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        Question Discharger for LiPos (3.85V for storage & long life)

#1 Joe Minton Aug 10, 2006 01:41 PM

Discharger for LiPos (3.85V for storage & long life)
 
I don't have a lot of money and would therefore like to get the longest life out of my LiPo battery packs.

I do not push them past 1C very often and haven't hit LVC for a couple of years now. All my packs are either equipped with individual cell connectors or soon will be. I own & use an AF "Blinky." I use Motocalc and a Whattmeter; I know what the loads are and select battery/motor/prop combinations that optimize performance against stress on the batteries.

Now, if I read it right, LiPos last longer if stored @ 3.85 Volts. I currently store my batteries (week or two at-a-time) fully charged (Apache "Smart Charger 2500").

Questions:
1) Would I get significantly longer service from my LiPos if I kept them at 3.85 Volts when storing for one or two weeks?
2) If there is an advantage, is there a discharger or discharge technique available that I can use?

Thanks Guys -- isn't the internet wonderful. I've learned so much about this hobby of ours on RCGroups!

Joe Minton

#2 everydayflyer Aug 10, 2006 02:14 PM

Simpliest cheap solution is to fly to approx. 3.7-3.8 per cell and then just add a small charge let them et 30 min. check and add more of needed.

TP-101C charger will charge to 3.85 per cell but no cheap.
A Triton can be used in the NiCd / NiMH discharge mode and set the correct cut off volts for the battery such as 11.5 for a 3S but in reality you have to set like for like 11 then let them spring up and then keep adjusting.

The 3.85 per cell does not need to be exact. 3.75 -3.9 if fine and even 4.0 is OK for a couple of weeks or so.

Charles

#3 Joe Minton Aug 10, 2006 03:02 PM

Thanks for your reply, Charles. I really and truly value your continued input!

I don't want to do the things necessary to make sure my batteries are between 3.7 -- 3.9 volts when I land. I'm lazy and I'd rather fly than worry about such things. Since I don't overload or over-discharge my batteries, my concern is storage voltage for maximum life. I would prefer to worry about such things after I get home from flying.

I can't or don't want to afford a TP 1010C. I'll probably get something like the new FMA charger if and when I decide to buy yet another charger. I'd have to take my shoes off to count the chargers I've bought in the past 5-years.

Is there anyone out there (hello ----?) who has a circuit for a discharger (Radio Shack parts only, please) that would let me bring the charge level down to reflect 3.75 -- 3.9 volts per cell?

Again I ask:
Is there any real-world (practical) benefit from keeping LiPos in this voltage/charge level range when they are used within two weeks of charging?

Inquiring minds want to know ;o)

Joe

#4 hoppy Aug 10, 2006 03:29 PM

I don't have a whole bunch of Lipo's but the ones for planes currently being used are stored fully charged ready for the next use which typically ranges from a day to weeks. The ages range from 3 months to 3 years. Packs which don't have a plane at the present time are stored at 3.85V/cell. So far, so good. Probably not the best but what else can you do when your flying escapades are determined by the weather (and heat). They all "seem" to be performing as good as ever but then I don't push them.

#5 Joe Minton Aug 10, 2006 03:35 PM

hoppy:

I'm with you! I consider my batteries to be a source of power and their maintenance is not an end-in-itself. I just want to know if there is an easy/cheap way to leave them at 3.7 -- 3.9 volts between flying sessions. And -- is it important enough to worry about.

Cheers,

Joe

#6 everydayflyer Aug 10, 2006 04:04 PM

Well you could use this circuite as astarting point if you are a DIY type.

http://www.uoguelph.ca/~antoon/gadgets/cycler.htm

I mkae no special effort to end flights with cells at between 3.7 and 3.8 . My ESCs are set to provide these levels and if a flight is cut short the plane can / could still be used to dump the excess charge.

My first LIPoly charger was an Apachee 2500 however my second one as well as the third was a Triton. They will do any battery maintaince necesary. I have added a buch more chargers but they are icing on the cake.

IMO everyone needs at least one good all purpose charger/discharger / cycler.

A fairly cheap alternative if your LiPoly have taps would be to purchase a Hyperion LBA 6 and use it in the discharge mode. It has an LVC of 2.75V (any cell) ,which I feel is to low but a auto tail light can be used as a discharge load and the LBA 6 be used as a backup cutoff if you fail to stop the discharge at the correct point.

Charles

#7 hoppy Aug 10, 2006 04:11 PM

is it important enough to worry about.

Not to me...:)

If I was getting cell inbalance (which I'm not), or puffed cells (which I'm not), or cells coming down hot that use to fly cool (which I'm not), or cells that can't fly planes they used to fly (which I'm not), then I would be concerned about it.

Now there is another factor here which I should make clear....these are all cheap/inexpensive packs and not the state of the art 20C, super light packs available today and none are flown to LVC or used in applications that drive them over 120F.

I should also point out that I have not measured capacity on any of them since I bought them so I might be in for a surprise, if they start acting funny I'll run a capacity check.

#8 everydayflyer Aug 10, 2006 04:42 PM

Some good points hoppy. Expensive is relative and to some a $50 LiPoly is a big investment annd they really need to get a couplre of years and / or a 200 flights from it. Some figure that if a $50 only last one season and 100 flights they got their money's worth from it and better one will be available next year.
The big problem with with those who spend $400 to $1,000 or so on LiPolys for a single aircraft and perhaps only fly it 25 times or so a season and perhaps only 50 to 75 flight total. If such a LiPoly dies in one year then those flights are on the expensive side.

Improperly stored LiPolys develop high IR which is worse than capacity lose IMO. It matters little if a 1300 mAh still delivers 1200 mAh if it was able to deliver 10A at 3.5 volts per cell and now it drops to 3.0V or less per cell at that discharge level when less than 500 mAh have been delivered..

Charles

#9 hoppy Aug 14, 2006 07:35 AM

Now I Did It... Just 3 days after saying I have never had a Lipo go bad....

I took a relatively new (6 months, < 10 cycles) 15C continuous 3s 1000mah which was in storage at 3.8V/cell and charged it - ~500mah in....so far so good.
Went flying and the plane started loosing power after 3 laps of the field at under a 6A draw.
Charging after I got home showed about 200mah in and all cells within 0.01V.
A discharge at 1C showed ~500mah
A discharge at 2.5C gave ~250mah

Yikes

#10 defranci Aug 14, 2006 09:41 AM

Quote:

Originally Posted by Joe Minton
Is there anyone out there (hello ----?) who has a circuit for a discharger (Radio Shack parts only, please) that would let me bring the charge level down to reflect 3.75 -- 3.9 volts per cell.

Joe

Joe
I use a cheap and easy version of the "Dump'r". Went to the auto parts store and got a 12 volt trailer clearance light for $1.99. Soldered a connector onto it. It's not fast or elegant, but it is cheap and easy. Downside is that it may take a couple of hours if you have larger packs.
Don

#11 ChrisAttebery Aug 14, 2006 01:12 PM

Joe,

I posted a new thread for a discharger circuit in the DIY electronics forum. It has not been tested, as I am soliciting input for some of the parts. Hopefully, I can get one up a running fairly soon. Take a look.


Chris



Quote:

Originally Posted by Joe Minton
I don't have a lot of money and would therefore like to get the longest life out of my LiPo battery packs.

I do not push them past 1C very often and haven't hit LVC for a couple of years now. All my packs are either equipped with individual cell connectors or soon will be. I own & use an AF "Blinky." I use Motocalc and a Whattmeter; I know what the loads are and select battery/motor/prop combinations that optimize performance against stress on the batteries.

Now, if I read it right, LiPos last longer if stored @ 3.85 Volts. I currently store my batteries (week or two at-a-time) fully charged (Apache "Smart Charger 2500").

Questions:
1) Would I get significantly longer service from my LiPos if I kept them at 3.85 Volts when storing for one or two weeks?
2) If there is an advantage, is there a discharger or discharge technique available that I can use?

Thanks Guys -- isn't the internet wonderful. I've learned so much about this hobby of ours on RCGroups!

Joe Minton


#12 OneHump Aug 14, 2006 02:11 PM

This thread is outstanding. As always, Charles and Hoppy (among others) are making invaluable contributions to my education. I came off slimers a couple of years ago and have been pondering lipo for the past 18 months. I have a Mini Funtana I bought a year ago that still needs a battery, but I hesitate to jump in until I'm well-educated. As you can see from my profile, I've been around a long time, but rarely post.

I'd like to thank Joe for a great topic and everyone else for providing such great info. Now let's hope I don't burn my house down! ;)


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