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Clarification of Lead Acid Battery Charging Voltages Required Please

rtbomp

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I'm familiar with stages of charging a (12v) lead acid battery, e.g. 13.8v float charge, then 14.4v boost 14.6 equalize etc.

Are these figures discrete steps, or does it mean that float charge is 13.8v to 14.4v, then boost is 14.4v to 14.6v and so on?

Also, I've read in a couple of places that the charging chemistry starts at 12.9 volts, so the charging voltage has to be at least this figure. If I have a 12.6v charger* connected to a battery there is current flowing, is this just passing through the battery? Is it causing any harm?

*The charger is in fact a PV panel connected to the (12v) battery (via a blocking diode), the battery voltage will be sitting at about 12.4v before the sun rises, and then slowly increases until it reaches 12.9v where the increase in voltage slows down markedly. It will then carry on rising until I cut it off at about 14.7 volts

Thanks.
 
I'm familiar with stages of charging a (12v) lead acid battery, e.g. 13.8v float charge, then 14.4v boost 14.6 equalize etc.

Are these figures discrete steps, or does it mean that float charge is 13.8v to 14.4v, then boost is 14.4v to 14.6v and so on?

Also, I've read in a couple of places that the charging chemistry starts at 12.9 volts, so the charging voltage has to be at least this figure. If I have a 12.6v charger* connected to a battery there is current flowing, is this just passing through the battery? Is it causing any harm?

*The charger is in fact a PV panel connected to the (12v) battery (via a blocking diode), the battery voltage will be sitting at about 12.4v before the sun rises, and then slowly increases until it reaches 12.9v where the increase in voltage slows down markedly. It will then carry on rising until I cut it off at about 14.7 volts

Thanks.
A lifepo4 battery doesn’t really have a bulk float or boost charge. This chemistry has almost no internal resistance and as long as the amperage is within allowable specs it will stay close to its state of charge voltage as it is being charged. What it does have is a “knee” the voltage profile the voltage will rise rapidly for the first 10% ish of charge and then remain nearly constant until it is 90% ish and the the voltage rapidly increases till you reach 100% capacity. Charging directly from a solar panel is dangerous if your bms fails and can potentially damage the bms and is not recommended
 
A lifepo4 battery doesn’t really have a bulk float or boost charge. This chemistry has almost no internal resistance and as long as the amperage is within allowable specs it will stay close to its state of charge voltage as it is being charged. What it does have is a “knee” the voltage profile the voltage will rise rapidly for the first 10% ish of charge and then remain nearly constant until it is 90% ish and the the voltage rapidly increases till you reach 100% capacity. Charging directly from a solar panel is dangerous if your bms fails and can potentially damage the bms and is not recommended
Thanks but I was asking about lead acid batteries, not lifepo4, hence posting in the "Lead Acid" section and putting "Lead Acid" in the thread title.
 
Charging the lead acid battery has basically 3 stages. Stage 1 is the period where the battery usually receives most of the charge. The battery accepts current from the charge source until the absorption voltage is reached. Depending on the charge current the battery will be charged to somewhere between 50% and 95% state of charge. A high current will get to the lower state of charge and a low current to the high state of charge.
The second stage is to complete the charge process, here the voltage is held constant at the absorption level for the absorption period. This period will be a few hours typically, the actual time depends on battery type, initial state of charge when the charge process started, and the charge current .
The third stage now considers the battery is fully charged and the voltage drops to a maintaining level, float voltage, to ensure the battery self discharge is compensated for. In many cases the charger will not completely charge the battery in the absorption period due to the many variables. Thus a high float voltage is often recommended. Over a long period, 10s of hours, the battery will be slowly charged to full.

With your very simple manual control solar charging, and I guess a low charge current, by the time you reach the target voltage of 14.7 the battery will be at a high state of charge. But it wont be fully charged and over time the battery may degrade.
Even a low cost PWM charger should have an absorption control where the voltage is held constant once the target voltage has been reached. This would be a useful addition to your system.
lead acid charge graph.jpg



Mike
 
Last edited:
Charging the lead acid battery has basically 3 stages. Stage 1 is the period where the battery usually receives most of the charge. The battery accepts current from the charge source until the absorption voltage is reached. Depending on the charge current the battery will be charged to somewhere between 50% and 95% state of charge. A high current will get to the lower state of charge and a low current to the high state of charge.
The second stage is to complete the charge process, here the voltage is held constant at the absorption level for the absorption period. This period will be a few hours typically, the actual time depends on battery type, initial state of charge when the charge process started, and the charge current .
The third stage now considers the battery is fully charged and the voltage drops to a maintaining level, float voltage, to ensure the battery self discharge is compensated for. In many cases the charger will not completely charge the battery in the absorption period due to the many variables. Thus a high float voltage is often recommended. Over a long period, 10s of hours, the battery will be slowly charged to full.

With your very simple manual control solar charging, and I guess a low charge current, by the time you reach the target voltage of 14.7 the battery will be at a high state of charge. But it wont be fully charged and over time the battery may degrade.
Even a low cost PWM charger should have an absorption control where the voltage is held constant once the target voltage has been reached. This would be a useful addition to your system.
View attachment 130863



Mike
Many thanks for your reply, I pretty much understand what happens during charging with a charger, but with a PV panel there are so may variables as you said. I didn't say much about my actual system (which is very experimental) to try to prevent the thread going off topic, but basically I charge from the panel until the charging voltage reaches about 14 volts and then automatically switch over to a PWM controller as I reckon it knows more about battery charging than I do! I'll post more about my project in a more suitable section. I don't switch the PWM on straight away as it takes more current than the panels provide at low light levels

I'm still intrigued as to what's going on in the battery up to 12.9v. With a basic transformer-rectifier charger with an open circuit voltage of 14.25 the voltage drops when you connect the battery (as you'd expect) and slowly creeps up as your graph shows, so either the internal resistance or the back voltage of the battery is increasing. I suspect the latter. so the battery must be taking some charge below the magic 12.9volts, perhaps?
 

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The equivalent simplified circuit of the battery is a voltage generator in series with a resistance. The resistor and voltage source have varable values, depending mostly on SOC.
There is no magic at 12.9 volts, any voltage over the resting battery voltage will cause current flow. However the battery internal voltage and internal resistance will rise, thus a resionable voltage difference between the applied charger voltage and the internal voltage generator must exist for an effective charge to take place.
If you consider the complete circuit of the charge process, again a very simplified version, you have the charger/ solar panel as a current source in series with a resistance connected to the battery internal resistance in series with the internal voltage generator , thus compleating the charge path.
There is plenty of indepth research published on the Internet regarding battery charging

Mike
 
Many thanks for your reply, I pretty much understand what happens during charging with a charger, but with a PV panel there are so may variables as you said. I didn't say much about my actual system (which is very experimental) to try to prevent the thread going off topic, but basically I charge from the panel until the charging voltage reaches about 14 volts and then automatically switch over to a PWM controller as I reckon it knows more about battery charging than I do! I'll post more about my project in a more suitable section. I don't switch the PWM on straight away as it takes more current than the panels provide at low light levels

Usually best to check manufacturer websites for the recommended charging voltages. For my 6V T105's, with 2 in series, I use 14.8V as bulk and leave it there using a power supply. This is at 10A. My large charger in the shop does the same with the voltage, the amps will be higher however. Once 14.8V is hit, the amps will begin to drop, this is absorption. . If you go higher amps, you will see the charging voltage be higher, the charger is working against the resistance of the battery but still held at 14.8V. Once I see the amps below 3A, I will switch to 13.5V for float.

I'm still intrigued as to what's going on in the battery up to 12.9v. With a basic transformer-rectifier charger with an open circuit voltage of 14.25 the voltage drops when you connect the battery (as you'd expect) and slowly creeps up as your graph shows, so either the internal resistance or the back voltage of the battery is increasing. I suspect the latter. so the battery must be taking some charge below the magic 12.9volts, perhaps?
You can charge from voltages lower than that. Internally, the battery plates are creating a chemical reaction where the sulfuric acid is being reconstituted from the lead sulfate on the plates. The opposite of what occurs when the battery is discharged. This takes electrons, the sulfate is the result of removing electrons. When you charge, you are adding back electrons which allows the chemical reaction to take place.

The internal resistance of the battery changes as more sulfate is converted back to acid and lead.
 
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