Float Voltage a Stationary Lead Acid Battery?

Ramussons

Joined May 3, 2013
1,395
Can someone tell me what is the Float Voltage to be maintained for a Stationary Lead Acid Battery? I remember that the Stationary Lead Acid Batteries (not the Sealed ones) were maintained at 2.2 volts/cell on Float. The Battery Banks of the Satellite Telecom Centres were 60 Cells floated at 132 Volts, the Float Cum Boost Chargers did a CC charge upto 144 volts ( 2.4 volts/cell) before switching to the 132 volts float.

This being what I know, why are we referring to 13.8 volts Float voltage for a 12 volt battery instead of 13.2 volts?

BuckRogers

Joined May 8, 2017
3
' why are we referring to 13.8 volts Float voltage for a 12 volt battery instead of 13.2 volts? '

We are not! In fact, all of the said voltages might be correct, because in the real world, the intended use of the batt is determing its charge and float voltages. A longer life is achieved by lower voltages and vice versa. So, if the batts are rarely cycled, the correct float voltage is 2,20, but can be increased to up to 2,30 volts. A good compromise would be 2,25 volts. That way, almost no water will be lost during floating, giving 13,20 to 13,80 Volts batt voltage, depending on the intended use. Charging voltage can be much higher, though. Batts which are deep cycled may be charged with up to 2,45 volts per cell, 14,70 volts per batt. This is acceptable if the voltage is reduced to the above mentioned float values after the charging process has completed.

Ian0

Joined Aug 7, 2020
9,472
Temperature compensation is important, otherwise it gases more as it gets warm, which makes it gas even more etc.
There is a school of thought amongst the UPS people that the battery should not be float charged, but disconnected and periodically topped up. I tested a 250kW UPS a while ago, which had eighty 6V 200AH flooded batteries, and that's how it worked.

BuckRogers

Joined May 8, 2017
3
@Ian0 that's incorrect to the max.

Rasmussons was asking about charge voltages for unsealed lead acid batts and the whys surrounding them. I already eplained that a battery which is rarely cylced should be float charged, because it minimizes water loss and prolongs battery life.

That won't be necessary in a sealed battery you are refering to.

Regarding the 'school of thought amongs the UPS people': UPSs never use unsealed batteries for three very good reasons.
One is called hydrogen. The other one is called installation angle. The last one is called dissipation.

Did you read Rasmussons' post at all?

A last word about flooded batteries: those are a special design where the electrodes are covered with a scrim. That makes
the batt more resistant to people who 'top it up' because they believe this is a normal fashion of charging battery.

It is not. The scrim saves the batt from losing water. That all it does. Float charging does the same.

Ian0

Joined Aug 7, 2020
9,472
@Ian0 that's incorrect to the max.
I’m not talking about sealed batteries either.
So was I, I was adding the comment about temperature coefficient which is important and had not previously been mentioned.
That won't be necessary in a sealed battery you are refering to.
Still not talking about sealed batteries. A flooded battery at 2.25V/cell (standard float voltage for VRLA) still loses water, although very slowly. Ask any classic car owner who has put us battery on 13.8V float charge over winter, and finds the battery ruined in the spring because it has lost its water.

Regarding the 'school of thought amongs the UPS people': UPSs never use unsealed batteries for three very good reasons.
One is called hydrogen. The other one is called installation angle. The last one is called dissipation.
So I was imagining the eighty batteries then? Flooded tubular-plate batteries, because VRLAs would never manage 2500 discharge cycles.
And VRLA batteries never release hydrogen? Why does it say on them “Do not charge in an unventilated area”?

A last word about flooded batteries: those are a special design where the electrodes are covered with a scrim. That makes
the batt more resistant to people who 'top it up' because they believe this is a normal fashion of charging battery.

It is not. The scrim saves the batt from losing water. That all it does. Float charging does the same.
No it doesn’t, it stops bits falling off the plate, and increases the endurance.
https://www.varta-automotive.com/en-gb/technology/efb-battery-technology
nothing to do with the water.

Have you ever come across a device called a ”fork lift truck”? Read its maintenance manual.

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BuckRogers

Joined May 8, 2017
3
Well, Ian0,

don't let the door hit you on your way out.

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Ylli

Joined Nov 13, 2015
1,083
Years ago, when I worked in the battery lab at Globe Union, we had a bunch of cells set up for accelerated life testing. They used a subset of Pb/Ca plates from their 2000 A-hr batteries, and at a temperature of (if I remember right) 120°F. We floated them at 2.18 volts.

Ramussons

Joined May 3, 2013
1,395
Been going through the discussions, but I still don't understand why some Batteries are floated at 13.8 while others at 13.2 Or am I missing something?
I've read some time back that while Stationary batteries use pure Lead in their plates, Automobile batteries mix Antimony / Bismuth because the plates need to be stronger to withstand vibrations. Does this change the Float voltage to 13.8 from 13.2?

Ian0

Joined Aug 7, 2020
9,472
Assuming it's not just due to the temperature compensation or adjustment errors. . .
Batteries either die of sulphation (if undercharged) or plate corrosion (if overcharged). The happy medium is about 2.27V/cell which gives the maximum life. The other factor is water loss. THis is mitigated by recombination in VRLA batteries, but in flooded batteries is only mitigated by a bloke with a jug of distilled water.
Higher float voltage = more water loss.
So, if the bloke with the distilled water is unreliable, better choose a lower float voltage, because the odds are that otherwise it will die from water loss before the sulphation kills it. If you can rely on the batteries being topped up, then the higher float voltage can be used and the life increased.
So, lower float voltage = lower maintenance, but slightly shorter life.
You are also correct about plate metallurgy. Antimony makes the plates harder but increases water loss. Calcium reduces self-discharge, but increases the tendency to plate corrosion.

Ylli

Joined Nov 13, 2015
1,083
Batteries with lead/antimony plates have a lower gassing potential than batteries with lead/calcium plates. The introduction of lead/calcium plates are what created the 'Maintenance Free' batteries because it greatly reduced gassing and therefore loss of water.

prairiemystic

Joined Jun 5, 2018
307
13.2V float is too low unless ambient temperature is high (above 25°C). Where did you get this value from?
edit: I found US Battery/Trojan oddly advises 2.17V (13.02V) float, and DIN 41773 is 2.26V +/-1% (13.38V), as the lowest float voltages out there.

Flooded lead-acid batteries have different charging voltage specs depending on the manufacturer and the use (stand-by vs cyclical). Although you can generalize somewhat and not cause damage, they are not all the same. The metals are different and proprietary: lead-antimony, lead-calcium, lead-selenium etc.
When you have a big investment in a stationary battery bank \$10,000's of dollars you will pay attention to the millivolts and millilitres.

Flooded lead-acid batteries float voltage is given as a range 2.25-2.35V usually for 25°C/77°F. For stationary batteries i.e. 13.5V is very common for equiv. 6 cells.

Some newer charger technology uses a sub-float voltage i.e. 13.2V with regular jump to 13.8V say once a day for 15 minutes. This is supposedly to minimize water loss and plate corrosion. But I've heard of failures (sulphation), short battery life so it might just be "another patent". Because float charging is a waste of energy, there are technical papers on how low you can go or use short-term float charging- but you must use an electron microscope to see if the plates are suffering (sulphation) crystal buildup.

All charging voltages are temperature corrected. It depends on the manufacturer, but it seems to vary from -4.0 to -6.3mV/°C/Cell for flooded.

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