Storing "Somewhat Charged" LiPo's

[Tech 68]

I've done my best to research this, both here and out on the web, but can't seem to find what I'm looking for. So it's time to ask folks that have more experience with Lipo's than me.
Since I need to do some final avionics/interference testing, and as accessing the batteries aren't the easiest to do, what do folks think about Lipo's charged to about 80 percent capacity being left that way for a week or so? They're 800 mAh 2s' for the Stratologger and the Proton. Need to make sure the GoPro and Mobius Maxi adjacent to them and the EggFinder (2s) and BigRedBee (1s), 48 inches in front of them in the nosecone don't cause any issues when I vacuum test the full-up system.
8 VDC should be plenty for test firing matches, I would think... but just bringing all the batteries up from the customary 7.6 VDC "storage" charge at 0.1 Amp charge rate may take me a while, not to mention the assembly and testing.
Since I'm extremely aware of the fire hazards, I bought a metal toolbox suitable for storing/transporting the AV bay and will keep it on the tile kitchen floor until I'm done and can discharge the batteries back to 3.8 VDC per cell and put back into ammo box storage.
The RC airplane folks say full charge for not more than a couple of days and storage levels for up to 6 months, but I can't find anything for "in between".
So any thoughts on what I'd like to do at 80 percent?

 

[BEC]

80% is better than 100%. Most recommendations are for ~60% for storage, as you've found.

The risk is shorter life, mainly, especially with cheap Chinese cells. That said, I have some Korean cells (Kokams) that are 18 years old now that weren't stored at "storage" levels that are still quite usable, or at least seem to be. I haven't asked them to power any of my planes for many years, so I don't know how well they pull a load anymore. I haven't flown a plane using them pretty much since I got back into rocketry in 2009.

They aren't going to spontaneously combust if you store them at 100% or 80%, they're just going to eventually puff up and require you to get rid of them. Even puffed, they will still likely work for short discharges (like firing an igniter)....but charging them requires some extra care.

 

[roc_boy]

I'd have no problem doing it myself (I've got some RC LiPo experience). When dealing with the "care & feeding" of LiPo batteries, IMHO there's 2 aspects to consider - #1 is safety, and #2 is doing what's best to keep your batteries healthy, so they continue to give you their best possible performance.

An undamaged pack is relatively safe, regardless of charge. I do agree with you though, and store all my LiPos in vented ammo cans - it's cheap insurance. The biggest risk with these things is while charging (I'm a big advocate of never charging unattended), or at any time if a pack is damaged. Storing at 50% charge is not as much for safety as it is for the health of the battery - LiPos tend to continue to provide peak output for more cycles if they rest at close to 50%.

I don't like putting a battery away more than overnight at 100%, but I have no problem leaving a LiPo less than full charge but above 50% for as much as a couple weeks. I'm not a racer who's squeezing the last few mAhs from every pack, but I haven't noticed any loss of performance.

 

[manixFan]

When I am testing electronics and know I won’t consume much power, I just leave the batteries at storage charge, do my testing, them restore them to storage voltage when I’m done. In many instances there will be so little voltage drop that I almost don’t need to worry about charging them back up.

Just something to consider,


Tony

 

[Titan II]

Not a problem.

 

[Tech 68]

Many thanks to all for the advice...
I feel more confident moving ahead, now.

 

[Rocketjunkie]

I currently use 9V batteries but am considering LiPos. However, I leave the 9V in and may not use it for a year or more. I just check and make sure the voltage is 9.2V or higher. Then I replace the battery. I would like to use the LiPos the same way. With the redesign of the Duracell 9V from 6 AAAA cells to a stack of rectangular cells, The ONLY remaining 9V is the Energizer Ultimate Lithium at $10 each It's almost cheaper to replace a 1S or 2S LiPo each flight If any one has a method of keeping the LiPos happy with this kind of usage pattern, I'm all ears

 

[manixFan]

I currently use 9V batteries but am considering LiPos. However, I leave the 9V in and may not use it for a year or more. I just check and make sure the voltage is 9.2V or higher. Then I replace the battery. I would like to use the LiPos the same way. With the redesign of the Duracell 9V from 6 AAAA cells to a stack of rectangular cells, The ONLY remaining 9V is the Energizer Ultimate Lithium at $10 each It's almost cheaper to replace a 1S or 2S LiPo each flight If any one has a method of keeping the LiPos happy with this kind of usage pattern, I'm all ears

One issue that all LiPo users face is maintenance. If you store your cells at the correct voltage, they can last years. I use a lot of 1S cells, so I had to make sure my charger will handle those - many only work with 2S or larger. I did have to make several custom wiring harnesses to connect my cells to the charger. It's easy enough to test the cells with either your charger or an inexpensive tester. If you have a charger that shows you the internal resistance of your batteries, you can use that to track their health, or you can measure it using a resistor and voltmeter. (Plenty of examples of how to do this are available - just google 'how to measure internal resistance of a battery'). Lipos are good for hundred's of cycles typically, way more than we would ever use in our hobby. So typically they will 'age out' before we use them up. I write the month/year on all of mine with a sharpie and have found that 3 years is pretty easy to get, and I have some that I still trust after 5 years. I would test the voltage every few months to make sure they aren't self discharging.

The biggest risk to battery health we have in our hobby is discharging below the 'safe' voltage level and damaging the battery. Generally if you discharge a LiPo below 3v/cell, you can damage it, or at least lose some capacity. A lot of electronics designed for LiPos will auto shut off or give a warning on low voltage, but not much of our stuff does. I've drained cells to zero by leaving them in a tracker that was turned on, etc. If that happens you can sometimes recover the battery, but it won't be the same as before.

Another concern is physical damage due to a crash or CATO. If punctured, LiPos can catch fire. I've started enclosing mine in a 3D printed battery box to help try and protect them in case of a bad flight.

LiPos are not as simple as a 9 volt, but they are available in almost any capacity and form factor. I use several oddball sizes for my small minimum diameter rockets where a 9 volt would not work at all. If you are only going to use them in one or two rockets, they might not be worth the hassle. I use them for trackers and all my altimeters (except for a few legacy ones) and have finally got a good system going. I've also found that buying a multipack brings the cost per cell down so even if I don't use them more than a few times, they are still competitive with 9 volts.

I have found Hobbyking.com a good source for LiPo gear, just make sure you choose the US warehouse on their site.

Good luck,


Tony

 

[Rocketjunkie]

@manixFan Thanks. However, Hobbyking is a real PITA to navigate. Ads pop up RIGHT where I'm trying to narrow down the choices such as size and number of cells. Then it loses my place whenever the ad goes away and I have to start again! FUBAR! I'll order from another site if I get into LiPos.

 

[manixFan]

@manixFan Thanks. However, Hobbyking is a real PITA to navigate. Ads pop up RIGHT where I'm trying to narrow down the choices such as size and number of cells. Then it loses my place whenever the ad goes away and I have to start again! FUBAR! I'll order from another site if I get into LiPos.

I guess the adblocker I use hides all those, I've never even seen them. I just tried the site, and other than little windows in the lower left that don't block anything and go away after a few seconds, it just looks like a regular site. What browser and ad-blockers do you use?

The batteries I use with the Raven:

https://hobbyking.com/en_us/turnigy-nano-tech-150mah-1s-25-40c-lipo-pack.html
are only $2.89, (about half the price anywhere else) but like so many things, are on indefinite backorder. At that price, even if I only get a few uses out of them they are cheaper than 9 volts. The ones I use with the Strataloggers are:

https://hobbyking.com/en_us/turnigy-nano-tech-300mah-2s-35-70c-lipo-pack.html
and sell for $4.99 each. At least those are in stock. I could use those with the Raven if I added a current limiting resistor in series with the e-match.


Tony

(For what it's worth, I use Safari on a Mac with Adblock Plus, Ghostery, and use the built-in Safari pop-up window blocker. For watching Youtube, I use Firefox with UBlock Origin. And no need to shout!)

 

[Rocketjunkie]

I guess the adblocker I use hides all those, I've never even seen them. I just tried the site, and other than little windows in the lower left that don't block anything and go away after a few seconds, it just looks like a regular site. What browser and ad-blockers do you use?

I use Firefox with AdBlock+ It blocks all the ads except the one on the lower left that pops up for a few seconds. When it goes away, all the filters I have set are gone. That one covers the left pane where you can select filters on the items shown.

 

[manixFan]

I use Firefox with AdBlock+ It blocks all the ads except the one on the lower left that pops up for a few seconds. When it goes away, all the filters I have set are gone. That one covers the left pane where you can select filters on the items shown.

I call that kind of design ‘sales prevention’. You do have to wonder if the website designers ever try to use the site when they are done with it. I didn’t experience the reset issue you mentioned, but that would make the site unusable as you describe. Hopefully you can find a good deal in the batteries elsewhere. Spend plenty of time researching a charger if you go with LiPos, I didn’t and ended up having to buy a second one to get all the features I wanted.


Tony