USB device charging question

[PNW_Steve]

Hello Everyone,

i ran across a discussion regarding leaving devices such as my cell phone, hotspot and tablets plugged in continuously. The consensus was that leaving such devices plugged in continuously can damage the device. They suggested using a 120v "lamp timer" to cycle the power to the charger.

What is the reality? Can leaving the devices plugged in all of the time damage them? If so, is there a "smart" charger that I could leave plugged in continuously?

Thanks.

[bob-n]

Leaving modern devices plugged in all of the time will not damage them, but it will reduce battery life by some unknown amount. LiIon batteries last the longest if they are kept at approximately 50% charge. Keeping it at full charge reduces lifetime. A realistic balance is to only charge to 80%.

I don't know of a smart cell phone charger that does that, but you can achieve that by using a simple windup timer. If the phone needs an hour to get to full charge, set the timer to 30 minutes, and you will be close enough to 80% charge.

By the way, the same thing goes for deep discharge. If you can avoid discharging below 20%, your battery should live longer.

It's hard to know how much battery life you gain by following these rules. Will it live for 4 years instead of 2 years? Will this add a few weeks to the life? It all depends on too many things that we don't know. For example, we don't know if your particular cell phone maker included some buffer in the charging algorithm to increase battery life or if they went all-out to get the longest talk-time.

Some older generation cell phones had overheating problems when left plugged in and operating. I don't know if all modern phones have solved this problem but I haven't heard of this problem in the past few years. You can tell by feeling the phone. If it is getting quite warm, don't do it. Heat will reduce battery life faster than anything else. Heat will also shorten display life and other things.

[SunEagle]

Leaving modern devices plugged in all of the time will not damage them, but it will reduce battery life by some unknown amount. LiIon batteries last the longest if they are kept at approximately 50% charge. Keeping it at full charge reduces lifetime. A realistic balance is to only charge to 80%.

I don't know of a smart cell phone charger that does that, but you can achieve that by using a simple windup timer. If the phone needs an hour to get to full charge, set the timer to 30 minutes, and you will be close enough to 80% charge.

By the way, the same thing goes for deep discharge. If you can avoid discharging below 20%, your battery should live longer.

It's hard to know how much battery life you gain by following these rules. Will it live for 4 years instead of 2 years? Will this add a few weeks to the life? It all depends on too many things that we don't know. For example, we don't know if your particular cell phone maker included some buffer in the charging algorithm to increase battery life or if they went all-out to get the longest talk-time.

Some older generation cell phones had overheating problems when left plugged in and operating. I don't know if all modern phones have solved this problem but I haven't heard of this problem in the past few years. You can tell by feeling the phone. If it is getting quite warm, don't do it. Heat will reduce battery life faster than anything else. Heat will also shorten display life and other things.

My motorolla e6 cell phone tends to get warm will going through a long usage period. It will also get warm during a fast charging cycle. So IMO they really haven't solved the battery temperature issue.

[Ampster]

I have started using an inductive charger that seems to turn off when my Samsung battery is full. It is too soon to tell if it will have any impact on battery life.

[PNW_Steve]

Any thoughts on the best solution for a battery powered device, USB charger that you want powered 24x7 from a 12 volt or 24 volt source?

[Mike90250]

There is a pile of them (12V) on Amazon and Ebay. And some low end charge controllers have USB 2.0 outlets on them But nothing I can recommend from experience, sorry.

[gbynum]

I've not looked at any of those in our cars to measure it, but WOULD EXPECT the better ones to incorporate switching regulators, poorer ones to use series pass regulators. At 12V, that would be about a 50% difference in wasted energy.

[PNW_Steve]

I've not looked at any of those in our cars to measure it, but WOULD EXPECT the better ones to incorporate switching regulators, poorer ones to use series pass regulators. At 12V, that would be about a 50% difference in wasted energy.

I had assumed that they were all switching regulators. I have a wealth of devices that use USB chargers. I'm going to have to make sure that my chargers are all switching supplies before we venture off grid.

Circling back to the original question: I have been told that leaving phones etc plugged in 24x7 can damage the device. Is that common enough that I should be concerned? I currently have a phone and a hot-spot their chargers 90+% of the time.

[bob-n]

Sorry, there is no clear answer. It all depends.

In the past, cell phones have been plugged in, charging, and operating, and have overheated and died. We may not have exact root cause failure analysis, but the presumption is that some phone was engineered for peak charging current and also for peak operating power, but not both at the same time.

We just don't know if that is true for your phone.

We do know that the risk is heat. If your phone will operate at a significant load (such as you talking on a cellular call) as well as charging at the same time, and not over-heating, you're good. Some amount of heat is normal, but if the back of the phone is uncomfortably hot, then the insides could be too hot for reliable operation.

People generally consider somewhere around 120F being the point of uncomfortable. Yes, this is a sweeping generalization based on non-scientific measurements. If you have a better way to evaluate this, I'd love to hear it.

Onto the less important question, a linear regulator that puts out 0.5A (USB-1 full load) and drops 7V (12V - 5V) will have to dissipate 3.5 watts. That's a lot of heat, and probably too much for a small, sealed plastic box. So it is extremely unlikely that any 12V USB chargers are linear regulators. With cars running at 14.5V, that same linear regulator charger would have to dissipate 4.75 watts, so even less likely. This goes even more for modern cell phone higher-current chargers which can be 1A, 2A or even more. So I really doubt that any 12V chargers are linear regulators. The cost of the box for that much power dissipation would exceed the cost of a switching regulator.

[PNW_Steve]

I have built, used and repaired many power supplies using linear regulators. I don't do that sort of work anymore and have not kept up. I had it in my head that linear regulators were dinosaurs.

Now I have another project.... Can anyone point me to switching USB charger that will run on 24 VDC? The ones that I have for a project are an unknown and I am trying to cut power use as much as I am able. Every watt counts.

[Ampster]

.....
Circling back to the original question: I have been told that leaving phones etc plugged in 24x7 can damage the device. Is that common enough that I should be concerned? I currently have a phone and a hot-spot their chargers 90+% of the time.

It probably depends on the phone. The only way to be sure is to measure Amps while charging, then when phone is at 100% and then again after unplugging phone. All I know is the light goes off on my inductive charger when battery is full.

[Mike90250]

You could likely find a DC-DC 90% + efficient converter and set the output to 5.1V and wire that to a USB hub and use that as a charging station.

[PNjunction]

The biggest problem that I run into is those that leave those devices ON while charging. This prolongs the time needed to fully charge, and the act of discharging at the same time as the unit is charging is not healthy for the battery.

This is seen often in the public-safety handheld radio industry, where the user doesn't read the fine print about the charger to "Turn the radio OFF" when charging. Same applies to cellphones, hotspots and tablets. A few months of this type of abuse results in pretty quick capacity loss.

To go the extra mile, don't hot-swap into a charge or discharge. Give it a few minutes of rest at least for the chemical reaction to slow down in one direction or the other.

Non-LFP batteries (the 3.7v nominal, charge to 4.2v types) don't like to be held at full charge for long periods of time. A few days maybe, but all the time? No. That only encourages secondary reactions, which contributes to longer charge cycle times.

TIME is the key element here. What do I do with a hotspot or tablet that I want to be usable, yet not sit at full charge all the time? Just operate it for a few minutes so it drops from 100% charge down to say 90%. That's good enough to be practical without getting too nerdy about it.

Likewise, don't let them sit around fully discharged, or near full discharge either. Moderation is key, just pump some charge into them to near half-way and call it a day. Check them every 2 months or so.

Heat aggravates these conditions, so adjust accordingly. But first, make sure you have the device OFF when charging!