Charging Lead acid Batteries

[sunil21]

I am trying to charge few lead acid batteries.
few 4.5ah 6v - sealed lead acid battery, a 35ah and 100ah lead acid battery

I have charged the 6v batteries many times before (not properly actually - connecting a 7.5 volt supply to it directly)

I searched on internet and found lot of documents and most of them somewhat differ from each other

voltage level (charge level)

on one site i found below data for 6v (SLA ) battery

100% - 6.30 ...... 25% - 6.0, .... 0% - 6.0

for a 12v battery
100% - 12.6 , 0% - 11.90 etc..

(the above written data, I read it in a web site 1 year back and i cant find it now so may be a slight different)

I have a 100ah Lead acid battery which I have hooked to a solar charge controller and a single panel. The panel is only 120

watts and when it fully charges the battery, controller shows 13.5V and floats. now I doubt it fully charges and i read the

manual and it says never let the voltage drop below 11.0v (the charge controller has set it initially to 10.7v.).Also the manual says to bench charge it once every 3 months of so.. when I connect this battery to 6 amps or more it drops to around 12v(may be due to internal resistance). ignore all the documents, based on your experience tell me what are the SOC (state of charge) and difference between SLA and LA?

I recently bought a CC/CV buck converter on ebay (it says 12amps, but the IC, I checked is rated for 8amps only).I set it up in this way..(for 6v SLA) - connected a 12v 2amp as source, set output voltage to 8v, CC to 500ma (with my multimeter) connected the output terminals to the input of the battery with a single 1N5408 diode beween the output of the buck and the postive of battery (must be around 7.5v after the diode). I have connected the voltmeter directly to the battery terminals.First it is cc mode and then goes to CV, the question is when I stop , what should be the value of voltmeter

 

[D.A.(Tony)Stewart]

This site has all the answers you need.


Flooded Cell 12V batteries are usually 11.5~12.5 float for near 0 and 100%
CV is usually 14.2

For SLA it is temperature sensitive for CV, so check with OEM specs.
Also the SLA CV level is a tradeoff between lifespan (13.9V) and capacity (14.5V for 6 cells)

ESR rise with drop in SoC and also aging and sulphation of the battery which can sometime be corrected by a 15 minute equalizing charge and/or high speed pulse over DC charge. ESR imbalance is a result of acid s.g. imbalance.

 

[Audioguru]

If a lead-acid battery voltage goes as high as 7.5V for a 6V battery then it is overcharging and its electrolyte is boiling away. I think the power diode will reduce the 8V to 7.2V then it will charge like half of my car battery.
Follow the charging instructions printed on the battery.

 

[sunil21]

on my 6v SLA it is written , cycle use 7.25 - 7.45v, initial current - less than 1.2Amp, Standby use - 6.8-6.9v.
So to charge this battery with the buck converter, I must set CV - 7.40 V(to 7.90v - .50V diode drop, which I can check and calculate) , CC - 450ma then I connect to battery (which we assume has 40% charge). so time passes,initially for few hours it will run on CC mode and and voltage slowly increase and finally reach 7.4voltage (on buck), how long do I keep it like that (what must be the reading of voltmeter hooked directly to battery?) , then disconnect and change it to float for some time( I read it is 13.5 to 13.8 for 12v battery, so must be 6.75 to 6.9 for a 6v battery) ?

 

[Warpspeed]

The figures vary because the measurements are taken in very different ways.

If a battery has been sitting for many hours, being neither charged or discharged for many hours, the figures you gave are pretty right.
Fully charged 12.6 fully discharged 11.9
That is what you should see in the morning if the battery sits idle overnight.

The other extreme is when its working pretty hard, it can be connected to a current limited charger with a voltage of maybe 14.2 for perhaps ten to fifteen hours, but NOT PERMANENTLY. This is for reasonably fast charging where slight water loss may be acceptable for a flooded battery.
Its far too high for a sealed cell, it will dry out fairly quickly if charged at that voltage.

Under heavy discharge, the voltage may fall to about 10.0v where the load must be disconnected. When the load is removed the voltage will spring back very slowly to something much higher. Often 12.0v or slightly more after several hours.

The third situation is where the battery is permanently connected to a battery charger in standby float mode. The voltage suggested for this is usually in the range of 13.5 to 13.8 depending on battery size, type, and average temperatures.
Its a good voltage to charge fully sealed cells, but it can take a very long time to reach full charge capacity.

The standby float mode 13.5v to 13.8v is high enough to very gradually come up to 100% over a very long time, but not so high that you start losing water from the battery.

So the voltage recommendations you see depend on how the battery is to be used, the type of battery (sealed or flooded) and under what conditions the voltages are measured.

 

[sunil21]

Another question I have is, lets take 12V,35Ah battery for example. I have it charged fully, so the battery voltage increases from 12.0V ( for example) to 13.5v on full charge(may be). So I disconnect the charger and keep it Idle. so the voltage at terminals must slowly reduce from 13.5v to 12.7 (or 12.65) , which may take a day or so. again I connect another battery of the same capacity to charger and voltage increase from 12.0 to 13.15v and I disconnect. Then the battery voltage may go down from 13.15 to say 12.46v gradually. During this time - between 13.5v to 12.6 or 13.15 to 12.46 how do I get the accurate SOC.
in both cases the voltage may go to value 13.0v. can I get it by connecting a particular amp load to the battery.
I have this question because I parked my car in the garage and after 12 hours not using it, I thought of checking the battery for SOC, and it read 13.1 volt, what can I assume from that?

 

[Warpspeed]

Twelve hours idle, and 13.1v ?
Never seen anything like that.

 

[BradtheRad]

on my 6v SLA it is written , cycle use 7.25 - 7.45v, initial current - less than 1.2Amp, Standby use - 6.8-6.9v.

( I read it is 13.5 to 13.8 for 12v battery, so must be 6.75 to 6.9 for a 6v battery) ?

Yes, that computes.

The battery reaches a higher voltage when charged at a higher rate. From experience you'll know what to expect with a particular battery.

As a general rule, avoid causing gassing of electrolyte. You'll need to spend a few charge sessions watching to make sure if this is happening.

 

[Audioguru]

I have never used a sealed lead-acid battery. Does it have a vent to avoid an explosion if it is over charging?

 

[D.A.(Tony)Stewart]

All ones I have used have pressure release valves using a rubber dot on top. But if installed on a sealed enclosure as we once did, then a small teflon vent seal is added to the enclosure to allow H2 to vent but not H2O.

4% H2 with any amount of O2 is the Lower flammability limit just below the explosive limit.

 

[Warpspeed]

They all have vents of some kind to prevent the build up of pressure.

Water loss through electrolysis is a big problem, because there is minimal water in the paste electrolyte to begin with.

Once that has gone, the battery dries out, and its life is over.
The charging voltage absolutely must be kept below the gassing voltage, which means a very slow recharge rate.

Even then, these are going to be expendable expensive short life batteries.

 

[sunil21]

Twelve hours idle, and 13.1v ?
Never seen anything like that.

Yes, after your comment I thought "something wrong with my measuring?", so I just checked and it reads 13.1v. I parked it around midnight on Wednesday (around 12:00 - Thursday morning), so it is now about 36 hours. The 35amp battery in question is my car battery which is around 3 1/2 years old, which had no known issues until recently when I went to service the car , they told me - the battery is low in charge and If we add water to it , it will loose all the charge - so leave it here and we will charge it and it will take 24 hours. But I was in a hurry and told them I will come later and left the station.

One week later I had a long journey with my friend and when I was waiting at the destination for few hours, I used the A/C fan , it drained the battery completely and if failed to start, so my friend had to push from behind to start it.

The question is "something wrong with 13.1v reading ?"

 

[Warpspeed]

Try replacing the battery in your digital multimeter.
The cheapies can read high just before the multimeter dies.