charge efficiency of lead-acid batteries @ 80% SOC

mike95490

Here's a new report from SNL (Sandia National Laboratories - not a TV show:D )


excerpt

ABSTRACT
Knowledge of the charge efficiency of lead-acid batteries
near top-of-charge is important to the design of small
photovoltaic systems. In order to know how much energy
is required from the photovoltaic array in order to
accomplish the task of meeting load, including periodic
full battery charge, a detailed knowledge of the battery
charging efficiency as a function of state of charge is
required, particularly in the high state-of-charge regime,
as photovoltaic systems are typically designed to operate
in the upper 20 to 30% of battery state-of-charge. This
paper presents the results of a process for determining
battery charging efficiency near top-of-charge and
discusses the impact of these findings on the design of
small PV systems.

http://photovoltaics.sandia.gov/docs/PDF/batpapsteve.pdf


Figure 2. Incremental charge efficiency is dramatically less than overall charge efficiency at the higher states of charge.




Clearly, the use of assumed charge efficiencies in
the range of 80% will not result in a fully charged battery
when this battery is expected to operate in the upper 20%
of it’s state of charge. It is expected that these results will
hold up well for other deep-cycle flooded lead-antimony
batteries as well.

--

OUCH !!

BB.

Re: charge efficiency of lead-acid batteries @ 80% SOC

Every time I read one of these detailed battery test reports--I end up wondering how any of this stuff works at all...

Batteries are the worst form of equipment for energy storage, except for all those other forms that have been tried from time to time. :roll:

With a nod to:

Churchill's famous dictum: "Democracy is the worst form of government, except for all those other forms that have been tried from time to time." (from a House of Commons speech on Nov. 11, 1947)

-Bill

Vic

Re: charge efficiency of lead-acid batteries @ 80% SOC

Interesting, thanks Mike, for posting this. Vic

mike95490

Re: charge efficiency of lead-acid batteries @ 80% SOC

Wow, I'm now looking again at
LiFe and Nickel-Iron batteries

Nickel-Iron vendor:http://www.zappworks.com/nife.htm
While they spout lifetime battery, looks like 5-7 years is most one could get out of them (2500 cycles) and then they claim - just change the electrolyte.

http://www.zappworks.com
http://forum.solar-electric.com/showpost.php?p=53551&postcount=5 40% loss?
http://en.wikipedia.org/wiki/Nickel-iron_battery
http://en.wikipedia.org/wiki/Potassium_hydroxide


Lithium Iron Phosphate Needs cell balancers, BMS
http://en.wikipedia.org/wiki/Lifepo4
http://www.a123systems.com/

jonr

Re: charge efficiency of lead-acid batteries @ 80% SOC

Old but pertinent. So you want to charge to full charge sometimes (to avoid sulfation). But you also want to avoid doing it any more often than necessary (since it is so inefficient). So the options (at least in my case) are;

significantly oversize the panels and live with the inefficiency
use a generator once per week to give all the batteries a full charge
a rotating preferential treatment scheme (manual or automated) that would allow a single battery to get a full charge every n days
lithium batteries
find a way to shift almost all loads to sunlight hours

vtmaps

Re: charge efficiency of lead-acid batteries @ 80% SOC

mike95490 wrote: »

Here's a new report from SNL (Sandia National Laboratories - not a TV show:D )

Not so new.
I downloaded that Sandia document based on Crewzer's reference in this link:
http://forum.solar-electric.com/showthread.php?p=724#post724

A couple of years ago I started to care less about efficiency of power generation and storage.... Panels got so cheap that it's easy to just "add another panel", rather than fine tune a system for max production efficiency. Example: if you're losing 200 watts in a long combiner-to-controller cable, it might be cheaper to add another panel than to run a new heavier cable.

Unfortunately, batteries are more expensive, so efficiency in consumption is more important than ever.

--vtMaps

PNjunction

Re: charge efficiency of lead-acid batteries @ 80% SOC

Most consumers don't realize that charging is not a linear function since they have no idea about how bulk vs absorb works.

For instance, many reviews of various cheap automotive chargers with digital displays (NOT my favorite, nor used here!) complain about how the charger is always stuck at 80% and takes such a long time to reach 100%. They were expecting the same incremental charge rate during bulk to apply during absorb, and return the charger thinking it is faulty.

Even PowerSonic agm manuals show that the typical charge for agm's is a 60/40 (bulk/absorb) amount of time , ie Amperage consumed / charge current * 1.78 gives a rough estimate of recharge time, which ALSO takes into account the need to return at least 110-120% of what was consumed.

Of course one solution to this issue is $$$ lithium batteries in all their various sub-chemistries, which charge right up to about 90-95% bulk, and an amazingly fast absorb for that last 5-10%, if absolutely needed at all. (subject covered well elsewhere here.

South Africa

Re: charge efficiency of lead-acid batteries @ 80% SOC

I have a question.

Assuming Victron, Tristar, Outback (in other words not the cheapies) MPPT on one hand.
On the other hand, chargers that need AC and not the cheap automotive chargers with digital displays.

In a non-off grid scenario, in other words you have AC and solar, which charger will charge batteries the best over time: Solar or the AC types?

vtmaps

Re: charge efficiency of lead-acid batteries @ 80% SOC

South Africa wrote: »

In a non-off grid scenario, in other words you have AC and solar, which charger will charge batteries the best over time: Solar or the AC types?

Most battery manufacturers recommend a charging profile that cannot be achieved by solar chargers. Therefore the RE industry has pretty much standardized on the IUoU charging profile. One very important feature of the IUoU profile is that multiple chargers (solar, wind, generator, etc.) can be used together on a single battery bank and they will all regulate if their absorb setpoints are the same.

Here's a good explanation of charging profiles by stephendv:

stephendv wrote: »

The differences are because of different charging profiles - different applications use different charges, there's a standard DIN naming convention for these, like IUoU, IUIa, WoWa, etc.
The one used by all renewable energy chargers that I know of is IUoU:
I = Constant current (Bulk)
Uo = Constant voltage with a timeout (Absorb)
U = Constant voltage (float)

The profile posted by Trojan is similar to what I've seen for some forklift batteries and is the IUIa profile:
I = Constant current
U = Constant voltage
Ia = Constant current until the final voltage is reached, then charging is stopped.

As far as I can make out, the choice of the charging profile has more to do with how the battery is used, rather than the type of battery. In my view, this is why Trojan and forklift batteries reference the IUIa profile: because that's the charging profile used for traction batteries. Battery manufacturers who focus on producing a "renewable energy" battery tend to reference the IUoU profile.
Similarly, when looking at the manuals for forklift batteries not a single one lists the bulk, absorb, float values, because no forklift charger uses that profile. They tend to use the WOWa profile which is a "taper" profile, basically a long slow decreasing current charge curve. Again, that appears to be because that's the best way to charge a forklift battery which is at 20% SoC at the end of it's shift, and has 10 hours available before the start of the next shift. And it doesn't mean that that's the only way to charge a traction battery. If the batt is used in an RE system (i.e. relatively shallow discharges) - then an IUoU profile seems fine.

To answer your question, the inverter/chargers (magnum, victron, outback, xantrex, etc) are excellent chargers, but limited to using the IUoU profile.

There are plenty of other high quality battery chargers available that do NOT use the IUoU profile. These types of chargers expect to be the only thing connected to the battery... they usually do NOT work well with loads or other charging sources. The reason is that they need to control the current (not the voltage) to the battery. They assume that whatever current they are producing is equal to the battery current. If there are any other currents to or from the battery, these chargers get confused.

--vtMaps

South Africa

Re: charge efficiency of lead-acid batteries @ 80% SOC

Thanks vtmaps, it is as I suspected, batteries prefer constant charging and sunlight is not a constant. BUT, I must admit that solar chargers are doing a pretty darn fine job with all the complications they need to accomodate.

As a experiment, I recently added another load to my system. It is a good quality AC charger dedicated to charge lead acid batteries that are solely used for lights at night, instead of waiting for the solar charge controller to first charge them before I can switch on the load for the remainder of the day. Load is between 600w to 800w per hour.

So I took cheap 2nd hand 36ah (24v) lead acid batteries that gives me +-5-10min under load to cater for intermittent clouds, connected them to a controller / inverter. The faster they get to 100% SOC, the faster I can switch on the AC loads. YES, the batteries are WAY undersized for the system (930w array) and are going to be destroyed but then, they where on their way to be recycled any way. :-)

System is geared to use +-2993 hours of sunshine per year. For the hours with no sun, their is the grid, which is cheaper than batteries. Grid tie is still illegal here.

To accommodate for the additional losses due to AC charging, like you said: Panels got so cheap that it's easy to just "add another panel".

stephendv

Re: charge efficiency of lead-acid batteries @ 80% SOC

mike95490 wrote: »

...
particularly in the high state-of-charge regime,
as photovoltaic systems are typically designed to operate
in the upper 20 to 30% of battery state-of-charge. This
paper presents the results of a process for determining
battery charging efficiency near top-of-charge and
discusses the impact of these findings on the design of
small PV systems.

Maybe I'm missing something, but I don't see how this has any practical implications because most systems have more than enough PV to make up for the low efficiency during high SoC charging.

Common PV sizing calcs say you need enough PV to charge the battery from 50% SoC to 90% in 1 day assuming a fixed 80% charging efficiency.
So that array should have no problem at all charging the battery from 80%-100%, even if the charge efficiency is only 40%.

jonr

Re: charge efficiency of lead-acid batteries @ 80% SOC

vtmaps wrote: »

One very important feature of the IUoU profile is that multiple chargers (solar, wind, generator, etc.) can be used together on a single battery bank and they will all regulate if their absorb setpoints are the same.

If charger 1 bulk charges at C/10 and charger 2 bulk charges at C/10 and my battery is not supposed to be bulk charged at anything over C/10, they won't regulate properly.

There are plenty of other high quality battery chargers available that do NOT use the IUoU profile. These types of chargers expect to be the only thing connected to the battery... they usually do NOT work well with loads or other charging sources. The reason is that they need to control the current (not the voltage) to the battery. They assume that whatever current they are producing is equal to the battery current. If there are any other currents to or from the battery, these chargers get confused.

An external current sensor fixes this and the bulk problem. More chargers should use one.

Blackcherry04

Re: charge efficiency of lead-acid batteries @ 80% SOC

stephendv wrote: »

So that array should have no problem at all charging the battery from 80%-100%, even if the charge efficiency is only 40%.

I see that more a issue, of Time ( Sun Hours ) and the Batteries ability to absorb the available current. The plate alloy, type battery ( AGM-FLA ) and they amount of gassing and the temperature of the battery are all factors. Having 200 amps available, but if you can only transfer 10 amps, you don't have much.

I have this issue right now with a bank of Surrettes, They exit absorb at > 2 % with a SG level of 1.220. The only problem is that it takes 20 hrs to raise them to 100% because they will only accept 8 amps for most of that time. The only thing you can do is live with a bank of less than 100% capacity in the 50-80% range and EQ them once a month and pray.

jonr

Re: charge efficiency of lead-acid batteries @ 80% SOC

Blackcherry04 wrote: »

The only thing you can do is live with a bank of less than 100% capacity in the 50-80% range and EQ them once a month and pray.

Or provide a rotating preferential charge to one battery. Ie, the other batteries can supply the 20 hours of power needed to fully charge one battery.

Blackcherry04

Re: charge efficiency of lead-acid batteries @ 80% SOC

jonr wrote: »

Or provide a rotating preferential charge to one battery. Ie, the other batteries can supply the 20 hours of power needed to fully charge one battery.

Call me dumb, but I don't see where that gives you any advantage in a multiple battery string. Seems like one battery faces the same limits, it can only accept as much current as it will without raising the voltage. Even raising the voltage has a limit somewhere.

I have tried charging two strings separately ( two sources ) and then combining them for discharge, I didn't think I gained much as the lower bank dropped faster and brought the other one to it's level.

jonr

Re: charge efficiency of lead-acid batteries @ 80% SOC

It gets each battery fully charged (20 hours) every 4 or 8 days. That's a whole lot better for the batteries than never getting fully charged. Or even monthly full charges.

Blackcherry04

Re: charge efficiency of lead-acid batteries @ 80% SOC

jonr wrote: »

It gets each battery fully charged (20 hours) every 4 or 8 days. That's a whole lot better for the batteries than never getting fully charged. Or even monthly full charges.

Give it a shot, maybe your onto something. Post back and let us know how it works. I'v tried everything else I can think of.

Cariboocoot

Re: charge efficiency of lead-acid batteries @ 80% SOC

I'm so glad I live in Canada where every system I get my hands on comes out working and no one has to worry about these theoretical details found only in laboratories and other venues of the unlucky.

BillBlake

Re: charge efficiency of lead-acid batteries @ 80% SOC

Supposedly a smaller 'Slave Bank' of the NEW Generation Nickel Iron Batteries coming

early this July, 2014 should be able to fix everything -

Now and for ALL TIME.

Doesn't matter how small the charging array is as long as they get charged sooner or Later.
Never any sulfation or any other problems.

They can baby and pamper your Conventional Battery Bank as you run the Slave Bank continuously
80% Dead. This is what they expect and they 'Kind of Likes it' :-)

You would just use an Inverter and an AC Charger every X number of days
same as if you had a wall plug.

These new Next Generation Ni-Fe Batteries (made right here in America)
make me think of the old Song

'I'm So Excited'
by
The Pointer Sisters.

They only promise you a 20 Year Float Life

but if you punish dem and run them 80% DOD
(most every night)
or every cycle

then you get 24.65 Years out of them.

But wait.

Their new Dealer raised it to 30.1 Years

running them 80% Dead.

When I said something it went down to 27.397 Years the next day
at 80% DOD.

This Company has invested years and millions.
They have not one but 3 Presidents and a Lot of other Officers.

It sounds like the Biggest Game
ever talked
in Storage Batteries.

At least to Old Bill's ears. I have talked with a number of them.

I dare not say anymore at this point until I hear back as to what happened to
my Last Post from 5/24/14 where I mentioned how to find out more.

So far I'm the only person that I know of really looking into this modern marvel
and alive to talk about it :-)

After all they ARE taking money and orders for them right now.
Not next year.


Bill Blake

Blackcherry04 wrote: »

Call me dumb, but I don't see where that gives you any advantage in a multiple battery string. Seems like one battery faces the same limits, it can only accept as much current as it will without raising the voltage. Even raising the voltage has a limit somewhere.

I have tried charging two strings separately ( two sources ) and then combining them for discharge, I didn't think I gained much as the lower bank dropped faster and brought the other one to it's level.

Cariboocoot

Re: charge efficiency of lead-acid batteries @ 80% SOC

New Battery Technology:

Let us know when it's real and retail.

"Amazing breakthroughs" occur every day with wonderful promises of solving all mankind's woes.

Then they never show up in the stores due to some "unforeseen" fatal flaw such as not being economically viable, not actually working, or just plain being another scam.

All you have to do is look at how NiFe batteries have been around as long as lead-acid, but almost no one uses NiFe because they aren't very practical really. No matter what promises are made. Lithium batteries have come with big promises too, but so far remain expensive and impractical as well.

I think we may need a category of "Strictly Theoretical" to put all these grand new technology threads in to so that people who actually want a working system that doesn't require 8 years of university education to run it can easily ignore the grandiose designs for the financially independent hobbyist.

zoneblue

Re: charge efficiency of lead-acid batteries @ 80% SOC

Sounds like Californias, shale oil "miracle":
http://www.businessinsider.com.au/eia-monterey-shale-2014-5

They suddenly realise its ony 5% as good as they thought.

Cariboocoot wrote: »

Let us know when it's real and retail.

"Amazing breakthroughs" occur every day with wonderful promises of solving all mankind's woes.
...
I think we may need a category of "Strictly Theoretical" to put all these grand new technology threads in to so that people who actually want a working system that doesn't require 8 years of university education to run it can easily ignore the grandiose designs for the financially independent hobbyist.

westbranch

Re: charge efficiency of lead-acid batteries @ 80% SOC

And this one. The graph shows the decline seen in some Frack'd wells. Not as rosy as some hope.

http://climatecrocks.com/2014/01/07/why-fracking-is-not-all-that/

jonr

Re: charge efficiency of lead-acid batteries @ 80% SOC

NiFe batteries work OK (but not as great as the hype) but are expensive and don't seem to have much potential to come down. Lithium batteries are close to equal cost to lead acid and show a nice downward price trend. So I expect Lithium batteries to become quite popular in solar applications over the next five years.

zoneblue

Re: charge efficiency of lead-acid batteries @ 80% SOC

In NZ we used to produce 75% of our electricity from hydro sources. Thats now dropped to 55% or lower. And projected to drop further. But is the govt thinking about how solar could meet the shortfall instead of gas? Even though PV and hydro are a match made in heaven, even though NZ's solar resource is significantly better than germanys?

http://www.mfe.govt.nz/publications/climate/policy-review-05/html/page4-1-1.html (pie charts half way down)

In a word, no. They dont get it.