How critical is the float charge

[Raul]

My recently purchased rolls s480 says to float at 2.19vpc ,52.56v @ (48). I got the controllers set at 2.3vpc , 55.2v @(48). If the system is cycled and after sun down will go to rest or supply loads anyways the voltage will drop below 2.19vpc . My question is , if during the day I get full bank at 10am and then it floats till evening at a higher then recommended voltage will it do any harm?
Many thanks

[Mike90250]

There are 2 types of "float"
1) Daily, cycle float. (SOLAR service) batteries are used and recharged daily. This float voltage is slightly higher than -
2) Standby service float: for batteries that are only used once a month or so, higher float voltage over days of time, will slowly boil away electrolyte. (UPS, Exit Lights....)

So which float voltage is the 2.19vpc listed as ?

[Sunking]

Float does not hurt the batteries, it makes them last longer. As Mike eluded too Float for Solar is higher, thus all the other voltages. The problem with solar is there is not enough time in the day to go through all 3 stages of charging in the normal manor to get the batteries charged up to 1.255 – 1.275 specific gravity.

As for what voltages to use, throw the manual away. You need to set the voltage to what your hydrometer tells you what fully charged is. Voltage cannot tell you that. For example Rolls has a technical document out for Solar Users to set voltage to 2.4 volts to prevent Deficit Charging.

Secondly you might want to get an up to date manual. You should be charging at 57.6 to 60 volts depending on temps. 2.3 vpc is way too low. You may even need to go higher if your specific gravity does not register 1.255 – 1.275 when charging stops at the end of day. Anything less is an Undercharged Battery, and nothing kills a battery faster than being under charged.

So get your temperature correcting hydrometer out, and put the manual away. Float if your batteries ever get fully charged is a good thing but a rare event for solar systems. It would mean your equipment would run off solar power during the later part of the day day instead of the batteries , and saves the batteries for night like it is suppose to do.

[Raul]

Thank you both for your responses. I set bulk/absorb to 2.4vpc and temp sensor in middle of the bank,and sg is 1.265 measured with a temp corrected hydrometer . This is a new bank less than 5 cycles and rolls lists only one float voltage for this particular battery s480 @ 2.19vpc. From what Mikes says I kind of understand now if you float for weeks on end aka storage/standby float at 2.19vpc as per manufacturer but if cycled daily then a 2.3vpc will be fine until winter : then probably will switch to max smoke to make the most of the narrow window charging time.

[Sunking]

Raul perhaps this might help bring things into focus. There are two basic ways to use Lead Acid batteries. Float Service, and Cycle Service.

Float Service is what utilities like phone companies use. Another name is Emergency Back Up. The batteries are put onto a Float Charger. The equipment using power gets it power from the Charger called a Rectifier. The voltage of the rectifier is set to the Float Voltage of the battery. So under normal operation the batteries are fully charged up and not taking any charge current, not are they discharging. They just set there 100% charged up and Float. The rectifiers are doing all the work and supplying power under normal conditions. The batteries can remain at Float Voltages for years waiting to be used.

When there is a power interruption, naturally the Rectifiers loose power and can no longer supply power. However the batteries are already On-Line and takes over. The equipment never sees a power interruption and keeps on going like nothing ever happened. When power is restored either by generator or normal utility, the rectifiers come back on. The rectifiers are sized (current) to run the equipment and recharge the batteries. Now at Float voltage normally 2.2 vpc, it can take up to 16 to 24 hours to fully recharge the batteries. Not because the current is too low, but because the voltage is low. In fact most utilities charge rate current is close to 1 to 2 hours. So one would think is should be able to charge in 2 hours, but that is not how it works. The Bulk is done in 1 to 2 hours (80% SOC), but at 2.2 vpc it takes another 16 to 24 hours to saturate to 100% SOC. So the downside of float charging is it takes a long time. The upside it is the safest, kindest and most gentle way to charge a lead acid battery, and can keep a battery fully charged up waiting to be used.

Cycle Service is just as the name implies. You are going to charge up the batteries, and use them everyday. So a Float Charger would not be fast enough for a daily Cycle Service as it takes up to 24 hours to fully Float Charge a battery. Not acceptable right? To charge a lead acid battery fast requires a Constant Current (Bulk) and a Constant Voltage (Absorb) phase. Now in reality there is nothing different between a Float Charger and 2-Stage CC-CV charger. Only thing that is different is the Voltage set point. They both operate the exact same way. When you buy a charger it has a Current rating of X amount of amps. For the fastest possible charge time, you want the current rating to be C/8 for Flooded batteries, and C/4 and higher for AGM. You have Rolls Sseries and they can be charged @ C/6. Check the manual.

So say you discharge the batteries 50%, and apply a C/6 charge current to a 600 AH battery or 100 amps of charge current. Your battery is going to take 100 amps for about 2 hours and the battery SOC will be roughly 80% SOC. The voltage of the battery will be roughly the set point voltage of say 2.4 vpc. When that happens the CC phase ends, and CV starts when the charge current starts to taper off. As the battery saturates the current tapers off toward 0 amps. When the charge current tapers off to C/33 or roughly 18 amps some 6 to 8 hours later, the battery is fully charged, and then the charger either turns off, or reduces the Voltage to FLOAT voltage of 2.2 vpc, Next morning your Fork Lift or whatever gizmo you have is ready to go with a fully charged battery.

OK did you catch the part of how long it takes to charge a battery with Two Stages? Is that or could that be a problem with Solar? Damn right there is a problem, there are not enough Sun Hours. What I described above is using a commercial AC powered charger where you have unlimited power 24 hours per day. You should be able to get through the Bulk Phase by late afternoon, but not enough daylight for Absorb to saturate the battery to 100% SOC.

So what can you do?

To start off you raise the Voltage set point from say 2.4 up to 2.6 vpc. This forces the Controller to stay in Bulk or Constant Power mode longer, so that when you get to the Absorb phase, the battery is close to 90% SOC instead of 80%. That means shorter Absorb time. In the end you may not get to 100% SOC by sunset even with the voltage set as high as it can go.

So here is the takeaway. The battery Owners Manual give you a range of voltage for Bulk/Absorb voltage set point. Something on the order of 2.4 to 2.6 vpc. It is not an exact number, it is a range. So what do you do? Simple you let your hydrometer tell you what the correct voltage is. If at the end of the day your specific gravity is too low, raise the voltage. If it is to high, lower the voltage. Once zeroed in, don't relax and forget about it. As the seasons change so will your voltage set point.

Lastly you want to error on the slightly Over Charge side of the Knife Edge, not the Under Charged side. Most will find if they use a Hydrometer will find out there is no voltage high enough to get their batteries fully charged. That means their system is undersized. Even with a properly sized system, you will fall short in Winter Months. That is why you bought a generator.

Hope that helps.

SK

[Raul]

Derek , you have my deepest respect for taking your time to share from your experience to help others . Thank you, and yes you did save me quite few thousands of dosh as I studied your posts and I could see from afar that you been in the industry for some time. I have slightly oversized the PV to one sun hr for the winter and that comes as a bonus in the summer as I get spare power without cycling during the day. Also I sized to 6 times daily autonomy on the battery but also have a good auto start main diesel generator and a smaller spare one in case of a fault to keep the batteries in good shape .

[Sunking]

Derek , you have my deepest respect for taking your time to share from your experience to help others . Thank you, and yes you did save me quite few thousands of dosh as I studied your posts and I could see from afar that you been in the industry for some time. I have slightly oversized the PV to one sun hr for the winter and that comes as a bonus in the summer as I get spare power without cycling during the day. Also I sized to 6 times daily autonomy on the battery but also have a good auto start main diesel generator and a smaller spare one in case of a fault to keep the batteries in good shape .

Thank you, I try to help. I just have no patience for ignorance. You did you rhomework and understand the challenges. A skill most do not have, simple problem solving. You are a wise man. You are set to go. Now go buy you a $10 hydrometer dammit, it wil be the most valuable tool you can own and save you many more thousands of dollars. .

Let the Fools and their Money part ways. Lord knows there are lots of people waiting for them to empty their pockets. It is like going to Las Vegas where you get over charged for everything, and robbed blind at the tables. When you get home, you cannot wait to start planning your next trip. Both perpetrator and victim are tickled pink. Go figure that one out. They call it Lost Wages for a Reason.

[Raul]

I have 2 hydrometers a hydrovolt and a draper but don't like either so I ordered a durrite as they supposed to be very accurate and reliable .

[lj4041]

I'm new to this Solar energy thing but I'm 100% sure I will be buying solar panels for my home in the next 30 days. I have 3 different Solar Companies offering me to buy their Solar system for my home -- I will be living in my home for at least 30 years - I want a good system for a wholesale price --- Which one should I choose? - All 3 companies offer a 25 year warranty on their panels.
My annual electric usage for my home is about 12,500 kWh -- I want to replace 98%-100% of my electric annual usage


#1. System price $23,959 by La Solar Group (1 solar edge 7.6W Inverter with Optimizers , 24 Suniva 335 Watt panels
#2 System price $19,500 by a wholesale contractor - 30 - 260 -watt Ecosolargy modules (poly) 30 - 215 watt Enphase Micro Inverters - 7.8 kw
#3 System price $23,263 by a wholesale contractor - 25 Hanwah Q CELLS USA- 325 Watt modules -8.1 DC and inverter 8.2

Which system and panels would you advise??

[Farlander]

@ SK,

So glad I found this post and read about how you set your charge parameters for off grid PV batteries. I recently installed an off grid 660 Ah AGM batt bank (2x4 12V EV8D deep cycle) in the Caribbean where the temp ranges from 75F-85F (25C to 30C) and I had set the outback charge controllers at 57.6 absorb (2 hrs) and float at 53.2 (both low ends of the range, battery spec sheet attached, absorb range 57.6 - 58.8, float range 52.8 to 55.2). I realize after reading your post that I'm likely not getting enough charge into the batts as they are cycled daily down to around 47.6V. We only have a 6.6kw PV array. I recently changed the settings to 58V absorb, and increased absorb time to 6 hrs, per your advice above. I also set the float charge up to 54.6V. There will be virtually no time when there aren't people on site consuming power.

I have a magnum PAE 4448 which is a great unit for inverting, but the generator charging feature is pretty bad, or I'm incapable of figuring out how to set it up. Unfortunately, the magnum battery SOC monitor was damaged, so for now the generator charger is using voltage only for reference. Above 51.2, the inverter charger defaults to float mode, which doesn't help very much with getting bulk amps in. I was considering raising the "float" on the generator charger to 55.2.

Two questions, will 58V be too high of a voltage set point to reach before going into a 6 hour absorb period, and is raising the generator float voltage the best way I can actually get more charge from the generator? I have a 12kW diesel caterpillar 240V generator tied to the inverter.

Thanks in advance!

[Mike90250]

Argh

cycled daily down to around 47.6V

That's an expensive mistake, you are destroying your batteries if that is an accurate voltage.

Higher ambient temps call for lowering the charge voltage a smidgen. Usually the BTS (Battery Temperature Sensor) for your brand of controller does that automatically,
But your issue seems to be you may need longer absorb times, less overnight load, larger battery or more solar. 90% of your charge comes from the Absorb portion of the cycle, and very little actually comes from FLOAT. Float is sort of the "finishing touch" not the major portion of charge,

For now, till some Magnum expert comes along to show you how to adjust the voltages, I'd run the genset for an hour late night, just before you go to bed, to push some charge into the batteries so they are not so low in the morning, Maybe an hour at midnight, and another hour shortly after dawn, before the PV kicks in,

Often, battery spec sheets are determined by Grid Backup use, where batteries sit on float for weeks at a time. Look for settings for CYCLE USE to use instead,
Cycle use Initial Charging Current: 99A,2.40-2.45VPC
Float use 2.20-2.30VPC
but 47.6 v is killing the batteries,

Your ABSORB voltage should be around 2.42V x 24 cells = 58.08V for several 3 or 5 hours, When CHARGING amps drop off below 30A (that's about 10% of the 330ah capacity)
then you can transition to float. Be sure you measure CHARGING amps, not total amps going to battery & loads,

[Farlander]

The 47.6V Low battery cutoff voltage assumes an inverter load of around 10 amps DC. Not sure how to calculate the voltage drop effect to adjust to battery Voc. Will raise the LBCO back up to 48.? . Looks like I'm good at the 58V bulk / absorb. I do not have the ability to "end absorb" based on return amps, because of 2 different controllers, not communicating, and no temp sensor. If I could go back I would do the entire system with outback and a mate, but this was our first off grid install on it was on a super tight budget. We adjusted the absorb time to 6 hours. I hope this won't damage the battery bank, but I can bet that there will often be several days at at time where the batts stay between 50-75% SOC. You are correct about the 33 amps return current (it is temp corrected in the data sheet) unfortunately I am in the U.S. and this location is Honduras and I do not have the ability to monitor it every day. No SOC meter, no data monitoring, mismatched equipment, and no temp sensors on the CCs.