[Noisebridge-discuss] [HBRobotics] High density 2S LIPO with Charge/Discharge cutoffs - Brainless Battery Pack?

Taylor Alexander tlalexander at gmail.com
Thu Sep 15 03:06:20 UTC 2011


Oh, and there is some good info and fun eplodey videos here:
http://www.rchelisite.com/lipo_battery_charging_and_safety_guide.php

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.
-Taylor

On Wed, Sep 14, 2011 at 8:02 PM, Taylor Alexander <tlalexander at gmail.com>wrote:

>
>
> On Wed, Sep 14, 2011 at 4:54 PM, Jake <jake at spaz.org> wrote:
>
>> It looks like it DOES contain a charging control circuit after all:
>>
>> "..automatically cutoff once the pack drops to 2.85v per cell ensuring the
>>  battery will not be damaged if you accidentally leave your transmitter
>>  switched on. The voltage protection circuit will also restrict charging
>>  to 4.2v per cell ensuring the pack can never be over charged."
>>
>>
>> http://www.hobbyking.com/hobbyking/store/__19481__Turnigy_4000mAh_Spektrum_DX8_Intelligent_Transmitter_Pack..html
>>
>> -jake
>
>
> 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.
>
> 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.
>
> My real question was what kind of charging cycle LiPo cells like, and if
> simple current-limited DC is enough.
>
> I checked out the datasheet for a simple charging chip:
> http://www.sparkfun.com/datasheets/Prototyping/Batteries/MCP73831T.pdf
>
> 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.
>
> 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.
>
> 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.
>
> 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.
>
> 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.
>
> 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.
>
> 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.
>
> And to be safe, throw the thing in a fire-safe container the first few
> times you charge it. Cheap insurance against assumptions.
>
> Good luck!
> -Taylor
>
>
>
>
>
>
>
>
>> > For all intents and purposes, a 9v DC source should be all that is
>> > needed to charge this pack.
>>
>> I don't know specifics, but I'm inclined to believe that isn't the case.
>> The protection circuits are only there to prevent overcurrent and
>> over/under voltage conditions. They don't do any charge current regulating
>> and I'm pretty sure LiPo cells want that. At least, I do know that even
>> though all of Sparkfun's single cell battery packs have protection
>> circuits, they always say you need a lipo charge circuit and I've never
>> heard anything about charging a LiPo off straight DC. Given how handy that
>> would be, I'm inclined to believe I would have heard something about that.
>>
>> So like I said, I don't know anything for sure, but if you don't either,
>> then there's probably more to it. Either way its something you should
>> definitely be sure of before trying.
>>
>> -Taylor
>>
>> > As long as you only need to provide 7.4 volts at 8 amps, you should be
>> > fine.  This pack only has two leads.  That's for voltage in and out.  No
>> > LIPO charging equipment required.  My expectation is that there is some
>> > circuitry in the pack that does this for you.
>> >
>> > On Wed, Sep 14, 2011 at 11:04 AM, William Garrido <
>> > blackspartan117 at gmail.com> wrote:
>> >
>> >> Nice! This would be great to drive my motors. I doubt my charger for
>> LiPo
>> >> would work, anyone know if a small LiPo charger for 2S? I have my
>> >> chargers embedded on my robot.
>> >>
>> >> Or even my MC, would get rid of the need for a stepup! Too bad its not
>> >> 6AH.
>> >>
>> >>
>> >> On Wed, Sep 14, 2011 at 10:10 AM, Corey McGuire <coreyfro at
>> coreyfro.com>wrote:
>> >>
>> >>> Need to replace NICAD/NIMH packs, but are afraid of the complexity of
>> >>> LIPO or added charger cost?  This pack might fit your bill.
>> >>>
>> >>>
>> >>>
>>
>> http://www.hobbyking.com/hobbyking/store/__19481__Turnigy_4000mAh_Spektrum_DX8_Intelligent_Transmitter_Pack..html
>> >>>
>> >>> Sparkfun and other places have single cells that have such cutoff
>> >>> circuitry to protect your single cell from being charged too high or
>> >>> discharged too low, but this is the first 2S solution I've seen. 2S
>> LIPO is
>> >>> a sweet spot because it nearly perfectly matches the voltage
>> characteristics
>> >>> of 6 Cell NICAD/NIMH packs, which are common everywhere.  This could
>> be a
>> >>> drop in replacement for such needs.
>> >>>
>> >>> One draw back, it appears to provide only 8 amps of current.  This is,
>> no
>> >>> doubt, because the circuit that regulates charge is in line with
>> whatever
>> >>> you plug it in to.
>> >>>
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