I have just acquired a new appliance fitted with a lithium ion battery. The instructions say there is no requirement to discharge before charging (and indeed a deep discharge is harmful), no memory effect and no reason not to leave the unit on charge continuously.
OTOH I understood the life of a battery was defined by the number of charge cycles. Does this not mean that repeated charging will adversely affect the life of the battery?
Yes, but it isnt as black and white as that, particularly when charging when the device isnt mostly discharged.
Yes, but not leaving it charging when it is fully charged with a good charger.
Maybe the charging circuit will only charge the battery when its needed not all the time. Most batteries hate dumb chargers. I used to have an old dustbuster and the rechargables in that only lasted a couple of years as the charger seemed to be designed with price in mind rather than longevity of the cells. Brian
A rough rule of thumb is that it's based on capacity used, in other words two charge cycles from 50% used equals one cycle from empty. It's not actually a linear relationship, and there is also a strong correlation to termination voltage - ie stop charging when they reach 4.1V and you get a lot more cycles than the usual voltage of 4.2V per cell, which is a lot more than 4.25V, etc.
fig 5 here has that graph
If the charger/BMS is any good it should switch off once 100% SOC is reached, at which point it doesn't matter if it's left plugged in.
On that note, a lot of people overthink this. There's already something managing your battery, so it won't kill it if you use it like your phone - just plug it in and charge it whenever. It's only when you're trying to stretch the number of cycles that this kind of macromanagement matters.
Theo
Scott submitted this idea :
My Laptop, which is in very regular long daily use, is almost always on charge when it is in use. I only unplug when I switch it off, or briefly run on battery when I move it.
I very occasionally run it on battery, just to check the batteries capacity - it is near good as new.
You should never deep discharge these batteries, if you want a good life from them, always run on mains when possible.
The old Dustbusters® had NiCd batteries, and you probably kept them fully charged for convenience rather than discharging them.
If it is a decent quality battery, don't worry. Just enjoy it.
Think of a phone. Most are charged when needed - and every night. Batteries on many of those last the life of the phone.
Most decent appliances that use these will prevent deep discharge.
but a partial discharge only counts as a partial cycle, in effect the life of the battery is worn out by (number of cycles * depth of cycle) and it tips towards preferring frequent, small charges.
Dave Plowman (News) was thinking very hard :
I agree and my laptop does. I can only recall testing it once, when I first took delivery, but I think it warned at 30% and auto shut down at
20%, unless I forced it not to.
This is an example of overthinking it. The BMS has a low-end cutoff voltage, it might be somewhere between 2.5V and 3.0V per cell. It then coulomb-counts energy in and out of the battery. But we don't know what '0%' SOC actually means - does 2.5V mean 0% SOC or is it a higher voltage?
It's quite common for a BMS to say that the battery is empty, when in fact there's 10-20% of the rated capacity left in it - it doesn't let you recover that energy specifically to prevent deep-discharge.
TL;DR: just use the thing, don't try and micromanage the battery. Keeping it cool when (dis)charging is probably the most useful thing you can do.
Theo
Its not actually that simple
1/. Leaving it on charge will result in almost no charging being done once charged, so ins not especially harmful.2/. The longest life is an unused half charged battery. So if the job is finished leave it semi flat if its going to be out of use for a few months
3/. Yes cycles do wear the thing out but the evidence for part discharge/recharge is sketchy at best.,
And there is a big difference between an intelligent device like a phone and a stupid one like a cordless drill on what the device can do to maximise the life of the battery.
The other problem is with inductive chargers like with the latest mobile phones. They do tend to get rather worse battery life because of the way they charge.
When did you take delivery?
Many lithium drill batteries have a microcontroller inside that manages the battery - you need that inside the BMS to perform balancing. Batteries are not dumb any more. (Though there are better and worse BMSes, the power tool batteries I've seen look like the better end of the spectrum)
There's a limit to what it can do, of course - it can't tell you to drill slower - but then iPhones limiting performance for the sake of the battery didn't go down too well either.
I think the issue there is that they keep topping up the battery to be fully charged at its high-end voltage limit, which is more stressful on the battery than letting it gradually run down through use.
Theo
But something like a phone can be much more intelligent again, particularly with inductive charging.
(Though there are better and worse BMSes, the power tool
Only because some decided that Apple was fudging the warranty and not replacing the defective battery design with a fixed one.
Yes, but clearly a more sophisticated system like a phone could be much more intelligent about that and would be able to keep a record of how the phone is used so that it wouldn?t bother to always top up the battery every time that was possible and so do better on battery life.
Yes I have on in a little mp3 player, when I leave it doing noting I tend to leave it after I've used it for a while. Its interesting though that if you leave it plugged into a usb power source it does get slightly warm so I tend not to do this now. Brian
Have something to add? Share your thoughts — no account required.
Ask the community — no account required