Battery charging

[Takis]

Hello fellows. I'm new to this forum and a newbie to solar energy.
i'm located in Greece. Most of the time sunny and it's a pity not to harvest the sun.

My system consists of
Renogy Rover 40amps mppt charge controller.
Canadian solar Solar panel 320w 24v 9.3amps
2x varta dual purpose proffetional lsf 105 batteries 12v 105ah.
24v system.
Custom (not mine) 24v inverter 1200w.

The maximum surface I can use for solar energy can fit 4 of the above solar panels. And that's my current limit.

I heard that you can charge the battery bank using the 10% of c20 and not to exceed that limit.

So, my first question is : can I charge batteries with lower current amperage than 10%. If not, what are the consequences ?

My second question (dilemma) is about expanding my battery bank and at later time, expand my solar panels. Is it better to go for 4x6v rolls https://www.off-grid-europe.com/roll...290-6v-battery or expand my vartas so as to get almost 300 - 400 ah ? Maybe I'm too new to handle that rolls batteries as they deserve.

[Mike90250]

Do not upsize your batteries till you have enough solar to charge them properly

The consequence of not charging your batteries with enough amps, is the recharge can take several days, and the batteries sitting at low state of charge, will begin to sulphate.
The other reason is, the electrolyte in a battery will tend to separate (stratify) over time and the proper amps in recharge "stirs" up the electrolyte.

[robeyw]

This is a reply to the sticky closed thread Are You Killing Your Batteries?

The problem is real but the solutions offered are only partially successful while inefficient and hard on the batteries.
Looking at https://batteryuniversity.com/learn/..._battery_table
Charge methods Constant voltage to 2.40–2.45/cell, float
at 2.25–2.30V/cell. Battery should stay cool; no fast charge possible. Charge time 14–16h.
And again at https://batteryuniversity.com/learn/..._to_prevent_it
Lead acid must periodically be charged 14–16 hours to attain full saturation. An alternative solution is using lithium-ion, a battery that prefers a partial charge to a full charge.

I have enough experience with old style low cost lead batteries including Exide Golf Cart batteries to say that what I propose below works though it may not be optimum. To do this, the total battery should be divided into two equal voltage sections and paralleled in normal use. As many light gauge jumpers as possible can be placed from one section to the other to maintain the two sections as close to the same as possible and the current in these jumpers can be checked with a clamp on amp meter to check for bad cells. The batteries will never be fully charged on a daily basis. I set the absorb voltage to 2.45/cell and absorb time to 3 hours. The absorb phase will never complete in winter. The result is the sulfate accumulates and becomes increasingly difficult to remove by charging so about every month after the charging day when the battery is well charged, I disconnect one string from the system and charge it for an additional 12 to 16 hours at 2.4 to 2.45 V/cell using power from the other (supplemental power can be added to the string running the system as needed) then check the SG. (actually, after 2 to 4 hours at 2.45V/cell I water cells if needed then at the end of charge check SG). If it shows less than full charge a second saturation charge should be performed soon and if there is no improvement, the latest SG should be recorded as full charge for that cell. SG is not as meaningful indication of state of charge after a series of partial charges due to possible stratification of the electrolyte but the extended charge removes stratification.

[Mike90250]

This is a reply to the sticky closed thread Are You Killing Your Batteries?

The problem is real but the solutions offered are only partially successful while inefficient and hard on the batteries.
Looking at https://batteryuniversity.com/learn/..._battery_table
Charge methods Constant voltage to 2.40–2.45/cell, float
at 2.25–2.30V/cell. Battery should stay cool; no fast charge possible. Charge time 14–16h.
And again at https://batteryuniversity.com/learn/..._to_prevent_it
Lead acid must periodically be charged 14–16 hours to attain full saturation. An alternative solution is using lithium-ion, a battery that prefers a partial charge to a full charge.

I have enough experience with old style low cost lead batteries including Exide Golf Cart batteries to say that what I propose below works though it may not be optimum. To do this, the total battery should be divided into two equal voltage sections and paralleled in normal use. As many light gauge jumpers as possible can be placed from one section to the other to maintain the two sections as close to the same as possible and the current in these jumpers can be checked with a clamp on amp meter to check for bad cells. The batteries will never be fully charged on a daily basis. I set the absorb voltage to 2.45/cell and absorb time to 3 hours. The absorb phase will never complete in winter. The result is the sulfate accumulates and becomes increasingly difficult to remove by charging so about every month after the charging day when the battery is well charged, I disconnect one string from the system and charge it for an additional 12 to 16 hours at 2.4 to 2.45 V/cell using power from the other (supplemental power can be added to the string running the system as needed) then check the SG. (actually, after 2 to 4 hours at 2.45V/cell I water cells if needed then at the end of charge check SG). If it shows less than full charge a second saturation charge should be performed soon and if there is no improvement, the latest SG should be recorded as full charge for that cell. SG is not as meaningful indication of state of charge after a series of partial charges due to possible stratification of the electrolyte but the extended charge removes stratification.

Running lead acid for 30 days before completing an absorb cycle, is really sulfating your batteries a little bit at a time. Sometimes an EQ cycle or 3 can reverse some of the sulfation but generally, after 36 hours, the crystals harden to the point they are not readily dissolved back into solution, hence the term Sulfated.

When batteries are abused to the point they start to stratify, the high acid concentrations attack the lower parts of the grid much faster and more sediment builds up in the sump, and eventually shorts the cell out, rendering it useless.

I do not recommend undercharging 2 banks for a month, and then trying to get them "fixed" with a lot of generator run time.

Instead, I suggest a person invests in enough PV to ( even in winter ) get the batteries fully charged at least 2x a week. Even if you burn fuel in a generator, that $10 of fuel is cheaper than ruining a set of batteries. The generator strategy I prefer, is to run the generator in the early morning, and use it to Bulk charge and the first half of Absorb. Then about 10AM, there is often enough sun to finish absorb and save generator fuel.

[robeyw]

What does completing an absorb cycle mean? A saturation charge can take 12 hours the day after the previous saturation charge. I am not talking about a 3 hour EQ but a slower charge. While I agree that achievinga high SOC frequently is desirable to maintain capacity the recovery procedure I gave has worked and makes the battery more practical. The reason for charging one bank with another is to avoid the long generator run time by more rapidly charging the bank in a lower SOC if needed. I can say from SG measurements on several batteries over years that the slow charge method brings up the SG to the extent that active material is on the plates when done in the time frame given. Batteries do differ in the voltage requirement and it may be more reliable to charge at atleast .005C and expect to use atleast .1C of the source battery. If my experience was as bad as you represent, I would have completely abandoned Pb.

[Mike90250]

I'm going to refer you you to the Morningstar charge description. It's accurate and to the point. They are better wordsmiths than I am.

https://2n1s7w3qw84d2ysnx3ia2bct-wpengine.netdna-ssl.com/wp-content/uploads/2015/10/ProStar-MPPT-manual-MS-001537_v1.0.pdf

page 37 & 38
basically, you choose what your Bulk voltage will be. Battery Mfg information for your battery and service type ( Float or Cycle duty ) is a good starting point

The first part of the charge cycle is BULK, where the battery is charged till it reaches that voltage.

Absorb is also defined by many battery Mfg's, and it's a good starting point, but each installation is likely going to have to adjust the duration for their conditions.
Absorb holds the voltage defined in Bulk, for a specific time, usually at least 120 minutes It can be terminated either by time-out or a set reduction in charge current.

Float is the final voltage where the battery is held at a nominally charged voltage, but without be actively charged.

Equalize is a controlled overcharge, which will gas the cells that are full, and continue charging the individuals that are not quite full, and possibly correct some sulfation issues..

In reality, after a couple days, you measure the battery SG and if it's low and you cannot add more time to the Absorb stage, you add more voltage to the Bulk setting, the effect being to cram more power into the battery according to the time you have. I have summer settings for my batteries to save water, and winter settings to jam as much power as I can in the short periods of sun.

here is their chart:


4StageCharging.png

[robeyw]

When batteries are abused to the point they start to stratify, the high acid concentrations attack the lower parts of the grid much faster and more sediment builds up in the sump, and eventually shorts the cell out, rendering it useless.

A single partial charge will result in some stratification. My question is how to make it less harmful? My batteries have a full charge SG of 1.3 when new and so far I have been able to maintain it, but how much good would it do to lower the full charge value to something like 1.265? The apparent full capacity would go down but if I never discharge to below 40% (discharge 60% of capacity) to preserve the plates, I think it is not important. Some batteries have a full charge SG as low as 1.2 but I they have more electrolyte so I can only go about a third of the way there due to cell geometry.

[Mike90250]

Are you confusing Stratification with Sulfation ?

When flooded lead acid batteries are somewhere between 80-90% full, sulfation starts. after about 48hours, the crystals harden on the plates, both reducing the SG, and obscuring active plate area, both contribute to capacity loss.

Tall case batteries, without a good Absorption -> Float charge, will Stratify from the acid mix not getting stirred up from vigorous bubbling. The bubbling only happens when the battery is over 80% full

[robeyw]

[QUOTE=Mike90250;n410004]Are you confusing Stratification with Sulfation ?

QUOTE]
NO, but I have not experienced the hard sulfation you refer to. If I had it I would not be maintaining the full charge SG. My question is based on the general idea that you previously expressed that higher SG increases corrosion and that stratification increases SG in the plate area, so how much would be accomplished by lowering the full charged SG from 1.3 to 1.265, considering that the stratification will be increasing the SG in the plate area above the full charge SG a large part of the time?

[Takis]

Sealed lead acid battery could face those problems ?

[Mike90250]

Sealed lead acid battery could face those problems ?

SLA don't stratify, but can suffer from sulfation. They also cost more, and don' generally last as long, one overcharge starts venting gas, which is lost forever.

[robeyw]

SLA don't stratify, but can suffer from sulfation. They also cost more, and don' generally last as long, one overcharge starts venting gas, which is lost forever.

It depends a lot on the specific battery. I have pure lead Gates SLA cells that I purchased as used surplus more than 25 years ago in service though most have not lasted that long. These long life cells are NOT float charged, I think that increases corrosion and leads to open circuits. Their capacity is certainly reduced, impedance and self discharge increased but they have been at a low SOC for significant periods of time and often go 2 months without being charged so the hard sulfation is not that much of a problem.

[qrper]

Why back in the early 90s I was a Trojan battery dealer. I told my customers two things when they inquired about lead-acid batteries.

1. Lead-acid batteries rarely die of old age—they're usually murdered.
2. Only the rich can afford to by cheap batteries.

That was back in the day when you could buy a new Trojan T-105 for around $40-$60 bucks.

[Takis]

Now this price is a dream.

[qrper]

Now this price is a dream.

I feel the pain, too!

That's why I dropped out of the Trojan dealer network. Even my dealer prices were so high, I couldn't sell batteries. The last run of batteries was in '98 and '99 for the big Y2K thingiee that never happened.

I moonlighted for a local mower shop. They sold the General Electric Eltrac (?) tractors. There were six 6 volt golf cart batteries in them. When the cost of replacing those got so high, people simply scrapped the tractors out. They loved 'em, but couldn't afford $900+ for battery replacement