How soon after adding water can I get a reliable Specific Gravity reading?

[SurPopp]

Hi,

Yesterday I topped up my Rolls 6 CS17P batteries with water for the first time since I got my system up and running, and had to add close to half a liter water per cell.

The whole system consists of 8 batteries at 48V, an Outback GVFX3048E inverter, an Outback FM80 charge controller and 12 250W panels wired in four series. As I don't really have any loads to speak of yet the SG is usually between 1.260 and 1.270 when I check it (about weekly) with about an hour of bulk and 1-2 hours of absorb (at 57.6 V) and the rest of the day at float at 52.6 V, except today when it was significantly lower around noon time (around 1.220- 1.230). I should also add that I'm in Uruguay enjoying long summer days with lots of sun right now.

Could it be that my SG readings have been inflated because of low water levels (not low enough to show the plates, but low) and I have under-charged the batteries by cutting down the absorb time from 4.5 hour recommended by Rolls to 1-2 hours that I found was enough to keep the batteries at 1.265 with the current load?

How long would you say it will take before I can have a reliable reading of the Specific Gravity given that the batteries don't see a lot of action at the moment (running a laptop and a couple of cell phones plus a fridge that uses about 0.5kWh/day)?

[Sunking]

After adding water, you must complete a full charge cycle and most likely an EQ cycle. You measure the SG after a full charge cycle to determine if an EQ cycle is required which in most cases it is. You monitor SG every 30 minutes in an EQ cycle to determine when to stop. There is no time limit. It takes what it takes and most likely assuming you EQ is going to be a 20 to 30 hour process.

[SurPopp]

Thanks Sunking for the quick reply! I'll have a look tomorrow night again and see what the hydrometer says.

[Sunking]

Thanks Sunking for the quick reply! I'll have a look tomorrow night again and see what the hydrometer says.

You are welcome. If your source for charging is Solar, do not get your hopes up. When I say a full charge is meant as a complete Bulk-Absorb cycle from an AC stiff source to fully saturate the battery. Solar is not capable of doing that under most conditions. Especially this time of year. It is all but impossible to perform an EQ charge with solar. .

[SurPopp]

Well, we're off-grid so it is 3 kW solar panels and a small generator (2 kW). However, this time of the year is actually summer here in Uruguay so we have plenty of sun.

The batteries have been in absorb for about 5 hours (since 8 AM actually, we're not really using a lot of power at the moment) at 57.6 V, but the current fell to < 4A after an hour or so. Would it make sense to start the generator right away or is it better to use it to keep the batteries in absorb after sunset?

And should also start cranking up the voltage as you suggest in the killing your batteries post I suppose. The reason I haven't done so is simply that it seemed to work fine (SG around 1.265-1.270 in the evening) until I watered the batteries yesterday...

Again, thanks for your help!

[Edit: corrected absorb voltage from 56.7 to 57.6]

[Sunking]

And should also start cranking up the voltage as you suggest in the killing your batteries post I suppose. The reason I haven't done so is simply that it seemed to work fine (SG around 1.265-1.270 in the evening) until I watered the batteries yesterday...

Again, thanks for your help!

You are welcom. You set the voltage to satisfy the hydrometer. It is a never ending job of adjustment as the seasons change.You may find 6 months from now you have to set the voltage higher. Only your hydrometer knows.

[Mike90250]

It boils down to watts out vs watts in. Winter has shorter, less intense days, so you have to up the voltage to jam enough watts in to recharge, At least the low temps help preserve the batteries in the higher charge rates.

[SurPopp]

Thanks both Sunking and Mike, your help is much appreciated!

The SG is slowly rising in the batteries after I extended the absorb time. Two days with 10 hours absorb has brought up two of the batteries to 1.265-1.275 but the remaining six haven't made it higher than 1.245-1.260 yet.

Some battery history: I got the batteries (Rolls/Surette 6 CS17P, 48V/546 Ah) from England in May and shipped to Uruguay where it took until end of July before I got them through customs. Early August I put them on the generator (2 kW) and charged them the best I could for about 4-5 hours two days in a row. At that time I didn't have a hydrometer (both! broke during the container transport) so I don't know the SG after that charge. A few weeks later I put up a series of three panels (750 W) and charged the batteries two three times a week and used the system to charge laptops, cell phones and some power tools batteries. However, now I know that the current from those three panels was too low to properly charge the batteries.

Now that I have all panels (3 kW) up and running, the current falls quite rapidly during absorb and yesterday for example it was down to 4A after an hour (but the SG still far from what it should be). I assume that could be caused by sulfate build up due to the shallow cycling?

Should I continue with absorb (now I'm trying 58.8V for six hours) or would it be better to try an EQ to bring the six low batteries up? Or, dreadful thought, will I have to disconnect the batteries and figure out a way of loading them onto the truck and take to to town and find someone with a big enough charger to bring them up? The latter option will be a real hassle so if you think either of the first two alternatives (or something else of course!) would work I'd be happy to hear about it.

Thanks again for all your help!

[Sunking]

Should I continue with absorb (now I'm trying 58.8V for six hours) or would it be better to try an EQ to bring the six low batteries up? !

First do you understand What Bulk, Absorb, Float, and EQ are? When it comes right down to it only means Constant Current and Constant Voltage aka CC/CV.

Bulk is Constant Current or in the case of Solar Constant Full Power from the panels. Think of Bulk as MAXIMUM SMOKE meaning ever watt the panel is capable of making at the time of day and conditions? You need to understand this so you understand what I am about to tell you.

Absorb, Float, and EQ are the exact same thing, Constant Voltage. The only difference between the Absorb, Float, and EQ are the voltage set points. EQ would be the highest voltage, followed by Absorb, and last is Float. The charge current in CV modes can be from 0 amps, up to MAXIMUM SMOKE. Depends on the SOC of the battery and the voltage set point of the charger and open circuit voltage of the battery. I am going to stop there and tell you how to get the most out of your panels.

Turn all of the voltages in your controller up to as high as they will allow you to set them. You want MAXIMUM SMOKE anytime the panels have Sun Light shinning on them. This will force your controller to stay in a Constant Current mode as much as the panels can produce given the solar conditions at the moment in time. Leave the voltage set high until you reach your target Specific Gravity. Ignore any voltage you see on the battery, just keep pumping current in until SG is where it should be.

Once you get fully charged then relax the voltages. to normal ranges. You said you have Rolls battery right? Like any battery there is no correct voltage to use for Bulk and Absorb voltages. Every battery manual tells you that, but you just dot catch it. What the manual tells you is a range of voltages per cell. But keep in mind those recomendations are made assuming commercial AC power and chargers which have unlimited time and power. You do not have a stiff source with solar. You have a soft source of unknown power for just a few short hours each day. So when you look at your Rolls Operator Manual it tells you Bulk/Absorb is 2.35 to 2.5 volts per cell. On your 48 volt battery is a range of 56.4 to 60 volts. EQ is a range from 2.58 to 2.67 vpc or 61.92 to 64.08 volts.

So today crank the voltage up as high as it will go but no higher than 64.08 volts. Keep it there until your specific gravity is where you want it. Then start relaxing the voltage down gradually until you find the sweet spot where at sunset the specific gravity is at 100%. If it is too high, lower the voltage. Never rest because as the season change, so will your voltages. In fact in winter, there maybe no voltage high enough to ever get 100% charged. That is one reason why you need a generator. You may find you have to leave the the voltage set to 64 volts. If that is the case, be prepared to run the generator a lot. FWIW that tells you your panel wattage is to low, and/or you are using to much power.

Ignore Bulk, Absorb, and Float. Set them all to the same voltage on a solar system, and let your hydrometer tell you what the voltage should be, not an Owners Manual

[SurPopp]

Thanks Sunking, not sure how many similar replies I've read from you here (but plenty!) and before all this started I told myself that this, at least, is something I'll keep in mind. But apparently I haven't. Being new to all this I wasn't aware of the CC/CV modes and how Bulk and Absorb/Float/EQ could be categorized into them.

I'll get to town a get some more water and start boiling away.

Thanks for the guidance!

[Sunking]

Your welcome. Go read this thread I just posted. It might help you.