[RE-wrenches] Higher LVR to force full re-charge in small OGPV systems
Eric Youngren
ericyoungren at gmail.com
Fri Jan 3 21:34:12 PST 2014
Thanks Jay, That's a similar line of thinking to an off-line reply that
Dan sent me. I will paste in some of his advice below and look at an
example.
While I do still think its a great idea to set the LVR high enough so that
the batteries are sure to get a full charge after every LVD situation, I
see now that the LVD is probably the more important setpoint to consider if
the primary goal is to give the batteries the longest life possible.
In a nutshell, to get the longest life we need to limit the maximum regular
cycling to 15-20% DOD. The big question is, can we do that with an LVD
setpoint only, and what should that be.. ? And what are the pros and cons
and potential pitfalls of too high or too low of an LVD? And what about as
the battery ages?
Here's an excerpt of Dan's suggestion:
"Using the battery SOC voltage curve (provided by the mfgr); the average
[typical] discharge rate; and the nominal ambient temperature, estimate the
battery voltage for 20% DOD -- and limit it to that with the LVD.
Hopefully the loads are small, which will help reduce the effects of
something called the "Peukert" effect on the battery discharge (yes, it's a
real term), and will result in better accuracy of the battery voltage as a
SOC indicator."
I looked at some discharge curves on batttery spec sheet pdfs that I have,
I will attach a captured image below for a typical 120Ah AGM 12V battery
that are commonly available in Africa. The 20hr discharge rate is
0.05C, or 6 amps. I think that is a conservatively high estimate of the
max load for these small 12V DC only systems (specified as 3 x 15W CFL
lamps and phone charging) The 20hr (6A) discharge curve crosses the 12.5V
line at roughly the 8 hour mark, which would mean 48AH removed from the
battery, or about 40% DOD. So, 20% DOD would be the 4 hour line,
which looks like it'd be up around 12.6V or even higher, right? (its a
small graph and hard to read)
Now, 12.6V just seems ridiculously high to set as an LVD. I've seen
plenty of brand new resting batteries at 12.6V with no loads at all.
Right now I'm leaning towards something closer to 12.4 to 12.5V for the
LVD, those seems like reasonable numbers that will prevent deep discharges
but not become a nuisance by turning off the lights early every night.
What do you guys think?
Or is this getting too much into social engineering and should I pick a
lower LVD closer to 12.0 and keep the high LVR? Yes, they will probably
be without lights for a few nights after every LVD situation, but wouldn't
that be good feedback that will encourage them to conserve better to avoid
that situation again? The high LVR will ensure that the batteries get up
to 13.8 or 14V after the really deep discharges and hopefully help give
them a reasonably long life..
Thanks for all the good ideas and suggestions!
Eric
On Thu, Jan 2, 2014 at 5:37 PM, jay peltz <jay at asis.com> wrote:
> Hi All,
>
> I've been wondering about the LVD set point for this project and I'm
> asking what you all think.
> So far 12.0v has been chosen for the LVD and 13.8 LVR.
>
> But IMHO as the PV might do 15-20% at best avg per day, I got to thinking
> that at 12v, its around 40-50% SOC, so it might be the system could be off
> for a few days as it recharges.
>
> If the LVD was higher then it would bounce around at at higher SOC with a
> lesser chance of off for days and hopefully at least some lights every day.
>
> curious what you think,
>
> thanks,
>
> jay
>
>
> On Jan 2, 2014, at 2:38 PM, Dave Palumbo wrote:
>
> Allan,
>
> The Full River sealed battery line also has charging voltages similar to
> the numbers for the Rhino’s. We have had good results with many Full River
> battery banks, no premature failures to date. Certainly these are higher
> charging voltages than the early VRLA batteries we saw years ago.
>
> David Palumbo
> Independent Power LLC
> 462 Solar Way Drive
> Hyde Park, VT 05655
>
>
>
> *From:* re-wrenches-bounces at lists.re-wrenches.org [mailto:
> re-wrenches-bounces at lists.re-wrenches.org] *On Behalf Of *Allan Sindelar
> *Sent:* Thursday, January 02, 2014 4:01 PM
> *To:* eric at solarnexusinternational.com; RE-wrenches
> *Subject:* Re: [RE-wrenches] Higher LVR to force full re-charge in small
> OGPV systems
>
>
> Eric,
> Not having previously heard of Rogue Engineering, I looked them up on the
> web.
>
> I would be cautious above using the Rhino-10 product you have identified.
> According to the product manual for it at
> http://rogue-engr.homestead.com/Manual_1950-162.pdf, it is a 10A PWM
> controller labelled and described as being for "SLA" - sealed lead acid
> batteries. Yet its specs include a 14.8V bulk voltage - what they call a
> "top=off" voltage - and a 14.1V float voltage. 14.8 is well up into the
> gassing range for a LA battery. Perhaps they expect that with a 10A maximum
> charge capacity, the current will be insufficient to force gassing, but
> these values fly against all conventional wisdom.
>
> Does Rogue Engineering know something that the battery industry doesn't?
> Or are they simply, as their name suggests, "going rogue"? And has anyone
> on this list used this company's products?
> Allan
> *Allan Sindelar*
> *Allan at positiveenergysolar.com* <Allan at positiveenergysolar.com>
> NABCEP Certified PV Installation Professional
> NABCEP Certified Technical Sales Professional
> New Mexico EE98J Journeyman Electrician
> Founder, *Positive Energy, Inc.*
> A Certified B CorporationTM
> 3209 Richards Lane
> Santa Fe, New Mexico 87507
> *505 424-1112 <505%20424-1112> office 780-2738 cell*
> *www.positiveenergysolar.com* <http://www.positiveenergysolar.com/>
> On 12/31/2013 2:34 PM, Eric Youngren wrote:
>
> Thank you Allan, Dan and Ray for your suggestions. This is the kind of
> feedback I was hoping for!
>
> I agree that the 75W of PV with 120AH of sealed battery is too low of a
> ratio of PV to storage and will not help the batteries last 7 years.
> Unfortunately those parts of the design are pre-determined in the bid specs
> and I don't think they will allow us to suggest alternatives now. We need
> to win the bid first, then maybe we can suggest some changes..
> MPPT would be great but it's just not in the budget for these little
> systems, and like Allan says these systems won't ever experience the cold
> daytime conditions where MPPT really shines.
>
> Looks like Rogue Engineering in Englewood, CO will be able to customize
> their Rhino-10 PWM controllers with the LVD and LVR setpoints we choose.
> I'm leaning towards Allan's suggested values of 12.0 for LVD and 13.8 for
> LVR. I am fairly certain that many of the systems will be run til the LVD
> shuts them off, so 12V will ensure they will not be too deeply discharged,
> and forcing them to wait until 13.8V before reconnect will give the
> batteries a chance to get back up to a full SOC before the cycle is
> repeated. Yes they will experience shut-downs, but I think that over
> time this approach will help them learn to live within the capacity of the
> systems and give the batteries the longest life possible. That, or they
> will figure out how to bypass the load controller and run directly off the
> batteries!
>
> I like the Midnite MNBCM battery SOC meter. It's currently the best
> 'dummy light' system monitor out there for small systems like this. We
> used them last year for over 1,000 small systems powering computers in
> schools in Tanzania and people seem to understand them easily. We will
> include them in the systems we are offering for this tender.
> Thanks again folks! Lets hope we win the bid!
>
> Eric
>
>
> On Tue, Dec 31, 2013 at 12:01 PM, Ray Walters <*ray at solarray.com*<ray at solarray.com>>
> wrote:
> I'll chime back in on a few points:
>
> To MPPT or not: It all gets down to the budget: I find the tipping point
> to be around 250 to 400 watts. My small systems usually don't have MPPT,
> and as soon as I get into more than a pair of modules, I'm using the
> cheaper 60 cell modules with MPPT.
> I like Blue Sky's new PWM 30 amp controller that also includes a digital
> meter.
>
> Temp compensation: it all depends on the temperature variation. If its
> warm year round, just program the charge settings lower, and as Dan said
> KISS.
>
> Metering: Midnite's Battery Capacity Meter was specifically designed for
> the African market. Very simple LED "smiley face, frowny face" reading,
> It's cheap, and it actually does a decent job of tracking SOC. Its not a
> Trimetric, but its way better than a volt meter too.
>
>
>
> R.Ray Walters
> CTO, Solarray, Inc
> Nabcep Certified PV Installer,
> Licensed Master Electrician
> Solar Design Engineer
> *303 505-8760* <303%20505-8760>
> On 12/31/2013 11:48 AM, Exeltech wrote:
>
> Eric .. Allan .. and all ...
>
> Allan wrote:
> Re an MPPT controller: Nope, I wouldn't. First, this is equatorial west
> Africa.
> It's a tropical climate with hot modules most of the year. MPPT offers the
> greatest benefit when the voltage delta-T is greatest: cold modules and
> empty
> batteries.
>
> *** While Allan is correct about deriving maximum MPPT benefit when the PV
> are cold, at 75W per panel, and 120A-H battery, the PV are undersized for
> all but very light-duty system usage. Obtaining the maximum possible
> charge current will help lengthen battery life. If two 12V 40W PV in
> series could be economically utilized (vs. one 75W panel) .. it's worth
> considering.
>
> Second, any controller with MPPT is more complex, and thus more expensive,
> than a simple PWM controller.
>
> *** Hence my comment at the top of the suggestion list: "... if budget
> and circumstances permit...". If not - go with a basic PWM unit and other
> cost-saving measures. If they're going to hold your feet to the fire on
> the "seven year" battery performance requirement, you're going to have to
> limit the depth of discharge, and also institute other life-enhancing
> practices.
>
>
> Re temperature compensation:....
>
> *** Temperature compensation in this application may be a toss-up between
> fully recharging the battery and obtaining maximum battery life. The
> choice here could go either way. If it's always warm (or hot),
> compensation may not be needed. In fact, it may be beneficial to help
> maximize the recharge if the controller thinks the battery is cooler than
> it really is, which in turn would push the charge voltage set-point up a
> bit. Not knowing the environment where the systems will be installed makes
> this decision an unknown . Also .. and keeping in mind that there are
> elevated equatorial areas where the weather can be cool .. compensation may
> or may not be a consideration.
>
> To me the other features of the C12 - four adjustable settings, removable
> knobs and 15-minute reserve - were more important than MPPT.
>
> *** There is much to be said for the KISS principle.
>
> Re an LED SOC display: agreed just as Dan stated. Is there one that can be
> programmed to lie a little? (The best of these I ever knew was the Photron
> Batterylite, gone since around 1998. I wish someone would resurrect that
> tiny gem.)
> *** With several hundred units in the mix, I'd think a small design house
> may be interested. As long as you're at it, how about adding a low-voltage
> disconnect option into the monitor design .. and perhaps even an audible
> alarm (with user reset). Each feature adds $ and complexity. If the users
> will routinely look at such an LED monitor and use power accordingly,
> neither of these features would be needed.
>
>
> Eric .. there's no singular (or simple) answer to your question. If the
> project is going to be awarded on a strictly low-bid basis .. and they're
> adamant about seven year battery life .. you're going to have difficulty
> meeting their budget AND achieving their system goals.
>
>
> Happy New Year to all....
>
>
> Dan
>
>
>
>
> --------------------------------------------
> On Tue, 12/31/13, Allan Sindelar <*allan at positiveenergysolar.com*<allan at positiveenergysolar.com>>
> wrote:
>
> Subject: Re: [RE-wrenches] Higher LVR to force full re-charge in small
> OGPV systems
> To: "Exeltech" <*exeltech at yahoo.com* <exeltech at yahoo.com>>,
> *eric at solarnexusinternational.com* <eric at solarnexusinternational.com>,
> "RE-wrenches" <*re-wrenches at lists.re-wrenches.org*<re-wrenches at lists.re-wrenches.org>
> >
> Date: Tuesday, December 31, 2013, 10:51 AM
> Dan and Eric,
>
> Now I'll respectfully argue with some of Dan's suggestions:
>
> Re an MPPT controller: Nope, I wouldn't. First, this is equatorial west
> Africa. It's a tropical climate with hot modules most of the year. MPPT
> offers the greatest benefit when the voltage delta-T is greatest: cold
> modules and empty batteries. No chance for the former, and we're trying to
> get around the latter. If these systems were going to a cold climate I'd
> agree with you. Second, any controller with MPPT is more complex, and thus
> more expensive, than a simple PWM controller. Given the expressed
> competitive-bid budget limitations, MPPT is not a smart place to allocate
> costs. Dan is right about the weak C/rate. Spend the money (if you can) on
> a second or larger module.
>
> Re cool batteries: agreed, although I don't know how to achieve this.
>
> Re temperature compensation: as much as I push it strongly here, it may
> not matter in this case, and may be a hindrance. What's the annual ambient
> temperature swing? Again, if it's equatorial lowlands the temperature may
> stay around a constant 80º all year. If so, TC is irrelevant. And they
> occasionally fail, which can cause settings to mess up. Once again, decide
> if the added cost is worth it in this application.
>
> Re an LED SOC display: agreed just as Dan stated. Is there one that can be
> programmed to lie a little? (The best of these I ever knew was the Photron
> Batterylite, gone since around 1998. I wish someone would resurrect that
> tiny gem.)
>
> To me the other features of the C12 - four adjustable settings, removable
> knobs and 15-minute reserve - were more important than MPPT.
> Allan
>
> Allan Sindelar
> *Allan at positiveenergysolar.com* <Allan at positiveenergysolar.com>
> NABCEP Certified PV Installation Professional
> NABCEP Certified Technical Sales Professional
> New Mexico EE98J Journeyman Electrician
> Founder, Positive Energy, Inc.
>
> A Certified B CorporationTM
> 3209 Richards Lane
> Santa Fe , New Mexico 87507
> *505 424-1112* <505%20424-1112> office 780-2738 cell
> *www.positiveenergysolar.com* <http://www.positiveenergysolar.com>
>
> On 12/30/2013 7:32 PM, Exeltech wrote:
> Eric,
>
> Allan's recommendations on both points are excellent, and spot-on.
>
>
> I would add the following suggestions for your controller and system if
> budget and circumstances permit:
>
> 1) Use a MPPT model controller to enhance the recharge rate under
> less-than-ideal conditions. The PV is already undersized for the battery
> you indicated (below).
>
> 2) If you get the job .. install the batteries in a location that will
> keep them as cool as possible. Heat is just as detrimental to battery life
> as is failure to fully charge.
>
> 3) Temperature-compensated charging will enhance battery life. However,
> the 75W PV vs. 120 A-H battery allows for only a C/20 recharge rate at
> best. This is not conducive to full recharge on a regular basis unless the
> loads are very small and are lightly used. If the budget allows for
> opportunity to double the PV wattage .. it would be wise to do so.
>
> 4) A simple "stop-light" style LED voltage display won't consume much
> power, but can go a long way toward providing a visual indicator for
> state-of-charge, especially if it's adjustable, and can be be biased toward
> a higher SOC (in essence, make it fib a little, and indicate a low SOC
> sooner than than is actually the case). Worth considering as an option ..
> but easily omitted to reduce cost and/or simplify the system.
>
> I wish you success on winning the project.
>
>
> Regards,
>
>
> Dan
>
> --------------------------------------------
> On Mon, 12/30/13, Allan Sindelar <*allan at positiveenergysolar.com*<allan at positiveenergysolar.com>>
> wrote:
>
> Subject: Re: [RE-wrenches] Higher LVR to force full re-charge in small
> OGPV systems
> To: *eric at solarnexusinternational.com*<eric at solarnexusinternational.com>,
> "RE-wrenches" <*re-wrenches at lists.re-wrenches.org*<re-wrenches at lists.re-wrenches.org>
> >
> Date: Monday, December 30, 2013, 6:48 PM
> Eric,
> Yes, one of the classic problems with small systems and less educated
> users. Just a small contribution: it would seem to me that in this case the
> LVD setting is as key as the LVR. What if the LVD was set as high as
> possible - say, 12.0 V with sealed batteries, along with a 13.8 - 14.0 V
> LVR. The system would still be run until it shuts off, but the battery
> remains at a relatively high SOC. The next day the system will return to
> operation, but with a much higher range of SOC. The users will still
> experience shutdown, and will over time become familiar with when to expect
> it and how to live within its capacity, but the battery life will be
> protected.
>
> This approach seems counter-intuitive, but the more I think about it the
> more sense it makes. The system would also return to operation sooner
> following cloudy weather - in fact, it would provide a minimal amount
> during each of the cloudy evenings.
>
> Is the C12 still made? It would still seem ideal if so: bulk, float, LVD
> and LVR, all adjustable, with removable knobs and a 15-minute reserve
> button after initial LVD shutdown. Rock-solid reliable, too, in my
> experience.
>
> I'd be interested in what approach you select. This intrigues me.
> Allan
>
> Allan Sindelar
> *Allan at positiveenergysolar.com* <Allan at positiveenergysolar.com>
> NABCEP Certified PV Installation Professional
> NABCEP Certified Technical Sales Professional
> New Mexico EE98J Journeyman Electrician
> Founder, Positive Energy, Inc.
>
> A Certified B CorporationTM
> 3209 Richards Lane
> Santa Fe , New Mexico 87507
> *505 424-1112* <505%20424-1112> office 780-2738 cell
> *www.positiveenergysolar.com* <http://www.positiveenergysolar.com>
>
> On 12/30/2013 5:00 PM, Eric Youngren wrote:
>
> Hi Wrenches,
>
> We are bidding on a project to provide several hundred small (75Wpv, 120AH
> battery) 12V DC off-grid solar home systems for rural villages in West
> Africa. The project planners have requested that the systems be designed
> and built to provide 7 years of battery life. That seems crazy optimistic
> to me but I'm trying to design the systems that will give the batteries a
> fighting chance of lasting that long.
>
> In my experience with these types of installations, the system users have
> little or no understanding of how to properly manage their batteries and
> usually no metering or SOC indication to help them even if they knew what
> to look for. So, the default control strategy becomes: run the loads
> until the Low Voltage Disconnect (LVD) turns them off, then wait until the
> Sun returns and the voltage rises to the Low Voltage Reconnect (LVR)
> setpoint (around 12.5V is a common default) , upon which point the cycle
> repeats, with the result that the battery bounces between LVD and LVR,
> almost never reaches a full SOC, and the batteries are lucky to survive for
> maybe two years. After that the system will provide a little power during
> the day while the Sun is shining but the batteries will quickly crash below
> the LVD after the sun goes down.
>
> So, we want to offer a controller with a high LVR setting that will ensure
> the batteries reach a full recharge after each and every LVD incident. I
> know the C-12 has an adjustable LVR setpoint and I see one Chinese brand
> (Manson) that can do it. Does anyone have any other suggestions of small
> (<20A) PWM controllers that can be adjusted to not reconnect until 14V or
> so? Price is going to be a factor in this project so they need to be
> low cost.
>
> Any other suggestions? What would you use for the optimal LVD and LVR in
> this situation? These are rural household, DC only systems with
> primarily lighting and small device charging loads. I know that this
> strategy will mean that they might not have any access to battery power for
> up to a few days if they hit the LVD during the rainy season and there is
> not enough sunshine to get the battery charged in a single day. My
> thinking is that experience will provide good feedback that will encourage
> them to conserve and manage their batteries to avoid the LVD situation as
> much as possible. That's the idea, anyway. I'd appreciate any advice
> from you all. Thanks!
>
> Wishing you all a happy and productive 2014!
>
> Best energy,
> Eric
>
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