Thread Tools
Dec 14, 2017, 10:40 AM
67,000rpm
Thread OP
Discussion

Any way to restore old, lipos which have developed high internal resistance?


I have a pair of 6-year old 3s 3000mAh 35C li-po packs with less than 70 cycles on them. They've had a very gentle life, being balance-charged at 1C with an imax B6 and discharged in a 1:10 car with a 60A brushless ESC with cutoff set to 3.0V/cell. Average runtime was 20 minutes per charge so average discharge current would be in the region of 3C or so (possibly with peaks of up to 20C).

They've been stored, fully charged, for the last 2 years. The IR is now so high that it is difficult to charge and discharge at more than 1-2 amps without hitting the 4.2V/3V cutoffs respectively. An 0.6A discharge to 3V/cell yields a capacity of just 700mAh.

So, is there anything that can be done to improve their performance? Cycling them in a certain way, freezing them... all ideas are welcome .

P.S. I've already bought replacements (currently being shipped from China). I'm only interested in trying to revive them for the sake of science.

P.P.S. Out of curiosity, I also tested my 15-year old NiCd "racing packs" from my first ever RC car (we're talking late 90's model). It's made of six sub-C sized Nickel-Cadmium cells in series to give 7.2V, 1400mAh. Mine are branded Tamiya, with the cells themselves made in Japan by Sanyo. They've been stored, fully discharged, for the last 5 years or so. And after a quick charge/discharge cycle yesterday turns out they still had a capacity of at least 700mAh! Seems old technology ages much better than cheap lipos!
Sign up now
to remove ads between posts
Dec 14, 2017, 10:53 AM
Registered User
No there is no way to reduce the IR. Once it's high, the battery can still be used at a reduced C rating though.
Dec 14, 2017, 10:57 AM
Registered User
I've found that the most effective means of restoring battery performance is to open my wallet, get out my Amex or Visa card, place an order with HobbyKing (or other merchant), and then wait for the brown van to show up. Sounds like you're well on your way here as you have replacements on the way.

Honestly, your packs are done and there is no way to improve discharge performance. At least you've learned that storing at full charge will greatly accelerate degradation. When not in use, lipolys should be between 3.70-3.90 volts per cell. Also, they should only be charged immediately prior use for best longevity.
Dec 14, 2017, 11:21 AM
Registered User
And they should be stored in a cool place.
Dec 14, 2017, 11:52 AM
Space Coast USA
hoppy's Avatar
I like to store between 3.7-3.78V to decrease any potential for fire.
Dec 14, 2017, 12:46 PM
67,000rpm
Thread OP
Quote:
Originally Posted by mrforsyth
I've found that the most effective means of restoring battery performance is to open my wallet, get out my Amex or Visa card, place an order with HobbyKing (or other merchant), and then wait for the brown van to show up. Sounds like you're well on your way here as you have replacements on the way.

Honestly, your packs are done and there is no way to improve discharge performance. At least you've learned that storing at full charge will greatly accelerate degradation. When not in use, lipolys should be between 3.70-3.90 volts per cell. Also, they should only be charged immediately prior use for best longevity.
Indeed I have . Hope they actualy arrive!

Interestingly, with these cells, even when they were newer, the degradation was fast and drastic enough to notice it even after a few months months, no matter how they were stored.

I've heard that freezing the pack could help - does anyone have any experience with that?
Dec 14, 2017, 01:43 PM
RELAX. You'll live longer
785boats's Avatar
Not only has leaving your packs fully charged in storage aged your packs prematurely, but so has completely discharging them down to 0% capacity each time you use them.
The lvc set at 3.0v is way too low. You should try and leave about 20% capacity in the pack after a run.
If you want your new packs to last longer, set it higher.
This chart may help you see what you are doing to the packs.
Dec 14, 2017, 02:13 PM
The Mad Titan
DC1138's Avatar
Quote:
Originally Posted by hoppy
I like to store between 3.7-3.78V to decrease any potential for fire.
Yep, me too. I aim for somewhere below 3.80 when I'm done.
Dec 14, 2017, 02:40 PM
Registered User
If you want long life, set your max discharge to end up with a resting voltage after recovery of about 3.8Volts. You don't want to freeze them (although storing below zero deg C won't hurt them. I store mine in my beer fridge at about 40 degrees F. You should let them warm up a bit before charging. NEVER fly until the voltage sags down enough to prevent flying. I time my flights and I have adjusted my timer to get my packs at reasonable storage voltage when I land. That saves me the hassle of having to put them at storage when I get home and it only shortens my max flights by a minute or two.
Dec 14, 2017, 04:15 PM
Space Coast USA
hoppy's Avatar
3.8V/cell (resting) (about 60% discharged) - a very safe level giving a long cycle life.

3.7V /cell (resting) (80% SOD) is still a safe level giving more flight time at the cost of cycle life.
Dec 14, 2017, 04:31 PM
Registered User
Increasing termination above 3.70V resting will have very little effect on cycle life but decreasing charge voltage to 4.10V will have a pronounced effect on cycle life. Thus, if one is willing to give up some flight time to gain more usable cycles, it's best achieved by reducing charge voltage to 4.10V/cell (or less) rather than increasing post flight resting voltage above 3.70V/cell.
Dec 14, 2017, 11:29 PM
Space Coast USA
hoppy's Avatar
DoD vs Cycle life - I'm sure I've seen DoD vs cycle life graphs showing a relationship.

Here's few.
https://www.google.com/search?q=lith...h=712&dpr=1.25
Dec 14, 2017, 11:56 PM
Registered User
Similar research demonstrates that reducing DoD from 80% to 60% yields ~50% increase in cycle life whereas limiting charge voltage to ~90% (4.10V/cell) of available capacity will yield a 100% increase in cycle life. Thus, more benefit is gained by limiting on the 'top' end. Limiting on both ends will gain maximum cycle life benefit if one is willing to sacrifice some flight time.

I typically charge to 4.10-4.15V/cell and then terminate at ~3.75V/cell resting for most of my models. The only batteries that I charge past 4.15V and discharge to ~3.70V resting are my Turnigy Graphenes that I use in my racing quads as flight times are preciously short (~2-1/2 minutes max). Fortunately, they have been extremely durable and have never let me down.
Dec 15, 2017, 04:49 AM
67,000rpm
Thread OP
Quote:
Originally Posted by 785boats
Not only has leaving your packs fully charged in storage aged your packs prematurely, but so has completely discharging them down to 0% capacity each time you use them.
The lvc set at 3.0v is way too low. You should try and leave about 20% capacity in the pack after a run.
If you want your new packs to last longer, set it higher.
This chart may help you see what you are doing to the packs.
Agreed, but aren't those numbers for open-circuit voltage after resting? My ESC measures the voltage under load (I believe with some delay, eg. it stops if voltage is below lvc for ~2 seconds) - so a low LVC in the region of 3.0-3.4V is needed as otherwise it would trigger way too early. (Also consider that the load profile in an RC car is harder on batteries than the load on an aircraft since cars have very frequent surges of intense current during acceleration, followed by periods of low current during coasting or braking - whereas for flying you have relatively steady current demand to keep the thing in the air).
Dec 15, 2017, 05:07 AM
67,000rpm
Thread OP
Quote:
Originally Posted by mrforsyth
Similar research demonstrates that reducing DoD from 80% to 60% yields ~50% increase in cycle life whereas limiting charge voltage to ~90% (4.10V/cell) of available capacity will yield a 100% increase in cycle life. Thus, more benefit is gained by limiting on the 'top' end. Limiting on both ends will gain maximum cycle life benefit if one is willing to sacrifice some flight time.

I typically charge to 4.10-4.15V/cell and then terminate at ~3.75V/cell resting for most of my models. The only batteries that I charge past 4.15V and discharge to ~3.70V resting are my Turnigy Graphenes that I use in my racing quads as flight times are preciously short (~2-1/2 minutes max). Fortunately, they have been extremely durable and have never let me down.
I've seen that research before, and studied the graphs on Battery University. I arrived at the same conclusion as you that limiting the top end drastically improves cycle life (probably because it limits electrolyte oxidation and battery corrosion), and limiting depth of discharge also helps (probably because less DoD means less ion movement so less strain on electrodes from dimensional changes). However, I'm unclear about what happens at the bottom end - is it depth of discharge that harms cycle life, or is it the region at the bottom end?

In other words, if I keep the DoD constant at, say, 30%, would I get more life by oscillating the battery between 20%<-->50%, than I would by oscillating the same battery between 40%<-->70%? The DoD is the same in both cases, but the former uses the "bottom" region while the latter uses the "middle" region. Personally I think the former would give longer life because it avoids the longevity-sucking environment of high voltages (i.e. high charge levels), but I don't have any data to back up my claim.


Quick Reply
Message:

Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion Internal resistance in small lipos: why so high? zovirl Batteries and Chargers 4 Nov 12, 2016 01:28 AM
Discussion Internal resistance high kuya- Batteries and Chargers 7 Jul 08, 2016 03:45 PM
Discussion Proper way to measure internal resistance in a Lipo? Rich in ILM Batteries and Chargers 7 Nov 09, 2013 10:07 PM
Discussion If a lipo pack builds up a lot of internal resistance is there any way to fix it? randall1959 Batteries and Chargers 15 Aug 25, 2006 12:26 AM
Which high capacity sub-C have the lowest resistance ? oded mazor Batteries and Chargers 15 Jul 01, 2003 06:26 AM