How would I estimate the time to charge FLA from 50% SOC using a generator?

[Living Large]

Proposed goal: estimate time to charge an FLA battery bank from 50% SOC to 100%, to determine generator run time after several cloudy days.
Assumption: there hasn't been sun, and no sun is coming for another day or two.

Please remember - I have no experience with solar, and am trying to estimate a system and visualize modes of operation and real world consequences.

I found a graph that purports to represent a charge curve for FLA batteries, attached below. My particular bank would be 800Ah @ 48V, and I would charge at C/10 or 80 A in bulk mode. Now I am at 50% DOD, and I want to charge. From the graph, which may or may not represent an actual battery, it appears the energy transferred in the absorb mode is about half that in bulk mode. The energy transferred in float by comparison is relatively small.

An armchair analysis of this generalized graph would be I need to add 50% of capacity, 400Ah - about (2/3)400 in bulk and (1/3)400 in absorb. In bulk, the time would be (2/3)400 / 80 = 10/3 or 3.3 hours. In absorb, (1/3)400/ (2(80)) = 5/6 hours = .83 hours

So that is about 4.1 hours +/-. Would I extend this time by maybe 10% to account for battery efficiency? That's about 4.5 hours. Is this a rational estimate of the run time needed?

Now the question of float comes up. It doesn't seem efficient at all to run a generator for hours to float the batteries. Might I plan to start early in the morning with generator charge from 50%, and then by noon or early afternoon, shut off the generator and allow the PV at 10% or less output to float the batteries?

Am I anywhere near a reasonable estimate, or have I made serious errors? I did ignore loads on the system, which won't be an issue other than once the generator is shut off. The only question I see there is if the PV in low output would generate enough to float the batteries and power needed loads. Is full float critical? If by evening when I start loading the batteries with lights, short runs of a microwave for cooking, etc, the fact I didn't float fully an issue in terms of cycling? It doesn't seem you can ever count on a full uninterrupted charge cycle unless you have two banks.

Thanks in advance!

[thastinger]

You would just bulk up the battery bank with the charger and not do the absorb or float cycle.
With a good CC, you'll almost always get a little something from the panels even on the worst days.
I only use my generator to EQ the battery bank, I wait for a forecasted sunny day then start it in the early morning, get the battery bank to the bulk level then use PV to finish the eq.

You're going to need a 4.5K or larger gen to do the 80A charge.

[Living Large]

You would just bulk up the battery bank with the charger and not do the absorb or float cycle.
With a good CC, you'll almost always get a little something from the panels even on the worst days.
I only use my generator to EQ the battery bank, I wait for a forecasted sunny day then start it in the early morning, get the battery bank to the bulk level then use PV to finish the eq.

You're going to need a 4.5K or larger gen to do the 80A charge.

Hey, thanks for the comment. Useful. I was on the right track, perhaps. I guess it might be optional to leave the generator on at the beginning of absorb, as needed by the conditions (how much power is coming from the PV.

Yup - I am considering the EcoGen, though I have been warned about using propane from another user here. I want a quiet generator, and it is the only one I have found so far that is quiet and puts out the power I believe I need. I guess I could build a shack, but the Ecogen can be freestanding. That is another thread - how to prepare for temps of 20 deg F and colder.

[thastinger]

First of all, those standby gens are not quiet, far from it. Secondly, there is nothing "eco" about running a gen on propane but the 100lb tanks shouldn't freeze up on you if you have to go that route. Just keep in mind that you'll have 300 bucks in the setup by the time you buy and fill a single 100lb tank. (I have 2 of the 100lb tanks I use for other purposes), and you'll have to buy new tanks every 10 years. You'll be surprised how little run time you'll get with a gen under that load and fed from a propane tank.


Yamaha makes a 4500 generator and they are quiet and reliable (I have a few hundred hours on our 3000ISeB we use for our camper). I don't know the power requirements of your charger though, I assume it is 240V input? If so you would have to go to the EF6300 to get a 240V outlet.
UScarb sells a tri-fuel kit for these gensets, you can even buy a generator directly from them with the tripower kit already installed. The good thing about these gens is they only run as fast as they need to meet the power demand, so they are much more efficient than the generacs which run at 3650 RPM regardless of load.

[thastinger]

I found the specs on the ecogen, it says 52dB and that is pretty quiet, maybe it is less because it is smaller than the normal standby gens. It also says it is 120 only though, so I assume your charger is 120.

There is 23 gallons of propane in a 100lb tank, which you probably won't get all of it if it is cold outside. Propane costs more per gallon and does not have the BTU energy contained in gallon of gasoline which is cheaper. So you'll use more of it and get less power from it, then you have those tanks to take and get filled etc. Big PITA.

I'd still prefer the EF4500 with the trifuel kit, this way you have the option for gasoline as well and running it off gasoline will be more economical than running it on propane. I'd keep one 100lb propane tank for backup but run off gasoline primarily.

[Living Large]

I found the specs on the ecogen, it says 52dB and that is pretty quiet, maybe it is less because it is smaller than the normal standby gens. It also says it is 120 only though, so I assume your charger is 120.

There is 23 gallons of propane in a 100lb tank, which you probably won't get all of it if it is cold outside. Propane costs more per gallon and does not have the BTU energy contained in gallon of gasoline which is cheaper. So you'll use more of it and get less power from it, then you have those tanks to take and get filled etc. Big PITA.

I'd still prefer the EF4500 with the trifuel kit, this way you have the option for gasoline as well and running it off gasoline will be more economical than running it on propane. I'd keep one 100lb propane tank for backup but run off gasoline primarily.

I'd have a larger tank - at least 250 gallon, and possibly larger. I was wondering if burying is an advantage in terms of temps, though the regulator and load would be above grade. You aren't the first person to question the propane option.

OK - you gave me something else to look at (EF4500). I'll check it out. The Ecogen is 120/240 as I recall. The thing about propane is I have it already for the stove and hot water and proposed boiler for radiant heat.

[Sunking]

FLA batteries should be charged no faster than C/8. To find the optimum size generator to burn the lest amount of fuel in the shortest amount of time take your total battery capacity in watt hours and divide by 4. Example for a 48 volt 400 AH has a capcity of 19,200 wh / 4 = 4800 Kva @ 240 Vac. The battery would require a 50 amp charger for a C/8 charge rate.

LPG is the preferred choice of fuel for off grid systems. Second choice is NG then diesel. Gasoline should be avoided.

[Living Large]

FLA batteries should be charged no faster than C/8. To find the optimum size generator to burn the lest amount of fuel in the shortest amount of time take your total battery capacity in watt hours and divide by 4. Example for a 48 volt 400 AH has a capcity of 19,200 wh / 4 = 4800 Kva @ 240 Vac. The battery would require a 50 amp charger for a C/8 charge rate.

LPG is the preferred choice of fuel for off grid systems. Second choice is NG then diesel. Gasoline should be avoided.

More great info. Thanks. I was unaware of this generator sizing guideline.

I have had a few people cautioning against LPG, because of behavior at 20 deg F and below. I believe the concern is the vaporization rate in BTU's is lowered substantially. For example, maybe a little more than half what it is at 60 deg F. I believe this could lead to fuel starvation under the right circumstances - a small tank and/or a tank that is mostly empty. I was planning for a buried tank of I would imagine a 250, 500 or larger gallon capacity.

[thastinger]

You'll be fine with a buried tank

[Living Large]

You'll be fine with a buried tank

Thanks. By the way, I do realize there is nothing "eco" about the propane generator route. I'd love to use the grid if it were available, and skip all this. It is, but it will cost $90K+. I'm retired, and while babysitting batteries with bubbling acid and worrying constantly about their state of charge and if the sun will be shining will give me something to do, I have to say I am not particularly looking forward to it at this point in the process.

In response to something you pointed out earlier - the Ecogen is 120V, but there is a 240V conversion kit that isn't too expensive that allows you to get the full rated power.

[paulcheung]

Proposed goal: estimate time to charge an FLA battery bank from 50% SOC to 100%, to determine generator run time after several cloudy days.
Assumption: there hasn't been sun, and no sun is coming for another day or two.


An armchair analysis of this generalized graph would be I need to add 50% of capacity, 400Ah - about (2/3)400 in bulk and (1/3)400 in absorb. In bulk, the time would be (2/3)400 / 80 = 10/3 or 3.3 hours. In absorb, (1/3)400/ (2(80)) = 5/6 hours = .83 hours

So that is about 4.1 hours +/-. Would I extend this time by maybe 10% to account for battery efficiency? That's about 4.5 hours. Is this a rational estimate of the run time needed?


Thanks in advance!

I don't know which brand of battery you going to use, but according to Rolls battery Manual your estimates are far out. It take me 4 hours to absorb my battery bank.

The following is copy from Rolls battery manual,

Bulk Charge
The first part of the charging process is the bulk charge. This is when the maximum
amount of current flows into the battery bank until a desired voltage is reached. The
recommended maximum current is 15% of the AH capacity of the battery bank based
on the 6 hour rate. A lower current can be used, but this will prolong the charging time.
Bulk charge voltage set points are outlined in Table 2.

Absorption Charge
Arguably, the most important part of the charge cycle is the absorption charge. Since
the bulk charge only recharges the battery bank to an 80% level, the absorption charge
completes the charging cycle. Most chargers on the market have a timer that allows the
user to adjust the duration for the required time to return the battery to full charge. In
order to set the correct time, a simple calculation is required. With the help of the 20 AH
capacity, you can figure out the remaining charge required for the battery bank.

As stated previously, the bulk charge brings the bank to an 80% charging level. The remaining 20% of the capacity is a function of time and current. The charger will maintain current level until the bulk set point has been reached, then the charger switches to the absorption timer. The current levels start decreasing as the internal resistance in the battery increases. Assume over the time of the absorption charge that 50% of your maximum charge current will be available (this is factored into the equation). 0.42 = (20%/50%) + 5%. 5% is added due to losses.

Absorption Charge time
Where : T = 0.42 x C /I T = ABSORPTION CHARGE TIME C = 20 hr RATED CAPACITY (of the Bank) I = Charging Current ( 10% of C20 max) 0.42 = ( 20%/50%) + 5% (5% is added due to losses)
Example 2 Banks of 6 V S-530 models 20 hr rate = 400 x (2 strings) = 800 ah I = 10% of 800ah = 80 amps. If charger is 60 amps max, 60 is used T = 0.42 x 800/80 = 4.2 hrs OR T = 0.42 x 800/60 = 5.6 hrs

http://support.rollsbattery.com/support/solutions/articles/654-battery-user-manual-flooded-agm

[CraziFuzzy]

You have to realize that a generator is going to be most efficient at a particular load point. Sometimes this information is published, sometime it is not. Ideally, to get the most watts for a given amount of fuel, you should do as much of your charging as possible at or near that load point. If you set up your charger to use that for the bulk rate, then you are going to have the most efficient setup. As you leave bulk (current limited), and go into absorption (voltage limited), the current will drop as SOC increases. At some point during that drop, it will become quite a bit more wasteful, and you'd be better off shutting down the generator and starting over a bulk at a later time. A generator's lowest efficiency is usually at minimum load/idle. Every so often, you DO want to fully charge them, but that may end up happening naturally on a sunny day. Then, every few months or so, you should do an equalization charge, and really top them off.

Now, if you happen to have other loads that could be fed into (a water pump with a expansion large tank, etc), you could use them to keep the generator in it's more efficient range, and not let those extra watts go to waste.

[Living Large]

I don't know which brand of battery you going to use, but according to Rolls battery Manual your estimates are far out. It take me 4 hours to absorb my battery bank.

The following is copy from Rolls battery manual,

{large snip}

OK - what that calculation example and your experience is saying at a glance is that absorb alone may take the time I calculated, and I may have to add 3 more hours. Maybe 7 to 8 hours total to get to float. I feel a bit foolish, in that I have been through the Rolls manual and forgot that section on charge time is there.

One person here, if I recall correctly, suggested shutting the gen set down at the end of bulk. The citation you gave suggests absorb is the most important phase of charge. So I'd either have to keep the gen set on for the beginning of absorb, or be able to shut it off if enough PV is available. It seems it will depend on if my charging day is a nice sunny day, or a cloudy one.

You have to realize that a generator is going to be most efficient at a particular load point. Sometimes this information is published, sometime it is not. Ideally, to get the most watts for a given amount of fuel, you should do as much of your charging as possible at or near that load point. If you set up your charger to use that for the bulk rate, then you are going to have the most efficient setup. As you leave bulk (current limited), and go into absorption (voltage limited), the current will drop as SOC increases. At some point during that drop, it will become quite a bit more wasteful, and you'd be better off shutting down the generator and starting over a bulk at a later time. A generator's lowest efficiency is usually at minimum load/idle. Every so often, you DO want to fully charge them, but that may end up happening naturally on a sunny day. Then, every few months or so, you should do an equalization charge, and really top them off.

Now, if you happen to have other loads that could be fed into (a water pump with a expansion large tank, etc), you could use them to keep the generator in it's more efficient range, and not let those extra watts go to waste.

Yup - I have in this research exercise learned about maximum generator efficiency, and have already considered that it would be wise to size the generator with it in mind for exactly the reason you state. I have briefly looked for a curve for the Ecogen - the one I am planning to use at the moment, and have not found one yet. The only thing in the data sheet is it uses 59% more fuel at 100% load as at 50% load. Something I read was leading me to believe it may be too small - a thread here gave a guideline for generator size, which I am sure is based to some extent on efficiency. It was something along the lines of Wh capacity of batteries/4. For me, if I have a 730 Ah bank, x 48 = 35040 Wh. Divide by 4 = 8760W. Ecogen is only 6000W. I was guessing gen set power during bulk would be about 4000W. I know nothing about the optimal loading (which I need to look into more) but it stands to reason it may be below the mid-point of the capacity of the gen set. On the other hand, once into absorb, the power will be dropping over the 4 hours. So you have to balance the bulk power needs with the first portion of absorb and try to find a sweet spot. Thanks for reminding me I have to look at this more. Another factor is some gen sets can't be run for hours on end, and some can. Air cooled versus liquid seems to determine it.

Incidentally, the house does have a large expansion tank - at least compared with mine. It's 52 gallon, though I am told one can have a bank of them. It had occurred to me to plan to run the well during charging, since the pump will be my largest load.

[Mike90250]

...

One person here, if I recall correctly, suggested shutting the gen set down at the end of bulk. ....

In the BULK stage, you are putting in all the power you can, to get the battery up to ~80% At 80%, absorb takes over, and the amps slowly begin to drop. If on a generator, as the amps cut back, you fall off your peak efficiency, and start burning excess fuel that is not giving the most charge into the battery for the buck.
If you run the genset in the morning, and bulk the batteries, any sun for the rest of the day, does the absorb part of the cycle, or you can if you want, switch to a smaller generator for absorb.