When using multiple batteries, how close should they match in capacity? I know not to use old with new batteries, because of the difference it causes.
I tested some Tenergy 18650s and got 2475 to 2658 in reserve capacity, tho voltage was all the same.
At what point should they not be used together?
Just depends on the application …
The more current you want , the better balanced they need to be …
So if you plan on buying 20A or 30A 18650 and then running them at 6 amps … You might want to be sure they are reasonably well balanced .
Some of the high discharge capable 18650 are now being used for RC applications , and current can easily hit 20 amp or more …
Things to watch for …
Final charge ( voltage ) termination
Resting voltage
Discharge Matching :
This is very interesting , ( Discharge matching ) As the batteries may be well matched at low discharge , a variance may develop at higher discharge rates …
Also , variance may creep in at a certain voltage level …
So just because they are matched well early , you need to watch out for what happens at lower voltages …
( I am going to build a balance charge cradle 2s ( 2 cells ) to test 2 batteries at the same time ( Pair ) …
I would suggest if you plan on running 3 batteries , you build a 3s cradle for testing purposes … And so forth …
This makes testing so much easier … And removes a lot of guess work .
There will be report / post when I have all the parts for the 2s cradle ( 2 x 18650 ) …
With all the super high performance lights , there should perhaps be better battery testing …
Thanks for the reply. I am using the BT-C3100 charger, and my multiple lights are the TN32-TN30,TK75 and a few others, but it was the TK75 that concerned me as I have a couple extensions I may use at some point. My main batteries (stored unused) are 3400 protected Pansaonics. Have about 50 of them…. not sure using 12 batteries at one time is a good thing.
I didn’t know how much variance there could be before they would be considered unmatched.
The capacity is not the ideal indicator how well they match
the capacity and internal resistance has to be close
you cant mix a high drain 3000mAh cell with a low drain 3000mAh
Hmmmm .
If the batteries are within 0.1 volt depleted , consider them well matched …
Anything over 0.3v variance is not fantastic …
At issue is rested voltage and reverse charging …
Lets say you drain your 18650 @ 3 Amp continuous …
What may happen is that one battery might be @ ( some point ) 3.8v whilst the other is say 3.5v … ( This would be under a 3 amp load or some such )
When you turn the light off , the batteries will normalize ( resting voltage )
Also what may occur is some reverse charging , one battery charges the other …
This will happen in parallel … And it can happen in series …
Now a lot of flame an smoke used to happen when people mixed non rechargeable CR123 batteries … ( Full with empty cells )
How many people used a Multi meter when reloading their Surefire ? ( Not many )
Another issue is battery quality …
The higher the current drain , the better the batteries need to be …
Because what can happen is a battery can fail under load …
I have tested my fair share of batteries , and with some cheaper batteries I found they could not maintain a 3 amp discharge …
What I saw on the charger was a rapid drop in voltage until the voltage hit the cut of limit ( 3v ) …
When this happens it happens very quickly …
In a series , one battery failing could result in revers charging and a smoke moment …
It’s all a little like owning a car … That 100HP family sedan is easy to drive and relatively safe .
That 1000HP monster in the garage takes skill and dedication to drive safely .
Story time :
When I was in high school , I remember this crash . ( Porsche 911 Turbo )
Apparently a young woman won the Lottery … ( Happens to some )
So she went and got herself a 911 Turbo …
Those early 911 Turbo cars were notorious for killing the unskilled .
Anyhow , she picked up the car and whilst driving home lost control ( driving in a straight line ) and hit a power pole killing her instantly …
I know because riding home from school that afternoon I saw the wrecked 911 …
And then of course it made the news ( I used to watch the news back then ) .
No … It would have to be extreme variance …
a imbalanced pack will put strain on the worst battery / cell … And this cell / battery will be the one to fail first .
Depending on the current , it could take 5 years of use to get there .
This stuff happens when the battery fails ( essentially )
Reverse charging can happen to batteries in series …
So what apparently occurs is a over discharged battery can be damaged and reverse polarity creating a internal short ( something like that ) … They call it reverse charging …
Batteries in series wont reverse charge (? Hmmmmm ? ) , but if there is an imbalance ,
the bad battery may end up being over discharged and damaged possibly becoming dangerous to recharge …
Just imagine a triple 18650 in parallel …
Say its a hot rod , with 9 x XM-L emitters capable of pulling 10 amps or more …
Thats 3.3 amp per battery …
You just spend $250 on a flashlight and you cheap out on the batteries …
In use one battery fails and gets sucked bone dry ( reaches very low Voltage ) … As a result is damaged …
It does not flame out in use , but being put in a charger , the damaged battery shorts internally and flames out burning down your house . ( Here in lies a very real and present danger )
Can a li-ion in parallel create a short ? ( Most people seem to think no ) But why not ? ( A short is a short ) So if polarity is reversed
Now that 2x . 3x . 4x 18650 in series or what ever can also suffer from damaged batteries …
But the added danger is reverse charging ( which is more or less a short ) and the consequences of that …
In general , series = one big battery
Parallel is a bunch of batteries operating independently of each other …
So in series a bad battery would be seriously stressed and possibly damaged
Whilst in parallel each battery would operate independently , and a bad battery would be far less stressed by what the other batteries are doing as it would do its own thing
( supply what current it could ) …
Obviously take things far enough and its bad ju ju …
Have I explained this properly ?
In a nutshell … a Li-ion can be damaged …
It can reverse polarity or suffer a internal short as a result of being damaged . ( or both )
Where this stuff happens a lot is RC battery packs …
Very high discharge rates … ( up to 100 AMP in some applications )
And most batteries are not ratted for that discharge …
Unfortunately I am all over the place explaining this … So probably hard to follow … ( My bad )
Butt RC is where a lot of flaming is occurring …
1) Badly balanced packs
2) Very high discharge rates
3) Improper recharging
4) Poor battery maintenance / checks
Depending on the current draw , you either need to worry or not !
Anything over 2 Amp , I would want to make sure the batteries are healthy .
A great way to test batteries is as a pack rather than independently .
So if you run 3 x 18650 ( 3s ) , I would get a battery cradle and set it up for balance charging . ( 3s in this case )
Then with the batteries fully charged I would do a discharge test at the current you plan to use …
So if you are going to run 3 18650 or 12.6v fully charged …
So set your charger to the current and voltage for the pack and discharge …
Most charger will allow you to view each individual cell voltage through the discharge …
This makes it easy to see any variance in voltage … As well you should be able to see at what point if any a battery begins to drop voltage faster . ( usually means less capacity )
I have a cheap 2s for my buggy ( RC )
First thing I did was balance charge it …
Then run a discharge test …
I charged the battery @ 0.5 amp till it was fully charged .
I then discharged it to check capacity as well as how well the cells were matched …
Discharging to 6v @ 0.5 amp one cell was 3.5v and the other cell was 2.5v … ( not a good showing )
Recharged the battery @ 0.5 amp till its fully charged ( Balance charged )
Then discharged once again @ 0.5 amp to see at what point the imbalance kicked in
All was good till about 3.66v ( individual cell voltage )
By the time one cell was 3.63v the other was 3.53v … ( 1120 mAh discharged )
I stopped the discharge right there and then , and went back to charging the battery @ 0.5 amp …
I did not cycle the battery again …
But rather went to bed , and then threw the battery in my WLtoys 12428 Crawler buggy …
Cells were about 3.222v and 3.183v … After use ( some what rested )
From another forum ( RC )
I use a battery voltage monitor that has an alarm and monitors the individual cells in the pack …
I set the alarm for 3.5volt … so it also allows me to monitor the battery whilst in use … + Not over discharge it since it is a cheap pack ( and not that great )
I always balance charge at low current ( 0.5 amp )
Anyhow , hope I have helped more than confused …
There really is quite a bit to know if you want to …
Yep , I was explaining things wrong …
Too tired and in a hurry to get to bed ( its like 1.00 am ) , I should be asleep …