A Simple Question About LiFePO4 Batteries

On 05/15/2018 01:35 PM, wrote:
I got my simple answer about the apparently non-existent larger size LiFePO4 battery.

Regarding the ensuing discussion, I didn't go the LiFePO4 route a few years ago even though the sentiment at the time seemed to "it's a no brainer; just do it." The current sentiment seems to be a bit more nuanced, which is good to know. Thx, all, for the candid inputs.

Chip Bearden


Right, some people are discovering that comparing preposterously
optimistic data sheet numbers of lithium batteries against actual
performance of SLA's isn't a valid comparison. Not everybody of course.

On Wednesday, May 16, 2018 at 4:16:35 PM UTC-7, kinsell wrote:
On 05/15/2018 01:35 PM, wrote:
I got my simple answer about the apparently non-existent larger size LiFePO4 battery.

Regarding the ensuing discussion, I didn't go the LiFePO4 route a few years ago even though the sentiment at the time seemed to "it's a no brainer; just do it." The current sentiment seems to be a bit more nuanced, which is good to know. Thx, all, for the candid inputs.

Chip Bearden


Right, some people are discovering that comparing preposterously
optimistic data sheet numbers of lithium batteries against actual
performance of SLA's isn't a valid comparison. Not everybody of course.

Some valid comparisons on this page;

http://www.craggyaero.com/lifepo_battery.htm

Richard
www.craggyaero.com

My observations were purely subjective:Â* With SLA batteries, I would run
out of power in 3-5 hours.Â* With LiFePO4 batteries, my volt meter still
showed good voltage after 5 hours.Â* No hard numbers, I know, but I got
the performance that I required.

On 5/16/2018 5:16 PM, kinsell wrote:
On 05/15/2018 01:35 PM, wrote:
I got my simple answer about the apparently non-existent larger size
LiFePO4 battery.

Regarding the ensuing discussion, I didn't go the LiFePO4 route a few
years ago even though the sentiment at the time seemed to "it's a no
brainer; just do it." The current sentiment seems to be a bit more
nuanced, which is good to know. Thx, all, for the candid inputs.

Chip Bearden


Right, some people are discovering that comparing preposterously
optimistic data sheet numbers of lithium batteries against actual
performance of SLA's isn't a valid comparison.Â* Not everybody of course.



--
Dan, 5J

I want to concur with Dan.
I fly out of Minden and we cruise between 15,000 and 17,999. It is COLD at that altitude.
My lead batteries would lose voltage as the temp dropped. After three hours at altitude they were lucky to put out 11 volts. Of course, when the batteries warmed up alter landing they had full voltage.
The LiFePO4 batteries do not seem to affected by cold soak. They work the whole flight at full voltage.

I never paid attention to the voltage vs temperature thing, but my
engineering training says that you are correct, Dr. Guy.Â* As to the full
voltage thing with the Li batteries, yes, they hold full voltage...Â*
Until they don't, and then they simply turn off, they don't run at
reduced power output as the SLAs do.

With the SLA batteries, I always used two batteries on a typical flight
in the Rockies, usually switching at around 3 hours.Â* With the LiFePO4,
I would easily run 5 hours and never have to switch, though, since I had
a separate tail battery, I kept a fully charged battery there.Â* And, as
I've said before on this group, after watching the batteries carefully
while charging on the concrete floor of my garage, I took the big step
after a year of leaving them installed in the glider with the proper
charger attached and plugged in.Â* Never a problem.Â* Your results may vary.

On 5/17/2018 6:29 AM, wrote:
I want to concur with Dan.
I fly out of Minden and we cruise between 15,000 and 17,999. It is COLD at that altitude.
My lead batteries would lose voltage as the temp dropped. After three hours at altitude they were lucky to put out 11 volts. Of course, when the batteries warmed up alter landing they had full voltage.
The LiFePO4 batteries do not seem to affected by cold soak. They work the whole flight at full voltage.

--
Dan, 5J

On Thursday, May 17, 2018 at 5:29:29 AM UTC-7, wrote:
I want to concur with Dan.
I fly out of Minden and we cruise between 15,000 and 17,999. It is COLD at that altitude.
My lead batteries would lose voltage as the temp dropped. After three hours at altitude they were lucky to put out 11 volts. Of course, when the batteries warmed up alter landing they had full voltage.
The LiFePO4 batteries do not seem to affected by cold soak. They work the whole flight at full voltage.

This LiFPO battery shows a significant a temperature dependence of voltage:

https://www.batteryspace.com/prod-specs/9055.pdf

Basically, all chemical reactions are very temperature dependent.

Tom

Interesting - the discharge voltage goes down with lower temperatures - isn't that the opposite of lead-acid? At the least, lead-acid needs higher CHARGING voltages when it's cold.

How well does a microAir radio, which is finicky at low SLA voltages, do with a "12V" LFP battery, which offers a slightly higher voltage on the discharge curve?

On Friday, May 18, 2018 at 8:05:47 AM UTC-7, wrote:
Interesting - the discharge voltage goes down with lower temperatures - isn't that the opposite of lead-acid? At the least, lead-acid needs higher CHARGING voltages when it's cold.

How well does a microAir radio, which is finicky at low SLA voltages, do with a "12V" LFP battery, which offers a slightly higher voltage on the discharge curve?

I have a MicroAir and it works great with a LIFEFO4 battery.

Richard
www.craggyaero.com

No, pulling power from any battery is a chemical reaction. Most reactions work better the warmer they are (to a point).
When I do discharge testing on large UPS batteries, the assumption is 77*F, there are charts to offset the rate/runtime based on battery temp.
Low temps give less amperage and shorter run times.

Part of why an old car battery has issues when it gets cold.

On Friday, May 18, 2018 at 11:05:47 AM UTC-4, wrote:
Interesting - the discharge voltage goes down with lower temperatures - isn't that the opposite of lead-acid? At the least, lead-acid needs higher CHARGING voltages when it's cold.

How well does a microAir radio, which is finicky at low SLA voltages, do with a "12V" LFP battery, which offers a slightly higher voltage on the discharge curve?

Those voltage curves apply to 0.5C discharge, i.e. a two hour discharge rate. The primary reason for the change in behavior at low temperature is increasing internal resistance, not decreasing open circuit voltage. Compare to similar curves for SLA batteries, if you can find them (they'll probably be at slower discharge rates).

Under real world glider use, LFP batteries perform better than SLA batteries in cold conditions. But don't recharge below 0 C.

T8