Oh, and there is some good info and fun eplodey videos here:<div><a href="http://www.rchelisite.com/lipo_battery_charging_and_safety_guide.php">http://www.rchelisite.com/lipo_battery_charging_and_safety_guide.php</a></div>
<div><br></div><div>Also note that the HBRobotics list got dropped from this discussion. Possibly because I am not allowed to post to that list. Someone may want to copy them if they need to know.</div><div>-Taylor<br><br>
<div class="gmail_quote">On Wed, Sep 14, 2011 at 8:02 PM, Taylor Alexander <span dir="ltr"><<a href="mailto:tlalexander@gmail.com">tlalexander@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
<br><br><div class="gmail_quote"><div class="im">On Wed, Sep 14, 2011 at 4:54 PM, Jake <span dir="ltr"><<a href="mailto:jake@spaz.org" target="_blank">jake@spaz.org</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
It looks like it DOES contain a charging control circuit after all:<br>
<br>
"..automatically cutoff once the pack drops to 2.85v per cell ensuring the<br>
battery will not be damaged if you accidentally leave your transmitter<br>
switched on. The voltage protection circuit will also restrict charging<br>
to 4.2v per cell ensuring the pack can never be over charged."<br>
<div><br>
<a href="http://www.hobbyking.com/hobbyking/store/__19481__Turnigy_4000mAh_Spektrum_DX8_Intelligent_Transmitter_Pack..html" target="_blank">http://www.hobbyking.com/hobbyking/store/__19481__Turnigy_4000mAh_Spektrum_DX8_Intelligent_Transmitter_Pack..html</a><br>
<br>
</div>-jake</blockquote><div><br></div></div><div>As it says, that is a voltage protection circuit. Lithium cells are damaged if they go outside the range of 2.8v-4.2v. As the pack charges, the voltage increases. That circuit physically cuts off the charger once the pack has been charged up to 4.2 volts. Its a standard lipo protection circuit.</div>
<div><br></div><div>That doesn't do anything to regulate the *current* flowing into the cell. Which, as Corey points out, needs to be regulated to less than 1C.</div><div><br></div><div>My real question was what kind of charging cycle LiPo cells like, and if simple current-limited DC is enough.</div>
<div><br></div><div>I checked out the datasheet for a simple charging chip:</div><div><a href="http://www.sparkfun.com/datasheets/Prototyping/Batteries/MCP73831T.pdf" target="_blank">http://www.sparkfun.com/datasheets/Prototyping/Batteries/MCP73831T.pdf</a></div>
<div><br></div><div>And figure 2-17 on page 9 shows a typical charging cycle for a 180mAh cell (beware, voltage on the left is not charge voltage, its cell voltage. The right side shows charge current.) It shows that they put a constant 100mA into the cell for most of the cycle, with current dropping off towards the end.</div>
<div><br></div><div>Figure 2-18 shows a charge cycle for a 1000mAh cell. Its slightly more complex - quickly ramps up to 500ma and then immediately starts dropping off.</div><div><br></div><div>Unfortunately if you just load some voltage across the cells, you'll get lots of current at first, and then it will quickly drop off as the cell voltage increases.</div>
<div><br></div><div>Luckily, if you get a current-limited power supply that is, I'd say, in the 0.5C or less range, you'll probably be fine. I would put a current meter on it though and verify whats happening. Most even cheap multi-meters seem to have current measurement.</div>
<div><br></div><div>As Corey points out, it is being sold for devices that normally don't deal with LiPo charging, so its not totally unreasonable to assume that they designed it with that in mind. However, when it comes to LiPo, assumptions can be dangerous and can result in your house being burned down. Nowhere on the product description does it explicitly describe how to charge it, so its really not sufficient to assume anything. If you burn your house down, they will point this out.</div>
<div><br></div><div>After looking at that LiPo charger datasheet and understanding how these are normally charged, you can have a little more confidence that it will be okay. You have to understand though, those charge cycles always ramp down the current at the end, and a constant DC source will not do that. We can *hope* its okay, and I would guess its *probably* fine, but I'm not a battery engineer so I can't tell you. My guess is it will wear out the cell sooner, but not cause a fire.</div>
<div><br></div><div>Anyway, just trying to point out - don't assume things when the result could literally mean an explosion if you are wrong. I agree that its probably totally fine, but I only agree with that after looking into how LiPo charging normally works. Maybe you guys already knew that - but I didnt.</div>
<div><br></div><div>And to be safe, throw the thing in a fire-safe container the first few times you charge it. Cheap insurance against assumptions.</div><div><br></div><div>Good luck!</div><div>-Taylor</div><div><div></div>
<div class="h5"><div><br></div>
<div><br></div><div><br></div><div><br></div><div><br></div><div><br></div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div>
> For all intents and purposes, a 9v DC source should be all that is<br>
> needed to charge this pack.<br>
<br>
I don't know specifics, but I'm inclined to believe that isn't the case.<br>
The protection circuits are only there to prevent overcurrent and<br>
over/under voltage conditions. They don't do any charge current regulating<br>
and I'm pretty sure LiPo cells want that. At least, I do know that even<br>
though all of Sparkfun's single cell battery packs have protection<br>
circuits, they always say you need a lipo charge circuit and I've never<br>
heard anything about charging a LiPo off straight DC. Given how handy that<br>
would be, I'm inclined to believe I would have heard something about that.<br>
<br>
So like I said, I don't know anything for sure, but if you don't either,<br>
then there's probably more to it. Either way its something you should<br>
definitely be sure of before trying.<br>
<br>
-Taylor<br>
<br>
> As long as you only need to provide 7.4 volts at 8 amps, you should be<br>
> fine. This pack only has two leads. That's for voltage in and out. No<br>
> LIPO charging equipment required. My expectation is that there is some<br>
> circuitry in the pack that does this for you.<br>
><br>
> On Wed, Sep 14, 2011 at 11:04 AM, William Garrido <<br>
</div><div>> blackspartan117 at <a href="http://gmail.com" target="_blank">gmail.com</a>> wrote:<br>
><br>
>> Nice! This would be great to drive my motors. I doubt my charger for<br>
LiPo<br>
>> would work, anyone know if a small LiPo charger for 2S? I have my<br>
>> chargers embedded on my robot.<br>
>><br>
>> Or even my MC, would get rid of the need for a stepup! Too bad its not<br>
>> 6AH.<br>
>><br>
>><br>
</div>>> On Wed, Sep 14, 2011 at 10:10 AM, Corey McGuire <coreyfro at<br>
<div><a href="http://coreyfro.com" target="_blank">coreyfro.com</a>>wrote:<br>
>><br>
>>> Need to replace NICAD/NIMH packs, but are afraid of the complexity of<br>
>>> LIPO or added charger cost? This pack might fit your bill.<br>
>>><br>
>>><br>
>>><br>
<a href="http://www.hobbyking.com/hobbyking/store/__19481__Turnigy_4000mAh_Spektrum_DX8_Intelligent_Transmitter_Pack..html" target="_blank">http://www.hobbyking.com/hobbyking/store/__19481__Turnigy_4000mAh_Spektrum_DX8_Intelligent_Transmitter_Pack..html</a><br>
>>><br>
>>> Sparkfun and other places have single cells that have such cutoff<br>
>>> circuitry to protect your single cell from being charged too high or<br>
</div><div>>>> discharged too low, but this is the first 2S solution I've seen. 2S<br>
LIPO is<br>
>>> a sweet spot because it nearly perfectly matches the voltage<br>
characteristics<br>
>>> of 6 Cell NICAD/NIMH packs, which are common everywhere. This could<br>
be a<br>
>>> drop in replacement for such needs.<br>
>>><br>
>>> One draw back, it appears to provide only 8 amps of current. This is,<br>
no<br>
>>> doubt, because the circuit that regulates charge is in line with<br>
whatever<br>
>>> you plug it in to.<br>
>>><br>
>>> --<br>
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