Deeper Discharge of Batteries

plongson · January 2013

We've been off grid for over a year and I have slowly changed many settings on the system to where it operates very smooth and is very reliable. The periodic EQ's, the gravity, absorb time and all those parameters seem to be real close now.

I do however have my auto-genstart set at 85% SOC and on a daily basis the batteries never get below this level. We are very conservative with power and when the sun shines, and we are making electricity, the genset never comes on. It reached 100% SOC just about every day on sunshine alone.

There are however those times in the winter when we have limited sun and the system does dip to that 85% SOC start set point.

I've read about the problems of not discharging enough and I've been wondering if I should start lowering my SOC set point to maybe 80% or even 75%. I imagine the generator would probably never come on at those settings.

I know voltage is not a good indicator of battery level, but when the 85% SOC point is reached I see voltages right around 48v. This is of course under load, and why I have been reluctant to lower that setting a bit.

Whattya think??? Oh, you can see my details in my signature...

Paul

niel · January 2013

Re: Deeper Discharge of Batteries

it probably wouldn't hurt it dropping it down to 80%, but if it were me i'd most likely leave it alone. remember that 48v under load is different than 48v at rest several hours.

Blackcherry04 · January 2013

Re: Deeper Discharge of Batteries

Opinions will vary here for sure. Batteries are expendable, it's about time ( years ), cycles, replacement cost. Your in the best 3 years now while they are new before the capacity starts to fade. The strategy you use today, will change over time. You have to think of fuel costs, when you do use the generator and maintenance costs. Lifestyle and your loads come into play.

If it was mine I'd drop it down some and then measure your Put-back capability. As long as you can get back to float a couple times a week, you'll be fine. The 50% DOD didn't fall from the sky. It's a point where all the tradeoff's come into play and converge, but it doesn't fit everyone. A 7-10 year old set of batteries are pretty ugly to deal with, what a relief it is the day you get the new ones.

Cariboocoot · January 2013

Re: Deeper Discharge of Batteries

Yes, every situation is different. The 50% rule-of-thumb comes in to play because of replacement, then need for great recharging capacity to bring it back up in a day, and the fact that batteries habitually drawn down that far may just give up one day.

Personally I think systems balance well at 25% DOD (75% SOC) daily use. I'm not a big fan of auto-gen start but would not want to run the generator needlessly as fuel costs a lot of money. Therefor a kW from a gen is more expensive than from the panels.

Usually to run/not run is a choice based on predicting whether or not the panels will be able to accomplish the job. You don't want the gen starting because you're at 75% @ 8:00 AM on a sunny day. But rigging up a gen start that can predict the weather ... I don't know. Clock to tell the time, something to measure panel current, history data to say "normally it's 'X' Amps by now" and then have the gen start if it's <X ... it's possible.

Anyway I'd let them cycle 25% as a matter of course, and have the gen come on if the SOC gets below 75% or if the under load Voltage drops to 48.

Vic · January 2013

Re: Deeper Discharge of Batteries

Hi Paul,

I believe that Flooded batteries should be cycled below 75% SOC, on occasion. It IS good for them, and builds/maintains Capacity. On the banks here, do this manually, on occasion -- about once per month as time permits. This is a time-consuming process, as the discharge needs to be monitored frequently, and the recharge must be begun on the same day. With larger banks it can be very time consuming.

I know that you travel a lot, Paul, so AutoGenstart is probably a must for you. But, when you will be around the home for a few days, it would probablly benefit the batteries to cycle them more deeply on occasion. As much as I hate to do it, I shut off the PV input (really switch the CC Mode to OFF) for three or so days to allow the bank to discharge to about 80% SOC, and take the bank lower yet on a following morning by running AC heaters for about four hours to get closer to 50%, then start the recharge -- usually from PV.

As you noted, voltage is not a great indicator of SOC, but there is little else one can do for AGS.

For daily recharge, believe that 80 - 85% SOC is fine for AGS, just let the bank see a lower SOC every 3-4 weeks would be good, IMHO. Opinions will differ on this.

Member, Sphephandv has been doing some very interesting stuff with external microcontrollers commanding MN Classic CCs. Appears that he does not allow the CC to recharge the bank (or at least not fully), depending on inferred SOC, and the weather forecast is also a variable. Pretty innovative stuff, in my book.

Looks like you are doing well with your batts, but mixing up the SOC where the recharge begins will help overall battery health in my opinion. Good Luck, Vic

Photowhit · January 2013

Re: Deeper Discharge of Batteries

I would suggest allowing them to discharge a bit more. The big Rolls batteries should be fine and save some gas. Kinda curious what loads you have when your seeing 48 volts, at a percived 85% SOC. You have a large robust battery bank and I would think it would have to be a serious load to see voltages that low at 85% SOC.

NorthGuy · January 2013

Re: Deeper Discharge of Batteries

If you look at the specifications, Surrette promises for your batteries the cycle life depending on the discharge

At 80% SOC - 2000 cycles or 5.5 yr of daily cycling
At 70% SOC - 1707 cycles or 4.7 yr of daily cycling
At 60% SOC - 1493 cycles or 4.1 yr of daily cycling

If that's true, then by cycling to 80% you use 0.050% of battery life per day, but by cycling to 70% you use 0.059%. Assuming you do 70% for 4 month and 80% for the rest of the year, you will use 0.059*120 + 0.050*245 = 19.33% of your batteries per year, and they will last 5.2 years. Thus, you will shorten your battety life by 5.5-5.2 = 0.3 yr = 4 months. So, by avoiding using the generator completely, you will have to replace batteries about 4 months sooner.

The question is whether the cost of fuel and generator amortization is worth those extra 4 months of battery life.

Of course, this is all theoretical and real life may be very different from Surrette specs.

BB. · January 2013

Re: Deeper Discharge of Batteries

Monitor your charging current too... Lead Acid batteries tend to be come "less efficient" at charging in the 80-90% (and higher) range (lots of gasing, higher charging voltages, more waste heat from charging, etc.).

I would log the charging current and when the current falls significantly below 50% of rated generator capacity, I would start thinking about shutting down the genset and set the charging complete with solar panels (during winter, you could let the generator go >90% SOC a couple times a week during bad weather/heavy power usage).

Starting the genset at 85% is probably too high... Unless it is a pretty small genset with a good sized AC Battery charger and you are loading it significantly (over 50% of rated Genset Load).

If you are charging 85-95% SOC with a genset, you might be "wasting" a fair amount of fuel (unless you have other AC loads that are sharing the genset at the same time).

All about "balancing".

-Bill

plongson · January 2013

Re: Deeper Discharge of Batteries

Thanks all, for some very good suggestions. Now the system is essentially dialed in, it's time to take it to the next level and make more efficient use of the genset and as it has been suggested, go deeper on the batteries. My first step is to go down to 80% and watch closely.

My genset is huge, 21 kw Kubota diesel that is in the design plan of the place primary for pumping water from a deep well for fruit trees. Unfortunately it is inefficient at charging duties because of the size. On the other hand, I never worry about overloading it...LOL. It burns about .5 gal/hr at the loads I throw at it. I can do and EQ, bake a turkey in the oven, have every light on in the house, pump water for the trees and run a frigg'n welder and it never even sounds like it has a load.

Photowhit: As Vic mentioned, my job takes me away for a week or so at a time and the place is basically on autopilot until I get back. So far it has worked fabulous, but that means during the winter when we're not there to stoke the stove, I rely on a propane forced air furnace to keep it a 55*f. and that's where the draw comes from pulling down the voltage. I also had to set the thermostat in the garage heater (where the batteries are) to 40F because the temps have been so cold locally. It hit -16f a week ago and never got above 10f for a week.

I monitor the system remotely with the Midnite status panel app and just bought the Magnum MAGWEB-e to further keep an eye on things.

Vic really worked with me early on coaching me on the finer points of managing the system and to him I am very thankful. I think I'm on the right track here due in no small part to the knowledge and experience of all of you.

THANKS! and I'll let you know how it works out.

Paul

westbranch · January 2013

Re: Deeper Discharge of Batteries

Paul, do you have a way of monitoring the temp in the battery box?
It might be interesting to match that with the temp external to the bat box. You might be surprised at how warm they stay fromthe charging they get. My old system was in an unheated but insulated 4x8x8 room and at -20*C it was ~5* above freezing in the batt room on a sunny day.
you might be able to not heat that space individually.

plongson · January 2013

Re: Deeper Discharge of Batteries

Ya know, I do. The Magnum has a thermometer and the Midnite has a battery temp sensor. I can set them up for an in/out comparison.

westbranch · January 2013

Re: Deeper Discharge of Batteries

can you poll/record those 2 while you are gone?

plongson · January 2013

Re: Deeper Discharge of Batteries

I can on the Midnite as long as it's on and minimized, and it will log. On the MAGWEB, I've yet to install it. Probably this weekend but the example I saw, it was also capable.

vtmaps · January 2013

Re: Deeper Discharge of Batteries

plongson wrote: »

My genset is huge, 21 kw Kubota diesel that is in the design plan of the place primary for pumping water from a deep well for fruit trees. Unfortunately it is inefficient at charging duties because of the size. On the other hand, I never worry about overloading it...LOL. It burns about .5 gal/hr at the loads I throw at it. I can do and EQ, bake a turkey in the oven, have every light on in the house, pump water for the trees and run a frigg'n welder and it never even sounds like it has a load.

Do you know about 'wet stacking' ? It sounds like your generator may be heading that way. --vtMaps

Photowhit · January 2013

Re: Deeper Discharge of Batteries

plongson wrote: »

T... but that means during the winter when we're not there to stoke the stove, I rely on a propane forced air furnace to keep it a 55*f. and that's where the draw comes from pulling down the voltage.

That must be some big fan, in comparison, my 24V 800 Ah battery bank might get to 24V if I run the microwave(900watt) in the morning after a night running the A/C on thermostat, I'd guess that's about a 80% SOC, though don't have a monitor on it. In comparison you have a 48V 800Ah battery and dropping to 48V running a fan!!!

plongson wrote: »

I also had to set the thermostat in the garage heater (where the batteries are) to 40F because the temps have been so cold locally. It hit -16f a week ago and never got above 10f for a week.

My bet is your batteries in the garage haven't gotten below 10f, 0 if it's a stand aloan garage. Even at -20 they have about 1/2 the rated capacity. Their being cooler might contribute to the low voltage reading, but the 48V reading sounds hinky to me, unless that's one heck of a fan/motor.

waynefromnscanada · January 2013

Re: Deeper Discharge of Batteries

NorthGuy wrote: »

If you look at the specifications, Surrette promises for your batteries the cycle life depending on the discharge

At 80% SOC - 2000 cycles or 5.5 yr of daily cycling
At 70% SOC - 1707 cycles or 4.7 yr of daily cycling
At 60% SOC - 1493 cycles or 4.1 yr of daily cycling

If that's true, then by cycling to 80% you use 0.050% of battery life per day, but by cycling to 70% you use 0.059%. Assuming you do 70% for 4 month and 80% for the rest of the year, you will use 0.059*120 + 0.050*245 = 19.33% of your batteries per year, and they will last 5.2 years. Thus, you will shorten your battety life by 5.5-5.2 = 0.3 yr = 4 months. So, by avoiding using the generator completely, you will have to replace batteries about 4 months sooner.

The question is whether the cost of fuel and generator amortization is worth those extra 4 months of battery life.

Of course, this is all theoretical and real life may be very different from Surrette specs.

Well, my six L-16 batteries are now into their 11'th year, still going strong. Lucky or could it be that over the years, they're only taken down roughly 5 or 10% per day? And even less than that since the mini-micro hydro came on stream. Example: the sun set here a good 14 hours ago, and at the moment with neither of the two freezers, the water pump, nor the fridge running, the battery voltage is 14.8 Of course the batteries are at minus 7*C at the moment, so it takes higher voltages to put anything back. Still on a daily bases, when the sun comes up, within minutes (in the winter as the sun is so far south it hits my vertical panels as soon as it pops up over the horizon) the MX-60 switches to float.
Perhaps I'm just very lucky, perhaps these "Power Battery" flooded wonders are of an unusual chemistry. I know the SG has been considerably higher than what seems to be usual for other batteries spoken of on this forum, and they've been that way since new. Couple years ago my cousin purchased two Surrette L-16 batteries and the SG was so much lower than mine that I thought they were no good. Yet they continue to function fine even though they get very little or no use between the times his wife abuses them. I've checked their SG every 6 months or so when I get to his camp, (he still can't find the time do it himself) and it's always more or less the same. Low compared to mine, yet bubbling if the sun is shining, and apparently fully charged..

plongson · January 2013

Re: Deeper Discharge of Batteries

There isn't any problem with the fans or sizing. It's probably the way I'm looking at the history. What I'm referring to is the lowest voltage recorded on the Magnum remote during the time I was away. Usually it is between 48 and 49 somewhere with the lowest ever recorded 47.9vdc

I've been striving to keep the garage and battery area above 32f, for the battery's sake and to keep the main room above (main living area) warmer. What temp do I absolutely want to keep the batteries above?

As far a the concern over wet stacking the gen-set. That too is no problem. I have investigated that on and off over the years and there is no indication of this phenomena. Wet stacking has a tell-tale "DROOL" of wet soot at the exhaust pipe joints and this unit is dry right up to the exhaust manifold.

I think the modern computer controlled diesel engines with electronic injector pumps etc are less pron to wet-stacking. But you are correct, diesel's really like to be loaded and run MUCH better/happier working hard.

Paul

ChrisOlson · January 2013

Re: Deeper Discharge of Batteries

plongson wrote: »

I've read about the problems of not discharging enough and I've been wondering if I should start lowering my SOC set point to maybe 80% or even 75%. I imagine the generator would probably never come on at those settings.

Everybody is different, but we cycle our batteries from 50-80% SOC for days at at time. Our system is auto-gen start too. With time I have learned how to set the gen start parameters so it doesn't start the gen as long as the bank gets to 80% SOC in a 24 hour period. One of two things will happen every week to 10 days - either we'll get a good day when the wind blows like crazy and catches them up with a full absorb - or the weather will turn worse and the system will finally start the generator to catch it up.

Our batteries don't seem to mind being worked at 50-80% for up to 10 days at a time. And we waste a lot less power than most folks do in charging losses because the most inefficient part (along with gassing and water loss) is taking them from 85% to full charge. My experience in the past has been that batteries that live their entire lives on the top end tend to die an untimely death. Batteries don't live their lives in 24 hour cycles like humans do.
--
Chris

plongson · January 2013

Re: Deeper Discharge of Batteries

Hey Chris, Thanks for posting. Reading your sticky about using the genset amp start parameter really got me going on perfecting that setting on my system. Great write-up...I'm going to play with it a bit this weekend as a matter of fact.

Ya, I'm more convinced than ever that deeper discharging has more benefits than always keeping them fully topped off. With everything...moderation...but I'm going to play with the 75%-80% range for now. My problem is if I'm out of town on a job I cant throw in a life line or change a parameter so i have to take it in small steps.

The fewer times that genset starts in the dead of winter the better I like it, and when it starts I wanna get it loaded up.

ChrisOlson · January 2013

Re: Deeper Discharge of Batteries

plongson wrote: »

Ya, I'm more convinced than ever that deeper discharging has more benefits than always keeping them fully topped off. With everything...moderation...but I'm going to play with the 75%-80% range for now. My problem is if I'm out of town on a job I cant throw in a life line or change a parameter so i have to take it in small steps.

That's the thing - everybody's "normal" loads are different so the setting parameters on when to start the generator are different. If you're like us where our normal evening time loads in the winter run at 1,000-2,000 watts for several hours, even with a fully charged bank the voltage drops below 48 under load. So if I had the generator set to start at 48 volts after two hours the darn thing would be running every night - and it wouldn't need to be running. We only very, very rarely see the generator run for battery charging. I would say better than 95% of the time it's for load amps, or peak load management, to keep the inverter below its continuous rated power.

When the generator does start for battery charging it's usually because of the 15 minute timer - and usually because the bank is down too far to carry a heavy load that is below the load amps threshold, but high enough to cause a good voltage sag due to the bank being close to 50% SOC. The only time it ever starts for battery charging on the 24 hour timer is after several bad days where the bank one day doesn't make it to 80% SOC. I have that 24 hour timer set low enough so that if the bank reaches that level on an "average" day that experience has shown me it's to 80% and that's all we need to make it another day.

To some this would be considered "abusing" the batteries. But they seem to like it. We got one of those little MidNite Battery Capacity Meters and the 10-100% SOC scale I take with a grain of salt because it's really just a LED type volt meter and doesn't show real SOC unless the batteries have been at rest for several hours. But the three LED's on the left are ones I pay attention to. If that goes from green to yellow then I know we've been "floating" the bank from 50-80% SOC too long and I start watching it to see if the gen auto-start is going to take over and "fix" it. With the settings I'm using now, just about with dead reliability within three days of when that light turns yellow either the wind turbines will fix it on a windy day, or the generator will start and fix it.

We probably use a lot more power than you do (we consume about 30 kWh/day in the winter), but I know for a fact that if I used the "traditional" settings that most people would use that our generator would be running every single day for battery charging - and we'd be wasting a lot of fuel doing it that doesn't have to get wasted.
--
Chris

Deeper Discharge of Batteries

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