Matching PV for 2000ah batteries.

[jestronix]

Hi All,

Im looking at purchasing two 12v 2000ah battery banks. Im wondering what the minimum charge rate would be? For independent banks that is.

[Sunking]

Depends on your location. In theory you have 2 x 12 x 2000 48 Kwh of storage capacity. Spread over 5 days or 20% DOD per day gives you around 6400 wh per day use. If you live in an area with a winter Sun Hour Day of 3 hours means the minimum solar panel wattage is 3200 watts. If you configure the system at 24 volts means a charge current of 133 amps.

This would mean you would need 2 very expensive 80 amp MPPT charge controllers operating in parallel.

[Mike90250]

another theory is 5-10% of the battery pack amps, should be available for charging.

So with a 2,000A battery that's 200A (10%) - 100A (5%) needed for charge. If they are gel/AGM that don't need EQ, you can drift closer to the 5% range. Flooded, you need the amps for monthly EQ to stir up the electrolyte (de-stratify it).

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But, what I'd suggest, is that you are going to use an inverter, to run these in series, and up the voltage. Your "power" or wattage will stay the same. But you can reduce your wire sizes (100A wires are pretty expensive - big fat hunks of copper!) and # of charge controllers. 24 or 48V are popular sizes.

There is also a problem with many batteries in parallel, where its difficult to have them share the load equally. In series, that problem goes away.


Here's a great article about connecting batteries on the diagonal
http://www.smartgauge.co.uk/batt_con.html (connect batteries on diagonal )

and some other battery stuff:

http://www.windsun.com/Batteries/Battery_FAQ.htm (short & sweet)
http://www.batteryfaq.org/ (very large)

[jestronix]

Thanks for the info, wow those are some big amps to keep these batteries up and running. At this point I can only throw about 20 amps charge into the batteries. Im looking to only grab one 12v bank at this point, 2000AH, they are second hand and a good deal for $500. 5 years service (well looked after) 1 year sitting. Mate of mine (solar installer) is going to give them a big boost charge for 24 hours before I take them away. He knows the history of the batteries, as he set the system up.

Im wondering what sort of life cycle I will see out of them if i cant properly equalize them?

It will only have a small amp hour per day usage, so 40 to 60AH a day. Can throw around 100AH a day back in. It probably wont change much from that.

Even if these batteries after 5 years or so drop to like 300AH holding capacity its not a problem, at this point looking for about 5 years usage, any more would be bonus.

I suspect they will stratify big time, are there other ways to de-stratify? Can you agitate them some how? Giant eye dropper

Interestingly I know someone that has had two 60watt panels on a 500AH flooded set for 10 years, only small loads though.

For $500 for 5 years of use Id be happy, or am I better off getting a 100AH AGM ? Maybe two for $650. But not much for those rainy weeks with 200AH.

[Sunking]

Im wondering what sort of life cycle I will see out of them if i cant properly equalize them?

Just exactly how would you put an equalization charge on them? Not possible to use solar PV to equalize. To Equalize you have to apply the EQ charge on a fully charged battery for 24 hours. The only battery you can EQ is flooded lead acid.

[Mike90250]

Just exactly how would you put an equalization charge on them? Not possible to use solar PV to equalize. To Equalize you have to apply the EQ charge on a fully charged battery for 24 hours. The only battery you can EQ is flooded lead acid.

Huh ?

200A needed for EQ. It would take a huge array, close to 13,000W. Possible, but not practical.

An EQ cycle, after full charge, should only be 1-3 hours. De-Sulphating takes a long time, and it's not very reliable.

If you could use a hydrometer on the batteries, you could use it to stir or slosh the electrolyte around, and de-stratify it that way, but that's a PITA. (Pain In The A...)

[Sunking]

Huh ?

200A needed for EQ. It would take a huge array, close to 13,000W. Possible, but not practical.

Mike an EQ charge is not a current charge, it is an constant voltage charge. Here is a pretty good White Paper on the subject.

[Mike90250]

I think there is a disconnect here. A Equalization Cycle (EQ) is a overcharge, applied to a flooded cell battery bank, with the intent of vigorous gassing (to destratify) the batteries, and to bring up to full charge, any batteries in the string with a low charge. This requires about 5-10% of the amps of the battery pack, to get the cells stirred up. This is only supposed to happen for an hour or 3 at the most.

What the white paper was , was for VRLA (sealed) EV batteries, which are never supposed to gas and bubble. That is a very different type of EQ, than chargers for solar batteries are programed to deliver.

This becomes a concern in some installs in summer, when panel voltage falls, and you cannot complete an EQ cycle. To push the amps thru, to bubble the cells, you need higher voltage than normal.

I guess I missed where the OP was using sealed batteries, and thus a more cautious style of EQ.

[Sunking]

I guess I missed where the OP was using sealed batteries, and thus a more cautious style of EQ.

Mike if the OP has a sealed battery type, then no EQ can be done. More later when I have time.

[Mike90250]

More later when I have time.

OK, I'm about to go off grid for the weekend, solar is going in, and batteries next week. I need to find out where i'm off, before i toast a bank of L-16's

[jestronix]

MMm seems even at 5% to EQ these batteries, Im going to need a minimum of 50A going in.

Could i use another battery set to do an EQ on the larger set? Two 8v golf cart batteries at 16v into the larger bank, only thing would be to some how control the amps to 50. In theory this would work. mmmm

[Sunking]

Ok Mike I have a little more time to discuss this. My comments are only valid for flooded lead acid batteries and do not apply to either Gel or AGM. Gel batteries should never be used in a RE applications, and AGM or special purpose batteries to be used in sub artic temperatures or mounting orientation is other than up.

You mentioned you have L-16 batteries which I assume are Trojan, so the exact voltages I mention only pertain to all Trojan flooded lead acid (FLA) battery products Generically it can apply to most FLA technologies but the actual voltages vary by manufacture due to plate alloy make up.

To start there are about 3 charging algorithms for lead acid batteries:
Float/Equalize
2, 3 and 4 stage.

Each of these algorithms share common techniques.

Float/EQ
This algorithm is the domain of stand by emergency battery service. For example telephone battery plants and uninterruptible power supplies. Float/EQ is a constant precession voltage algorithm. Depending on the plate material in Float operation the voltage is held between 2.18 to 2.35 volts. For a Trojan 2.25 volts for float, and 2.35 for EQ. A float charger will be rated something like 24 volts @ 100 amps. The way they operate is the rectifier supplies the power for the equipment up to 100 amps. The batteries are tied to the charge bus to remain fully charged and take over in the event of a commercial power failure.

So lets say the battery goes on discharge and is discharged to 50% DOD before power is restored. The 24 volt plant voltage would have fallen from 27 volts down to about 23.8 volts. When the rectifiers come on they will deliver full current (called current limit) to recharge the battery., and the voltage will fold back to match the battery impedance to say roughly 24 volts or maybe even a little bit lower. As the batteries charge up the voltage starts to rise, eventually to the set point of 27 volts. At that point the current will start to taper off until the battery is fully recharged some time later. Once full charge is achieved the rectifier current will be whatever the equipment demands plus battery charge current to over come the very small self discharge rate..

EQ mode works the exact same way, except the voltage goes up depending on battery plate alloy but in the range of + .1 volt per cell. So in that same 24 volt battery plant from 27 to 28.2 volts. When the switch is thrown the charge current will spike up briefly, very briefly until the surface charge is equalized with the battery impedance. Then the current will taper off and a constant voltage is applied until the EQ sequence is turned off. EQ times vary from 6 to 24 hours. Best method to determine EQ time is using a hydrometer to determine when the SPG has equalized.

2, 3, and 4 stage:

These are variants of each other with some float and EQ mixed in for stage 3 and 4. Since this is for RE applications I will only detail out the 2-stage process because that is the best algorithm where there are transient loads on the battery during charging. A 2-stage charger has the advantage because its circuits cannot be

[Mike90250]

I agree with nearly all, but the time needed for the EQ cycle, I think that when a bank gets a short 1-3 hr EQ monthly, it's easier on the batteries, than trying to to a years worth of EQ in one day. We'll just have to agree to diaagree on EQ.

[jestronix]

So does this mean I can use another battery bank to equalize another ? Also Im wondering what effect a daily discharge of 5% cycle will have on these batteries ? Would I need to equalize as often?

As for stratification Im happy to use the hydrometer to slosh the acid around a bit every two months or so.

[Sunking]

So does this mean I can use another battery bank to equalize another ?

Not in the way you are speaking of the voltage has to be constant voltage. If you were to place a set of discharged batteries in parallel with a fully charged set of batteries, they will equalize in voltage and you would have two sets of discharged batteries. Equalize charges are an over charge condition.

Also Im wondering what effect a daily discharge of 5% cycle will have on these batteries ? Would I need to equalize as often?
.

Assuming you recharged after 5% discharge it will prolong the life of the battery. No you would not have to EQ as often. Your hydrometer will tell you when it is time to EQ.