LiFePO4 charging efficiency

[wookey]

I just measured my newish meanwell charger in 16S3P headway pack and was slightly surprised to find that fillin gup the pack after my commute used about 20%-25% less juice thancharging my old SLA pack. Is that expected - does LFP have much better charge efficiency than SLA? I also found that the meawell supplies are 90% efficient, which is very good indeed, given the price. The idle current isn't great though at ~8W - room for improvement there.

Findings:
714Wh put into pack. 790Wh taken out of the mains. Idle-current of meanwell supply at end-charge 7.8W

Total discharge measured by CA was somewhere in the 650-690Whreange (didn't actually write it down this time - will do so in the future). That suggests a charging efficiency of ~91-97%, which seems very good. Is that expected?

My SLA setup used to take about 1kWh from the socket. Up to 1.5kWh if I left it idling for a while. So that's at least a 20% imprvement. I don't know if some of that is due to the temp-controlled fan in the meanwell, vs. the permanent fan in my old charger - wouldn't expect it to make very much difference.

This seems to be an under-appreciated bonus of SLA->LFP upgrades.

 

[amberwolf]

Most likely the Li does take a lot less total power to fully charge vs SLA. I can't remember what the ratio was for my SLA, but on my NiMH it's about 1.5x in vs out.

 

[cell_man]

Isn't what you're seeing with the big difference in efficiency down to the peukerts effect on the SLA? I'm not saying the LiFePO4 isn't more efficient in this respect, it's just a possible explanation IMO. Regardless those are still pretty good figures on both the pack and charger. When you also consider the high efficiency of a typical motor and controller, I reckon we're onto a winner here

 

[dogman dan]

I would think it's more a more efficient discharge for the lifeop4. With sla's I used to get 10 miles out of 12 ah. With lifepo4, I get about 12-14 miles out of 12 ah. Different chargers matter too, It takes more AC watthours to charge my ping with a 5 amp charger than it takes with the 2 amp. So the 2 amp charger wastes a bit less of the power making heat.

 

[Lock]

Hey wookey... as you are comparing w/SLA ya might find this study of lead acid charging efficiency by Sandia Labs of interest:
http://photovoltaics.sandia.gov/docs/PDF/batpapsteve.pdf

Stuff like:
"These tests indicate that from zero SOC to 84% SOC the average overall battery charging efficiency is 91%, and that the incremental battery charging efficiency from 79% to 84% is only 55%."

Cheers
Lock

 

[recumbent]

..."These tests indicate that from zero SOC to 84% SOC the average overall battery charging efficiency is 91%, and that the incremental battery charging efficiency from 79% to 84% is only 55%."

Interesting data, I noticed that when charging my LIFEPO4 batteries, it seems the last 10% of battery space takes twice as long to charge than the first 90%. I'm using rough numbers here of course.

 

[dogman dan]

Apparently the last 10% is that way on many chemistries. lead acid for one, starts to boil away at the top of the charge and get hot. Nicads and Nimh get hot at the end of the charge. Any getting hot is gonna cost some efficiency. They seem to behave a bit like climbing a hill that gets steeper at the top.

 

[Lock]

Wookey's comments just got me thinking... as I've never thought of this efficiency over time/ % SOC business... found this for NiCad:
http://www.globalspec.com/reference/11104/121073/Chapter-7-8-Nickel-Cadmium-Cells-Charging-of-Cells

"Charging cells is quite efficient until approximately 80 percent charge has been accepted. Thereafter, charging efficiency falls off and, increasingly, gas evolution takes place."

Lookin' much better for NiCad versus what Sandia reported for lead acid. (100% eff to 60% SOC? Can't be!)

... but now I am curious about LiFePO4. Have seen no info so far showing charge eff as % SOC

Lock

 

[Lock]

I don't know if some of that is due to the temp-controlled fan in the meanwell, vs. the permanent fan in my old charger - wouldn't expect it to make very much difference.

Beyond the fans, might be that the SLA charger is older tech and the newer charger for the LiFePO4 is more eff in other ways too...
Lock

 

[wookey]

Lots of good info there - cheers. (haven't read the links yet).

My SLA charger uses the same SMPS chip as the meanwells but has a bit of extra logic to turn on 'full charge', and 'float charge' LEDs as required. So I'd expect it to be fairly similar and in fact if I could find wherre to tweak the output voltage it should charge my LFPs quite happily too.

I'm currently under-charging slightly as the meanwell max voltage is about 1V less than ideal. This may be helping efficiency if charge-efficiency tails off with LFP in the same way that is does for other chemistries.