Volts VS. Amp-Hrs

[zboss]

Originally Posted by glenn.225

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.

Yes... I agree that is the case based on the literature. I do see the charge go up slightly after the load is removed.

I don't understand the curve of voltage to amp hours. Is the curve linear or otherwise? For example, is the amp hour capacity between 12.10 and 12.11 volts the same amp hour capacity as between 12.60 and 12.61 volts or is the 12.60 to 12.61 volts difference have a higher amp hour capacity?

[skipmac]

Originally Posted by zboss

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.

Maybe I missed it but how are you determining that you are at 50% state of charge? If you are measuring by voltage while you are using the batteries that is NOT accurate. To determine approximately the state of charge by voltage the only way that is even close is to turn off ALL loads and stop all charging and let the batteries rest for a couple of hours or so. THEN, measure the voltage.

If you are measuring the voltage while you are charging or discharging the batteries that will not give you an accurate indication of the state of charge. Will show if the batteries are dead but not much closer than that.

Originally Posted by zboss

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%?

Not sure how exact it needs to be but a cycle would be discharging then recharging the batteries. How low is a discharge and how high is a charge. Have not seen that specified exactly but I would call 100 to 50 the max and min, and 80 to 50 good enough to count as a cycle but 90 to 60 or whatever. I don't think voltages need to be that exact.

[zboss]

Originally Posted by skipmac

Maybe I missed it but how are you determining that you are at 50% state of charge? If you are measuring by voltage while you are using the batteries that is NOT accurate. To determine approximately the state of charge by voltage the only way that is even close is to turn off ALL loads and stop all charging and let the batteries rest for a couple of hours or so. THEN, measure the voltage.

If you are measuring the voltage while you are charging or discharging the batteries that will not give you an accurate indication of the state of charge. Will show if the batteries are dead but not much closer than that.




Not sure how exact it needs to be but a cycle would be discharging then recharging the batteries. How low is a discharge and how high is a charge. Have not seen that specified exactly but I would call 100 to 50 the max and min, and 80 to 50 good enough to count as a cycle but 90 to 60 or whatever. I don't think voltages need to be that exact.

I am using the shunt. It is measuring quite a few different values including voltage, amps in, amp hours consumed, charge efficiency, and about another two dozen figures. The only ones I really pay attention to are overall running amp hours consumed, the current battery voltage, and the current amp utilization.

If you take all my systems out the equation... am I right to assume that if I have a 260 amp hour battery (based on 20 hour rating) and I go from 100% state of charge to 50% state of charge, and all else being equal, I should have taken 130 amps from the battery assuming I pulled no greater or fewer than the stated 20 hour rating?

Maybe a better questions would be...

1) what are you guys running your batteries down to percent wise
2) how many amps are you getting out?
3) What is your total capacity?

[skipmac]

Originally Posted by zboss

Yes... I agree that is the case based on the literature. I do see the charge go up slightly after the load is removed.

I don't understand the curve of voltage to amp hours. Is the curve linear or otherwise? For example, is the amp hour capacity between 12.10 and 12.11 volts the same amp hour capacity as between 12.60 and 12.61 volts or is the 12.60 to 12.61 volts difference have a higher amp hour capacity?

I think you are getting distracted by focusing too much on voltage as an indicator the state of charge of your batteries. Amps in and amps out as shown by any good battery monitor is by far the easiest way to keep track. If the settings drift a bit over time an occasional stay at a marina with a 24 hour charge and reset the zero point should keep you good.

Also, trying to measure voltage in the second decimal place is not going to be accurate. Very few voltage meters will be accurate to that level and even the ones that are will see a wide variation in the reading based on a little moisture or corrosion at the connection.

[glenn.225]

Originally Posted by zboss

I don't understand the curve of voltage to amp hours. Is the curve linear or otherwise? For example, is the amp hour capacity between 12.10 and 12.11 volts the same amp hour capacity as between 12.60 and 12.61 volts or is the 12.60 to 12.61 volts difference have a higher amp hour capacity?

I don't think there is a liner comparison at all. From experience a high load will pull down voltage quickly and lower load will pull down voltage less even though SOC is same. There may be some way of calculating this but why?? Just use SOC on battery monitor and forget the voltage. Works for me.

That is, unless you want to do a study on this and enlighten us all. I'm not near bright enough to even attempt something of this nature.

[skipmac]

Originally Posted by zboss

I am using the shunt. It is measuring quite a few different values including voltage, amps in, amp hours consumed, charge efficiency, and about another two dozen figures. The only ones I really pay attention to are overall running amp hours consumed, the current battery voltage, and the current amp utilization.

About how I watch my monitor. Doesn't hurt to look at the voltage. Will confirm that your charging source is working and that it's putting out proper voltage and if you have a battery going bad you would probably see voltage too low, too soon based on the amp hours used. But I think of voltage as a secondary piece of information, something as a double check on the amps in/out.

Originally Posted by zboss

1) what are you guys running your batteries down to percent wise
2) how many amps are you getting out?
3) What is your total capacity?

I have 6 X 6V batteries in series/parallel so about 640 amp hours. I top them off as much as I can when I can but general rule is try to never go below 50%. Trying to charge beyond 80% on engine or generator takes too long so unless you can plug the charger in at a dock or have wind or solar to add the extra the 80% is a typical max. But you certainly should try to top them off ever week or three and do an occasional equalization.

[TacomaSailor]

zboss

I think you are ignoring the comments made by some pretty experienced battery managers. And, you haven't answered a couple important questions that might help you understand your situation.

In a prior life I was a process control engineer setting up laboratory and refinery computer measurement and control systems. You can only solve problems when you absolutely know your measurements are accurate and precise. That means verification of the measurement by a device independent of the device you are questioning.

1. have you verified battery voltage with a precision DVM independent of your battery monitoring system? Measured directly at the battery terminals with no load attached to the batteries?

2. how do you manage to use 150 amp hours during a 12 hour overnight period? Your reefer compressor (8*9) plus lights (6*3) plus miscellaneous (6*3) is only about 110 amp hours. What else is using the power? The reason I ask is - have you verified that you are actually using the amps your monitor reports?

3. is it possible you have some loads (current draws) from the batteries that are not being measured by the shunt? This is the problem opposite of that posed in question 2. I turn off all the loads I am aware of and put a clamp on amp meter onto one battery cable. I then compare that reading (carefully zeroed) to what the monitor reports. It should be zero in both cases. I then start turning on loads, one at a time, and compare the clamp on results with the monitor results.

4. I and others who have managed large battery banks for many years, have told you that voltage is seldom used, and is not a very accurate indicator, for managing battery usage and status.

In reference to your questions about:

1) what are you guys running your batteries down to percent wise
usually about 15% (~90 amp hours) but occasionally 20%
2) how many amps are you getting out?
this question does not make sense - sometimes the batteries are delivery 90 amps for the microwave, at times 8 amps with the refrigerator and TV running, and lot of time 4 to 6 amps.
3) What is your total capacity? ~625 amp hours 6 x T-105

As I stated earlier - for 19 years and a lot of nights on the hook (at one point almost 1000 straight without hooking to shore power) - I hardly ever look at voltage on the lower end. I monitor amp hours discharged.

When charging the batteries I do monitor V to ensure I don't overcharge but do get them above 14.4 for at least 30 minutes a day.

Hitting the upper limit on a daily basis and keeping the Voltage up there for a while will do much more to ensure battery life than worrying about how far you pull the batteries down. Most cruiser batteries die from sulphation / under charging than do from over discharge.

Just my opinions but my battery banks usually last more than 7 years.

[mitiempo]

Lots of good information has been posted here for you.

There are a few things I would check first.

1. Is the shunt the first connection after the negative battery post of the battery bank? If it is not any items connected between the shunt and the battery negative post will not be measured by the monitor.

2. Is the monitor properly programmed? In particular for the bank size in AH - 520 - and for the correct Peukert value - 114 for the Trojan T-145 I believe but you can check with Trojan to be sure.

[d design]

If you dont trust your batteries that much than do your own tests and pull down your batteries to the rated AH and just whatch the voltage in case of emergency

[mitiempo]

Originally Posted by d design

If you dont trust your batteries that much than do your own tests and pull down your batteries to the rated AH and just whatch the voltage in case of emergency

While accurate I don't think that is necessary on batteries that are quite new and of a known AH rating.

[d design]

Originally Posted by mitiempo

While accurate I don't think that is necessary on batteries that are quite new and of a known AH rating.

End of story brother

[Maine Sail]

If these batts are new give them 20+ cycles before they start holding voltage better. That said loaded voltage is rather meaningless unless you've done the calculations and spread sheeted it for varying loads and SOC's...

See if your BM will tell you the average loads you draw over 24 hours? Then calculate the Ah capacity you should have at that load using your batteries Peukerts constant...

If you are below the 20 hour rate you will get more Ah capacity....

[zboss]

Originally Posted by TacomaSailor

zboss

I think you are ignoring the comments made by some pretty experienced battery managers. And, you haven't answered a couple important questions that might help you understand your situation.

In a prior life I was a process control engineer setting up laboratory and refinery computer measurement and control systems. You can only solve problems when you absolutely know your measurements are accurate and precise. That means verification of the measurement by a device independent of the device you are questioning.

No, not ignoring them, just confused. Is 50% draw down measured in volts or in amps?

Originally Posted by TacomaSailor

zboss
1. have you verified battery voltage with a precision DVM independent of your battery monitoring system? Measured directly at the battery terminals with no load attached to the batteries?

Yes and no. I don't have a 24 hour period where I can let the batts sit to do a proper specific gravity test. I have tested the batts using a volt meter once but this is a pain because of location and having to disconnect the batts from one another.

Originally Posted by TacomaSailor

zboss
2. how do you manage to use 150 amp hours during a 12 hour overnight period? Your reefer compressor (8*9) plus lights (6*3) plus miscellaneous (6*3) is only about 110 amp hours. What else is using the power? The reason I ask is - have you verified that you are actually using the amps your monitor reports?

We don't, as mentioned, we use about 100 amps over 24 hours. The 150 figure is the running total draw down since we last disconnected from shore power. We can't recapture the last 50 amp hours without hooking to shore power or running the engine for many many hours. We have no alternative energy.

Originally Posted by TacomaSailor

zboss
3. is it possible you have some loads (current draws) from the batteries that are not being measured by the shunt? This is the problem opposite of that posed in question 2. I turn off all the loads I am aware of and put a clamp on amp meter onto one battery cable. I then compare that reading (carefully zeroed) to what the monitor reports. It should be zero in both cases. I then start turning on loads, one at a time, and compare the clamp on results with the monitor results.

No, everything goes through the shunt. We had the entire system brought up to modern ABYC standards this summer. New everything. I will try as you suggest.

Originally Posted by TacomaSailor

zboss
4. I and others who have managed large battery banks for many years, have told you that voltage is seldom used, and is not a very accurate indicator, for managing battery usage and status.

In reference to your questions about:

1) what are you guys running your batteries down to percent wise
usually about 15% (~90 amp hours) but occasionally 20%
2) how many amps are you getting out?
this question does not make sense - sometimes the batteries are delivery 90 amps for the microwave, at times 8 amps with the refrigerator and TV running, and lot of time 4 to 6 amps.
3) What is your total capacity? ~625 amp hours 6 x T-105

As I stated earlier - for 19 years and a lot of nights on the hook (at one point almost 1000 straight without hooking to shore power) - I hardly ever look at voltage on the lower end. I monitor amp hours discharged.

When charging the batteries I do monitor V to ensure I don't overcharge but do get them above 14.4 for at least 30 minutes a day.

Hitting the upper limit on a daily basis and keeping the Voltage up there for a while will do much more to ensure battery life than worrying about how far you pull the batteries down. Most cruiser batteries die from sulphation / under charging than do from over discharge.

Just my opinions but my battery banks usually last more than 7 years.

I have to wonder why then that the manufacturers put so much emphasis on not running below certain voltages. Question #1 was... if you have 640 amp hours available, and you go from 12.7 volts to 12.1 volts (for example), how many amp hours are you using? Obviously, there would be some variations for large draws like the microwave you spoke about (you get less overall amps when putting a heavy load on but the manufacturers of batteries list a 20 hour rates and such) and I am trying to get an idea if I am way off on my utilization. The only thing that uses a lot of power is the fridge.

We only run the engine an hour a day. We have a 175 amp alternator (new) and Balmar 614 (new).

[Stu Jackson]

If you take ALL the input provided, here's what they're saying:

1. Your amp hour meter is more accurate than voltage.

2. Under a LOAD the voltage will sag, therefore, voltage is NOT a good idea for measuring battery state of charge.

3. With new batteries, cycle them as Maine Sail said, at least a few times if not the 20 he suggests to avoid the incompatibility of the voltage sag and the SOC on the battery monitor.

[skipmac]

Originally Posted by Stu Jackson

If you take ALL the input provided, here's what they're saying:

1. Your amp hour meter is more accurate than voltage.

2. Under a LOAD the voltage will sag, therefore, voltage is NOT a good idea for measuring battery state of charge.

3. With new batteries, cycle them as Maine Sail said, at least a few times if not the 20 he suggests to avoid the incompatibility of the voltage sag and the SOC on the battery monitor.

This is it in a nutshell. Short, sweet and to the point.