# Float Question

Registered Users Posts: 21
I am trying to understand the Float portion of a multi-stage charge controller. I understand the concept of bulk and absorption charging to get a drained battery up to its maximum charge state. My question is why Float is significantly lower than the full state of a battery. For example, I have two Sun Xtender 153 AH 12 volt batteries. The manufacturer's documentation says a full charge on this battery is between 14.2 and 14.4 volts. The Morningstar TS-60 charge controller I use can be configured for AGM and 14.4 volts, but their manual says that after it reaches a full charge on the battery it drops to a float level of 13.4 volts.

If you want a battery to stay at or near its maximum charge, why not have a float value of 14.4 volts for a 14.4 volt battery? If the battery drops to 13.4 volts it is at a significantly reduced capacity.

Thanks for any light you can shed on this matter for me.

Re: Float Question

14.4 volts is actively charging/at gassing voltage.

13.4 volts or so is below gassing voltage and high enough to take care of self discharge and some"corrosion"issues.

12.7 to around 13.0 volts is full charge for a resting battery (lead acid).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 21
Re: Float Question

Thank you Bill. The docs for my Concorde Sun Xtender battery says that the recommended charging voltage for this battery is 14.2 - 14.4 volts. I mistakenly thought this was the full charge level of the battery. What you shared makes great sense. I understand that a higher input voltage is needed to raise the charge on a battery. So if a full charge is 12.7-13.0 volts, the voltage range of 14.2-14.4 volts could push the battery to that level. When the battery reaches 12.7-13.0 volts, the charge controller simply maintains the battery's charge by applying a "trickle charge," or a charge slightly higher than the full resting voltage of the battery.

On a related note, you mentioned "gassing voltage." If I am understanding this right, the battery produces gases due to chemical reactions taking place while charging. Once it is fully charged no more chemical reactions occur and gassing stops. You mentioned that 13.4 volts (or thereabouts) will not produce gassing. This is true only if the battery is fully charged, correct? If the battery were below its full charge state, a 13.4 volt input charge could produce gassing because the battery would be charging. (I am trying to get the principle correct in my mind.)

Also, if the charge controller applied a voltage higher than 13.4 volts after the battery was charged, no more chemical reactions would occur producing gassing, so the voltage would be turned into heat, which is not a good thing. Is this correct?

Thanks again.
• Solar Expert Posts: 3,123✭✭✭✭
Re: Float Question
Thank you Bill. The docs for my Concorde Sun Xtender battery says that the recommended charging voltage for this battery is 14.2 - 14.4 volts. I mistakenly thought this was the full charge level of the battery. What you shared makes great sense. I understand that a higher input voltage is needed to raise the charge on a battery. So if a full charge is 12.7-13.0 volts, the voltage range of 14.2-14.4 volts could push the battery to that level. When the battery reaches 12.7-13.0 volts, the charge controller simply maintains the battery's charge by applying a "trickle charge," or a charge slightly higher than the full resting voltage of the battery.

On a related note, you mentioned "gassing voltage." If I am understanding this right, the battery produces gases due to chemical reactions taking place while charging. Once it is fully charged no more chemical reactions occur and gassing stops. You mentioned that 13.4 volts (or thereabouts) will not produce gassing. This is true only if the battery is fully charged, correct? If the battery were below its full charge state, a 13.4 volt input charge could produce gassing because the battery would be charging. (I am trying to get the principle correct in my mind.)

Also, if the charge controller applied a voltage higher than 13.4 volts after the battery was charged, no more chemical reactions would occur producing gassing, so the voltage would be turned into heat, which is not a good thing. Is this correct?

Thanks again.
You have it slightly backwards.
When the battery is discharged below about 80% SOC the applied current will go almost entirely to chemical reactions in the plates and electrolyte.
As the battery gets more and more charged, more and more of the supplied current will go to electrolyzing water (gassing) instead.
At 100% charge, any current above the self-discharge current will go entirely to gassing, but with the voltage at the proper float setting that current will be minimal.
If you temporarily raise the voltage well above the Float voltage, as you would for equalizing, the resulting current will all go to gassing and accompanying heating of the battery.

If you have a sealed battery, there is a maximum rate at which dissolved oxygen and hydrogen are able to recombine inside the battery. This process takes care of normal charging. But if you go beyond this rate, the resulting pressure will blow out the vent seals and vent electrolyte as well as gas out of the battery. Bye bye battery....
SMA SB 3000, old BP panels.
Re: Float Question

As inetdog says... Here is some more details:

http://www.powerstream.com/SLA.htm
Minimum voltage
Anything above 2.15 volts per cell will charge a lead acid battery, this is the voltage of the basic chemistry. This also means than nothing below 2.15 volts per cell will do any charging (12.9V for a 12V battery) However, most of the time a higher voltage is used because it forces the charging reaction at a higher rate. Charging at the miminum voltage will take a long long time. As you increase the voltage to get faster charging, the voltage to avoid is the gassing voltage, which limits how high the voltage can go before undesirable chemical reactions take place. The typical charging voltage is between 2.15 volts per cell (12.9 volts for a 6 cell battery) and 2.35 volts per cell (14.1 volts for a 6 cell battery). These voltages are appropriate to apply to a fully charged battery without overcharging or damage. If the battery is not fully charged you can use much higher voltages without damage because the charging reaction takes precedence over any over-charge chemical reactions until the battery is fully charged. This is why a battery charger can operate at 14.4 to 15 volts during the bulk-charge phase of the charge cycle.

The basic lead acid battery is ancient and a lot of different charge methods have been used. In the old days, when voltage was difficult to regulate accurately flooded lead acid batteries were important because the water can be replaced. The lead acid chemistry is fairly tolerant of overcharging, which allows marketing organizations to get to extremely cheap chargers, even sealed lead acid batteries can recycle the gasses produced to prevent damage to the battery as long as the charge rate is slow. We offer a range of chargers from inexpensive to very sophisticated, depending on the requirements of the customer, but all of the chargers we sell off-the-shelf are highly regulated sophisticated chargers that cannot overcharge the battery.

Cyclic versus Standby charging. Some lead acid batteries are used in a standby condition in which they are rarely cycled, but kept constantly on charge. These batteries can be very long lived if they are charged at a float voltage of 2.25 to 2.3 volts/cell (at 25 degrees C) (13.5V to 13.8V for a 12V battery). This low voltage is to prevent the battery from losing water during long float charging. Those batteries that are used in deep discharge cycling mode can be charged up to 2.45 volts/cell (14.7V for a 12V battery) to get the highest charge rate, as long as the voltage is dropped to the float voltage when the charge is complete.

Lots of information:

http://www.windsun.com/Batteries/Battery_FAQ.htm
http://www.batteryfaq.org/
http://batteryuniversity.com/

Eric/Westbranch posted a link to a 1922 battery repair manual. A very interesting read and look back almost 90 years at technology and mass production (near the end are some factory photographs).

THE AUTOMOBILE STORAGE BATTERY ITS CARE AND REPAIR

Despite the title, also includes information on storage batteries too (Farm Lighting Batteries).
Originally Posted by stephendv

... Victron has authored a great paper that covers some interesting material on charging forklift batts: http://www.victronenergy.com/upload/...yUnlimited.pdf

Department of Energy Lead Acid Handbook:

DoE_BatteryHdbk1084.pdf

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 21
Re: Float Question

Thanks Inetdog and Bill. That is some very helpful information and resources. I will study them for a while and hopefully get schooled on this subject.

Perhaps this information is in some of the links you provided, but could you tell me what voltage level for a battery such as the AGM one I have would be considered 50% charge, or fully depleted? I have ordered a Trimetrix battery monitor and it reports battery capacity in percent power left. I was wondering how it determines what is left. In looking at various inverters, I have noted that different ones will cut off when the battery gets to certain levels to protect the battery from damage. I believe one inverter model said it would cut off at 10 volts. That seems awfully low for a battery, even a deep cycle battery. What percentage of full capacity would a 10 volt charge be considered (roughly)?
• Solar Expert Posts: 3,123✭✭✭✭
Re: Float Question
Thanks Inetdog and Bill. That is some very helpful information and resources. I will study them for a while and hopefully get schooled on this subject.

Perhaps this information is in some of the links you provided, but could you tell me what voltage level for a battery such as the AGM one I have would be considered 50% charge, or fully depleted? I have ordered a Trimetrix battery monitor and it reports battery capacity in percent power left. I was wondering how it determines what is left. In looking at various inverters, I have noted that different ones will cut off when the battery gets to certain levels to protect the battery from damage. I believe one inverter model said it would cut off at 10 volts. That seems awfully low for a battery, even a deep cycle battery. What percentage of full capacity would a 10 volt charge be considered (roughly)?
I believe that with the Trimetric you have to input the battery bank size in AH and the charging efficiency (amps out over amps in to recharge the same amount). It then keeps track of the current coming and going rather than relying on the very weak correlation between battery voltage under load and battery SOC. You have to tell it once when the battery is fully charged so that it can start counting, and then recalibrate it from time to time as its count drifts one way or the other from reality.

Cheap battery monitors that just look at voltage are not worth even the small amount you pay for them since they mislead you.

The manufacturer of your specific battery will hopefully provide a table of resting battery voltage against SOC, since the voltages will vary a bit with different additives in the plates and different electrolyte concentration.
SMA SB 3000, old BP panels.
• Registered Users Posts: 21
Re: Float Question

I found the information I was looking for in the little four page document that came with my Sun Xtender battery. It includes the following table.

Re: Float Question

A "Full" to 1/2 Full battery is roughly 12.7 to 12.1 volts (i.e., ~3+ hours of resting at ~77F battery bank)...

http://www.solar-electric.com/deep-cycle-battery-faq.html[h=4]Here are no-load typical voltages vs state of charge[/h] (figured at 10.5 volts = fully discharged, and 77 degrees F). Voltages are for a 12 volt battery system. For 24 volt systems multiply by 2, for 48 volt system, multiply by 4. VPC is the volts per individual cell - if you measure more than a .2 volt difference between each cell, you need to equalize, or your batteries are going bad, or they may be sulfated. These voltages are for batteries that have been at rest for 3 hours or more. Batteries that are being charged will be higher - the voltages while under charge will not tell you anything, you have to let the battery sit for a while. For longest life, batteries should stay in the green zone. Occasional dips into the yellow are not harmful, but continual discharges to those levels will shorten battery life considerably. It is important to realize that voltage measurements are only approximate. The best determination is to measure the specific gravity, but in many batteries this is difficult or impossible. Note the large voltage drop in the last 10%.

[TH="width: 132, bgcolor: #FFFFFF, align: center"]State of Charge[/TH]
[TH="width: 132, bgcolor: #FFFFFF, align: center"]12 Volt battery[/TH]
[TH="width: 132, bgcolor: #FFFFFF, align: center"]Volts per Cell[/TH]

100%
12.7
2.12

90%
12.5
2.08

80%
12.42
2.07

70%
12.32
2.05

60%
12.20
2.03

50%
12.06
2.01

40%
11.9
1.98

30%
11.75
1.96

20%
11.58
1.93

10%
11.31
1.89

0
10.5
1.75

[h=3]Why 10.5 Volts?[/h] Throughout this FAQ, we have stated that a battery is considered dead at 10.5 volts. The answer is related to the internal chemistry of batteries - at around 10.5 volts, the specific gravity of the acid in the battery gets so low that there is very little left that can do. In a dead battery, the specific gravity can fall below 1.1. Some actual testing was done recently on a battery by one of our solar forum posters, and these are his results:
I just tested a 225 ahr deep cycle battery that is in good working order..
I put a load on it 30a for 4 hrs it dropped its voltage to 11.2
I then let it cool down for 2 hrs

then put the load back on again in 1hr 42 mins it dropped to 10.3v
35 mins under 30a load 9.1v (273w)
10 mins later max output current 11.6a 8.5v (98.6w)
5 mins later max output current 5.2 amps 7.9v (41w)
3 mins later 7.6v and 2.3a (17.5w)

This shows after it gets below 10.3 v you only have 35 mins of anything useful available from the battery.

Battery is now dead and most likely will not fully recover

The 10.5 volt minimum is not to protect the battery, but (in my humble opinion to protect the inverter's DC input state.

Inverters are constant power devices P=V*I

Put a 1,000 Watt load on a "perfect inverter" ignoring losses for the moment:

1,000 Watts / 14.5 volt = 69 amps
1,000 Watts / 12.0 volt = 83 amps
1,000 Watts / 10.5 volt = 95 amps
1,000 Watts / 8.0 volts = 125 amps

P=I2R == You double current, you have 4x the waste heat (wiring losses).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 762✭✭✭
Re: Float Question
..If you want a battery to stay at or near its maximum charge, why not have a float value of 14.4 volts for a 14.4 volt battery? If the battery drops to 13.4 volts it is at a significantly reduced capacity.

That is a GREAT question! It has to do with operational aspects of proper agm charging when using it in a solar application where one is cycling the battery, and not using it as a standby / backup battery.

I usually recommend setting both absorb and float to the same voltage in a solar cycling application with agm's for the following reason:

Normally, once the controller limits the voltage, and the battery starts to absorb and current starts dropping, a "full charge" is usually indicated by the current dropping to C/20, in this case, 153 / 20 = 7.6 amps. You are done.

BUT, in a cyclic solar application, you may not reach the end of absorb daily, and you'll walk down the capacity slowly over time as that 1% that doesn't get recharged hardens the sulfate on that area. Now your battery acts like it is only 1.51ah capacity. Repeat by not achieving a full charge, and you can see how it walks itself down.

A gross dental analogy is like not brushing your teeth 100%, and now plaque is forming under the gum, even though in the mirror you have a sparkling smile. Gross, but that's all I could come up with for now.

One way to compensate for not reaching a full charge on a regular basis is of course to use an ac charger or generator perhaps weekly or so.

The other way is to set your absorb and float to the same value. Some controllers are time limited, and will switch to float earlier than C/20, and there is not enough time in a lower voltage float to compensate. By artificially raising the float to the same value as absorb, it is like an extended absorb, with no float. We are not doing standby / backup where one does indeed want to use a lower voltage float.

Most agm's have a specification for "extended absorb", where one can remain in absorb longer than usual. Sometimes this is 4 to 8 hours beyond normal. This really helps when doing solar on a regular *cyclic* basis for agm's. Deka East/Penn has guidelines on this type of RE usage as well.

Keeping the battery at the absorb voltage for very long times will create grid corrosion. However, that is tempered by the fact that we have nature's on/off switch when it gets dark daily. And, most agm's are murdered by undercharge faster than by mild grid corrosion, so we pick the lesser of two evils, keep the absorb and float voltage the same, and try to keep the agm happy that way with an extended absorb trying to achieve a fuller charge under less than ideal solar conditions. It is a tradeoff.

Note that it is mandatory, especially in an extended absorb operation like this to be using temperature compensation, preferable measured at the battery terminal, and not just ambient. Still, ambient is better than nothing.

Of course this assumes your batteries are in good condition, and in your case, I hope well balanced. Normally I'd be tempted to place each one on a charger *individually* for a preventative-maintenance routine once in awhile, and then reassemble back into a bank for normal use. An Iota would be ideal for this.
• Solar Expert Posts: 3,741✭✭✭✭
Re: Float Question
inetdog wrote: »
At 100% charge, any current above the self-discharge current will go entirely to gassing

A bit of hyperbole there... some current will produce heat. The higher the charging voltage the more heat produced. --vtMaps
4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
• Solar Expert Posts: 3,741✭✭✭✭
Re: Float Question
In looking at various inverters, I have noted that different ones will cut off when the battery gets to certain levels to protect the battery from damage. I believe one inverter model said it would cut off at 10 volts. That seems awfully low for a battery, even a deep cycle battery. What percentage of full capacity would a 10 volt charge be considered (roughly)?

To elaborate a bit on what Bill wrote...

An inverter tries to supply whatever power the load requires. The battery provides the power to the inverter, and as the battery voltage drops the inverter draws MORE current (if volts goes down, then amps must go up... Power = volts X amps)

As the inverter draws more current (amps) it becomes less efficient (more current means more energy is wasted as heat) which makes it draw still more current to make up for the losses. Meanwhile, the battery is becoming less efficient as current increases (Peukert factor) which means more energy wasted and more current from the battery.

As this downward spiral continues, when the battery voltage gets down to 10 volts, the currents will exceed the current handling capacity of the inverter, and he inverter shuts down. The inverter's Low Voltage Disconnect (LVD) is to protect the inverter, not to protect the battery or its cables. You use a fuse or circuit breaker to protect the battery and cables.

A high quality, full featured inverter will have an adjustable LVD so that you can protect the battery.

--vtMaps
4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
• Solar Expert Posts: 3,741✭✭✭✭
Re: Float Question
I have ordered a Trimetrix battery monitor and it reports battery capacity in percent power left. I was wondering how it determines what is left.

The trimetric is a good monitor.

A battery monitor is often compared to the gas gauge in your car, but that is not a good analogy.

The gas gauge in your car actually measures the level of gas in your tank. A battery monitor doesn't measure the level of anything. It is more like the odometer in your car. The odometer counts miles and the battery monitor counts ampHours.

Suppose you have a car that gets about 25 mpg and has a 15 gallon gas tank. Suppose also that your gas gauge does not work. What do you do? You use your odometer. For example: After a fill up you drive 150 miles and you expect that you have used 6 gallons and have 9 gallons remaining in your tank.

If you fill up the tank again you can, as above, use your odometer to estimate your gallons remaining.

But what if you do not completely fill your tank. For example, starting from a full tank you drive 150 miles and then you add 3 gallons to your tank and then drive 100 miles and then add 4 gallons to your tank and drive 150 miles and then add 5 gallons to your tank and drive 100 miles. At this point you estimate that you have 7 gallons remaining in your tank, but that estimate is not too accurate because your mileage is not ever exactly 25 mpg. The only time you know exactly how much gas is in your tank is when you have just filled it up (or when you run out of gas).

So it is with your battery monitor. The only time it is exactly accurate is when you fully charge your batteries and reset the battery monitor to read 100% full. It can be very accurate counting the amphours in and out of the battery, but it can only estimate the state of charge based on what you tell it of the battery capacity and of the battery efficiency. It can read 100% (by counting amps into the battery) before the battery is fully charged.

As just mentioned, when your battery is 100% charged you can reset your trimetric to read 100% SOC (state of charge), but how do you know when the battery is at 100%? With flooded cells you use an hydrometer, with AGMs it is necessary to have faith in the manufacturer's recommendations. One way to know when your battery is charged is to use 'end amps', and your trimetric can use 'end amps' to automatically reset itself to 100% SOC.

When you hold the battery at absorb voltage, the current into the battery declines until the battery is 100% charged. At that point, the current declines no further and since the battery is charged, all that current is just making heat, gas, and corrosion. That current is 'end amps'.

The trimetric automatically resets to 100% is based on end amps criteria that you set. For example, you may program the trimetric to reset when the voltage is above 14.2 for a minute while the current is below 15 amps.

Bottom line: the longer its been since a complete charge, the less you can trust your battery monitor. And you can't trust it at all if its not set up correctly. That said, battery monitors are valuable tools in estimating your SOC, but you have to understand their limitations.

By the way, there are a number of refinements that can improve the accuracy of battery monitors... for example, if the monitor knows the temperature of the batteries, it can adjust the Peukert factor and ampHour capacity of the battery. I don't find it necessary to have such refinements... by achieving 100% SOC and resetting the trimetric at least twice a week my trimetric stays accurate enough to meet my needs. Also, I have flooded batteries so I can easily 'audit' the performance of my controller and my trimetric.

--vtMaps
4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
• Registered Users Posts: 21
Re: Float Question

That was a great response vtmaps! Thanks for the explanation. I always like a good analogy. My brain seems to relate to them better.

By the way, I do have the temperature sensor to hook between my Morningstar TS-60 PWM charge controller and the batteries. That should keep the charger maintaining the battery bank at optimum levels. I don't know whether the Trimetric 2030RV battery monitor has an option for battery temperature sensing. I don't recall reading that it does.
• Solar Expert Posts: 3,123✭✭✭✭
Re: Float Question
vtmaps wrote: »
A bit of hyperbole there... some current will produce heat. The higher the charging voltage the more heat produced. --vtMaps
No hyperbole at all. Some of the power will go to heat, but all of the current has to either find an internal leakage resistance or else electrolyze water. There is no other chemical path that can carry current from one plate to the other.
Whether the current goes to normal chemical reaction or to electrolyze water, the voltage will be higher because of the internal series resistance of the battery. And any voltage applied beyond the minimum needed for that chemical reaction will indeed be going to heat. (The terminal voltage of the rested battery rises with SOC because as the concentration of H2SO4 rises the voltage required to drive the reaction increases. Once you get to maximum SG, any increase in voltage will because of the internal resistance of the battery and the applied current. Plus some small effects from "surface charge" which is a different chemical reaction.)
SMA SB 3000, old BP panels.