LiFePO4 charge/discharge efficiency?

[rruff]

What I'm referring to is W-hr out vs W-hr in at typical camper rates... somewhere around C/5 or less.

I've seen articles claiming that LiFePO4 is very efficient in theory, like near 100%. But... I've been playing with some old cells and I'm only getting ~80%. Granted these are old and not pulling full capacity either. Plus there must be some losses due to heat dissipation.

I'm sure many of you have meters and are measuring input and output. What do you typically see?

Thanks!

 

[frater secessus]

At moderate charge rates, voltages, and SoCs I'd expect I'd expect more losses from the BMS itself, from passive balancing bleed-off, battery warmers, and wiring than charging losses in the cells themselves. IMO any actual inefficiencies would get lost in the noise.

It's common to program in 99% efficiency into monitors that require that info for their calculations. Now if someone is in love with the idea of charging/discharging at 1C and baking their pack at 14.6v all day every day I'd expect electrolyte breakdown, overworked balancers, and similar processes to dissipate some of the charging power as heat.

Anecdote from a logger: "The overall (charge/discharge) coulomb (current*time efficiency) of prismatic LFP batteries is around 99%. From my logged data over a period of around eighteen months I have 99.4%." -- source

I've been playing with some old cells and I'm only getting ~80%. Granted these are old and not pulling full capacity either.

Sounds like those are EOL in the formal sense, and not representative of LFP in general.

 

[rruff]

Sounds like those are EOL in the formal sense, and not representative of LFP in general.

Thanks, that's what I'm trying to figure out. Seems like they are not worth fooling with.

 

[wb8vyn]

What I'm referring to is W-hr out vs W-hr in at typical camper rates... somewhere around C/5 or less.

I've seen articles claiming that LiFePO4 is very efficient in theory, like near 100%. But... I've been playing with some old cells and I'm only getting ~80%. Granted these are old and not pulling full capacity either. Plus there must be some losses due to heat dissipation.

I'm sure many of you have meters and are measuring input and output. What do you typically see?

Thanks!

keep it simple
DC current using large welding cable and a simple amp and volt meter with a known load, say a toaster that can be forced into the on mode over time.
Mine is 1k (one thousand watts}.
P (power In watts} divide I (amps) equal R resistance). (I X E =P in watts) (E divide I = R resistance)
Anyway- a 1kw load for an hour nets 1 kilowatt hour. Estimate.
A 100 watt solar panel via 12 volt controller under ideal conditions will net about .5 to .6 kw which means under normal conditions (not ideal) two days to recharge a 12 volt flooded battery after toasting toast for two people.
From a practical stand point, considering that you don't want to over use your batteries,
thereby shortening their useful life, use 4 100 watt panels minimum with three or four flooded batteries.
After a 2 year test under normal living use, 4 100 watt panels with 3 deep cycle batteries in parallel and an mppt controller, in a northern state (Michigan) everything is still cooking along very well.
Original cost was about $850.00. via Amazon which included a 2500 watt pure sign wave inverter for 110 volt AC needs.
It's easy to divide the cost per year for off grid electric so far.
My plan is to replace the flooded batteries every 3 years although the warentee on the batteries is 10 years. ( I bought blems) At last check I can replace them for about $70.00 each at today prices.
Common sense tells me that I'll over use the batteries once in a while, so, I'm not going to over think and I am not an engineer by any stretch. What I have works well for me a single old man living in a camper trailer enjoying life on the cheap.
I have been thinking about adding 2 more panels but thinking makes my head hurt so now I am forced to take another nap.

 

[frater secessus]

keep it simple

OP is talking about charging efficiency of a specific chemistry, not overall system functionality. IOW, for a given Wh of charging how much ends up actually stored in the cell for later use? With lithium chemistries the answer is typically >99%.

This is considerably higher than lead chemistries. Rolls-Surrette, for example, says their AGM and FLA charging efficiencies are ~80% and gel 85%. I would have expected FLA to be somewhat lower but they're the experts.

With lead the lion's share of losses occurs in Absorption and Float. Bulk charging can be something like 95% efficient, which is why the carbon-foam Firefly batteries were all the rage before lithium prices came down. The FF could be run in the "Bulk charging zone" (my term, can't remember theirs) for weeks with only ocassional trips up to Absorption for maintenance. Very fast and very efficient charging. But Li does most of that better and FF went bankrupt.

It's easy to divide the cost per year for off grid electric so far.

My average cost over the past 1,491 days is $.59/kWh. Expensive compared to grid power, cheap out here on this hilltop in AZ.

I have been thinking about adding 2 more panels

IMO with lead-chemistry banks "too much solar is never enough".

 

[bullfrog]

Just to add some perspective, Ticaboo Utah generates their own power using diesel, probably fifty homes at a cost of last I checked $.58/kWh.

 

[KarlH]

@rruff Here's an article that might help answer your question:

https://batteryuniversity.com/article/bu-808c-coulombic-and-energy-efficiency-with-the-battery
It's complicated because both rate and state of charge can cause the efficiency to be higher or lower.

Powerstream also has a pretty good online tech library:

https://www.powerstream.com/tech.html