charge up properly. They have an upper limit of 4.2v...they are not designed to be charged up past that.

So if you take your "11.1v" LiPo and charge it's going to charge to 12.6v. If you've got a "dead" cell that's not charging up properly...and don't have a smart charger that will 1) prevent overcharging any given cell, 2) detect that there's a problem cell in the pack...and you just charge it up to 12.6v, you'll be driving the remain 2 good cells WAY past their upper threshold. Can't be good right?

*** A quality smart charger and balancing system (often built into the charger) is a MUST HAVE for LiPo batteries. ***

When you rapidly drain a battery (any battery)...the chemical reaction produces heat as a by-product. The heat produced can often be sufficient to "boil" the contents of the battery...damaging 1+ cells. So it wouldn't be a good idea to try to charge that now damaged pack right?

When you "normally" drain a don't want to take any given cell past 3v ('cause then you have issues when charging it up). That's where the PCB comes in.

For a "11.1V"'ll cut off ~9v (3 cells, 3V/cell). For a 7.4V'll cut off at 6v (2 cells, 3v/cell). Obviously you can't use a 11.1v PCB for a 7.4v battery...cause even at 2 cells x 4.2v = 8.4v max charged up it'll trip the circuit and cut power. Guess what will happen if you use a "dumb" 7.4v PCB for a 11.1V battery?

Now, in practice...when your battery (any type of battery) starts to run out of power you should be able to notice a drop in ROF, and slowness in the trigger response and a longer "wind up" of your gears in the mechbox. When that happens, your battery is almost depleted. How fast it gets there is dependant on your battery, your rifle/setup and how much/fast you shooot. When I notice this drop off, I stop using the're just over draining the battery and performance sucks anyways. LiPo's tend to drop off quicker and more suddenly than other batteries.

Now, in reality...some guys are so stupid that they might as well stick their heads in a hole and have their buddy fill it in. They'll never notice how their rifle is performing, they'll not take care in checking how it's charging, how it's changing over time, etc... Those dummies should be using LiPo's...and perhaps even they shouldn't be shooting at other people (that may be overly harsh ).

Also note...some PCB's that are just simply "in-line" between the motor and battery simply detect the overall voltage and not the voltage of any one specific cell. They are cheap...and should be avoided like the plague. To be effective a PCB should be monitoring the voltage of each cell of a battery pack.

Additionally...batteries are basically bags of layered chemicals. Hopefully they are produced consistently and held to high quality control standards. In much QC to you think goes into a $12 discount, no-name battery that you're buying off some faceless dude on e-bay?

Each cell can last/behave/react a bit different from another. Each cell has multiple contact points that can be damaged by/though use and handling.

Due to the consequences of mishandling/use of LiPo's...they require that their users understand what they are, how they behave, how they'll react and how to care for them.

Same should go for NiMh and NiCd batteries...but they are not likely to burst into flames and kill everyone in your building.

Last edited by m102404; April 21st, 2010 at 09:09..
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