quote:

Originally posted by Bill N.Y.:
I installed an 1800 watt inverter and left it to power my 120v fridge. After 27 hours, 65-85 degree range, open and close the fridge 12+ times, turned off the inverter for 1/2 an hour, my batteries are now at 12.46 volts which places it at the "80 percentage of charge" range.

I checked the Interstate Battery Website and it states "To get the most charge-discharge cycles out of a deep-cycle battery, it should not be frequently discharged below a specific gravity of 1.190 as measured by a battery hydrometer."

That 1.190 specific gravity places the Battery voltage at 12.20 volts or at the "50 percentage of charge" range.

Question: Fill in the blank.
I think that I'll kick on my charger at the "___ percentage of charge" range.
My answer is 65 or 12.32v

Yes-No... What would you do? Thoughts?

I know that this is Bill H's favorite topics. .



I'll tell you... I'm like a kid in the candy store with this inverter. I should have done this awhile ago!

To begin with, the 50% SOC rule is just one spot on a slanted line on a graph. It is a good rule of thumb, but other factors can shift it higher or lower. 60% will make your batteries last longer, but you might need more batteries to last between charging, costing more money, adding more weight and taking up more space. 40% will shorten battery life a little, but could allow a smaller, cheaper and more compact battery bank. A smaller battery bank could be charged more often, but that would cost more in fuel.

It's all a trade off. My Trojans have gotten down to 40% a few times in their 9 years of use without any apparent problems. They still test good on my Midtronics conductance tester, so are showing no signs of age.

But, to get to your question on the 65% SOC, your usage, battery bank, and charging preferences need to be considered.

To minimize genset run time, and its attendant noise, pollution and fuel consumption issues, charging above 80% becomes undesirable as the charge tapers off, especially with a big genset.

So, you need to work backwards from 80% SOC with how many amp hours you will be using. When you run your genset should be factored in, as well. For example, no one will hate you if you run your genset when others are, as in the morning for coffee and microwaves. Some folks also have a run period around dinner prep time. Some campgrounds even have similar generator hour rules.

Depending on your usage and battery bank size, you could end up charging only in the morning or every other morning.

So, as I often say, "It depends."

This is a little behind us, as solar usually does all the charging and managing.

I should add here that it is best (if you can) to use the voltage chart that matches your batteries. They are not all the same.

BTW, what is your battery bank? Your numbers are making me think ahead to refrigerator replacement time, which is a Damoclean sword for any RV owner. Do you have a Kill A Watt? Power consumption numbers on that fridge would be interesting.

Duh, I re-read you first post and completely missed that you did turn off for 1/2 hour!!

I am TIRED--changed the caster shims today physically demanding to say the least! Using 3 foot breaker bar with a 6 foot pipe will tell how bad the rust was on the "U" bolts.

Certainly agree about checking voltage for state of charge, I sorta do that--

With a 200 amp alternator and now the RS3000 that can charge up to 150 amps (although I use the C/5 charge rating) so I limit it to 80 amps still doesn't take long to recover.

Looking at adding the Xantrex alternator regulator that converts the alternator to a 3 stage charger, was thinking about designing one but found it on their website. Not going to re-invent the wheel!

quote:

Originally posted by bill h:
BTW, what is your battery bank?

That third battery, slightly out of view, to the left is a Group 31 that is used for the starting circuit.



I have 2 of these batteries installed. Brand new, less than 4 months old.

quote:

Originally posted by bill h:
Your numbers are making me think ahead to refrigerator replacement time, which is a Damoclean sword for any RV owner.

The Fridge is a household unit Bill

quote:

Originally posted by bill h:
Do you have a Kill A Watt? Power consumption numbers on that fridge would be interesting.

No Bill, I do not. Sounds like I need another toy.

quote:

Originally posted by Bill N.Y.:

quote:

Originally posted by MWrench:
Problem is that in order to use the battery voltage as a reference for discharge, all load has to be removed for some minutes and allow the voltage to settle, then what ever you read is an "indication" of the state of charge.

Sorry Ed, I should have stated - turn off inverter 1/2 hr and then checked voltage readings. I'll reword my post.

Trojan says wait 24 hours before reading voltage and applying the reading to the chart. My own experiments have showed me that (on awakening) waiting disconnected for 1 hour (while I walk the dog), taking a reading, and adding a tenth of a volt will yield about the same result. YMMV, so do your own checkout.

quote:

You have to admit... for most people, it is alot easier to check the voltage than it is to whip out the trusty hydrometer.

Yeah, between breakage, inaccuracy, acid holes in pants etc, the hydrometer is a pain. If you are going to use one, Freas is the most consistantly accurate. They will sell you a float that will work in many hydrometers.

However, I am a real believer in the utility and economy of a Tri Metric 2020. Not only is it supremely accurate, but it can pay for itself in battery management and tracking. As an example, if it detects a failing battery, you can buy the battery you want, at the price you want, where you want, when you want and replace it at your leisure.

Depending where you go, that can be a serious consideration, for time, money and inconvenience.

Ah, yes. I remember your posting on your battery bank. Since you replace them every year for free, a lot of my post is of little use to you.

It must be nice.

The reason I mention the Kill A Watt is that it gives total usage for a period of time. The power consumption on the label of the fridge does not reflect the duty cycle, which varies with temp and usage. For example, if all my beer is the same brand, I stand in front of an open fridge for lesss time than if I have to make a really important decision. Cheese, too.

I have had my T105s for 3 years now and they seem fine, I have used them( abused? maybe)When I was working on the inverter installation I had them fully charged and then disconnected for three weeks the voltage at the end of three weeks was 12.7 VDC same as when I took them off line.

Bill h, how often to you desulfate yours?

quote:

Originally posted by bill h:
Since you replace them every year for free, a lot of my post is of little use to you.

It must be nice.

It is

But, that being said, I still wanted to hear yours, and some other thoughts on what is done for battery care and charge. One day, I might have to keep these as I will not sell them if they go past the 8 month mark. Besides, even if they are free, that's no reason to abuse them. I want to sell good stuff, not worn out junk.

quote:

Originally posted by bill h:
The power consumption on the label of the fridge does not reflect the duty cycle, which varies with temp and usage.

Exactly, it varies with temp and usage.

quote:

Originally posted by bill h:
For example, if all my beer is the same brand, I stand in front of an open fridge for less time than if I have to make a really important decision. Cheese, too.

Oh... I see

quote:

Originally posted by MWrench:

Bill h, how often to you desulfate yours?

Not very often. Perhaps two or three times in their life of 9 or 10 years. I put some EDTA in them when they were very young. They are always kept at float by the solar. If not used, I run them down a little and let the Iota charge them back up vigorously about once a month to prevent sulfation from sitting at float.

Batteries are emotional, and need love and attention. Sitting unattended, whether discharging or at float is not good for them.

If we use Tim's figure of 7 amps per hour for 24 hours, that is 168 amp hours. How long will it take the generator and charger to put that back into the battery? In my case, my Iota DLS75IQ starts out charging a 50% pair of T-125s at 36 amps, dropping down to 27 amps after one minute and 25 amps after 5 minutes, and then stays there until the batteries get up around 80% or so. 25 amps is pretty close to the 12% of capacity that Trojan recommends, so the Universe is in harmony.

Figuring a charge rate of 25 amps per hour, it would take my setup around 8 hours to put that back into the batteries, with estimated charging losses. Quite obviously, I would need a charger that pushed more amps into the batteries, or a bigger battery bank to accept a higher charge rate gracefully. Doubling my battery complement to four T-125s would mean that my Iota would put 50 amps into all four. That would mean that I could replace the power used by the fridge in only four hours. A more sophisticated (and larger) charger, with battery temp monitoring could charge at 100 amps (double the 12% rate) if everything goes well. That would cut the genset daily time down to 2 hours just to replace what the fridge used. However, battery life gets shorter with faster charging.

Now, these figures are based on estimates of a duty cycle of 33% or so. If the duty cycle is less, then the figures are different. A Kill A Watt would tell us for sure exactly how much power the fridge used in a 24 hour period.

Another way to go would be to have the expensive AGM batteries. They can take a humongous charge with no damage, so genset run time is way down. Their cost would be mitigated somewhat by less fuel consumption. Additionally, AGMs can withstand higher discharge currents with less shortening of life than flooded batteries. This has no relation to refrigerators, but does relate to microwaves, hair dryers and toaster ovens.

Bill, if we assume that your Cat batteries are ~ the equivalent of four T-105s, then we have over 400 amp hours. BTW, that is a reasonable assumption based on weight. Since the voltage readings corresponded to a usage of 20% of capacity, we can conclude that the fridge used around 80 amp hours. That is less than half of the 168 amp hours based on Tim's estimate. My own observation of an older, but similar, fridge was around a 33% duty cycle, as well, but was in a hotter environment. I suspect fridges have gotten more efficient in recent years, and may have a lower duty cycle than either Tim or I have observed on ours.

This is getting back to a need for you to measure your fridge with a Kill A Watt. It will measure only AC usage, with no regard for inverter losses, but it is a start. If you are willing to measure your fridge's usage, I will send you mine.

quote:

Originally posted by bill h:
This is getting back to a need for you to measure your fridge with a Kill A Watt. It will measure only AC usage, with no regard for inverter losses, but it is a start. If you are willing to measure your fridge's usage, I will send you mine.

Thank you Bill, that is very generous. I priced them out on eBay and found that my cost was $22.88 so I ordered one already. I'll report my usage after receiving it.

quote:

Originally posted by Bill N.Y.:
After 27 hours, 65-85 degree range, open and close the fridge 12+ times, turned off the inverter for 1/2 an hour, my batteries are now at 12.46 volts which places it at the "80 percentage of charge" range.

I started my inverter test at 9pm on 7/19. It ended at 3am on 7/22 with 12.28volts or "60% state of charge".

Question: "What are you doing up at 3am?" I just returned from a road call for UPS and decided to check it. I'll recheck the voltage a little later today to see if it recovered anymore on its own.

I have to admit, 54 hours running and it appears to be pretty close to the 60% state of charge mark. This makes me... "a very happy camper. "

Outside temps yesterday never exceeded the mid 60s as it had been a rainy and overcast day.

I know that my duty cycle of the fridge (battery usage) will increase at higher temps and more prolonged use of opening and closing. Still, I would have been happy at 24 hours.

As Bill H pointed out,

quote:

"no one will hate you if you run your genset when others are, as in the morning for coffee and microwaves."

I would suspect that my charger will get it topped off (relatively speaking) if I run my generator when everyone else is.

BTW: My charger is a WFCO WF-8875 POWER CONVERTER which means it's rated at 75amps.

Just for fun, keep running the batteries down to 50% and shut off the fridge. Then fire up the gen and charge up to 80% as you record amp readings.

There is a lot of variation in how fast chargers do this. It is more than just the amp rating. A lot of chargers will put out their rated amperage only into a very low battery, and it lessens as the SOC is higher. As I mentioned, my Iota puts out around 12% of the batteries' capacity at 50% SOC.

Hey Bill,

Before reading this, I took another reading at 3pm. That would give me 12hrs recovery without a charge. The voltage recovered to 12.34v which is around a 67.5% "state of charge".

I then turned on my circuit breaker for the charger while hooked to shore power, and saw my interior lights get brighter. Ok, charger working... Voltage reading now at 12.99 and slowly climbing after 5min.

quote:

Originally posted 07-22-2009 04:12 PM by Bill N.Y.:

quote:

Originally posted by bill h:
This is getting back to a need for you to measure your fridge with a Kill A Watt. It will measure only AC usage, with no regard for inverter losses, but it is a start. If you are willing to measure your fridge's usage, I will send you mine.

Thank you Bill, that is very generous. I priced them out on eBay and found that my cost was $22.88 so I ordered one already.

.

I'll report my usage after receiving it.

Did it come yet?

Why yes it did Bill. Fast shipping too.

Never did run the refrigerator test but have used it to watch my inverter climb to 130 before the inverter shut down due to internal overload protection. I've used it to measure a low voltage at a pole once and moved to another site.

Ordering up a new inverter soon.