charge efficiency of lead acid final 10%

[Kurt]

I have been having a bit of a discussion with others on another forum regarding the efficiency of flooded led acid battery's during the final stages of charge 90% to 100% SOC.

The common rule of thumb is flooded lead acid battery's are around 85% efficient when charging from 0 - 100 SOC. Though what I have found is when charging from 90% soc to 100% soc they can be as low as 50% efficient or less at turning DC whrs in into stored energy.

I made the post on a off grid forum as most people including myself that run there homes from flooded led acid battery's tend to use a shallow DOD each day to give long cycle life . Personally I use just 10% DOD but 20% - 30% is also common. Why I mentioned it was because how we off grid people use our battery's. We are always recharging them in there most inefficient range for the battery to accept a charge be it 70% - 100% SOC or in my case 90% - 100 SOC.

I can see this when I measure DC kwh in VS Dc kwh out on my shunts charging from 90% soc - 100% I am just over 50% efficient.

The thing is they all think I'm nuts and don't seem to get the concept :lol:

So the question is... Charge efficiency of flooded led acid from 90% SOC - 100% SOC. From what I'm experiencing and from what reports I have read is only around 50% efficient or less. Anyone know the science behind why the efficiency isn't linear across the range and drops off dramatically as it reaches saturation .

Kurt

 

[amberwolf]

I did a very quick google on your last phrases and thread title, and found a few ppages, but this one might answer it:
http://www.theoildrum.com/node/9499
unfortunatley I got lost in the numbers very quickly; guess I'm too tired to keep up.

 

[Kurt]

Its a interesting read though he is not in the same category as me .

50% depth of discharge is not unusual for my system.

He would achieve much better overall battery recharging efficiency as he is recharging from a much lower SOC.

Its that last 20% where things go down hill

here is a link to a very good report I found that hits right on what I am getting at. but it doesn't say why the top part of the charge is so inefficient.It doesn't go into any detail about the science behind whats happening to cause it inefficiency at the top end of charging.
http://www.wind-sun.com/ForumVB/att...01d861e3e44280&attachmentid=1137&d=1273073737

Kurt

 

[heathyoung]

Its something I have wondered about as I am planning to run hybrid/island at the moment (solar grid tied, with a mid-sized UPS (3KVA) and a 6 Kwh bank of SLA) that runs the lights and fridges at night off stored power due to the absolutely abysmal FIT here in NSW.

Lithium would be great, but the bank would be exxy to replace - 72V bank = 24 Lifepo4, would only consider 90AH as a minimum.

Lithium has a much higher charge efficiency - and a good lifetime when run at a lower SOC - 70%, so you don't need the 10% rule for long life = smaller battery banks.

 

[Ypedal]

How much of a factor does the charger play I wonder...

 

[Kurt]

Well my charge controller is a midnight classic 150 and its listed as 98% efficient.

I just think it has something to do with how the lead acid chemistry works as its starts to fill up it accepts the charge rate (all be it at a lower and tapering rate in absorb than when on bulk) Yet starts to actually store less and less what its being fed and accepting as actual reusable (stored energy).

I just wasn't sure of the Physics behind it .

There is also additional losses due to charge voltage vs (say 60v for my system at absorb) Then your pulling the energy out at 50ish volts. There is a 10% loss just in the voltage difference.

Kurt.

 

[iperov]

150 years discussion about efficiency of lead acid -_-

 

[bigmoose]

This might sound strange if you have never done it... but just contact these gents at Sandia who wrote the paper. Sandia is funded by tax dollars, and you will find their scientists refreshingly approachable. Carry in a professional manner, explain what information you are after and why. I'll bet you will be surprised by the quality of the response you get. Call the Sandia switchboard and ask to be connected to these guys. I would do one at at time, so if the first guy doesn't respond you can call the other in a week or two.

btw his email is jwsteve (at) sandia (dot) gov However the Sandia employee locator at http://www.sandia.gov/cgi-bin/emplloc?ename=Stevens doesn't show him there anymore...Surprisingly it shows Garth at (505)844-1722, but Garth has retired from Sandia and is now part time at Raytheon Ktech Corp, 1300 Eubank SE Blvd, Albuquerque, NM 87123 Switchboard (505) 998-5830

Good luck! Perhaps this will help you a bit.

John W. Stevens and Garth P. Corey
Sandia National Laboratories, Photovoltaic System Applications Department
Sandia National Laboratories, Battery Analysis and Evaluation Department
PO Box 5800, MS 0753
Albuquerque, New Mexico 87185-0753

 

[Punx0r]

IIRC achieving 100% SOC entails a fair amount of gassing, which I guess is where the energy is being lost.

What about if we consider a conservative float charge level (perhaps 13-13.2V) instead, where there is minimal gassing?

 

[Kurt]

What about if we consider a conservative float charge level (perhaps 13-13.2V) instead, where there is minimal gassing?

Float really only waists about 1/2 amp around 25w for my system to maintain 55v.That's only 150wh on a 63,000whr battery. Lets say your discharging the top 10% 6.3kwh .So You need to return that 6.3kwh the following day as a charge . Then the 0.150kw wasted as float (assuming 6hrs float, typical number for my system) That's only a 0.023% loss due to maintaining float for 6hrs. Float isn't actually charging the battery just maintaining the battery at 100% and overcoming self discharge

Absorb stage where that the charger switches to after the initial bulk stage reaches a set voltage . In my case its 60v. That's where you get some gassing. Though the battery's naturally needs less and less amps to hold that 60v as it starts to fill. I then have a (end amp) set point that triggers float . This is when the battery is taking less that 16A to maintain 60v.

Kurt

 

[Punx0r]

Wow, so you're loosing ~6KWh during the bulk phase of charging?

Sorry, don't know what to suggest as that's quite surprising. Heat generation isn't something I'd every considered in a gentle charging lead acid battery, but that energy must be going somewhere...

 

[wineboyrider]

NIFE batteries.....? 8)

 

[Kurt]

Wow, so you're loosing ~6KWh during the bulk phase of charging?

No NO wrong I never said that!

I usualy cycle my pack down around 3kwh dc (consumption) overnight (that's around 5% of its capacity) and it takes roughly double that 6kwh dc to get the battery back to 100% SOC so around 50% efficiency.

From what I have read once a flooded lead acid battery reaches 80% SOC things start to get very inefficient the more you charge to fill it to 100%. If I was to discharge my battery to almost flat and then recharge it to 100% SOC overall my recharge efficiency would be around 85% efficient. As it starts out at over 90% efficient for most of the bulk stage of the charge but then drops off dramatically in the final 1/4 of charging to 100% giving a average of 80 - 85% efficiency over the full range.

The issue I have is I am always working the battery in the top 10% so always working in the inefficient section of the charge cycle.

I hope that makes it clear.

NIFE batteries.....? 8)

When you have $15,000 of battery's its not something you want to toss away until you have the full service life from them. I would go for a lifpo4 pack 1/3 of the size/ capacity 20kwh instead of 60kwh. cycle it down to 30% each night instead of 10%. But I have quite a few more years of life in the lead acid to make do with for now.

 

[leffex]

The common rule of thumb is flooded lead acid battery's are around 85% efficient when charging from 0 - 100 SOC. Though what I have found is when charging from 90% soc to 100% soc they can be as low as 50% efficient or less at turning DC whrs

In that case as you say top-charging is less effecient.

I have been tinkering of a home-grid system to drive a heater 1kw/h plus fridge and direct heating for water w/ needed. This is as you say is not an effecient way of charging especially if it continues like that. If its only 50% efficient in that usable range i will have hard to get those millions of Watts i need each year... 20,000 watts per day(1kw/h solars in Sweden...)

Just changing batteries could increase full system charging effeciency by up to 80% (50-to 80/90)

Test: additional lithium battery (used for electricity in the house) charged from the lead acid batteries, most effeciency and longlife(max charge 80%, lithium battery should be smaller than lead and drained as little as possible to like 50%)

System overview:
Solars - lead acid batteries - lithium battery smaller pack - housegrid

I dont know how to calculate if this is a good idea. anyone?

 

[Kurt]

If you need to cover 20kwh each day you will need a lot more than 1000w of PV .

I have 4000w of pv and can only expect 5 full sun hrs a day over the year for QLD Australia with (lots of sun). Some days a lot less sun and some days just a little more. Basically my system has been sized to cover 12kwhconsumption each day. Although I usually use 6kwh or less.

I would 100% advise lithium - lifep04 over led acid. Especially if your building a new system. The only reason I am using flooded led acid is the system was only 2 years old and was already install at my home when I purchased it. 400ah of lithium at 48v would run rings around 1330ah at 48v of lead.

The lithium would be about 1/3 less expensive than the industrial spec lead acid cells including BMS. So its a no simple choice.

kurt

 

[RobertStevenson]

The charge efficiency drops off at top state of charge because the conditions become more favorable to side reactions as the battery approaches top state of charge (http://battery.berkeley.edu/Unprotected/9_27_07_PerformanceMetrics2.pdf). In particular free hydrogen ions combining with free electrons to form hydrogen gas and the electrolysis of water to hydrogen and oxygen gas. Heat is not an appreciable form of energy loss (in fact electrolysis is endothermic).

However the development of advanced lead acid batteries (e.g. UltraBatteries with a carbon / lead oxide negative electrode) has greatly increased the efficiency of lead acids, not because they are intrinsically more efficient, but because they can operate more or less continuously in partial state of charge and don't require the refresh cycles of previous lead acids.
See e.g.:
http://www.ecoult.com/ or
Furukawa, 2013. Development of UltraBattery, Furukawa Review No. 43. http://www.furukawa.co.jp/review/fr043/fr43_02.pdf)

These batteries can achieve 90% + efficiency over a long period since they don't experience permanent sulfation and side reactions to anything like the degree seen in conventional VRLAs. Commercial versions are available at large scales (MW) and very close at the domestic scale. Lifetime costs are expected to be low compared with traditional lead acids - not sure how they compare in price to Lithium, but they win out on safety and recyclability.