Battery lifetime if not fully charged each cycle

beamguy · August 2016

All the manufacturers recommend and spec their batteries for usage where they are fully recharged after each use, which can take 8 hours or more. I am contemplating a system where I try to minimize the amount of charging time (to preserve the lifetime of an expensive generator). I am wondering if anyone has experience with battery systems that minimize charging times. For AGM batteries I expect that I would not get far beyond the constant current "bulk charge" mode very often. So that would mean I charge the battery to about 80-85% each day.

I know the battery life will be less, but how much lifetime do I loose?

NOTE: I was just reading about a battery operated bus system where the electric bus is recharged at each station in 15 seconds. Yes that is right 1/4 minute. I likely cannot afford that type of technology.

Thanks

BB. · August 2016

I am not a battery engineer--There is a suggested method where batteries are cycled daily between ~50% to 80% state of charge daily, and recharged >~90% once per week. And, one vendor (Rolls?) has said to recharge >~90% (then equalize?) once every 4 weeks.

The theory, as I understand, is that lead acid batteries do not sulfate while actively being cycled. Nice thing is that lead acid batteries are very efficient while being charged below 80% state of charge (when charging >~90% State of charge, lead acid batteries are generating more heat and hydrogen+oxygen gasses at that time, plus gassing&equalization is hard on batteries too).

An alternative would be to get LiFePO4 batteries--They do not have any cycling issues (other than avoid over charging and over discharging will will kill these batteries). These batteries are not cheap.

Regarding genset usage--Sizing the genset to the battery bank+energy usage/needs is important too... Gasoline/Propane Gensets are fuel efficient when operated at ~50% of rated load (when operated at less than operated load, fuel flow remains not much below 50% of full power fuel flow).

For diesel gensets, It is usually considered best if you operate the genset at a minimum of ~40-60% of rated load to lessens the chances of "wet stacking", carbonization, and cylinder wall glazing. For Diesels, their fuel usage a lower loading is much more efficient vs gasoline type engines.

For charging your battery bank, you have the option of charging at ~20% rate of charge until the battery gets to ~80% state of charge. At ~80-90% SoC, roughly 10% rate of charge, and above 90%-100% SoC (charging and equalization), you only need ~5% rate of charge...

So--Depending on how deeply you cycle the battery bank, you may be able to run a smaller genset (10% is probably the optimum rate of charge--And if you are charging at high SoC and/or equalization, even 5% rate of charge is enough)...

I.e., if you have a large battery bank, you may need an 8kW genset (~1-2 Gallons per hour). Or with a smaller battery bank (or if you need a few hours extra after using solar panels during the day)--You might get away with a Honda eu2000i that uses ~0.25 to 0.10 gallons per hour.

I am big on "balanced" system design... Design the battery bank for your loads--Then design the solar+genset+etc. to "keep the battery bank" happy.

-Blll

jonr · August 2016

Trojan also says equalize lead acid monthly or based on some measurements:
http://blog.multitel.com/final-routine-test-battery-equalization
But I've seen other places where Trojan says only equalize based on measurements.

Since AGM shouldn't be equalized, my guess is that AGM should be brought to full charge more often, depending on temperature - weekly?

Marc Kurth · August 2016

Great advice already provided by the folks above.

Fast charging is something that I get involved with. I have industrial customers who need to charge extremely fast, because their downtime is measured in thousands of dollars per hour.

With the right battery arrangement and enough charger capacity, 160 minutes from 80% DOD with AGM's is actually quite achievable depending upon the parameters. Give us the basic info and we can come up with a plan!

What size battery bank?
How short of a time to you want to charge?
What do expect as a battery lifespan?

Marc

Marc Kurth · August 2016

I forgot to directly answer your question about battery life with an 80%-85% daily charge: Roughly 2 to 3 years without equalization, will result in suffering a 25%-35% loss of storage capacity with Concorde AGM's. Certain brands of AGM's will be much worse, some will close to the same.

Prior to solar becoming so darned cheap, that is exactly what most blue water sailors did: Run the battery bank down to 20%, then charge back up to 80%-85% and stop the diesel generator. They stopped because they saw the charging current tapering down, and realized that two more hours of genset operation was not efficient and there aren't any gas docks in the middle of the ocean.

With Concorde AGM batteries, the answer is to equalize them at an elevated voltage, to keep the batteries operating strong. Time interval of "how long", "how hard" and "how often" depends upon the application specifics, but it isn't complicated.

beamguy · August 2016

Marc Kurth said:

.... Give us the basic info and we can come up with a plan!
What size battery bank?
How short of a time to you want to charge?
What do expect as a battery lifespan?

I would be happy to have you draw up a plan. Here are the basic requirements:
I would like to draw 400 watt hours from the battery per day and then replace that energy in less than an hour of charging on typical days.I do not expect that we will fully charge the battery on these days.
Once a week or two we could run the generator longer to condition the batteries (i.e. bring them to full charge)
Right now I am looking at a single 12 volt 100 Amp-Hour AGM battery, but any economical battery would be fine. Is there an advantage to buying one of the more expensive brands?
If we need to take the battery out to a heated location in winter it is a long drive, so a sealed battery would be far more convenient.
The system is run daily for 12 weeks during the summer then shut down for the winter.
The existing generator is very expensive to maintain, so even if we replace the battery each year we will be ahead, but it would be nice if the battery lasted a few years.
Ultimately we will be faced with replacing the generator, and at that time we will likely consider alternatives that do not cost $10K, but that is what we have now.
Thanks,
Mike

Marc Kurth · August 2016

Based on that set of restrictions, you are fighting a losing battle. I suggest following the recommendations already given.

Get a cheap battery and replace it often, or get a small generator.

A good battery and high capacity charger to give you any real benefit in less than one hour of charging, will approach the cost of a small inverter based generator.

beamguy · August 2016

Marc Kurth said:

A good battery and high capacity charger to give you any real benefit in less than one hour of charging, will approach the cost of a small inverter based generator.

Thanks. I agree that a small generator could do the job, but so far I have not found one that has the convenience that we are used to. We just flip a switch on the wall and the generator in a separate building 100 yards away comes on. It runs on a 200 gallon propane tank, does not require fiddling with a choke, priming or anything else. In this day and age of cheap electronics I believe such controls should be a cheap addition, but I do not find any models with those features.

westbranch · August 2016

I think this is the outfit mentioned here in previous threads...http://www.propane-generators.com/remote_start.php

BB. · August 2016

Read some nice things here about the Honda em4000sx... Not sure if you can get a propane conversion, but it does have electronic engine control (including mixture control) and does not need a manual choke.

http://powerequipment.honda.com/generators/models/em4000

-Bill

SandyP · August 2016

beamguy said
Right now I am looking at a single 12 volt 100 Amp-Hour AGM battery, but any economical battery would be fine. Is there an advantage to buying one of the more expensive brands?

I would look at a ~200Ah GEL type battery that can accept a "high" rate of charge to replace your 400Wh in an hour of charging.
Two of these in series may do the job : http://www.hoppecke-us.com/tl_files/hoppecke/Documents/HO-US/solar.bloc_us0812.pdf
They also are "happy" operating in a partial state of charge condition as long as you do fully recharge them every 10 days or so.

jonr · August 2016

The above link seems to go to an AGM battery.

jonr · August 2016

Sounds like using 200 AH of AGM batteries with a ~.25C charge rate for ~3 hours every day and then one day every week for 7 hours (to get a full charge) might be reasonable. You could use a Honda EU2000i generator with a propane conversion. It might last 10 summers, then replace it. It's not remote start, but it is very easy to start. And quiet enough that it could be relocated to a more convenient spot.

littleharbor2 · August 2016

GEL batteries biggest drawback is their inability to take a "high rate" charge. Unless you want a bunch of swelled up bubble ridden gel. AGM is the type battery that works here.

WaterWheel · August 2016

Spoke with Crown battery rep this morning about maximizing life in my Crown 6v 395 amp/hr batteries with this hot weather.

Currently with the hot weather I'm absorbing at the 58.1v (before temperature compensation) factory recommended voltage but it's taking 5 hrs for the batteries to reach 1.260 SG (91% charge) with the lower temperature compensated absorb voltage . (1.277 is fully charged). We discussed cooling methods and he said that when cycle testing they keep the batteries in a water bath to help cool them with the constant cycling during testing and little time to rest between cycles.

He said they would like to see the a 1.265 SG in this hotter weather at the end of each absorb cycle as a compromise between not heating the batteries too much during the summer and recharging them enough. This is defiantly higher then BB's suggested 80% charge point and I strongly respect BB's opinions and knowledge. Crown tech suggested that I increase the absorb voltage to 58.5v during the warmer summer months which will hopefully reduce the absorb time a bit.

My batteries run between 87 degrees on a summer morning up to about 106 degrees at the end of absorb on a hot day in a fan ventilated box with the lid cracked open to further dissipate heat.

Just throwing the information out there since absorb SGs are mentioned on this thread.

Vic · August 2016

Hello WaterWheel,

You mentioned ... "Currently with the hot weather I'm absorbing at the 58.1v (before temperature compensation) factory recommended voltage but it's taking 5 hrs for the batteries to reach 1.260 SG (91% charge) with the lower temperature compensated absorb voltage . (1.277 is fully charged) ...

The function of Temperature Compensation is to increase the Vabs when the batteries are cooler than the Reference temperature when cool/cold, and to decrease Vabs when the batteries are warm/hot. The Reference temperature is almost always in the range of 25 - 27 C (about 77 - 80 F). This compensation value is to try to keep the regulated voltages, like Absorb voltage equivalent to the charging voltages at ther Reference temperature. If the Compensation value is correct for your batteries, the Vabs will be correct for the nominal temperature of the batteries, as measured by the Battery Temp Sensor (BTS).

Your batteries are warm to HOT, in my book.

You DO realize that many Hydrometer readings need to also be temperature compensated. You may be using the Hydrovolt, which appears to compensate readings for the electrolyte temperature.

...      "Crown tech suggested that I increase the absorb voltage to 58.5v during the warmer summer months which will hopefully reduce the absorb time a bit..."
This is often done to help get as much charging done is a shorter time, on RE  charged  (as from PVs) systems.   Higher Vabs settings will increase heating of the battery, all else being equal.

   ",,,   My batteries run between 87 degrees on a summer morning up to about 106 degrees at the end of absorb on a hot day in a fan ventilated box with the lid cracked open to further dissipate heat."

Also do have the battery banks here, in insulated plywood boxes. Have beer leaving the box cover completely open in the past eight years, or so, to allow the batteries to be ventilated. Use a box fan for each bank to circulate cool air, from the Window A/C in the Power Room, which tries to keep the batteries at about 70 F, on average in the summer.

Skipping some number of days that the batteries receive a full charge can help reduce the average battery temperature. Naturally, a full-charge plus an EQ still results in considerable battery heating, and it can require about one day to return the battery temperature to below the Reference temperature of 77F (25C).

To help assist in scavenging Hydrogen from the power room, a small amount of outside air enters into the bottom of the battery box, and exits via a flush, screened vent hole in the ceiling of the power room.

Just my read on things, Vic

WaterWheel · August 2016

Hi Vic,      I generally understand the temperature compensation and have the Bat/Temp Compensation set at -108nV/C which works out to roughly a .6v charging voltage drop for every 10 degrees fahrenheit.       Using the Hydrovolt I give it a few run throughs of battery temp electrolyte to allow it to temperature compensate so it should be fairly accurate with the hot batteries.

I've got one of those 48v battery box fans that turns on when the voltage is >54v and back off after voltage drops back to 52v and vents outside but occasionally I consider the "beer cooler" to chill things down in the battery box.     I'll probably just end up running a larger fan since water baths aren't a great option.

I only fully charge about once a month when I eq and instead try to get above 90% SOC most days although the Crown battery rep was recommending getting above 94% SOC even on these hot days.

Some day I hope to dig a cement lined battery box into the slope outside my garage so I can get some earth cooling....    another project for another day.

Battery lifetime if not fully charged each cycle

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