silly question on batteries

hillbilly · January 2009

So I've learned a lot from reading this forum and a few others for the last few years, and from paying close attention to my own off grid system. One thing that I've noticed is the (almost) consistent advice about NOT discharging batteries too deeply. Not just here, but in other publications too; it's sometimes so strongly discouraged that a poor newbie might be scared to put a load on the battery for fear of sulfation...
Anyways, I get all that, and am not arguing with that premise. But here's something interesting from Surrette's own website regarding charging and discharging routines:

>...If the battery bank is large in relation to the PV array (C/20 min) and loads are
>large then the batteries will require a higher voltage setting. Also the battery
>should be cycled deeply (i.e. to 50%) before starting an auxiliary charge source
>such as a generator. Once every three months the bank should be discharged to
>the low voltage set point before starting the generator. This is usually dependent
>on the cut-off of the inverter which is usually 11 volts on a 12V system. The
>batteries are designed to be cycled and a deeper discharge forces electrolyte
>deeper into the active plate material and helps open up fresh reaction sites...

I just thought it was interesting that I've never heard anyone else mention anything like this before (here or elsewhere). I've been hesitant to run my batteries down that low, particularly as we do sometimes get socked in with cloudy weather for a week or two in the winter... that would mean running the genny 4-6 times in a two week period, I don't feel like pulling the batt's down to 50% that many times just to charge them back up. Typically in those worst of spells, our batteries will go as low as 70-75% after 2 days of loads and no sun, perhaps at the lowest down to 61% (record low) if we hold off on running the genny another day or so.
So I've taken to sort of waiting until the batts get down a bit, perhaps waiting 3-4 days before starting up the genny. But, like I said, I've not been comfortable with the idea of dropping the batteries down to 50% or less... am I just gunshy? I've basically been sort of trying to mesh what I see as two opposing theories here together in some sort of a compromise. Furthermore, since I can adjust the low voltage cut out on my inverter, if I were to periodically drop the batts down to the LVD point I am not sure quite how low to set that voltage and/or what kind of discharge rate to use to do this.
What do the experts here think of all this? Any further ideas, advice... even peanut gallery remarks are welcomed

BTW, forgot to mention the batts are 8 Rolls 460's: 2 strings at 24V 700ah capacity. 2years old now, and seem to be doing ok from what I can tell.

icarus · January 2009

Re: silly question on batteries

HB,

I suggest that you read some of the following,,, pretty informative:http://www.batteryfaq.org/
http://www.batterystuff.com/tutorial_battery.html
http://www.windsun.com/Batteries/Battery_FAQ.htm#Lifespan%20of%20Batteries

On the other hand,,, if the surette site call for such,,,?
Tony

hillbilly · January 2009

Re: silly question on batteries

Tony, thanks for those links... but, I've read all that many, many times. I think I do get the basics on batteries and such. It's just that from the battery manufacturer, they provide a tid bit of info that I've never seen echoed anywhere else. Whereas the general concensus that deep discharges reduce battery lifespan is prevalent anywhere you ask...
Leaves me wondering, just curious if anyone else had further info or ideas on the matter... what would you do?
thanks

mike95490 · January 2009

Re: silly question on batteries

surette likes selling batteries. I could see a deep"er" cycle once or twice a year, "to open up fresh sites" but not on a regualr basis.

BB. · January 2009

Re: silly question on batteries

hillbilly wrote: »

even peanut gallery remarks are welcomed

Shall I move this to the "Hype" forum?

Probably some of the directions about battery discharge and generator use are collapsing several rules of thumbs and practical operational suggestions into one overall statement.

For example, I can understand the discharge down to near 50% before starting the genset... Generators, in general, have very poor fuel efficiency vs kWhrs supplied when running small (relative to genset ratings) loads... Many of the "cheap" 5kW noise makers, probably use as much fuel (gallons per hour) at 50% load as they do at 10% load.

Bulk up the batteries, at high charger current, for a couple hours, then shutdown and let the solar array take them up to the last 10-20%.

Which also takes into account another couple rule of thumbs... If, you plan on 3 days of no-sun, down to 50% capacity; the rule not to let a flooded cell lead acid battery go down below 50%; and the rule not to let batteries sit below 75% state of charge (25% discharged or more), or the sulfates will begin to harden.... When added together, you can get a rule that on the third day of no sun (50% discharge), you will want to start the genset to get the battery quick charged back above 75%.

I am not sure about the statement "...deeper discharge forces electrolyte deeper into the active plate material and helps open up fresh reaction sites..." Since this is from a battery manufacturer, it would be silly for me to disagree with somebody who makes their money designing and selling batteries.

Certainly exercising a genset every couple of months, and fast charging back up and equalizing is a good idea for mixing electrolyte.

The Rolls should have very heavy/thick plates and should withstand deeper cycling better than some other batteries with lighter plates.

Setting LVD for inverters--measuring voltages on batteries when under load (or charging) to determine their capacity is not very accurate or reliable. The LVD on inverters is, most likely, more of a protection circuit to prevent the inverter from drawing too much current (and failing at low DC input voltage) vs trying to protect the batteries themselvs.

I am a big believer in battery monitors. Especially useful for sealed batteries (AGM, Gel, Valve Regulated, etc.) where you cannot use a hydrometer.

There was another thread a few months ago:

Time to Question the 3 day Rule ?

Started by Nigtomdaw. In there are a couple of my posts, if you disregard the below 75% and sulfate hardening guideline--Just taking two banks of batteries--one 4x larger than the other, but otherwise equal loads and charge sources...

When you take the depth of discharge vs cycle life--within about 20% maximum differences--A small bank that is discharged to 50% over three days, vs a 4x larger bank that is discharged to 88% over three days, then both recharged... Looking at the cycle life, the small bank will die in 1/4 the life (deeper cycles, shorter number of cycles before failure), as a large bank (shallow cycling, 4x the number of cycles before failure)... However, because the small bank has 1/4 the number of batteries--you can afford to replace "4 small sets" for about the same price as replacing 1 large set over time.

So, factor in maintenance costs, inflation, cost of money, etc... It is pretty easy to make a case for few batteries and deep discharge vs lots/large batteries and less deep discharge.

You (Hillbilly) have seen the "3 day thread"... In the end, avoiding the battery killers (boiling dry, running dead, storing for months/years below 75% charge, contaminating cells with tap/river/well water or during hydrometer checking, not cleaning corrosion, parallel batteries with a bad cable or two, etc.)--the batteries will probably give good service even if the other rules of thumb are not followed religiously.

-Bill

hillbilly · January 2009

Re: silly question on batteries

mike90045 wrote: »

surette likes selling batteries.

Hah, that was my first thought when I read that too.

Windsun · January 2009

Re: silly question on batteries

Sulphation kills more batteries than over-discharging.

Letting a battery sit for long periods of time at only partial charge allows the crystals to gradually build up and reduce the capacity, even though the plates might still be almost intact.

One of the problems with off grid systems is that batteries often do not stay fully charged, at float level, for much of the time. It appears that the Surrette method is trying to overcome that.

hillbilly · January 2009

Re: silly question on batteries

Bill, thanks for the comments there. I suppose that could make sense about surrette making some pretty big generalizations. One thing that maybe makes this a bit more vague for our system is that our loads are not that big compared with our bank size; after 3-4 days of little to no sun the lowest our batteries have dropped to (according to the Trimetric) was 61% (4 days of almost no sun).
In fact a more typical situation in winter time is one where our loads ran a bit higher than normal, and/or we have several days of on-off sun and clouds. Thus after 3-4 days, the batteries never quite got back up to a complete full charge and we start the genny up for a half hour to an hour in the morning to "bulk up". Typically the batts are above 70% or even 80% in these cases, depends a bit too on what the weather is looking like.
I think for the most part we've been doing things fairly well as far as giving regular charges and EQ's and such. The batts use a little water every now and then, not so much since we added water miser caps. I wasn't too worried about their health per se, just curious to hear what other's thought about the suggestions to drop the batt SOC lower before connecting to a generator, and about giving them some deeper cycles now and then (exercise?).

Windsun wrote: »

Sulphation kills more batteries than over-discharging.

Letting a battery sit for long periods of time at only partial charge allows the crystals to gradually build up and reduce the capacity, even though the plates might still be almost intact.

One of the problems with off grid systems is that batteries often do not stay fully charged, at float level, for much of the time. It appears that the Surrette method is trying to overcome that.

Ok that first part makes sense, so dipping the SOC way down low every now and then right before charging it up shouldn't make too big of a difference overall on lifespan. The second part I'm not quite getting, maybe my brain's just slow today... it's been known to happen. I'm missing how discharging the batteries deeper would help overcome the problem of the batteries not spending as much time at a full SOC and in float?
anyways, thanks for the comments and suggestions (except the threat to throw this to the wolves in the hype section ; )

icarus · January 2009

Re: silly question on batteries

What I have always wondered is what are the consequences of having the battery 95% charged all the time.

We have ~ 400ah of capacity, and use ~30 ah/day (12vdc) Most every day the controller goes from bulk to absorb mode, but very rarely to float,,, there just aren't enough hours in our short winter day. The Tri-metric goes down to ~90% every day by morning, and comes back to almost 100% every day, even though the "battery full" light doesn't blink because it hasn't been at absorb long enough. In other words, I put in, almost every day just what I take out,,, ususally a bit more. So my batteries are almost never 100% charged, almost never less than 80% and rarely less than 90%.

After a few days of little sun, I will run the genny and the TC-20 for a couple of hours and that will bring them up to float on the charger scale, the controller scale and the Tri-metric. Question is if I use the 3 day rule, of leaving them "not fully charged" for 3 days (only 95-99% charged!) am I hastening sulfation?

As wind-sun suggests, the batteries almost can never get to "full charge" They are almost always under some load such that getting to and staying at float voltage is very difficult. For example, the CC might get to float early in the afternoon, so I turn on the Modem or plug in the lap top. Right off, the load drops the voltage to enough to below the float threshold. They are still charging from the panels,, but at a lower rate.

Tony

I do an eq charge every few months, but all the cells tend to have equal sg even before eq.

Cariboocoot · January 2009

Re: silly question on batteries

It should be pointed out that a battery's re-charge specs are dependent on the battery. That is why you'll find different numbers for absorb/float volts & time, as well as equalization requirements. The best you can do is trust the manufacturer's specs on the particular battery. The 'generalized' information is just that: generalized. It applies when no manufacturer's data is offered (which can be often).

In any case, it is impossible to keep a battery 100% charged, to charge it perfectly every time, or to extend its life indefinitely. Some of them will stand up to 'mishandling' better than others, but eventually all will fail. That's why in another thread I asked about battery recommendations: nothing better than the experiences of others to give you the real skinny on the quality of a product! That's one of the best things about this board.

BB. · January 2009

Re: silly question on batteries

At least from what I have read, sulfates begin to harden around 75% or lower state of charge... So, as long as you are 80% and above, then there should not be that problem.

I have read that keeping batteries near 100% increases corrosion on other parts of the batteries. Keeping them hot (from charging) increases corrosion and bubbling tends to abrade the plates and corrosion points, etc.

In the end, I think avoiding the "known battery killers" is better than trying to figure out the last 5% of what to do better.

-Bill

wild01 · January 2009

Re: silly question on batteries

Something that rarely gets mentioned is that motive power (forklift) batteries behave very differently then traditional beep cycle batteries. they are much heavier construction and are designed to be abused. I run my house on a forklift battery and have run into several problems when using certain chargers designed for regular deep cycle batteries. My battery is made by deka, and is designed to last 20+ years. it is rated at 2100 cycles at 80% depth of discharge, 4000 at 50% and 6000 at 20%. the down side is it self discharges @ 1% a day. one of the problems I've run into is that it has a high internal resistance, and requires a much larger current than a lot of chargers think it needs. (there is a warning right on the battery not to finish it @ less that 23 amps) I know that rolls batteries are very similar to motive power batteries, and come close to their life expectancy. if you do the math (and trust the factory ratings)
ah*0.8*2100 =ah*1680
ah*0.5*4000 =ah*2000
ah*0.2*6000 =ah*1200
you see that these batteries where designed to put out the most total power in a 50% discharge state and they put out more total power @ 80% dod than at 20%. Of course that also goes along with the fact that they are designed to be fast cycled and almost never sit around discharged. (a typical forklift will be run down to 80%dod every shift and recharged immediately.)

If I treated an L-16 like this I would fry it in a week, but forklift batteries are not even remotely in the same ballpark. my 700ah 12 volt battery weighs 588lb and requires a forklift to move around. (that irony is not lost on me) on contrast an L-16 rated at 420 amp hours weighs 121 lbs so 420/242=1.735 amp hours per pound @ 12 volts while 700/588=1.190 amp hours per pound @ 12 volts. so you can see just how much heavier built mine is. because of this trojan rates their battery @ 500 cycles at 50% dod

so my point?

most of the (solar)industry rules of thumb have beed developed using the industry standard battery (the L-16) which should never be discharged below 50% and while it is a much better battery than say an off the hardware store shelf trolling motor battery, it is not nearly as heavy duty as a motive power battery and cannot be treated like one. Conversely motive power batteries run on a (similar but) different set of rules.

the whole reason I switched to my current battery is because I was working in a machine shop where we abused the crap outta our electric fork lift every day 12 hours a day 7 days a week, and I asked a guy that had been working there for years how often they had to change the battery. he said, "that old thing, we bought it at auction 10 years ago, it already had the battery in it." when I looked at the battery it was manufactures in 1984. this was in 2005. I don't know how it was treated before we had it, but at least once a week we would forget to plug it in, run it all day the next day until it was so dead we had to tow it into the charger, recharge it @250 amps/hour and use it again in 3-4 hours. oh yeah and the guy who watered the thing use tap water! I figured if the battery could withstand that kind of abuse it was just what the doctor ordered for my house.

BB. · January 2009

Re: silly question on batteries

Forklift batteries are very rugged and can be very good when used for off-grid solar.

The major down side that I am aware of is that Forklift batteries are not as efficient as other storage batteries (probably because they out gas more hydrogen during the charging process than other deep cycle batteries--possibly because of slight chemistry differences and/or just because they are charged so hard in Forklift work).

NAWS does sell Forklift batteries too... And one time I did a back of the envelope calculation here, and ignoring possible less efficient charging (and costs of distilled water), using these batteries (and assuming 20 year life), were the most cost efficient battery out there.

I don't have any information on hand about water use and efficiency, so I do not know how much those impact the total life cycle cost.

-Bill

hillbilly · January 2009

Re: silly question on batteries

Great to see more information/ideas about batteries come forth. Wildone I should reiterate that my surrette batteries are of the "4000" serries, and NOT the industrial "5000" serries. So I don't think that the specs between our batteries are that similar (I wish).
Bill when you were calculating cost vs discharge and overall lifespan on batteries, were you also factoring in that with a bigger battery bank your loads would equate to a different discharge rate, and thus a higher rated capacity. Just random curiosity here:

My bank for example is rated 700ah at the 20hr rate, but 882ah at the 72hr rate, and 920ah at the 100hr rate. When using a lot of power our own rates are anywhere between 8amps and 20amps with 10-12amps being the most common. So in theory (wouldn't it be nice) our battery bank should be more like about a 900ah capacity? IF that were the case and we use roughly say 85-90amps would equate to a 10%DOD.

So you got me thinking about this, if we had gone instead with a battery bank that was 1/4 the rated size (at the standard labeled 20hr rate) that would be 175ah at the 20hr rate (8.75amps). In order to simplify the math for my brain let's say we discharge at 9amps for 8 hours a day. On the larger bank this would be roughly the 100 hour discharge rate, and so we get (920AH-72AH= ~8%DOD) ... or over three days would equal about 23%DOD
That 9amp discharge on the smaller bank would roughly equate to the 20hour rate, thus (175AH-72AH= ~41%DOD a day). So we'd be reaching an 82%DOD in just two days. I know that this is of course VERY hypothetical and overly simplified, but just for the sake of argument... it seems that there is a relevant factor there to be accounted for.

BB. · January 2009

Re: silly question on batteries

I was real simple... Assumed probably 7 year life for the "cheap" batteries, and 20 year for the expensive set. Assumed that both were the same Amp*Hour (~6x daily load, 3 days no sun to 50% discharge).

The other way was to assume one small bank and one bank 4x as large. I used the generic number of cycles vs depth of discharge and found that within 20%, the 4x larger bank lasted ~4x longer.

So, if you have to replace 1 bank every 5 years, or replace a 4x larger bank every 22-24 years, the costs are very similar--but one guys complains he is always replacing batteries, and the other says "I have had the same bank for 20 years"... Just how you look at things.

The math is pretty easy (total cost over 20-25 year system life). And, if you want, divide the total system cost by the amount of power you will be able to use in 20-25 years--gives you $/kWhr.

-Bill

icarus · January 2009

Re: silly question on batteries

I made a similar calculation when I bought my current set of 4 t-105's to replace the previous l-16's I guessed that the lower price of the t-105's was a better bet than the bigger, more expensive l-16's. I figured if I got more than 5 years our of the t-105's they would be cheaper than getting 10 years out of the l-16's

My first set of t-105 is still on a seasonal string after ten years. The l-16s are down to 3 of the original 6 after 12 years,,, still on a seasonal string.

The t-105's are a lot easier to move around too!

Tony

wild01 · January 2009

Re: silly question on batteries

what I did for mine was buy a used forklift battery off of ebay.
it was a bit of a gamble but I got a good deka forklift battery for $350 and another 300 to ship it. (if going this route expect to pay as much in shipping as you do for the battery) I figured even if it was a little abused it was still a much better bet than a standard L-16

for instance there's one on ebay right now out of kentucky. 1350 buy it now 36 volt 680 amp hour 6 hour looked up battery specs @ deka website 806 amp hr 20
so if you figure freight at 1350 (just a guessor drive a pickup down there and rent a forklift to unload it @home) 2700/6=$405per 6 volt 806 anp hr battery, if you shop around you might find an L-16hc for $300.00 2 of those would equal over 800 amp hours. so I figure the forklift battery on ebay is a good gamble, but then again I am cheap and willing to take a risk.

BB. · January 2009

Re: silly question on batteries

Also look for posts by user "Adas". He is in Hawaii and has build a couple rather large off-grid systems using recycled forklift batteries.

Size/weight of the forklift batteries can be a killer if the building access is poor and you have no way of moving them around.

-Bill

RCinFLA · January 2009

Re: silly question on batteries

It sounds like Rolls is proposing a method to deal with some level of sulfates harden by undermining the sufates with a deep discharge. It can break off and eventually work its way to bottom of battery.

I would not consider this an optimum solution as it eats away available plate material and reduces the specific gravity of electrolyte by locking it up in sulfate crystals so less sulphuric acid is made on recharge. The Rolls battery have thick plates so there is material to spare.

If you can bring a battery to full charge at least once every two weeks and do the recommended equalization schedule with a generator this would be best for lifetime of battery.

The last 15% of recharging is very inefficient in energy conversion. Best if can be topped off by solar panels.

It may boil down to spending money on fuel or spending money on replacing batteries more often.

adas · January 2009

Re: silly question on batteries

Aloha, Frank, aka Adas here. My forklift batteries still going strong. Here are some of my brief tales:
Although I never calculated, I think I get about 1/3 to 1/2 the capacity of the "professional" batteries (rolls, etc). but who cares they are free or very cheap!

1: I just revived a 24v (about 550 amp) battery a few days ago that was in the back of my warehouse dead (well total 5.3v is dead) for the last 6 years. I added just enuf water to cover the plates and used my PV panels to charge it. Now it holds 25.2 v when left standing after an hour.

2: I have 2 systems. One is of 4x850ah 24 volt batteries (one has a bad cell from the beginning. This system runs grinders, mig welders, fridge, lights for 2 hours in the morning (we start at 6 am), charges a gem electric car and an electric forklift. I also send some PV voltage to the office with underground wire to a second FX60 Outback in the office periodically.

The office uses 4x36v 650 ah batteries that are tapped off at 24v. . Some of the cells are bad, so I jumper out the bad cell. (still have 2-3 cells spare.) ( I make jumpers from flattened copper water pipe and use 5/16 lag bolts to attach) Also make my bus bars from flattened copper pipe melted into pvc connectors to insulate). In days where we had minimal sun for 4-6 days or so I have run down as far as 21v. (I had no choice in the matter)
All these batteries were from a freight company that had 5 or so electric lifts and used and abused these batteries until they had no life left (truth is that they mostly had no water. some took as much as 2 gallons per battery.

So yes they are rugged and forgiving and I know I have spoken hearasy above, but still going strong and have not turned on the generator for the plant or office/home for one year now. Not tempting fate, and open to more suggestions on getting more life out of these batteries, although my recycle yard friend will call me when someone brings in a "good junk forklift battery" and just swap out mine for his when mine really die. Just call around and you will probably get one free.
(BTW I was one of the few on Oahu who had power when the island went out, Obama included) a couple of weeks ago.

Frank

pmcall · January 2009

Re: silly question on batteries

Wild Bill,

You inspired me to take a chance as well. I bought a 48 volt GNB battery for $900 and the shipping is another $750. Don't know the amp hours, but the thing weighs 3000+lbs ( buyer claims he bought it used for his forklift, but battery was too big). I am either blowing $1600 dollars or saving ~$4000 dollars ( figuring about $5200 for 16 L-16's)

I have a question about the connectors. The picture on eBay for the battery I bought shows a plastic connector on the end - the cables coming out of my systems have an "eyelet". I am thinking about getting the other side of the forklift connector and making a pigtail to the eyelets, maybe connect with a bolt and a nut. Curious as to what you did here? Thoughts?

wild01 wrote: »

what I did for mine was buy a used forklift battery off of ebay.
it was a bit of a gamble but I got a good deka forklift battery for $350 and another 300 to ship it. (if going this route expect to pay as much in shipping as you do for the battery) I figured even if it was a little abused it was still a much better bet than a standard L-16

for instance there's one on ebay right now out of kentucky. 1350 buy it now 36 volt 680 amp hour 6 hour looked up battery specs @ deka website 806 amp hr 20
so if you figure freight at 1350 (just a guessor drive a pickup down there and rent a forklift to unload it @home) 2700/6=$405per 6 volt 806 anp hr battery, if you shop around you might find an L-16hc for $300.00 2 of those would equal over 800 amp hours. so I figure the forklift battery on ebay is a good gamble, but then again I am cheap and willing to take a risk.

mike95490 · January 2009

Re: silly question on batteries

Even though you have a 8000AH battery, you only need to have the connections handle up to the max amps your fuse is.

I have not seen the style with "eyelets" you are speaking of, but if you can obtain a firm contact, you should be ok. A way to test, hook it all up, and then throw the heaviest load expected on your inverter for a couple of minutes, then feel the cables, working your way to the battery connects (careful - they might be hot) and feel for hot spots. It's a crude way to check, but will find a grossly bad connection, and you save power if you don't have high resistance hot spots.

Use good grade electrical parts for the connections, copper or tinned copper. Stainless nut and bolt can CLAMP copper-copper, but do not expect stainless to conduct electricity, it's much higher resistance than copper. And remember all the houses that burned from aluminum wire in the 70's

pmcall · January 2009

Re: silly question on batteries

pmcall wrote: »

Wild Bill,

You inspired me to take a chance as well. I bought a 48 volt GNB battery for $900 and the shipping is another $750. Don't know the amp hours, but the thing weighs 3000+lbs ( buyer claims he bought it used for his forklift, but battery was too big). I am either blowing $1600 dollars or saving ~$4000 dollars ( figuring about $5200 for 16 L-16's)

Looks like at the very least I have a project on my hands. The battery came in today, and I am reading 31 volts out of it.

silly question on batteries

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