Test results LiFePO4 glider batteries after 6 years

On Monday, February 20, 2017 at 6:32:36 AM UTC-8, glidergreg wrote:
On Sunday, January 29, 2017 at 1:41:07 PM UTC-6, SF wrote:
I started using K2 LiFeP04 batteries in my glider in 2010. I have had excellent results with these batteries. The electronics, especially the radio, enjoy staying above 12V, and I've never run out of power inflight with them.

This winter after taking some of the instruments out to have them calibrated & upgraded, I took one of these batteries with me to the shop to power everything up to see if all the magic smoke remained in the equipment after it was reinstalled. Imagine my surprise when nothing came on, and I found the battery at 10.5 volts.

This battery was charged after its last usage 1-1/2 months ago and left in a cabinet. Since these batteries are not supposed to self discharge that fast I decided to charge, and test them using two batteries in parallel on a 2.04A load. My six year old 9.6AH K2's were down to 5.4 Ah each, 10.8 Ah total.

I purchased two new Stark Power SP-12V9-EP LiFeP04 batteries and wired them in parallel on the same 2.04A load, and got 8.67 Ah each, 17.34 Ah total.

The tests were run until the voltage under load dropped to 11.5 V, not the 10.5V level where the battery management board turns off the output. There are probably some additional Ah's left in there after 11.5 V but not much, these batteries drop off pretty fast after 12.5V. The stark batteries are listed at 9Ah each.

The LiFePO4 batteries are advertised as having a cycle life of 2,000 cycles. Over the six years they were in use, I probably put less than 400 cycles on these. So obviously they also degrade with age and not just the number of charge/discharge cycles.

I still recommend these over the old sealed lead acid batteries. I also advocate annual testing to see if they still have the capacity you need, rather than waiting until they surprise you in flight.

SF

Richard just checked my 15 Amp and 12 Amp LifePo4 batteries purchased in 2013 from Bioenno, both are at 13.30 Volts waiting for the season to start. See you in Nephi

You have to check the fine print on their web page "battery allows for extraction of 90% of the rated capacity"

Richard

On Sunday, February 19, 2017 at 12:50:04 PM UTC-5, SF wrote:
I'm game, I'll repeat the charge discharge process a few times and report back.

SF

What cut off voltage are you using in your tests?
Also what discharge rate.
Thx
UH

On Wednesday, February 22, 2017 at 7:01:10 AM UTC-8, wrote:
On Sunday, February 19, 2017 at 12:50:04 PM UTC-5, SF wrote:
I'm game, I'll repeat the charge discharge process a few times and report back.

SF

What cut off voltage are you using in your tests?
Also what discharge rate.
Thx
UH

With the West Mountain Radio CBA IV I use:

1 or 2 aH and 10 volts for the cutoff on the CBA IV. If you set the cutoff at 8 volts you will see if the battery BMS works.

You can use up to 100 watt about 6 aH with this device after that you go above 100 watts and need an amplifier.


Richard
www.craggyaero.com

I used 2.5 amp as the discharge rate, was about 45 watts, and used a cutoff of 10 volts. Also using a CBA IV.

David

I was using 11.5 volts as the voltage to stop testing at. After 11.5 v not much left, and didn't want to over discharge for a test. Don't completely trust the bms board.

Sf

Are LiFePo4 batteries not rated using a standard 1C discharge rate like SLA batteries? So, for a 10ah battery the rated capacity would be achieved at a discharge rate of 1a. 2.5a seems on the high side for a glider, even one with a fair number of electronic goodies. My ship draws not too much over 1a with FSG71, Trig, Clearnav, Flarm, and CNv. No?

P3

On Wednesday, February 22, 2017 at 10:20:46 PM UTC-5, Papa3 wrote:
Are LiFePo4 batteries not rated using a standard 1C discharge rate like SLA batteries? So, for a 10ah battery the rated capacity would be achieved at a discharge rate of 1a. 2.5a seems on the high side for a glider, even one with a fair number of electronic goodies. My ship draws not too much over 1a with FSG71, Trig, Clearnav, Flarm, and CNv. No?

P3

I'm testing at very close to 1 amp using six 2 ohm resistors that actually measure 13 ohms total and plan on using 12 volts as end point. I doesn't seem like there is much useful capacity past that point. I'll report my data back.
UH

On Wednesday, February 22, 2017 at 9:20:46 PM UTC-6, Papa3 wrote:
Are LiFePo4 batteries not rated using a standard 1C discharge rate like SLA batteries? So, for a 10ah battery the rated capacity would be achieved at a discharge rate of 1a. 2.5a seems on the high side for a glider, even one with a fair number of electronic goodies. My ship draws not too much over 1a with FSG71, Trig, Clearnav, Flarm, and CNv. No?

P3

Eric, a 1C rating for charge and discharge would be 9 Amps for the batteries we are discussing. LiFePo4 will do that all day long but I doubt you would get 1,000+ cycles out of the battery. RC flight batteries are routinely sujected to 20-30C discharge rates (these are LiPo's) and they rarely make 100 cycles. They can also be aggressively charged at 3-4C. Wonder what that number would be for a Tesla battery on the quick charger. Compared to the older chemistries such as NiCd an NiMh, the new ones can sure take a beating..
Herb, J7

On Thursday, February 23, 2017 at 5:44:33 PM UTC+3, wrote:
On Wednesday, February 22, 2017 at 9:20:46 PM UTC-6, Papa3 wrote:
Are LiFePo4 batteries not rated using a standard 1C discharge rate like SLA batteries? So, for a 10ah battery the rated capacity would be achieved at a discharge rate of 1a. 2.5a seems on the high side for a glider, even one with a fair number of electronic goodies. My ship draws not too much over 1a with FSG71, Trig, Clearnav, Flarm, and CNv. No?

P3

Eric, a 1C rating for charge and discharge would be 9 Amps for the batteries we are discussing. LiFePo4 will do that all day long but I doubt you would get 1,000+ cycles out of the battery. RC flight batteries are routinely sujected to 20-30C discharge rates (these are LiPo's) and they rarely make 100 cycles. They can also be aggressively charged at 3-4C. Wonder what that number would be for a Tesla battery on the quick charger. Compared to the older chemistries such as NiCd an NiMh, the new ones can sure take a beating.

Tesla's own "Supercharger" stations supply up to 120 kW (depending on the size of the car battery and it's existing charge state), and take 20 minutes to charge to 50%, 40 minutes to charge to 80%, and 75 minutes to 100%.

So the initial twenty minutes of charging is at an average of 1.5C and next 20 minutes at an average of 0.9C. So, not very extreme on the batteries.

Normal driving uses around 320 - 340 Wh/mi (200 - 210 Wh/km), so at 120 km/h a 75 kWh battery is being discharged at 0.32C, or a top end 100 kWh Tesla at 0.24C.

What's a decimal among friends?