What floating current do you see?

[CharlieBauer]

I have a 5 year old AAP H8/49 AGM in a car that doesn't get driven much. I had been charging it every 2 months with my CTEK 4.3 but was getting concerned with the low SOC, so I decided to put it on full time.

Float voltage is confirmed as 13.6v as it should be.

I have it plugged into a smart plug with energy monitoring. It has been on for 7 days and I am seeing consumption of about 2.3 watts. A day or two ago it was 2.5 watts. It fluctuates up and down a little but seems to be on a very very slow downward trend. I saw it dip below 2 watts today.

I understand that the smart plugs wattage isn't the correct number to use to calculate current as it is measuring how much the charger is consuming rather than how much it is flowing to the battery.

But after several days, what would one expect to see? Is this the kind of current / watts that the battery needs to stay full, or is it still slowly topping up? Note that it is consuming 60 watts a day and is a 95AH battery, so several days of this after being mostly charged by earlier stages is substantial.

 

[shanneba]

This is from the battery testing sticky at the top of the sub forum-

Battery Charger Testing Results

Like a number of us on here, I have a number of battery chargers. I keep a bunch because I have a number of cars that dont get year-round use, and I also have some for maintenance. Because there is now an Automotive Electrical subforum, and because there are a lot of good questions...

bobistheoilguy.com

 

[tmorris1]

There is no way to tell from your setup. The only way to measure would be to out an ammeter between the charger and battery.

2W @ 120V is only about 15mA, so I would guess most of that current is running the charger.

 

[Davejam]

2.5W @ 12V is about 0.2amps. I think that's ok.

 

[JHZR2]

2.5W @ 12V is about 0.2amps. I think that's ok.

And the charger is probably in a very inefficient range of operation with a bunch of switching losses. So I would anticipate only half of the 2.5W is reaching the battery. As said above, you really need an ammeter between the charger and battery.

 

[CharlieBauer]

Thanks for all the responses.

I probably should have been clearer that I was more interested in the trend in current over a longer period of time. The graph above is great but the float is only for a day.

I had read elsewhere that after about 10 days, somebody saw their current reduce, which suggested that a 13.6v float was topping up the battery.

So let's say that the CTEK 4.3 floating current is 0.125 amp (based on the chart above and close to what I'm seeing). Is this sufficient to add capacity to a battery? It seems to be about 40w a day which is 1200w a month.

 

[gathermewool]

Whatever you’re seeing for wattage is likely what it takes to maintain 13.6VDC + whatever your ECM is drawing + hear losses in the charger (A non-zero amount). I’ve seen disconnected batteries drop to ZERO amps while in v-mode of regulation.

If the charging scheme of a “smart” charger cuts off voltage too soon then a float voltage of 13.6 might increase SG and SOC. For a fully charged battery, float voltage will not add capacity UNLESS it is also desulphating to increase effective surface area in the battery. I don’t know much about that though.

The unit of energy used is w-hr, not w. At 2W constant over 24 hrs, that’s 48 W-hrs or 0.048 kW-hr used. That’s around a penny per day where I live.

 

[circuitsmith]

I recently floated the battery in my Tucson, removed from the vehicle, using an adjustable power supply.
Floating at 14.5V, the current fell to 230mA after about 90 minutes and stayed there for an additional hour.
Then an equalizing charge at 16V and ~500mA for 15 minutes.

I used to do this every 3 months, since I drive only 1-2 times per week.
Now I use a 10W solar panel to keep the battery topped up; and do a bench charge once a year.

I've seen a new battery fall to under 100mA, and at 9 y.o. ~400mA, all at 14.5V in a cool (60-65F) basement.

 

[CharlieBauer]

Update:

Towards the end of 10 days of floating at 13.6v, power consumption dropped slightly but not significantly. Averaged about 2.25 watts towards the end vs 2.5 watts at the beginning.

At Day 10, the charger moved to stage 8 where it charges if voltage drops below 12.7v. I have not seen the battery drop below 12.7v and power consumption at this stage has been 1.5 watts with very little fluctuation.

 

[Dave9]

Some of those electricity usage meters are inaccurate at very low draw, I'd put a multimeter in series (between charger/battery) and measure current to the battery.

 

[wrcsixeight]

The more important figure as to amperage acceptance/ state of charge, is when the battery is at Absorption voltage, not float voltage.

The ONLY way to determine true full charge on an AGM is to when the amps decline to a certain level at absorption voltage, generally 14.4 to 14.9v, depending on specific AGM. That is also for a 77f battery.

Float current into the battery, not the total current feeding the charger, will vary between batteries, and vary on the same battery as it ages.

My previous Northstar AGM a 90 amp hour group 27 , when new, would be considered full, when amps required to maintain 14.42 volts fell to 0.4 or less.
But if held at 14.42, amps would eventually taper to 0.0x

When this battery had accumulated ~ 1200 deep cycles over the next six years, amps would never taper below 0.6. They would bottom out and then start rising as the battery began heating rapidly. If at this point I lowered voltage to 13.6v, amps would be right around 0.35, and never decline any further.

Using a manual charger, an adjustable voltage power supply, means I could not just set to 14.7v and forget about it, so when I could not be there to lower voltage, I would just set it to 13.6v. When I would return and be able to monitor it and lower voltage when amps tapered to proscribed level, I often found even 12 hours at 13.6v would still leave the battery less than ful and 30 mintes to 4 more hours were required at 14.7v for amps to taper to prescribed levels.

Anyway, if one watches an ammeter and a voltmeter on a battery from day one it will behave far different than it will the week before it fails to start the vehicle.

In general the current required to maintain float voltage, when at high states of charge will increase as the battery ages, but there needs to be a known starting point as a basis for comparison.

Lifeline/ Concorde, which make arguably the best deep cycle AGMS on the market, have a good PDF writeup, and while they prescribe a lower absorption voltage than other agms, they recommend that the battery be considered full once amps taper to 0.5 per 100Ah of capacity when held at absorption voltage. There is no way to determine a full state of charge, by amperage, when held at float voltage. The lesser amperage it accepts is indicative of a higher state of charge, but one needs to bump it back to 14.4 to 14.7v and see the amps the battery accepts at that electrical pressure.

https://lifelinebatteries.com/wp-content/uploads/2015/12/6-0101F-Lifeline-Technical-Manual-Final-5-06-19.pdf

Most of my close observances of lead acid battery behavior is on batteries intentionally cycled.
A battery that is only ever used as a starting battery will age differently, and perhaps behave differently when it reaches end of life from living chronically undercharged, x% from true full, combined with age, rather than a battery cycled to 50% then brought to a true 100% SOC.

Watching battery voltage is only somewhat enlightening.
Watching how much amperage flows into a battery at a voltage, or watching how much voltage a battery can maintain while supporting a X amp load, is very enlightening.

Those really interested in guestimating battery health/ state of charge, should have not only a voltmeter on their charger's dc output, but an Ammeter as well.

One of these inexpensive RC wattmeters is very easy to hook inline on the DC output and will not affect charger operation as long as one can join wires competently and the charger's wires do not have a battery temperature sensor on them.


As for looking at an AC Kill a watt meter feeding a DC charger, well the charger is likely only 80% to 85% efficient, and the true efficiency likely varies whether it is putting out near it s rated output or a tiny fraction of it when iholding 'float' voltage after having achieved and held absorption voltage.

 

[CharlieBauer]

Thanks wrcsixeight.

The CTEK has "pulsed" a few times to keep voltage above 12.7v. But without logging equipment, it's hard to know exactly what it is doing.

After the battery rested several hours, I turned on the headlights, which uses 70w, and the voltage dropped and held at 12.12v. It then recovered to 12.8v.

I put the charger back on (on AGM mode) and it ran somewhere between one and two hours in the absorption stage before moving to the float stage.

Rather than let the CTEK do whatever it does in pulse / maintenance mode, I've scheduled the smart plug to turn on daily for four hours. That way, I can be sure the battery will rest long enough to dissipate surface charge and that the charger will go through the absorption stage, feeding the battery the maximum voltage and amps that the charger is capable of.

 

[JHZR2]

I recently floated the battery in my Tucson, removed from the vehicle, using an adjustable power supply.
Floating at 14.5V, the current fell to 230mA after about 90 minutes and stayed there for an additional hour.
Then an equalizing charge at 16V and ~500mA for 15 minutes.

I used to do this every 3 months, since I drive only 1-2 times per week.
Now I use a 10W solar panel to keep the battery topped up; and do a bench charge once a year.

I've seen a new battery fall to under 100mA, and at 9 y.o. ~400mA, all at 14.5V in a cool (60-65F) basement.

14.5V isn’t what you would call floating.

There is no way to determine a full state of charge, by amperage, when held at float voltage. The lesser amperage it accepts is indicative of a higher state of charge, but one needs to bump it back to 14.4 to 14.7v and see the amps the battery accepts at that electrical pressure.

Exactly. Float voltage should ensure a constant state of charge, replacing the losses associated with self discharge, without any sort of cyclic discharge/charge or other condition. It should be an equilibrium state. The fact that the battery does accept a charge is because that current is making up for the self discharge and other activation losses associated.

But after several days, what would one expect to see? Is this the kind of current / watts that the battery needs to stay full, or is it still slowly topping up? Note that it is consuming 60 watts a day and is a 95AH battery, so several days of this after being mostly charged by earlier stages is substantial.

I think you mean 60Wh/day, and thus it would take about two weeks to use 1kWh and be charged 10c. Hardly substantial.

Batteries will not last forever, regardless of how well you treat them. Especially automotive batteries that aren’t maintained constantly in an air conditioned space. As much as I’m a fan of keeping up after them, especially in cars with parasitic draws, there is diminishing returns. If the consumption is such a concern, put the thing on a timer, and recognize that you’ll keep the battery in a decent range, even if not perfect, for the long run.

 

[CharlieBauer]

I think you mean 60Wh/day, and thus it would take about two weeks to use 1kWh and be charged 10c. Hardly substantial.

Batteries will not last forever, regardless of how well you treat them. Especially automotive batteries that aren’t maintained constantly in an air conditioned space. As much as I’m a fan of keeping up after them, especially in cars with parasitic draws, there is diminishing returns. If the consumption is such a concern, put the thing on a timer, and recognize that you’ll keep the battery in a decent range, even if not perfect, for the long run.

By "substantial", did not mean to imply cost (although a CA kWh is more like 30c than 10c), but in relation to battery capacity.

As you say, there are diminishing returns and my intention was to just leave it plugged in and let stage 8 do it's thing and let stage 7 float for 10 days any time the charger was removed. But wrcsixeight has convinced me of the benefit of absorption charging, so I've decided to put it on a daily timer to ensure it goes through the absorption stage and I can still forget about it.

 

[JHZR2]

By "substantial", did not mean to imply cost (although a CA kWh is more like 30c than 10c), but in relation to battery capacity.

As you say, there are diminishing returns and my intention was to just leave it plugged in and let stage 8 do it's thing and let stage 7 float for 10 days any time the charger was removed. But wrcsixeight has convinced me of the benefit of absorption charging, so I've decided to put it on a daily timer to ensure it goes through the absorption stage and I can still forget about it.

Hopefully you can realize the other side of that story. Higher voltages accelerate plate corrosion. Higher voltages accelerate dissociation of water to H2 and O2, which not all will recombine if not in a valve regulated battery. Additionally, the volume of PbSO4 vs other lead forms is different, thus the plates and dendritic structures will change in volume, aiding in loss of active material. I pan some cases, shedded material will build up and create a conductive internal short that will discharge the battery.

Theres more to it than just putting it on a timer. Sure, if you drove the car daily, it would discharge and charge, it not necessarily for the time duration and schedule. That’s the beauty of the float charge. It sits at a lower voltage that doesnt pulse the plates, expand the plates, shed material, corrode the materials, split water, etc.

While a timer isnt a bad idea, once a week is more like it. There are diminishing returns, and it’s not clear what improvements you will truly get doing this daily. At that point just let it float. In reality you’ll never miss the fraction of an Ah or even few Ah difference. But you may well feel a difference as impedance grows and capacity is lost trying to ge that extra Ah.

 

[CharlieBauer]

Hopefully you can realize the other side of that story. Higher voltages accelerate plate corrosion. Higher voltages accelerate dissociation of water to H2 and O2, which not all will recombine if not in a valve regulated battery. Additionally, the volume of PbSO4 vs other lead forms is different, thus the plates and dendritic structures will change in volume, aiding in loss of active material. I pan some cases, shedded material will build up and create a conductive internal short that will discharge the battery.

Theres more to it than just putting it on a timer. Sure, if you drove the car daily, it would discharge and charge, it not necessarily for the time duration and schedule. That’s the beauty of the float charge. It sits at a lower voltage that doesnt pulse the plates, expand the plates, shed material, corrode the materials, split water, etc.

While a timer isnt a bad idea, once a week is more like it. There are diminishing returns, and it’s not clear what improvements you will truly get doing this daily. At that point just let it float. In reality you’ll never miss the fraction of an Ah or even few Ah difference. But you may well feel a difference as impedance grows and capacity is lost trying to ge that extra Ah.

Thanks. I'm going with daily for now to see if there is consistency with what the charger does / what the battery accepts. wrcsixeight cautions that smart chargers don't fully charge and I've seen a few odd episodes, so I'd like to see if there are any differences one charge to the next.

The first run of 4 hours, a day after turning the charger off, resulted in total power consumption of 20wh, so it does seem to be full.

I get your point about doing this once a week instead, and I'll move towards that frequency. Easy enough to do with a smart plug.

 

[JHZR2]

Thanks. I'm going with daily for now to see if there is consistency with what the charger does / what the battery accepts. wrcsixeight cautions that smart chargers don't fully charge and I've seen a few odd episodes, so I'd like to see if there are any differences one charge to the next.

The first run of 4 hours, a day after turning the charger off, resulted in total power consumption of 20wh, so it does seem to be full.

I get your point about doing this once a week instead, and I'll move towards that frequency. Easy enough to do with a smart plug.

You don’t have sophisticated enough equipment to make that determination, and even then, what difference is it going to make?

Will you not replace your battery? Do you have a method to determine impedance growth? Capacity loss? How will you discern the difference between that and temperature effect?

The reality is that nice theories are nice theories, and impractical practices are just that. Diminishing returns are a real thing, and you’re still a victim of calendar. You’re not a telecom site making revenue. You don’t count coulombs or have a fancy cycler.

But whatever floats your boat!