What does "fully charged battery" mean?

[booster]

It is a new year, and I am sure there will be some folks that are planning to update or replace their batteries and/or charging systems so I think this is a very valid question to ask everyone, as it goes to the heart of battery, charger, monitoring, and use in our vans.


I think we may find that there are a lot of different ideas, based on what we see in various discussions of related topics.


As an example, one of the very common comments made in other discussions is along the lines of "I do XXXXX and my batteries are fully charged", which in itself is a very relevant piece of information that could be very useful to anyone trying to improve their own system. IMO, the usefulness of the statement goes way down when there is no explanation of how it was determined that the batteries got "fully charged", as personal experience has convinced me that getting fully charged is not really very easy with equipment that is readily available to us at this time. Others will disagree with me on that. There are lots of discussions on this forum of different charging and monitoring subjects, so lots of interesting information is available by searching.


The fact that all of the discussions here stay around essentially forever is what makes it more important to give some detail to the subject, IMO, as folks will be searching in the future and could easily be confused by various claims or comments.


So, what does "fully charged batteries" mean to you in your real world camping? There are no wrong answers to this question



An example may be as simple as "I run a (brand and model) smart shore charger cycle and it goes to float, so the batteries are full".

[ManWonder]

It is intriguing to read "There are no wrong answers..."

i was under the impression one needs to use a battery monitor (a good one) and let the batteries sit for a good while - then take a reading with no load on them to determine state of charge... not so?

[MsNomer]

"Fully charged" means my TriMetric meter is satisfied the umpteen parameters I plugged into it, including a bit of overcharge, have been satisfied.

[hbn7hj]

Fully charged:
For lithium it means 14.5 volts and 20 amps acceptance for a 200AH battery bank.

For lead acid it means 14.4 volts and 4 amps acceptance for a 225AH battery bank

[markopolo]

My van AGM's: 14.3V & less than 1 amp acceptance for approx 400Ah bank. I can't seem to get to 14.4V & less than 1 amp with the PD charger so 14.3V & less than 1 amp will have to do.

For the Class A: 14.4V & less than 4 amps acceptance for approx 400Ah bank. There are some basic loads that I don't turn off so it is likely closer to 14.4V & less than 2 amps acceptance actually.

[breezeway95]

I have two 6v AGMs. Fully charged is a reading of 12.8 volts with no load and about 30 minutes after any charging source has been removed. 50% charge is 12.3v under the same circumstances. I try not to take the coach batteries under 50%.

[<<B-Guy>>]

Originally Posted by hbn7hj

Fully charged:
For lithium it means 14.5 volts and 20 amps acceptance for a 200AH battery bank.

For lead acid it means 14.4 volts and 4 amps acceptance for a 225AH battery bank

Interesting conversation! Maybe I can learn something!

Could you please elaborate on the terminology.

When you say 14.5 v and "20 amps acceptance for a 225AH battery," Does that mean that a fully charge battery could initially accept a load of 20 amps whilst still retaining a terminal voltage remains of 14.4v? If so, for how long?

Just trying to understand! ............. Thx.



Brian.

[michaelingp]

There are many charts on the Internet, and most seem to say OCV of 12.7 for flooded batteries and 12.8 for AGM. However, odd as it may seem, I believe it depends on the battery. I bought a flooded lead-acid battery for a friend's car, and the OCV was 12.96 and I did my best to remove any surface charge. Even a week later it was 12.90.

[hbn7hj]

Originally Posted by Wingeezer

Interesting conversation! Maybe I can learn something!

Could you please elaborate on the terminology.

When you say 14.5 v and "20 amps acceptance for a 225AH battery," Does that mean that a fully charge battery could initially accept a load of 20 amps whilst still retaining a terminal voltage remains of 14.4v? If so, for how long?

Just trying to understand! ............. Thx.



Brian.

Acceptance means charging current, not load current.

[booster]

Originally Posted by ManWonder

It is intriguing to read "There are no wrong answers..."

i was under the impression one needs to use a battery monitor (a good one) and let the batteries sit for a good while - then take a reading with no load on them to determine state of charge... not so?

The "no wrong answers" is intended to mean that this is not intended to be a discussion of the technical end of whatever methods are the best/most accurate, but only to collect information on how the various members are determining if their batteries are fully charged to put some context to the many times we see posts that describe batteries as fully charged but with no explanation of how "fully charged" was determined.



In keeping with the idea of no tech discussion here, I will send you a PM as I think you may have misinterpreted some of the monitor and voltage testing discussions.

[avanti]

I define "fully-charged" as when my Trimetric says "100%". However, I rarely think about it: I have 140 watts of solar and park outdoors. Good enough for me.

[<<B-Guy>>]

Originally Posted by hbn7hj

Acceptance means charging current, not load current.

Thanks - so does this mean, in the case described, that the battery in question if showing 14.5v is fully charged, even though it is still accepting a charge of up to 20 amps?

I always assumed that when the charging rate dropped to zero, that only then, the battery was charged to capacity. Maybe not?

Brian.

[hbn7hj]

Originally Posted by Wingeezer

Thanks - so does this mean, in the case described, that the battery in question if showing 14.5v is fully charged, even though it is still accepting a charge of up to 20 amps?

I always assumed that when the charging rate dropped to zero, that only then, the battery was charged to capacity. Maybe not?

Brian.

Normal charging rate for 200ah of lithium batteries is 95 amps in my system. When that charge rate drops to 20 amps, not zero, it is considered full.

With lead acid that number is .02 (corrected) of rated capacity which is 4 amps for a 200ah rated battery bank.

[booster]

Originally Posted by hbn7hj

Normal charging rate for 200ah of lithium batteries is 95 amps in my system. When that charge rate drops to 20 amps, not zero, it is considered full.

With lead acid that number is .2 of rated capacity which is 4 amps for a 200ah rated battery bank.

I think hbn7hj got a typo and means .02C for lead acid. Normally, the ranges for wet cells would be 1-3%C (1-3 amps per 100ah). AGM is normally much lower with many at .5%C (1/2 amp per 100ah). TPPL AGMs are sometimes as low a .1%C (1/10 amp per 100ah).



It is best to check with the manufacturer for the specs for you particular battery, and also realize the amps will go up as the battery ages.

[ponti33602]

I was told that our Roadtrek lithiums would be 13.6/13.7 for full charge.

[GallenH]

So, I have a few questions about all of this. Any help = greatly appreciated.

1. Charging voltage does not = battery voltage, right? Lifeline says to charge at 14.2-14.4, float at 13.2-13.4 and condition/equalize at 15.5 (all specs assuming a 12v battery. But they state that a fully charged battery is 12.8+. So my battery (not connected to a charger) would never end up at 14.4.....or would it?

2. So let's say a fully charged battery is 12.8V. As it ages, it loses capacity (that's a question of sorts). Does that mean that my 12V battery will always (if fully charged) read 12.8V? And when my 100AH battery gets down to 80% capacity will still read 12.8V when fully charged but only last 80AH?

Just wondering.....................thx.

[GallenH]

Sorry: My Lifeline = AGM

[booster]

Originally Posted by GallenH

So, I have a few questions about all of this. Any help = greatly appreciated.

1. Charging voltage does not = battery voltage, right? Lifeline says to charge at 14.2-14.4, float at 13.2-13.4 and condition/equalize at 15.5 (all specs assuming a 12v battery. But they state that a fully charged battery is 12.8+. So my battery (not connected to a charger) would never end up at 14.4.....or would it?

2. So let's say a fully charged battery is 12.8V. As it ages, it loses capacity (that's a question of sorts). Does that mean that my 12V battery will always (if fully charged) read 12.8V? And when my 100AH battery gets down to 80% capacity will still read 12.8V when fully charged but only last 80AH?

Just wondering.....................thx.

Good questions! Charging voltage is just when the charger is running and will be higher than the rested battery voltage. Just as you say for the numbers. When the the charger is taken off the batteries, they will slowly drop from whatever charger voltage they were at to their rested voltage, assuming there are no significant loads on them.


Lifeline, like many others, states 12.8v for a rested battery can be considered fully charged, but in reality many/most AGMs will read higher than that if in decent condition. As it turns out, I just did a test of this exact thing today with our Lifelines during a battery cooling fan test. The test ran from 45% SOC to full on the 100 amp Magnum charger, but I took the charger off when the batteries were at about 90-95% full. They had been charging at 14.3v for several hours at that point. The voltage dropped to about 13.3v over 30 minutes and then stopped dropping, so this would be very near the full rested voltage. I then ran the headlights on van from the coach batteries for a couple of minutes which dropped the voltage to 13.0v. The voltage recovered to 13.0v when the load was taken off them and stayed there. All of these voltages are significantly higher than the rested full voltage of 12.8v that Lifeline gives us (and the test was at 90% full), so they must say that to cover what heavily worn batteries would give. Our batteries are probably 4 years old now, IIRC. Since we control by amps to determine full, I know that the batteries were still accepting about 15 amps, so nowhere near the 2 amp full threshold.


The second question is tougher, as from what I have seen I think it can go either way. As the batteries go bad, they will always have less capacity, so that is a given, but I have seen batteries that came up to full voltage (they were wet cells that always only got to 12.7v) but did not have much capacity at all. I have also seen batteries that lost capacity and full charge voltage. I think in the real world, running a capacity test is probably the only reliable way to know how much capacity you have lost as voltage may not give reliable answer.

[booster]

The test mentioned above just got completed by reaching full charge based on getting to the float transition point of 14.3v and 2 amps to the batteries. The batteries are 440ah of Lifeline GC2 AGMs. This what I saw during recharge.



The test ran the batteries down quickly, in less than the 3 hours, to 245ah down which read 45% SOC. They were recharged on the 100 amp charger which flashes to 100 amps and settles at about 94 most times. The recharge started after only a few hours of being discharged, and at cool temps, so is a near best case recharge scenario.


Total recharge time was just a bit over 8.5 hours to full


By the end of the first hour, at 66% SOC, the batteries started to accept less than the charger could provide even though voltage had not yet got to 14.5v (was at 13.6v)


By the end of the second hour the voltage had gotten to 14.3v so in absorption and amperage tapering continuously.


It continues at 14.3v and tapering voltage for another 6.5 hours until the 2 amp transition threshold was met.


If the batteries had been taken to our low limit of 20% SOC, that would have added another 1 hour plus to get to the tapering amperage point, so 9.5 hours charge time.


If the batteries had been discharged more slowly or sat for a couple of days, it probably would have taken more recharge time by maybe an hour, based on some earlier tests done more slowly.


I think most AGMs would follow a similar pattern if they had a similar charger capacity to battery capacity ratio.

[Phoebe3]

Thanks for posting that, Booster! It's really helpful.