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Old 15-01-2014, 21:06   #1
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Volts VS. Amp-Hrs

Folks,

I am hoping there is a simply answer here... We put in new T-145 plus batteries this past fall with a total amp-hours of 520. My understanding is that you don't want to run down your batts past 50% ever, so in a 12 volt system this is 12.10 volts, right?

Our only big energy hog is our big fridge which is about 8 or 9 amps when running, and of course the autopilot drive. I also understand that if you draw down heavily you get fewer total amps-hours out of the battery vs. a lighter draw.

I was expecting to get about 260 amps hours out before hitting 12.10 volts but in fact I am only getting about 150ish amps out before hitting 12.10... so I have to ask, what is going on here?
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Old 15-01-2014, 21:20   #2
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Re: Volts VS. Amp-Hrs

Well could be several things.

Yes you are correct in that the faster you pull amps out of a battery the less the capacity. And a battery's amp hour rating is based on drawing power out at a certain rate.

Testing battery status by voltage is a tricky thing. The true state of a battery determined by voltage you have to check it after the battery has set without being charged or discharged for some time (hours). If you check voltage while there is a load on the battery it will show lower.

Also, are you sure your batteries were fully, 100% charged at the start?

Typical cruisers, unless they are at a dock on shore power most nights or run a generator for hours every day or a really good solar system seldom get their batteries to 100% charge. Off the grid it is common to run the batteries between 80% charge to 50% charge. The reason is it's fairly fast and easy to charge FLA batteries to 80%. After that it takes longer and longer to get less and less charge into the battery.

To have a better idea of what's going on, state of charge, etc you really need to use a battery monitor. Shows amps in and amps out so a lot easier to keep track.
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Old 15-01-2014, 21:26   #3
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Re: Volts VS. Amp-Hrs

Trojan T145 battery is 6 volt/260 aH.
Two connected in series would be 12 volts/260 aH.
FOUR connected in series/parallel would be 12 volts 520 aH.
Is the latter what you have?

260 aH discharge from four batteries (at the 20 HR rate) would be 50% discharge.
If you just have two batteries, 150 aH discharge is quite a bit more than 50%.
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Old 15-01-2014, 21:29   #4
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Re: Volts VS. Amp-Hrs

Other than the fridge are you running an inverter full time? How about lights and fans? You might try turning it all off when not in use thereby reducing the load to only what you are using now and as a result getting more time between charges.

Good luck
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Old 15-01-2014, 21:34   #5
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Re: Volts VS. Amp-Hrs

Answer a couple of questions... yes we have four batts...

Yes, we have a monitor... a very good one... Mastervolt Easyview with a digital shunt.

No, the batts are usually run between 52% and 85% when on the hook, but the amp hours mentioned are the running total draw down since we last charged to 100% from shore power. So, in the morning, we are seeing something like 150 amps consumed ("running total") and after charging for an hour we see about 40-50 amps consumed.

No, we are not running an inverter except during the evening. Here is our breakdown while on the mooring this past week:

Fridge 8-9 amps (on and off as needed, doesn't come on a lot)
Lights 1.1-2 amps at most if we leave them all on
Heater .2 amps
Inverter 1-2 amps
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Old 15-01-2014, 21:49   #6
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Re: Volts VS. Amp-Hrs

I guess I'm confused here. You say the fridge draws 8-9 amps per hour? What else is running? How long is it taking to get to 12.1 volts from a full charge? Are you sure of that 8-9 amps per hour on the fridge? If nothing else is running and in an hour you consume 8 or 9 AH then the fridge consumption would be verified, but if you have other stuff on in the background and I'm guessing you do then I would start by turning on only one system at a time to get an accurate figure for consumption. Only then will you know if the high rate is correct.
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Old 15-01-2014, 21:53   #7
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Re: Volts VS. Amp-Hrs

Heater only .2? That seems way low. I count 13 amps without the heater but without isolating each system how do you really know?
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Old 15-01-2014, 23:10   #8
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Re: Volts VS. Amp-Hrs

Skipmac provided a very important answer that seems to have been ignored.

The battery voltages that are specified as 75% or 50% are RESTING voltages which are measured after the load is removed and the battery has set with little load for a few hours.

My wife loves her 900 watt microwave and will run it for 10 minutes. During that time the poor batteries are delivering over 90 amps and the voltage gets pulled down to 12.1 or so. After turning the microwave off the voltage slowly rebounds and may get back to 12.6 or so within 10 minutes if the batteries were pretty full (12.8 V) at the start of the microwave usage.

I manage my batteries (625 amp hours) by using amp hours and pay very little attention to voltage. I try to keep the amp hours pulled from the bank to about 15% of the total capacity. Sometimes I will let the draw down reach -120 amp hours but it is rare for the batteries to get below -75 or -80 amp hours.

That system has worked perfectly for almost 19 years. My current battery bank is almost 5 years old. the prior two banks went over 7 years each.


How do you use 150 amps during the night? I cannot remember ever using that much power between 5 PM and 9 AM. Over a period of about 1500 nights at anchor we averaged about -80 amps at daybreak.

reefer on 35% of the time ~5 hours * 9 = 45
Lights 8 hours * 2.5 = 20
TV, Stereo, DVD 8 hours * 3 = 24

What kind of heater? Diesel / Propane / Electric?

Our Ardic diesel fired hot water / forced air uses 5 amps when running.

What kind of fridge uses 9 amps? I've had three different compressors for my 9.5 cubic foot fridge and the most efficient only used 6 amps (compressor and water pump for the compressor radiator)

Have you verified the amp hours are actually resetting to 0 when you reach a full recharge?

You example showed you were down 150 amps and recharged to where you were only down 40 amps. If you then pull another 150 amps out you'll be down to 190 the 2nd morning, 230 the third morning, etc.
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Old 15-01-2014, 23:37   #9
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Re: Volts VS. Amp-Hrs

Here is another problem to consider.

Is this a sensor problem and there is nothing wrong with your batteries?

My Link2000 battery monitor occasionally shows a battery voltage of about 12.2 or 12.1 V in the morning when the usage is only 50 amps out of a 625 amp hour bank.

When I measure the voltage directly off the battery posts I see 12.5 or 12.6 V. I ignore the problem and for the the next week or so the Link 2000 senses the voltage correctly. Then, I again find a low voltage.

All the sensor contacts are clean and the measurements for the 2nd bank are always correct as measured at the battery posts and displayed by the Link 2000. I can not isolate the cause of the problem but it is very easy for me to verify the voltage by directly measuring at the battery posts.

When you see 12.1 V - have you verified that reading by taking a reading directly at the battery posts with a good DVM?
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Old 16-01-2014, 00:23   #10
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Re: Volts VS. Amp-Hrs

I just checked my 625 amp hour bank. It was fully recharged at 5 PM at which time it was floating at 13.6V.

It is currently discharging at 7 amps and is down 41 amp hours with a battery voltage of 12.4 V.

I turned off the loads so there is no discharge and the voltage jumped to 12.52V in two minutes and 12.61 in 20 minutes.
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Old 16-01-2014, 02:24   #11
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Re: Volts VS. Amp-Hrs

There's a lot of stuff on here about batteries.

Batteries are a bit like life...you can never be sure how much you've got ! and the more you delve into it, the more complex it becomes !

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Old 16-01-2014, 15:35   #12
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Re: Volts VS. Amp-Hrs

Quote:
Originally Posted by zboss View Post
Folks,

I am hoping there is a simply answer here... We put in new T-145 plus batteries this past fall with a total amp-hours of 520. My understanding is that you don't want to run down your batts past 50% ever, so in a 12 volt system this is 12.10 volts, right?

12.6v+ volts is fully charged, 12.3v = 75% charge, 12.2v = 50% charge, 12.1v = 25% charge.

Source = Tim Murphy, Dec 2013 BoatUS magazine, article on DC electrical troubleshooting using digital multimeters. (I think some specific battery brands say 12.7 or 12.8 is fully charged.)

Discussion is about measuring system at rest, i.e., off charger and with no load for a while, and expecting different measurements at the battery vs. somewhere else on the boat. Latter would be affected by voltage drop stemming from distance, wiring size, etc.

If that helps...

-Chris
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Old 16-01-2014, 21:39   #13
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Re: Volts VS. Amp-Hrs

Quote:
Originally Posted by ranger42c View Post
12.6v+ volts is fully charged, 12.3v = 75% charge, 12.2v = 50% charge, 12.1v = 25% charge.

Source = Tim Murphy, Dec 2013 BoatUS magazine, article on DC electrical troubleshooting using digital multimeters. (I think some specific battery brands say 12.7 or 12.8 is fully charged.)
-Chris
Nope.. See Table 7 for charges...

http://www.trojanbattery.com/pdf/TRJ...UsersGuide.pdf

In regards to the questions regarding amp hours... its 150 amps hours running total... So, we started out with 0 amp hours used then disconnected from shore power... down to 50% charge... used 150 amp hours, then put 100 amp hours in each day using alternator... so we continually float between 50 and 150 amp hours total running draw, until such time we plug back in into shore power and can get back to 0 amp hours.

My particular fridge draws 9 amps when it is running. It does not run for a total of 60 minutes each hour, but how much it is actually drawing can only be counted if it is the ONLY thing drawing each day, which isn't going to happen. The problem is not the draw down... the problem I am asking about is:

520 Total amp hours available... 50% voltage would be 260 amp hours used (theoretically) but I am only getting 150ish amp hours when we reach 50% voltage.

In addition, they are new bats. I understand from the literature that the full capacity takes time to develop. Trojan says 50 to 100 cycles. What is a cycle? 50%?
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Old 16-01-2014, 21:51   #14
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Re: Volts VS. Amp-Hrs

Quote:
Originally Posted by ShaktisBoy View Post
Heater only .2? That seems way low. I count 13 amps without the heater but without isolating each system how do you really know?
Yes... about that. We have a diesel heater and the only thing that consumes power is a small 12 volt fan... like a computer fan. It runs pretty slow. Usually only uses about .1 amps. When we run it full out.. max .2 amps.

I am very very confident in my digital shunt and easyview accuracy.
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Old 16-01-2014, 22:08   #15
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Re: Volts VS. Amp-Hrs

The most accurate method of checking batteries is specific gravity. When using voltage, batteries have to "settle" awhile, as mentioned above some suggest over 24 hours with no load or charge.
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