Lithium Battery Storage

[John DeRosa]

It is time here in the upper US Midwest to put gliders away for the winter.  Part of that is to take my lithium batteries home for storage. 

I was reading a notice for sale of some FES lithium battery charges that included a "discharging assistant".  See https://groups.google.com/g/rec.aviation.soaring/c/1ZFo7gGEnbY. 

Per the manual for the discharging device (https://front-electric-sustainer.com/wp-content/uploads/2022/01/FES-Discharging-assistant-v1.33.pdf) it says to use this device coupled with a 1KW or larger DC load to discharge the battery to the 50% level for "longer time storage".  I have not heard of doing this before.  I was previously under the impression that storing a battery (SLA?) at full charge was best.

I did a bit of research and found this article (https://www.dnkpower.com/lithium-ion-battery-storage/) which says to "Charge 40% to 50% of the battery charge, and refrigerate in a dry environment. Cooler temperatures and less charge is conducive to maintaining the life of the battery, but too little charge can not be, because the battery will be self-discharge in storage, once the battery slowly run out of power, it will seriously shorten the battery life."  Other articles mimic this advice.

This is the first I have heard of this advice.  My question is not how to discharge the battery but how do you know it is at the 50% level?  With a lithium battery's rather flat discharge curve using the battery voltage won't easily work - plus what voltage is 50%?  

I suppose you could measure your battery's discharge time until it gets to the non-flat part of the curve, then recharge, and discharge to half of that time.  My simplistic battery tester (http://aviation.derosaweb.net/presentations/#batterytest) might help towards that end.

So I am stuck between deciding keeping the batteries on an appropriate "trickle" charger over the winter or to discharge to 50% and not recharge until the spring.  Your thoughts?

Thanks, John (OHM)

PS - What is the wife going to think when she opens the refrigerator and sees next to the pickles and mustard several lithium batteries?

[David Anisman]

What I do: simply fully charge and then discharge to ~50% of nominal capacity based on current and time at ~0.2C. Repeat every 3 months.

So, I fully charge my 9AH battery then discharge with a 25 Watt incandescent bulb and a timer at ~2A for 2 hours and 15 minutes.

No trickle charging during the winter but at the beginning of the season a connect the batteries to the charger for a couple of weeks straight to let the top balancing algorithm of the BMS do its thing.

I use the PowerSonic PSL-BTC-1290 9AH battery which includes BT connectivity and with the iPhone app I can see the state of charge of the battery. This is actually very nice but not necessary to implement what I'm doing as described above.

David

[David Anisman]

I started a discharge cycle based on my post above right after I sent it. Just finished the first battery. It took 2 hours and 10 minutes. See pic below.

Thanks for the reminder!

If I decide to go flying I will just recharge them and then repeat the process. I would not leave them on trickle the whole winter, that's the issue. The 50% does not have to be dead-on accurate. Any number between 40 to 60% is fine. It's at the extremes that the damage happens. I also don't plan to store them in the refrigerator, I don't believe the residual benefit is meaningful.

David

[John DeRosa]

I was not aware of these Bluetooth (BT) enabled batteries from PowerSonic.  https://www.power-sonic.com/batteries/lithium-bluetooth-series/

The app looks interesting for monitoring.   https://www.power-sonic.com/wp-content/uploads/2019/07/PowerSonic-APP-Operation-Instruction_July19.pdf

What charger are you using?  The ones from PowerSonic?  https://www.power-sonic.com/chargers/life-series/

Thanks, John (OHM)

[David Anisman]

I use the original charger that I got with the first LiFePo batteries I bought (Stark Power). It's not a branded PS charger (but we don't really know who really makes the PS charger). The output goes up to 14.6VDC and is continuously applied even when the battery is fully charged (defined as charging current <300mA).

The app makes it trivial to monitor voltage, current and state of charge remotely. I can easily monitor state of charge in the cockpit while flying or from my desk at home just to check. I can check how much current the glider panel is pulling, what is the charging current (when charging). It tracks the number of cycles (not important in our use case). It shows cell temperature (charging these batteries at or below freezing temps will destroy them). It shows the battery voltage at the BMS, but that's not particularly useful due to the characteristics of this battery chemistry.

David

[David Anisman]

I was looking again at the PS battery's spec sheet (which is excellent and comprehensive btw) and was reminded that this battery actually has low-temperature charge protection at -10+/-5C.  It will stop the charging when temps enter that range and restart it once it goes back up to 0+/-5C. It's more permissive than the usual hard 0C cut off therefore I assume they must have tested it under those conditions, so I take back my earlier comment about damage from charging in freezing temps with regards to this battery. However, not all LiFePo4 batteries implement this type of protection in their BMS.

https://www.power-sonic.com/wp-content/uploads/2019/04/PSL-BTC-1290-Technical-Document.pdf

David

[Moshe Braner]

Belatedly taking a look at RAS_Prime.  Hello!  Regarding storage of lithium batteries, it's been my impression that getting them to exactly 40% charge is not important.  What is important is to not store them fully charged.  Even 80% charge is much better than 100%.  So aim for anywhere near half-charged and don't worry too much about it.  They're going to die of old age after a decade or so no matter what you do.  Thankfully you can now get LiFePO4 batteries for a fraction of the price they used to fetch a few years back.  I bought some 6AH ones for $20 apiece, which is cheaper than lead-acid.  And yes they seem to have functional BMS.  And you can also now get higher capacities in the same volume, e.g., 10 AH usable capacity in the "brick" form factor that SLAs come in with about 7AH nominal capacity (more like 3AH usable when the SLAs are a couple of years old), and 16 AH in the wider form factor that used to offer 12AH.  No reason to buy SLAs any more, and I never get anywhere close to running out of juice even after two long flights (if I can't charge between them).  And if you don't have access to a charger designed for LFP, can use an SLA charger, it does work.  Maybe it won't bring it above 90% charge, but that may be more of an advantage than a disadvantage!

[Uli N]

Moshe - how does one determine the state of charge of a LiFePo4 battery for proper storage? Is there some kind of tester?

I bought a K2 charger with my batteries but it seems to bring them up to 100% and hold them there. I think I will change my strategy for the next set by not plugging them in to the charger until the night before I intend to go flying.

Uli

'AS'

[Chris Behm]

I use my R/C battery charger for all my charging needs. It has 4 ports and can charge a LiFe battery on one, and NiMH on another, Lead-acid on another, and LiPo on another, ALL AT THE SAME TIME! And it is AC/DC input. 

I would not recommend using a trickle charger, because they don't tell you what number of amps or milliamps you are putting back into the battery after use. Any decent charger should tell you that, or you are essentially "flying blindly". You can use this info to see if there is something wrong with the battery. Most trickle chargers won't tell you anything, and you will likely find that out on your own personal best flight, right Mr. Murphy??! If the battery suddenly only takes a fraction of what it should, you will notice that with an amp counting charger. Not with a dummy light.

So, back to your question, all the R/C battery chargers these days have a "storage" setting that generally either charges or discharges the battery depending on its current state of charge.

I recommend buying one of these types of chargers. There are MANY to choose from. If I recall correctly, the Storage voltage is 3.2V per cell in LiFe, so 12.8V for our 4S glider batteries.

Ultimate Guide to LiFePO4 Voltage Chart - Jackery

Kind Regards,

Chris Behm

304DD

[Chris Behm]

Yes, charging the battery before use is better than after, this is what I do. I don't bother taking the battery to Storage voltage and wouldn't worry about it if you fly say once a month or more often. Just use the 12.8v storage level if you have seasonal flying and won't be using the battery for a few months. My LiFe batteries in my RC gliders are my favorite to use, they are very unfussy about long periods of non-use.

Let me know directly if you want help picking a charger that counts the amps in and out. There are a LOT of them out there. Get a name brand with a long warranty period, and an "octopus" charge cord(s) and you can use it for many other charging needs.

Regards,

Chris 

304DD 

[Uli N]

Does anyone know, how the 'Status Indicators' on these 18V Lithium power-tool batteries work? I am talking about the row of LEDs, which light up when a button is being pushed and tell the user, if the battery is sufficiently charged for the job on hand. Could something like this be added to our batteries or even to the panel? Right now, I have no idea what the capacity of my batteries are.

Uli

'AS'

[John DeRosa]

Uli,

That is a great question for which I have no good answer.  Andy Blackburn is on this forum ... maybe he can weight in.

During the era of rechargeable batteries between lead-acid and up to ni-cd it was pretty easy to estimate how much capacity was left by using voltage.  The discharge profile of these batteries had a nice downward sloping voltage curve to use.  I had a single LED in my panel that went from green to yellow to red at some key voltage trigger points.  Easy!

Now, with lithium batteries the discharge profile is basically a flat horizontal line until it "falls off a cliff" and the BMS shuts down.  That same LED I spoke of goes from green directly to red in the blink of an eye.

Here is an interesting discussion about determining capacity remaining in lithium batteries using ultrasonics???  Sounds expensive.

https://www.reddit.com/r/explainlikeimfive/comments/wg4zl1/eli5_how_do_battery_charge_level_indicators_work/

Of course if you search on Amazon for "lithium battery meter" you get quite a few hits.  Do they just measure voltage or something else?  Expiring minds want to know https://www.amazon.com/s?k=lithium+battery+meter

I am going to reach out to an R/C aircraft guy who might know "an" answer (but probably not "the" answer)

- John (OHM)

[Mark Fisher (SpeedlightSystems)]

There are a plethora of battery management IC's that designers often refer to as a Fuel Tank. It basically  keeps a running balance of Current in (charging) and current out (In Use).
The Capacity of the battery is known in Amp Hours, so these LEDS give a coarse view on what Amphours is left in the battery as a percentage.  Five Leds might be 100 %. Two leds will be 40%.

Cheers

Mark

[Uli N]

Thanks for that info, Mark!

From what I gather, this system is something that lives with the battery like it does in case of the tool system batteries. Could this be somehow adapted to a panel-mounted system, which 'assumes' that every time it gets powered up, the battery is fully charged and has - in my case - 20Ah capacity? 

Uli

'AS' 

[David S]

Yes, there are many such devices on the market. Here’s an example:

https://www.amazon.com/ANKG-Precision-Measurement-Connectors-connected/dp/B07YF393ZH

https://www.amazon.com/Precision-Consumption-Performance-Backlight-Connector/dp/B0C39R8JQ5

Several members of our club have similar. It will do what you are asking for, but I have never seen one installed in a glider. People are using them primarily in their charging setups. On occasion they might temporarily connect it in the glider just to measure the power draw. 

Cheers,

   …david

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[Uli N]

Thanks, David - I had no idea these things exist! They seem inexpensive enough to give it a shot.

Uli

'AS'

[John DeRosa]

Uli,

I have one of these voltmeter/ammeter devices.  I typically use this to determine how much power each of my avionics devices are drawing, which is what David was speaking about.

But they are not for use in an instrument panel - too large unless disassembled.  Even then probably too large.  You could use one of these smaller devices that can be easily panel mounted.  About 2"x1" (48mmx29mm) => https://www.amazon.com/HiLetgo-Voltmeter-Ammeter-Digital-Meter/dp/B072BY4XZ7 

However, will this device help determine how much capacity you have left in your lithium battery's tank?  I would say not.  Because the performance curve of a lithium battery during discharge is so flat both the voltage and amperage readings will remain constant (until the very bitter end).  Not much help anticipating ahead of time when to switch batteries.

You want to know what is inside cordless drill batteries which somehow gives you metrics at 100%, 75%, 50%, etc.  

Mark mentions "...plethora of battery management IC's...".  Sounds great.  What are they?  Where do you buy them?  Is there a sample circuit available?  

Thanks,

John (OHM)

[Eric Greenwell]

One solution is to replace your battery with one (Power Sonic is one provider) that provides battery data (volts, amps, charge, etc) over Bluetooth to an app on your phone. Pilots that use them really like that feature. I don't have one in my glider, but I do like the one in my motorhome a Renogy LFP 12 volt, 200AH.

[Uli N]

All very good information - great discussion!

I went down that rabbit hole on Amazon and found this gizmo:

 https://www.amazon.com/dp/B0BGWY1ZDN/ref=sspa_dk_detail_3?pd_rd_i=B0BGWY1ZDN&pd_rd_w=Fup9I&content-id=amzn1.sym.0d1092dc-81bb-493f-8769-d5c802257e94&pf_rd_p=0d1092dc-81bb-493f-8769-d5c802257e94&pf_rd_r=JMFQGS9P7VZT5GXD447Y&pd_rd_wg=J2OnR&pd_rd_r=373e7d89-a3e2-4fdd-937b-272887c5dacd&s=hi&sp_csd=d2lkZ2V0TmFtZT1zcF9kZXRhaWwy&th=1

This seems to fit the bill since it does display Ah consumed plus the capacity remaining. In my case, I would not have to place that display on the panel but could put it near the batteries, where I can still see it.

Any comments from the electronics wizards are welcome before I order it. ;-)

Uli

'AS'

 

[Eric Greenwell]

I would use one with an internal shunt instead of the Hall-effect coil sensor, as Hall-effect sensors tend to drift. And I'd use one with a maximum current range close to my current usage in the glider; eg, my glider draws about 1.5 amps in flight, so I'd look for one with a maximum current of 5 amps, but even 10 amps would be much better suited than the 100 amp unit you have selected.

Eric

[Eric Greenwell]

The unit mentioned by David S (or one of the many similar ones) would be a better choice than the meter you selected, as long as you don't require a panel-mount shape. Another consideration: all these units draw current from the battery, so will eventually drain the battery, unless it's charged periodically, or the meter is disconnected. One unit claimed 7 ma draw, so .007amp x 24 hours = 0.168 AH, for a month = 5 AH. So, unplug the unit after flying (or charge and fly frequently), or put a switch in the circuit.

Eric

[Moshe Braner]

Yeah, I don't think it's worth worrying about the exact percent charge.  Just don't store the (lithium) battery "full".  So I too charge the day before I am planning to fly.  With my 12 AH battery and with my cockpit equipment a 6-hour on time consumes about 3AH, so it's still well above the ideal 40%, more like 70%, but that's OK.  I use an "imax B6" charger, that reports the AH going into the battery, and also allows me to do a test discharge which I do on my glider batteries once a year or so.  It is very versatile for many different types of batteries, and is inexpensive.

On Sunday, September 24, 2023 at 1:32:02 PM UTC-4 Chris Behm wrote:

[huang Elena]

Hi John and everyone,

Great discussion — and you’re absolutely right that storage practices can significantly affect lithium battery lifespan, especially over long winters.

For LiFePO4 batteries, the general rule is to avoid full charge during long-term storage. As you mentioned, keeping them around 40–60% state of charge (SOC) is ideal. Unlike SLA batteries, which prefer full charge to prevent sulfation, lithium chemistries age more slowly when partially charged.

Here are a few quick pointers based on our lab and field data at CM Batteries:

1. Storage Charge Level: Anywhere between 40%–60% SOC is fine — don’t stress about hitting exactly 50%. The key is simply not to store at 100%.

2. Temperature: Cooler environments (ideally 10–25 °C / 50–77 °F) slow down chemical aging. Just avoid freezing conditions or very high humidity.

3. Recharging Cycle: If stored longer than 6 months, check voltage and top up slightly if it drops below 13 V for a 12.8 V pack.

4. Chargers: SLA chargers can work with LiFePO4 packs that include a proper BMS, though they may not fully charge the pack (typically 90%), which is actually safer for storage.

We recently published a detailed guide that goes deeper into this topic — covering optimal voltage ranges, storage environments, and safety tips:
👉 Comprehensive Guide: How to Store LiFePO4 Batteries

So, in short — yes, discharging to around half is the right approach. It doesn’t need to be precise, and you don’t need any special “discharging assistant” unless you’re managing large packs for aviation or industrial applications.

Hope that helps clarify things, and happy (and safe) flying next season!

Best regards,
Elena Huang