State of Charge

[barnakiel]

Originally Posted by zboss

So, being told that we should not drop our state of charge below 50% (and trojan actually recommends 80% for best long life) I am wondering under what conditions that voltage is measured.

For example, when our fridge is running our voltage drops significantly but recovers significantly when it stops running. Does that 50% rule apply during the amp draw-down or without the amp draw-down?

- z

It is not about voltage. It is about how much the battery gets drained.

If you remove 50A from a 100Ah battery, you are at 50%.

As your 100Ah get older it will lose capacity, and so 50% may become 45, 40, 35 Amps in her later days.

b.

[sailorboy1]

seems some people are having a hard understanding the difference between a battery state of charge from its' voltage reading under various loads

A battery under load reading 12.2V doesn't mean it is at 50% charge. Hell if I use my Mr. Coffee maker at SOC of 95% the voltage during it will read 11.7V. Doesn't mean a thing!

[a64pilot]

Mr. Coffee pulls what, over 100 amps? A big drop in voltage is understandable, but a significant voltage drop with a 5 amp draw isn't, at least to me anyhow.
I guess what significant is need quantifying.
I'm thinking maybe a connection has corrosion, is loose or even undersized cables

[sailorboy1]

Originally Posted by a64pilot

Mr. Coffee pulls what, over 100 amps? A big drop in voltage is understandable, but a significant voltage drop with a 5 amp draw isn't, at least to me anyhow.
I guess what significant is need quantifying.
I'm thinking maybe a connection has corrosion, is loose or even undersized cables

Mr Coffee pulls 75 amps. At a 5 amp draw on my boat I see a 0.1-0.2V drop. But my point was that you can not talk about SOC and voltage under load in general. Yes you can kind of say that 12.4V with a 5 amp draw may be normal with a battery pretty well charged. But if you are getting 11.7V at the battery terminals with that draw there is probably something else going on and it probably means the batteries are not well charged

[Cavalier]

Originally Posted by FlyingCloud1937

... I use the 30-30-30 Rule.
Discharge at 30% of the 20 hr rate for thirty seconds, then wait 30 seconds. Then wait 30 seconds, measure the voltage with a calibrated DVM.

...wouldn't waiting 30 seconds and then another 30 seconds make it a 30-30-60 rule?

[zboss]

Originally Posted by barnakiel

It is not about voltage. It is about how much the battery gets drained.

If you remove 50A from a 100Ah battery, you are at 50%.

As your 100Ah get older it will lose capacity, and so 50% may become 45, 40, 35 Amps in her later days.

b.

But that seems to be one of the questions... 50% of what? From a volts perspective:

12.73 volts is 100% full
12.10 volts is 50% full

From an amp perspective my 520 Amp-hour banks is:

100% charged at 0 amp-hours consumed
or 50% charged at 260 amp-hours consumed.

In regards to the fridge drawing down that many amps... I am looking at old wiring as perhaps being the culprit. I noticed that my power line leaving the breaker box was a little warm. I've never felt a wire get warm like this so I am going to look for corrosion and brittle connections. Maybe just replace the whole wiring for good measure. I am pretty sure the unit was installed in 1987 when the boat was built so its had a good life. It still keeps meat frozen hard with ambient temps in the mid 90's F.

Also, I'm going to upgrade to a Carel controller and add a small circulation fan in the box.

[Terra Nova]

Originally Posted by a64pilot

Nobody else is wondering why the fridge makes a significant draw down on the battery voltage?
Fridge is 5 or so amps?

Good point.

Originally Posted by a64pilot

Mr. Coffee pulls what, over 100 amps?...

Not likely.

[zboss]

Originally Posted by a64pilot

Mr. Coffee pulls what, over 100 amps? A big drop in voltage is understandable, but a significant voltage drop with a 5 amp draw isn't, at least to me anyhow.
I guess what significant is need quantifying.
I'm thinking maybe a connection has corrosion, is loose or even undersized cables

Its closer to 8 amps BTW. With all our fans going and or macbook (i work from the boat), plus maybe the stereo (got to have music while I'm doing my work magic ), we easily are pulling down 12 amps.

We recently completed a light equalization on the batteries and are seeing much better behavior as far as the batts retaining the voltage after a night. We usually run a deficit of about 55 to 85 amps overnight. Before equalization we would see 12.10 to 12.15 in the morning with the fridge running, higher when not. Now we see 12.35 to 12.45 in the morning with the fridge running. Even then, I think I need to conduct a better equalization by running a steady current against them individually rather than as a bank.

The batteries had not been equalized since we got them last September and they rarely get "really" 100% (voltage wise) full unless we run the generator for a few hours in the morning and then let the solar do their thing.

Oh and to top it off... I recently discovered that my Mastervolt shunt is reading way low. In some cases 0.20 volts lower than the battery voltage checked with a decent RMS meter directly to the battery terminals. I've placed an order for a Balmar Smartgauge.

[sailorboy1]

[QUOTE=zboss;1604714

We recently completed a light equalization on the batteries and are seeing much better behavior as far as the batts retaining the voltage after a night. We usually run a deficit of about 55 to 85 amps overnight. Before equalization we would see 12.10 to 12.15 in the morning with the fridge running, higher when not. Now we see 12.35 to 12.45 in the morning with the fridge running. Even then, I think I need to conduct a better equalization by running a steady current against them individually rather than as a bank.

[/QUOTE]


To me that means your batteries in the past were NOT fully charged. I had the same thing happen to me 2 years ago when the Battery Monitor was saying one thing, but the voltages suggested something else. It turned out that the BM was wrong and it took a good charging and equalization to correct (and some reprograming of the BM so it would stop resetting early).

12.35-12.45V with a 8amp load and 85AH out sounds like good batteries to me.

I don't feel you need to do individual batteries really, just be sure the bank is at low acceptance current before going to equalize.

[Maine Sail]

Originally Posted by zboss

But that seems to be one of the questions... 50% of what? From a volts perspective:

12.73 volts is 100% full
12.10 volts is 50% full

For flooded batteries at 80F fully rested/disconnected for at least 24 hours (it often takes longer) this may be accurate but will be brand dependent.

I have two AGM's on my bench that have been there for three weeks 100% disconnected still at 12.97V... Shop is about 70F..... The flooded battery next to them has been off charge for two weeks and I just checked it at 12.76V

Originally Posted by zboss

From an amp perspective my 520 Amp-hour banks is:

100% charged at 0 amp-hours consumed
or 50% charged at 260 amp-hours consumed.

Unfortunately it is not that easy..

Your 520 amp hour bank can be considered full when your battery voltage under alt or shore charger is at 14.4V + (NOT FLOAT VOLTAGE!) and your net accepted current is less than 2% of bank capacity, but 1.5% is a better number to strive for.

his means (with charge source in absorption mode):

Battery Terminal Voltage Measures 14.4V or greater
Ammeter shows 7.8A or less flowing into the batteries

The bank can now be considered full..

Ah's consumed at 0 tells you nothing other than the monitor thinks the bank is full and is most likely misleading you..

What was the temp during discharge?
Whats is the charge efficiency of the bank?
When was the bank last capacity tested and the Ah capacity adjusted?
What is the Peukerts constant for the bank?

The rate at which you drew the capacity from the bank changes the SOC. So 260 Ah's consumed may not always be 50% SOC.

Draw 260Ah's at 200A and you will not be at 50% SOC
Draw 260Ah's at 4A and you will not be at 50% SOC

Also your 520 Ah bank is only a 520Ah bank for a short period of time. What is the real capacity?

I could go on and on and on.....

The only time you know you are full is when a shore charger or alt is at 14.4V+ and your bank is accepting less than 2% of capacity in charge current but preferably less than 1.5%.

If the bank was truly full it would look like this:


That is 0.1A acceptance on a 400Ah bank at 14.4V..

A good cheat is to charge to full then draw 40-50% of the capacity at a fixed current = to the 20 hour rate ("C" / 20 = 20 hour load) when the batteries are at 75-80F. Try to hold the current as steady as possible to not inflict any Peukert effect.

Stop and let the batteries rest for 24-35 hours 100% disconnected, still at 75-80F. What is the open circuit voltage? Does it agree with the SOC you should be at? If not a full capacity test is in order.

*WARNING: If you have solar or wind and an Ah counting battery monitor please STOP relying on "auto-synch". Use only "known full" resets, 14.4V+ less than 1.5%, and program out auto reset....... Manually resynching the monitor is the only way to maintain any semblance of accuracy with an Ah counter and solar/wind...

[zboss]

Thanks... I still cannot explain why my mastervolt shunt is reading the voltage low. When I connect directly to the batts I get a much higher voltage most of the time. I cannot imagine that mastervolt applied a processing algorithm to the voltage sampling, so it should be reading the same voltage as my RMS meter.

[ontherocks83]

Originally Posted by Maine Sail

Your 520 amp hour bank can be considered full when your battery voltage under alt or shore charger is at 14.4V + (NOT FLOAT VOLTAGE!) and your net accepted current is less than 2% of bank capacity, but 1.5% is a better number to strive for.
...

Can you elaborate on this a little? I can understand the 14.4v and 1.5% being full but after it hits that it eventually goes to float voltage. Why is float not considered full? I'm sure I'm missing something obvious I just don't know what it is.

[zboss]

Originally Posted by ontherocks83

Can you elaborate on this a little? I can understand the 14.4v and 1.5% being full but after it hits that it eventually goes to float voltage. Why is float not considered full? I'm sure I'm missing something obvious I just don't know what it is.

The recommended float voltage is 13.2 for our trojans. Full would actually be 14.8 volts rather than 14.4... at least that is my understanding.

I believe what mainesail is inferring is that you need to hit that high voltage.... 14.4 or 14.8 with minimum amps coming in to get the batts in a state of full charge vs. just a maintenance charge that the float voltage applies.

FYI. I installed my new SmartGauge tonight. It does not take 1 hour. Of course, it takes three hours because you need to take the whole boat apart, get all your tools out, make cuts, clean up the sawdust, etc..

I'm still leaving my Mastervolt shunt attached. We will need how they compare.

[Maine Sail]

Originally Posted by ontherocks83

Can you elaborate on this a little? I can understand the 14.4v and 1.5% being full but after it hits that it eventually goes to float voltage. Why is float not considered full? I'm sure I'm missing something obvious I just don't know what it is.

Many charge sources really suck at determining when your batts are full. They often enter float not based on anything other than time, or as I refer to it as an "egg timer".. Just because your charger has entered float it does not mean your batts are full. Full needs to be measured at 14.4V + for that method to work.

For example, at the same SOC, at 13.2V you may be accepting 1.5% but now bump the voltage to 14.4V and you may now be accepting 8-10%, if your charge source is like many and does "premature floatulation".....

[ontherocks83]

Got it thank you. That makes perfect sense now.