What is wrong with wiring batteries in parallel?

[jflorey2]

I wonder if the same issues occur with cells in series as in parallel. With cells in series as you say the same amount of charge has to go through all the cells. Unless all the cells in series have the same capacity and same charge efficiency some will end up being overcharged and some undercharged.

Exactly. So you charge enough that all cells are either fully charged or overcharged; the overcharged cells lose a small amount of water but otherwise remain fine and at 100% charge. There is thus a mechanism to keep them all fully charged and balanced.

I wasn't thinking about keeping the voltage the same but more about actively shifting charge from cells that have reached the absorb voltage early and during absorb actively pushing extra charge into the cells which are staying at a lower voltage rather than letting the voltage rise in some cells to the point where they outgas. That is why I specified active balancing.

If you balance so all strings get the same _current_ that would work; at that point each string gets the same charge, and is thus not much different than a single string in terms of life. But that's not how most balancers work, and it's hard to do.

However, again, you can't say "oh, that cell is at 2.35V so it's fully charged, any more charging and it will outgas." You can measure with a hydrometer (most accurate) or you can charge until you know it's at 100%. So traditional voltage balancing won't be as effective as it is for (say) lithium ion.

[karrak]

I was under the impression that overcharging LA batteries was bad for them due to grid corrosion, is this only a fairy minor issue?

By using external active balancing I would have thought you would be helping to avoid both overcharging and undercharging. Of course whether this is practicable or not has to be considered.

A question, is charging to say 2.35V/cell with a low termination current of say less than ~C/50-C/100 a reliable way to fully charge a LA battery?

Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger github.com/simat/BatteryMonitor/wiki
Latronics 4kW Inverter, homemade MPPT controller

[Sunking]

I was under the impression that overcharging LA batteries was bad for them due to grid corrosion, is this only a fairy minor issue?

For someone who claims to be a battery expert ask some really ignorant questions. Equalization is a controlled over charge and is only performed when specific gravity spread is greater than .030 among all cells. Only a daily cycled Pb battery gets EQ every couple of months. Pb batteries in Float Service never requires EQ. Corrosion of the grids are created by voltages above 2.45 volt @ 77 degree F for extended periods of time. That does not happen with a Solar system. Pb batteries charge at 2.4 vpc ... Grid corrosion is a problem with automobiles. Autos do not lower the voltage to 2.25 volts aka FLOAT.

So the answer to your question is: irrelevant and insignificant. EQ is only done when needed infrequently. Just like lithium batteries, fully charging and equalization stresses the batteries. The difference is Pb batteries are brutes and extends battery cycle life by dissolving lead sulfate crystals. Lithium on the other hand shortens battery life.

By using external active balancing I would have thought you would be helping to avoid both overcharging and undercharging. Of course whether this is practicable or not has to be considered.

Huh? Invalid question. There is no EXTERNAL BALANCING of Pb batteries. They self balance themselves. When a PB cell is charged, charge current still passes harmlessly through to the next cell in series to charge lower SOC cells.

A question, is charging to say 2.35V/cell with a low termination current of say less than ~C/50-C/100 a reliable way to fully charge a LA battery?

No. Nor can it be done with a lithium battery. You would never know when the cell is fully charged. Both charge the exact same way, there is no difference. On a 4S lithium or 12 volt PB you apply 14.4 volts and charge until charge current tapers to C/33 to C/20. Just for you that means 3 to 5% of C. At C/50 to C/100 you would never be able to detect full charge. Difference would be a lithium battery would be destroyed leaving a 4S at 14.1 volts after it charged up. Quite possible catch fire from thermal runaway. A lead acid battery would be just fine with it and need a drink now and then.

FWIW C/50 to C/100 is not a charger. It is a Trickle Charger. Any battery expert would know that. In any solar application minimum charge rate is going to be C/10 or higher.

[karrak]

For someone who claims to be a battery expert ask some really ignorant questions. Equalization is a controlled over charge and is only performed when specific gravity spread is greater than .030 among all cells. Only a daily cycled Pb battery gets EQ every couple of months. Pb batteries in Float Service never requires EQ. Corrosion of the grids are created by voltages above 2.45 volt @ 77 degree F for extended periods of time. That does not happen with a Solar system. Pb batteries charge at 2.4 vpc ... Grid corrosion is a problem with automobiles. Autos do not lower the voltage to 2.25 volts aka FLOAT.

I have never claimed to be a battery expert. I have only had limited experience with batteries during my engineering career. I would now say I am knowledgable when it comes to using LFP batteries as I have successfully set up two offgrid systems based around LFP batteries that have been running for nearly five years without incident and because of having to do research to check whether or not statements made by you are BS or not.

I was not talking about Equilising LA batteries, I was talking about limiting the absorb phase to limit the grid corrosion and water loss.

This is what Battery University has to say on the subject

"The correct setting of the charge voltage limit is critical and ranges from 2.30V to 2.45V per cell. Setting the voltage threshold is a compromise and battery experts refer to this as 'dancing on the head of a needle'. On one hand, the battery wants to be fully charged to get maximum capacity and avoid sulfation on the negative plate; on the other hand, over-saturation by not switching to float charge causes grid corrosion on the positive plate. This also leads to gassing and water-loss

You, yourself say the same thing regarding LA batteries being charged in cars.

I was stupidly hoping that by starting this tread that there could be a rational discussion about the different issues involved with paralleling batteries and how to circumvent them but it appears not to be the case.

No. Nor can it be done with a lithium battery. You would never know when the cell is fully charged. Both charge the exact same way, there is no difference. On a 4S lithium or 12 volt PB you apply 14.4 volts and charge until charge current tapers to C/33 to C/20. Just for you that means 3 to 5% of C. At C/50 to C/100 you would never be able to detect full charge.

More BS. Charge a 4s 12V LFP battery to 14.4V with a termination current of C/33 to C/20 and you get an SOC around 100%, do the same with a termination current of C/50 to C/100 and you get an SOC around 100%.

Difference would be a lithium battery would be destroyed leaving a 4S at 14.1 volts after it charged up. Quite possible catch fire from thermal runaway. A lead acid battery would be just fine with it and need a drink now and then.

Only true if the LFP battery is not top balanced. If it is top balanced the individual cell voltages would get to 3.525V and just sit there. The LFP battery would have to be very out of balance at the top end to get the slighest chance of thermal runaway which would only be possible at a cell voltage of ~4.5V!

Leaving both 12V LA and LFP batteries at 14.1 volts for extended periods would I think be bad for them.

FWIW C/50 to C/100 is not a charger. It is a Trickle Charger. Any battery expert would know that. In any solar application minimum charge rate is going to be C/10 or higher.

I was not talking about trickle charging, I was talking about charge termination current.

Minimum charge rate for solar applications is nearly zero at sunrise and sunset.

Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger github.com/simat/BatteryMonitor/wiki
Latronics 4kW Inverter, homemade MPPT controller

[notyoung]

In amongst the angst, there was a mention of the time it takes to *fully* charge a lead acid battery. Have you read this paper from Sandia National Laboratories?
http://www.otherpower.com/images/sci...Efficiency.pdf
The "Conclusions" section tells us much.

I've seen "Not more than two strings in parallel" many times, but then you're faced with the question "How long is a string?" DIY systems are likely to have a string length that fits on the available shelf.

My solar power is currently a "Wait until daylight" solar charged generator with 504AH of 12HX330FR AGM (six 84AH batteries cost me a total of $235 used in year 5 of a 10 year projected service life and I've had the use of and the opportunity to learn about those batteries while planning their future replacement - relatively cheap education and it appears that at least 3 of those batteries might make 10 years). The 2000 watt pure sine wave inverter powers the fridge, a few LED lights and internet access plus the furnace in winter, giving 10 to 24 hours of silent power depending on the season. In a longer outage, all 1600 watts of solar would be in place on 6 MPPT controllers (I like redundancy) to provide some level of power in the long term.

No impressive initials (BS Information Systems, but consider the connotations of "information"). Now retired for the fourth or fifth time and writing fiction:


MOD NOTE. Please do not attach advertisement links for your products to your post.

[jflorey2]

I've seen "Not more than two strings in parallel" many times, but then you're faced with the question "How long is a string?" DIY systems are likely to have a string length that fits on the available shelf.

Uh - typically no. String length is determined by voltage. A 48 volt system will almost always have (for example) 8 6V batteries (because 6*8=48.) If you could fit 10 6V batteries on the shelf, 2 would be useless, because the inverter typically won't be able to handle a 60V nominal (which means a 75V charge voltage.)

[notyoung]

DIY systems are often 12 volts (all the available 12 volt appliances and accessories) with multiple batteries in parallel. That is where you find the parallel "string the length of the shelf". Some of the 12 volt systems using 6 volt golf cart equivalent batteries are also "the length of the shelf". Nothing to do with detailed design or engineering in these small systems, just wherever the space and the $$$ cross.

My comments are an aside. The primary value of my original post is the Sandia report's conclusion that lead acid charging efficiency drops to 55% at a little more than 80% SOC. That means the last bit of charging really does take "forever".

[littleharbor]

The term string refers to series wired, batteries, in this case. A group of paralleled batteries, no matte how long, is not a string.

[jflorey2]

DIY systems are often 12 volts (all the available 12 volt appliances and accessories) with multiple batteries in parallel. That is where you find the parallel "string the length of the shelf".

Right. But that's not a string. A string is a series connected set of batteries.

The primary value of my original post is the Sandia report's conclusion that lead acid charging efficiency drops to 55% at a little more than 80% SOC. That means the last bit of charging really does take "forever".

Yep. And further, that keeps going - to get the last few percent you are at hideously low efficiencies because you are starting to dissociate water.