I know it is advised not to keep Li-ion batteries fully charged. But if we consider most modern rechargeable devices with lithium inside, they are designed to work when permanently connected to the power and having batteries charged to full or near full level at all times.
What would be the medium/long term effects of leaving MODERN li-ion batteries at full charge all times? How long before my 18650s starts to show considerable capacity loss? Considering that lithium technology has changed a lot in the recent years.
I’m asking for a powerful 250 watts RMS bluetooth Hi-fi speaker I’m building, designed to work permanently connected to power and charged via CC/CV DC converter. 12x Sanyo GA will be wired in 4S3P and a balance/protection board will be used. It must also retain a 80% of charge for when it needs to go portable.
If we go by the experience in our phones which use the same battery chemistry and having it go through the process of almost daily charge cycles, it takes about 3 to 4 years before I notice a noticeable capacity loss, so perhaps a fully-charged pack should degrade a little longer.
LiIon chargers do usual not float charge the batteries. They charge to a limit, turn off and wait for the voltage to drop, then they charger up to the limit again.
In many laptops this limit is adjustable and will be reduced if the laptop is on power permanently.
I have a laptop which I used about four years on AC power with the batteries removed from it. I put in battery pack once a month to use with it to drop down about 30% and then fully charge it. Then I removed the battery again. After four years the battery pack was in 93% capacity of original.
Good question
Notebooks always connected to power seemed to lose cell capacity faster then devices going through discharge/charge cycles.
Idk how it is now but still making a habit of regularly use them on cells even when they are not used as portable device.
Some laptops has a “battery preserve” function which maintains the battery charged to 80% max, but mine even with that function off has a severely worn down battery pack, wouldn’t last longer than 15 minutes without power!
For this speaker project I decided to go with the simple way: 16x Samsung 30Q batteries wired 8S2P, a 29.6V 20A battery protection & balance module, and a cheap 8S li-ion charger from ebay. The battery pack will have two outputs via a 6-pin 3-position switch: one is to powering the speaker with the battery, and the other one to charge the battery (but not at the same time), and off.
This is a new hobby I recently got into (sadly) and have already spent about the same on flashlights for the last 12 months. I will publish a build thread on the off-topic subforum, it’s not only one but 5 separated portable speakers project I’m working on, 3 cheap ones and 2 high end ones. The question this thread concerns is about the largest one, which will be more or less stationary. I think I’m sightly obsessed…
Used T430 battery stats (9x 18650 cells, 3S3P) for reference:
Mfg: LGC
Mfg date: 08.2012
First used date: 12.2012
Cycle count: 368
Design Capacity: 93.6 Wh
Full charge capacity: 61.28 Wh
Temp: 36C //currently idle
All data taken from TP Power Manager off battery MCU. Notebook connected non stop to AC (when not off-grid working on battery), has worked for 13k hours. Battery set to recharge to 85% when drops below 40. Charged to 100 at least half thousand times.
To sum up: retained battery capacity about 65% working in this conditions after 3.5y.
The folks who design and make phones, tablets, and the like know there are people who stay ‘plugged in’ wherever they can, so they design the charging in a way which doesn’t let that become a big problem. They’re going to avoid safety problems (big liability costs) and they’re going to avoid early battery failure (warranty costs, loss of customer to competitor). They can add another cell to compensate for a designed-in less than full charge if that’s what they need to do. Their customers are not expected to know anything about batteries and charging; that thinking has to be engineered in for them… Their battery charging parameters are far different that what we have with our discrete cell chargers .
We can’t add another cell and we want to access the full capacity, so fully charging to 100% capacity is what we get. Plus we’re going to be using many different cells for different capacity and resistance and terminating voltages. If we misuse our cells and have problems that is our fault, and there are no designed-in protections against that. We are expected to know what we’re doing and provide the thinking part since they can’t do that at a reasonable cost. An entirely different design parameter applies to us .
If we do the thinking well, we use the right charger and cells to get maximum performance without inducing unsafe conditions or unduly shortening cell life. Float charging to a low percentage of cell capacity probably doesn’t hurt much, Float charging to a high percentage of capacity does shorten cell life and possibly worse. If you really need to have full capacity accessible at all times such as what float charging is designed to accomplish then you need to use a different battery technology to achieve that goal . What the effect will be with LiIon is going to depend a lot on the cells being considered so no one answer can suffice. And since float charging LiIon isn’t done much in our usage there’s not going to be much info available for you. LiPo and NiMh would be more appropriate compact technologies to consider, while SLA and AGM technologies would be more optimum if there’s space available for their larger size.
If you’re “playing” design engineer the you have to do the thinking- all of it- for yourself. You are now the one responsible for any issues emanating from your choices. If your design harms someone now you’re personally liable for that, so whatever you choose to do keep a large safety margin in your design.