Lead Dioxide Clumps and Autonomy

vtmaps

I am trying to design a system (solar panel, charge controller, and battery) to power a small DC load continuously. I would like this system to run with minimal supervision and maintenance. I would like to size the battery (possibly AGM) to allow for five days of poor solar charging, and size the PV panel to charge the battery in one sunny day.

If the battery goes five days without sun, the daily depth of discharge is about 10%. When there are consecutive sunny days the battery may only discharge 5% overnight between charge cycles (especially in summer when nights are short).

According to the WindSun BAttery FAQ:

a battery that is continually cycled 5% or less will usually not last as long as one cycled down 10%. This happens because at very shallow cycles, the Lead Dioxide tends to build up in clumps on the the positive plates rather in an even film.

According to BatteryFAQ.org:

Avoid shallow (less than 10% DoD) discharges of deep cycle batteries because lead dioxide builds up on the positive plates.

Is it unrealistic to design a system with five days of autonomy (without an AGS)?
Thanks, in advance. --vtMaps

BB.

Re: Lead Dioxide Clumps and Autonomy

I don't know about the battery chemistry--but if you know your loads and need ONLY 5 days of backup power--I would be tempted to go with the AGM and discharge it to 20% state of charge (use a battery monitor to turn off the loads--should be more accurate than just letting the battery "go flat and die" type operation).

You will not have that many 20% SOC operations--so it should not lessen the overall life of the battery.

Also, this allows you to have almost 1/2 the size of battery bank AH capacity--Which means that your solar array is 1/2 the size for 1 day recharging.

What happens after 5 days of no-sun? Backup genset, you make a 200 mile helicopter fligh and refill the propane tank, or just wait until there is sun again?

I wonder if having two banks "A" and "B"--If you switch them once a month or so--Supports deeper cycling on daily usage--and you have the "backup" available (if A fails, or for an additional X days of storage).

-Bill

I should add--depending on the project, a different chemistry (such as NiCad) may be a better solution. Lots of stuff to reseach here.

RCinFLA

Re: Lead Dioxide Clumps and Autonomy

Lead dioxide build up is not fatal but it does eat up positive plates. This is one reason why positive plates are alway thicker then negative plates on deep discharge lead acid batteries.

Float charging voltage is a compromise between the negative plate desiring a higher float voltage to keep sulfate formation down and a lower desired float voltage on the positive plate that will only build up lead dioxide coating and create support grid corrosion.

Lead dioxide is what makes a positive plate a positive plate but too much coating is not a good thing. First it is a poor conductor so too much coating drives up battery internal resistance resulting in greater terminal voltage drop during high current demands. Second, as more of the positive plate is eaten away it can expose positive plate support grid to acid which under cuts the electrical connection to the plate's support grid.

Lead dioxide is a white powder that will 'dust off' and sink to bottom of battery if there is an excessive build up, as soon as a significant current load is applied to battery. Don't get too hung up on the positive plate being eaten away. It is a fact of life for a lead acid battery. The alternative, with a lower float voltage, is sulfated negative plate which is not reversible.

Many AGM's use pure lead plates without an alloy grid support structure so they don't suffer from grid corrosion. They get their structural integrity from the tight packing of the glass matt separator. Most large kW commercial standby UPS systems use AGM batteries which are on continual float charging.

65DegN

Re: Lead Dioxide Clumps and Autonomy

What size is the load?

vtmaps

Re: Lead Dioxide Clumps and Autonomy

65DegN wrote: »

What size is the load?

I knew someone would ask... The peak power draw will be about 6 watts. I am trying set up an answering machine and cordless handset for my telephone at home. I do not want this new power system connected to (or even near) any part of my main off-grid home power system... fear of lightning! My main power system is something I pay a lot of attention to, this phone power system is something I would like to (more or less) just set and forget. Also, the battery and CC will be in a crawlspace under the kitchen where the phone line enters the house, accessible but not something I want to check on too often.

At this point I am in research phase, I do not even know what voltage the system will be at... I would like to find a phone unit that works on 6 volts, but will probably end up with a 12 volt system and a DC voltage converter to drop the voltage to whatever the phone wants.
--vtMaps

techntrek

Re: Lead Dioxide Clumps and Autonomy

I assume you have a wired phone connection, and your worry is about a lightning strike on the phone line, travelling to your house and then jumping to your electrical system? If your "main power system" is a PV system it has a large lightning collection system too - the PV array sitting outside. So if your goal is saving your PV system from a strike on the telephone system down the road, I'm not sure you are gaining any "safety" by doing this. Even if your main power system is a genset sitting outside you have the same problem. If the strike on the phone line is close enough it can send streamers to your genset and/or PV, too.

If your goal is to prevent you from being electrocuted during a storm you already have the answer - keep your cordless phone away from the base during a storm and then you can make and take phone calls all day w/o worry of getting zapped.

I don't think setting up two separate power systems solves either problem.

vtmaps

Re: Lead Dioxide Clumps and Autonomy

techntrek wrote: »

I assume you have a wired phone connection, and your worry is about a lightning strike on the phone line, travelling to your house and then jumping to your electrical system? If your "main power system" is a PV system it has a large lightning collection system too - the PV array sitting outside. So if your goal is saving your PV system from a strike on the telephone system down the road, I'm not sure you are gaining any "safety" by doing this.

techntrek, My phone line has its own ground rod about 70 ft away from where my main electric system has its ground rod. When lightning strikes in my area, the two ground rods may have 1000's of volts between them. I have friends (both on and off grid) who have seen burnt insulation where their phone wires and their house wires are in proximity. I suppose another solution might be to connect the two ground rods by heavy gauge wire. In order to do that underground I would have to tear up quite a bit of landscaping, patio, driveway, etc, as well as buy over 100 ft of the heavy gauge wire.
I am no expert on lightning, and I'm not certain about this, but my instinct tells me that if the two electrical systems are completely separate there is much less chance that things can go wrong. Perhaps I am being too cautious. Any lightning experts out there want to chime in?
--vtMaps

BB.

Re: Lead Dioxide Clumps and Autonomy

Look for Handsets that have battery backup (usually, these will draw less power if you want to use external backup power). You might find such a base station that has an answering machine chip too.

Or, look into how much the "phone" company charges for Voice Mail.

And, there is the Google Voice option. Use their voice mail and the ability to have the "google number" auto dial one or several phones (home phone, cell phone) automatically.

We are really close to disconnecting our land line phone. I keep dropping features to reduce the costs, and the phone company keeps adding "requirements" to the plan that increase costs (started at $30 per month for "simple" home phone, dropped to about $13 a month, then they keep adding fees that now jack it up back to $20 per month. Mostly use it for alarm and the (very rarely used) fax machine.

Using an IP phone and cell phones mostly these days.

-Bill

vtmaps

Re: Lead Dioxide Clumps and Autonomy

BB. wrote: »

We are really close to disconnecting our land line phone.
<snip>
Using an IP phone and cell phones mostly these days.
-Bill

Bill, We have no broadband options, except perhaps satellite. We have no cellphone coverage. Dialup is a miserable option because we only get about 19k with a 56k modem. My phone company's Voicemail is $6 per month. So I am looking for a unit with answering machine and cordless handset. I've seen a few with energy star ratings and 9 volt wall warts.

But this thread is wandering off... Do you think that connecting the phone (and its ground) to an independent power system will protect my main system from differing ground potentials caused by lightning?
--vtMaps

BB.

Re: Lead Dioxide Clumps and Autonomy

I believe in connecting all items in/coming into the home should all be connected to the ground rod at the edge of the building (and usually water and gas pipes too).

Separate grounds can allow dangerous voltages between the two "earth grounds" (i.e., a lightning strike near one ground will raise the local earth voltage with respect to a ground rod 70' away (as energy dissipates through the ground).

In our area, power and telephone come in from the same pole--so both are exposed "in similar ways" to a local strike.

Here is one IEEE document describing lightning protection (PDF Download). I have not had a chance to read through the details--But it seems to agree with a common grounding system for all "stuff" inside your home too.

Got to go...

-Bill

vtmaps

Re: Lead Dioxide Clumps and Autonomy

BB. wrote: »

Separate grounds can allow dangerous voltages between the two "earth grounds" (i.e., a lightning strike near one ground will raise the local earth voltage with respect to a ground rod 70' away (as energy dissipates through the ground).

Bill, I think you are agreeing that I need an independent power system to power my phone. My home power system is located in my detached workshop. My phone service enters my house. The two ground rods are 70 ft apart. Therefore, I must create a new power system for the phone which is grounded to the telephone ground rod. The phone and its power system will not be in physical proximity to any of the general house wiring (the circuit breaker panel in my house is a subpanel of the main panel in the workshop).

So, getting back to the original post, do you have any advice for me on how to design a relatively low maintenance, low power system that has many days of autonomy and can handle the resulting shallow discharges?
--vtMaps

BB.

Re: Lead Dioxide Clumps and Autonomy

Years, ago, I purchased a 12 VDC in 3/4.5/6/9 volt adapter from Radio Shack (car 12 VDC in, selectable output). I wonder if you can connect this with a typical cordless phone base station (if not 12 VDC) and use a small array+AGM+charge controller.

Something like this Power Stream 12-30 VDC in and 1.5 to 12 volts in 1.5 volt steps out... Note that the converter only down converts, so if you have ~12.5-13.5 volts in it will only output ~11.5 volts out--should still be OK (so they say in the fine print).

Depending on the cost of your phone base station--you can decide if it can take "12 volts" (really 12-~14.5 volts for typical solar charger application) directly instead of using the above $20 down converter. Only takes ~10-20 mAmps with no output load.

You could use an AC charger from your main system inverter (if it makes sense) to recharge the base station "external battery pack" when you have AC on... But it would not give you more than a few thousand volts of lightning protection that a completely isolated system would.

-Bill

SolaRevolution

Re: Lead Dioxide Clumps and Autonomy

(Maybe this should be in a seperate thread)

Food for thought for lightning strikes:

Several years ago while I was living in a neighborhood with overhead utilities, the pole mounted transformer across the street took a lightning strike. I was loading my truck in the driveway under our house drop and saw the lightning run down to the service entrance goosenecks. It felt like someone hit me with a 2x4.

The house nextdoor was all electric and had no surge arrestors anywhere. They lost every electrical device, except for their dishwasher and one of their cable tv boxes.

We were in one half of a duplex and had an SOV connected to our main AC panel and a surge protected UPS power strip for my office with the phone and DSL lines connected. We lost only our 4 GFCI outlets and the UPS battery died within a week. I only had a mini-solar system in the yard which was isolated and was unaffected. The tennants in the other side of our duplex had a high quality surge arrestor for their entertainment system and lost nothing but 2 GFCIs. Our duplex was actually closer to the strike than the neighbors who lost everything.


Another time an off-grid client got a strike on tree 500' away from the main house. Problem was that the tree had a phone line running through it between the main house and the caretaker's cottage about 2000' away. The lightning came into the house by the fax machine. All "wall-wart" type transformers and 2 or 3 GFCIs in the several-building-compound got fried. The control panel for the swimming pool equipment got fried. The power system equipment was in a seperate building and had SOVs on the AC and DC. The power system displays froze but came back on and worked normally after restarting the system. The caretaker's cabin was fine.

I did a Cell tower system on a mountian top for verizon. SOVs on everything. #2 grounding ring around the entire site with everything conductive connected to it. It's not a matter of "if it gets hit" it's a matter of how often. All still good for 3 years now with no lightning related shutdowns.


The moral as I see it is: surge arrestors on everything (including phone and data that) you don't want to lose!

SolaRevolution

Re: Lead Dioxide Clumps and Autonomy

I wonder if an adjustable "rebulk setpoint" with a fairly low setting could keep the battery from "short cycling" daily. Set for 12.3 or so with very light loads it may not re-bulk every day.

It sounds like this would be a very small system and the equipment to make it happen could be cost prohibitive but you may be able to set an AUX relay to disconnect a contactor for the pv once the battery is charged and not reconnect until the voltage falls below a chosen setting. ?

BB.

Re: Lead Dioxide Clumps and Autonomy

AGM's should be better (the little I understand) with continuous float operation (physical compression of the positive plates by glass mat/construction). Probably would still need to replace the AGM battery every 3-6 years (when it fails to keep the phone up over night, over a couple days of bad weather, etc.).

If you have a fair amount of DC loads to qualify (Amp*Hours)--One of these meters is pretty cute. Allow to measure the "true" power used over a day, or several days of usage.

-Bill

SolaRevolution

Re: Lead Dioxide Clumps and Autonomy

Wow Bill,

Those little meters are great! I'll have to get them for my mini-PV systems:
http://villagetech.org/MiniPVSystem.aspx