When to run the Generator

Rngr275 · October 2012

Since I am really the FNG to the Off Grid world I am on a huge learning curve... and here is one that would have come up sooner or later.

Since Saturday there has been very little sun and my battery bank is slowly going down. @94% on Saturday, 85% on Sunday, and @ 79% when I left for work this morning. Now with Sandy headed here it looks like there will be another 2-3 days of little PV power. So my question is when and how to charge the batteries with my generator. Do I run it a few hours at a time to bump up the batteries everyday or other day. Run it till they batteries are nearly charged (90+% range), Run the generator until float stage is reached, run it into float, etc.

Just not sure what there perferred way would be.

As a side note It was recommended to me by my installer and Trojan to let my new batteries go down to the 50% SOC range to help break them in... looks like that won't be a problem.

Did I mention how cool it is not to worry about loosing power!!!

westbranch · October 2012

Re: When to run the Generator

considering the situation, you can do it anyway you need , objective is to get to absorb after you get home tonight, worry about float after Sandy is finished IMHO, then when the sun comes out gen set bulk and PV finish the charge.

Cariboocoot · October 2012

Re: When to run the Generator

Best option is to run it when it will do the most good. Running it when the batteries are mostly charged will use a lot of fuel for little power because the generator will be lightly loaded.

If you've planned your system for 25% DOD daily with a 25% reserve for the next day, then the third day you start the gen and Bulk up. If you're going to have a long, drawn-out period of no sun so the panels don't finish charging then you have to bite the bullet and keep it on until the batteries are fully recharged. This should be every three days, maybe four. It's something of a judgement call. If at all possible, save up the laundry for that "got to keep the gen running 'til charging is done" day to make the best use of its capacity.

I've just done this schtick this Summer with the panels off the roof for a week while I repaired the roof. Bulked the batteries in the morning for two hours, and again in the evening for one. Twice I let it run 'til Float was achieved. Nice thing about the inverter-generators is their low over-all fuel consumption under light loads; I would have use a lot more gasoline with a fixed RPM gen of the same Watt capacity.

Blackcherry04 · October 2012

Re: When to run the Generator

It's hard being the FNG.....lol. Now you have the 50%-80% of DOD to work with ( 30% of Capacity ). If your going to take the Bulk voltage back to the Absorb Trigger voltage It's a good time to use some opportunity loads. In my case it all depends, I know that have to take my Bulk Voltage back up to at least to 27.2 Bulk charging voltage ( actual battery voltage of 25.1 ) so that I will be at 24.1 ( 50%) in the morning based on my overnight loads, my absorb is 28.7. To me is not worth running the generator long enough to get to the Absorb voltage everyday. It is true you need to do that every 3-4 days, so I play it depending on the weather. Your's will be different and if you use a monitor system is in %'s you can use that if you know it mirrors your system.

With the number of batteries you have and a 45 amp charger, your going to have to test and see how long it takes to get to Absorb and decide how often you want to run a full cycle.

Ralph Day · October 2012

Re: When to run the Generator

Do you have a good hydrometer yet? I see you're talking about %age discharging via a monitor...don't forget that the only true state of charge measurement is via measurement of the specific gravity of your electrolyte. When you think you are at 80 % are you really? I stopped looking at the %age state of charge number on my monitor after the first year...it was never accurate even when calibrated. And especially don't rely on a meter to tell you soc during a generator run...sg, sg, sg (on those 3 or 4 day absorb runs especially). I was always low balling when to turn off the generator and didn't get as charged as I thought I was getting.

Ralph

unicornio · October 2012

Re: When to run the Generator

I usually trust my monitor (FLEXnet DC) to start the genset with the % of soc ... once properly set up I believe is accurate enough to make the decision to start the genset at 60% SOC and stop at 85% SOC with sufficient accuracy ... it is true that you need to configure the FLEXnet dC properly, and for that, you must use the hydrometer, and that the use of the hydrometer is necessary and essential to know with absolute accuracy the state of battery ...

I think it's best to use the genset at its best zone of fuel consumption/power curve, so I do not usually have it running beyond the first 30 minutes of absorption ...

I also think that sometimes (occasionally, depending on usage and battery and cycling) is very good to reach the 50% of SOC, and then return to do a full charge and equalization ... in these cases, (and if necessary due to weather conditions) I can afford to spend some fuel with little return, if i needed to care exquisitely my batteries ...

of course, I speak of those old times when my genset working properly!...;-)

ChrisOlson · October 2012

Re: When to run the Generator

Rngr275 wrote: »

So my question is when and how to charge the batteries with my generator. Do I run it a few hours at a time to bump up the batteries everyday or other day. Run it till they batteries are nearly charged (90+% range), Run the generator until float stage is reached, run it into float, etc.

Our system is all automatic and the inverter starts the generator as required. But sometimes it helps to take a look at automatic systems to see how they do it - because automatic systems typically take excellent care of the batteries.

I don't know how other folks do it, but I don't believe in floating batteries with a generator. Just bulk and absorb the bank, then shut the gen off. To maximize kWh/gallon of fuel burned it's good to have an auxiliary load for the generator. We use water heating (also automatic) to keep it fully loaded during absorb.

I'd say the two most important timers are the 24 hour and 2 hour.

The 24 hour timer starts the generator if the bank hasn't been above 27.0 volts (24 volt system) for 24 consecutive hours. During fair conditions the bank can usually make it to 27.0 volts once a day. Sometimes they will go for a full week barely making it above 27.0. Then a day comes when they don't. The generator starts and tops the bank off. The batteries like it because they haven't been really discharged that bad for the week, but they haven't been boiled either. The 24 hour timer takes care of that.

The 2 hour timer starts the generator if the bank voltage drops below 23.5 volts for 120 consecutive minutes. This charges the bank up in the event it drops below 50% SOC.

There's also 15 minute and 30 second start timers in the gen setup menu, however I've never seen either one get used because one of the other two takes care of the problem first. But theoretically, if you had a huge load applied to the bank the voltage could rapidly drop to the 15 minute timer threshold, or even the 30 second threshold, before the 2 hour timer had a chance to start the generator. So the 15 minute timer is set to 23.0 volts, and the 30 second timer is set LBCO (22.0 volts).

I think these basic principles can be applied to manually started generators too. At 79% SOC after three days, that's not really concern for starting the generator yet if it's been up to 85% in the last 24 hours. At least not in my opinion it's not - it would a be a waste of fuel. On day four, if it never reaches 85% then consider starting it and top them off. But it doesn't concern me too much to see our bank cycle from 60% - 85% SOC for a week at at time because the charging losses are a lot less - meaning more of your kilowatt-hours you generate with solar and wind are used in the house instead of being wasted in battery charging inefficiency constantly topping batteries off every day.
--
Chris

RCinFLA · October 2012

Re: When to run the Generator

If you want to maximize fuel efficiency then load gen at 50% to 75% of its continuous duty power level for constant current bulk phase of charging. This of course may be limited by your charger capability or battery AH size being able to consume that much gen power. This also depends on what type of charger you have, being whether it has a good power factor. Generators are rated for V*A, not true power watts so a poor power factor charger means larger gen for the same actual charge power delivered to batteries.

Just let gen run until absorb voltage is reached for max fuel efficiency. This will be about 75% to 85% SOC depending on % of AH you run the constant current bulking phase recharge current at. You will lose some ground (battery capacity) each day charging this way and if you go solely on gen for 5 to 7 days you might want to allow charging to go through absorb current drop off cycle to re-top off batteries every 5 to 7 days. Most battery metering systems need a full recharge cycle to accurately reset their 'Battery full' assumption even though they often reset to 'Battery full' when bulk voltage is first reached. Better meters require some amount of time be spent at absorb voltage to trigger full reset.

Depending on generator type anything less then 20%-25% VA rating loading will waste fuel. Air cooled gens consume a fair amount of fuel just pushing the engine cooling air with their constant 3600 rpm flywheel blowers.

If your gen VA rating is significantly higher then your charging/battery size consumption then plan to additionally AC load something useful that gets the overall load on the generator up above 50% rated during your charging period. (run washer, pump water, etc.)

Blackcherry04 · October 2012

Re: When to run the Generator

I guess I am the Automatic system with my Honda 2000 EU. Chris, the logic your system has is something no one could disagree with.

I also agree with Ralph, it doesn't really matter what you use, Volts, Amps In / Out or % of SOC. Knowing your SG and how it is calibrated against your measuring stick is what's important. I have had several Monitoring Systems over the years with shunts and all kinds of calibration offsets. Over time they all have drifted off the actual SOC. Temperatures change, battery's age, Charge AMP's change, I have my little spread sheet cheat sheet and a LED blue sea meter ( with 3/4 in digits and 2 decimal places ) and I measure volts these days. I can look at the Volts and tell exactly what I need to do based on the time of day and weather.

ChrisOlson · October 2012

Re: When to run the Generator

Blackcherry04 wrote: »

I have had several Monitoring Systems over the years with shunts and all kinds of calibration offsets. Over time they all have drifted off the actual SOC.

I would tend to agree. It seems all the new inverter auto gen starters are using battery SOC to determine when to start and stop the generator. The systems measure amps in and amps out and keep a running tally. Back 3-4 years ago I had a Trimetric and it seems like it was always off. Nowadays I have gone to just using an intelligent voltmeter in the form of a MidNite Solar Battery Capacity Meter mounted to the kitchen wall by the entry door where we can always see it. It don't have no numbers on it or anything that needs to be studied to determine the current status of the bank - just some LED's on it. We like it because we can tell at a glance from across the room where the bank is at. If it gets down to what my wife calls "Red 20" she gets worried if the generator don't start and put it back up to what she calls "Green 100". LOL!
--
Chris

vtmaps · October 2012

Re: When to run the Generator

Ralph Day wrote: »

Do you have a good hydrometer yet? I see you're talking about %age discharging via a monitor...don't forget that the only true state of charge measurement is via measurement of the specific gravity of your electrolyte. When you think you are at 80 % are you really?

I don't see how you could expect your hydrometer to be accurate at 80% SOC. At 80% SOC you probably have some stratification of electrolyte and what you are measuring is dilute electrolyte. I don't think you can trust SG readings until after the battery has been gassing for a while.

If my battery monitor says 80% and my hydrometer says 70%, I would trust the monitor. If (after charging) the monitor says 95% and the hydrometer says 90%, I would trust the hydrometer.

--vtMaps

Rngr275 · October 2012

Re: When to run the Generator

The %SOC from the FNDC is used as quick reference for myself knowing that the actual %SOC as measured with the Hydrometer typically reads ~ 10% lower. The batteries were not @ full charge when the system was fired up and according to Trojan the batteries may not get to full capacity until they cycled a number of times, and not just lightly (say 85-100%). They need some heavy cycling 40-100%.

Anyway made it home and the FNDC read~ 78% and I measure my test cells and they were @ ~ 1.19 (Temp Compensated). I fired up the genny and let it run about 2.5hrs but then I had to leave to take care of some Hurricane stuff. SOC was ~ 91% on the FNDC which is in the low 80% range on the Hydro. Looks like some sun in a couple days so I am trying to get down to ~ 50% (measured with SG) and the use Genny and sun to bring all the way up, giving the battery bank a long hard charge (does that sound pornographic) as recommended by Trojan. If the weather changes it will be up to the generator. It does take a bit... max inverter charge rate is 45amps into a 740Ah bank of batteries takes... well I am sure there is a way to calculate how long it takes to charge from different DOD's.

Funny thing when the inverter swithes to the generator all the clocks bump forward some random time... could have made me real early for work.

Blackcherry04 · October 2012

Re: When to run the Generator

Well. you can get away with a lot in the first 2-3 years in a Battery Bank's life. Once they get to half life, thats when the struggle begins. Keeping up with your Batteries reduced capacity and your Monitors readings keep you hopping. Learning about your system now will certainly help in the future.

Vic · October 2012

Re: When to run the Generator

vtm

Regarding Hydrometer inaccuracy due to stratification, without having had an Absorption stage, IMHO is quite a bit of lore.

For the TALL batteries here, the only time that I see stratfication, is just after water has been added to the cells without SOME Absorb, or high A.sorb V charging for just a few minutes.

One bank fairly often runs with PV input off for days and days. See NO precipitous drop in SGs. Just a slow SG decrease associated with the charge removed from the batteries, and perhaps some small amount of self-discharge.

Believe that MorningStar started this with an L-16 hotter charge V setting for the venerable PWM CCs a couple of decades ago. It is possible that "HC" batteries could be more prone to stratification, with the attendant 1.277 - 1.280 SG electrolyte. But, sure have NOT seen any indication of stratification here either with L-16s or other even "taller" banks of standard Deep-Cycle/1.265 SG construction.

It is possible that COLD batteries would be more prone to stratification, and vt, think that you are in a much colder climate than I, although your batts may be cozy warmish in the Winter.

The Hydrometer is MY friend ... Opinions, Vic

Cariboocoot · October 2012

Re: When to run the Generator

If the electrolyte is stratified it will show on a hydrometer as 'low SG' (because most of the acid content is at the bottom) however this is not an inaccurate reading because the battery is indeed in need of charging. You will not have higher capacity than read with the plates "half in acid, half in water"; you need all of the plates to be in the same % electrolyte solution.

Cold batteries are more likely to show signs of this, but cold climate does not necessarily equate to cold batteries. Battery temperature rarely gets near ambient if you keep the charging system active.

mtdoc · October 2012

Re: When to run the Generator

Rngr275 wrote: »

The %SOC from the FNDC is used as quick reference for myself knowing that the actual %SOC as measured with the Hydrometer typically reads ~ 10% lower.

Rngr,

If your FNDC is consistently reading 10% higher than your hydrometer readings then you probably need to adjust the battery charge factor down on the FNDC set up. Once your batteries are "broken in" as Trojan says then perhaps you will get better charge efficiency and you will need to readjust the BCF.

vtmaps · October 2012

Re: When to run the Generator

Vic wrote: »

Regarding Hydrometer inaccuracy due to stratification, without having had an Absorption stage, IMHO is quite a bit of lore.

Vic, are you saying that the absorption stage is what causes stratification? --vtMaps

Vic · October 2012

Re: When to run the Generator

vt,

For the TALL batteries here, the ONLY TIME that I see STRATIFICATION, is just after water has been added to the cells without SOME Absorb, or high A.sorb V charging for just a few minutes.

IS what I was trying to say.

sorry that I did not make it clearer.

Otra vez; Having a bank of tall FLA off charge for days, and with light loads on them, yields only the expected slow, small reduction in SGs, as MEASURED using a Hydrometer. If there was a propensity of them to stratify, would expect the measured SGs would fall at a greater rate, and to a lower level

Have done partial charges, using Bulk to about 55 V, and without an Absorption for a number of days, and see the SGs rise, as expected.

I have seen stratification just after adding water, also as expected, but only a few minutes of Absorption voltage seems to do a fine job of mixing, to expected SG levels. All of the measurements were performed with a Hydrometer.

This is what I was trying to say. In my personal experience, it seems to me that the stratification issue is more concern than problem.

So, IMHO, smallish discharges in one day, or over a few days, without ever getting to Vabs should not yield erronious SG readings due to stratification. This is based on my experience. That is all, Thanks, Vic

Cariboocoot · October 2012

Re: When to run the Generator

Vic wrote: »

This is what I was trying to say. In my personal experience, it seems to me that the stratification issue is more concern than problem.

Right you are, Vic; you will rarely see any significant stratification in a set of batteries that is being used. The Bulk cycle pushes the sulphor off the plates and sends the electrons back where they belong, the Absorb cycle finishes the job and stirs everything back up nicely.

But leave some of those tall case batteries just sitting about, even with a Float charge on them, and stratification can be an issue. Another reason why actually cycling a deep cycle battery, at least occasionally, is better than not.

ChrisOlson · October 2012

Re: When to run the Generator

Cariboocoot wrote: »

Right you are, Vic; you will rarely see any significant stratification in a set of batteries that is being used. The Bulk cycle pushes the sulphor off the plates and sends the electrons back where they belong, the Absorb cycle finishes the job and stirs everything back up nicely.

One of the tech support people at Surrette told me that insufficient charging voltage and absorb time (he called it deficit charging) will cause electrolyte stratification. When we first got our new bank I set absorb at 30.0 volts and could never get the SG above 1.250 and the batteries never used any water. So I called Surrette and the guy told me that because the batteries we got are lead-calcium I have to bump the absorb voltage up to 31.7. But because I had been deficit charging them for three months that I should immediately do a equalization at 33.0 volts because the electrolyte was stratified.

So I followed his instructions. It took 6 hours @ 33.0 volts to finally get the SG up to 1.275. They've been fine ever since with the Classics reset to absorb them at 31.7. We just had a week of poor solar and wind conditions and the batteries got really out of shape and never got fully charged all week. But today the sun came out nice and bright and the wind blew and we got over 50 kWh. The bank hit 32.0 volts (the batteries were a little cold) at 12:30 PM and dropped into float 3:40 PM. I checked a few random cells to see how they did and they were all at 1.275 after only 3 hours of absorb time.

So I can only go by what I know and what the tech support people at Surrette told me, but I do believe that the electrolyte can stratify on tall case batteries, even if they're being used, if they're not properly charged.
--
Chris

Rngr275 · October 2012

Re: When to run the Generator

mtdoc wrote: »

Rngr,

If your FNDC is consistently reading 10% higher than your hydrometer readings then you probably need to adjust the battery charge factor down on the FNDC set up. Once your batteries are "broken in" as Trojan says then perhaps you will get better charge efficiency and you will need to readjust the BCF.

I think part of the problem is that the SG was not checked initially and when the FNDC was turned on/initiated it assumed that the batteries were at 100% but the SG doesn'y reflect that. I agree though that the BFC could and probably does need adjusting. Down is the correct direction... right? and only a little I would expect.

On another note why does my battery voltage read 50.8v which is about full charge but the SG still calculates out to ~90% charge??? I have been scrathing my head in this one. Isn't the voltage reading on the MAte/FNDC the battery voltage?

Blackcherry04 · October 2012

Re: When to run the Generator

Rngr275 wrote: »

On another note why does my battery voltage read 50.8v which is about full charge but the SG still calculates out to ~90% charge??? I have been scrathing my head in this one. Isn't the voltage reading on the MAte/FNDC the battery voltage?

It would be nice if they did, but the clear answer they hardly ever do. Some have added a separate " Sense " line, but even those can be wrong. My Xantrex SCP is off .4 and thats not unusual. They use the Voltage at the inverter and it's effected by the wire , resistance and distance. Even the charger can effect it. Thats why I posted that I use a digital VM connected directly to the batteries and verified with 3 other meters. Again, knowing your system and using your own offsets help. All the rest of the Monitors are window dressing that may be right or may be wrong.

BB. · October 2012

Re: When to run the Generator

The "resting voltage" of a battery bank is a combination of SG level and battery temperature...

Cold batteries will read higher voltage. And batteries may be filled with higher or lower than 1.275 SG electrolyte (higher SG gives battery more storage capacity, a bit shorter life; Lower SG gives a bit longer life and is sometimes done for "tropical" locations to increase life too).

Also, as batteries age, the lock up the sulfur from the electrolyte in sulftation--which drops SG over time too.

-Bill

When to run the Generator

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