PWM Float stage problem

teopap · March 2020

Ok so this might sound dumb, but I've been looking weeks for an answer at various posts and no luck so far. My question is about a PWM controller reaching the float stage extremely fast and thus I can never charge my current off grid battery (140AH, flooded lead) more than 50-60%. I know that float voltage should not be more than 13.6 - 13.8V max but :

I have 3 x 150w 12V solar panels and a cheap 40A PWM charge controller (the same as this one) with float setting at 13.6V. So I have the battery discharged at around 40% (~12.0V). When connecting the solar panels at afternoon full sun it will reach 13.6v from 12V within 2-3 minutes! When it reaches 13.6V (as shown in PWM display) it has already entered float mode thinking that battery is fully charged ? At 13.6v it stops increasing and stays there with voltage numbers increasing and decreasing rapidly from 13.2v to 14.0V. Like every ~0.5 seconds is at 13.2v, then going to 13.4v then 13.6v, then 13.3v, then 14.0v then 13.6 and so on. I don't know if it has a problem or just its way showing that it's in float mode. (When the weather is cloudy and battery is deeply discharged it will rarely reach this 13.6 stage)

When it's been in that stage for 5 hours (starting at 1PM with full sun until 6PM) when connected the battery to inverter at 6PM it's displayed in inverter 12.3V (at PWM 13.2V - a fully charged battery value), the same value I get when using an on grid smart charger to fully charge it. BUT when applying a 70W load (like a 43'' LED TV) for one hour, the inverter will show 11.2V after one hour meaning that it lost 1 whole volt in just one hour (opposed to a 0.2v drop/loss when charged with a smart ongrid charger). Meaning that the battery when PWM charged is used it is not charging (the 13.2v was just a surface charge value) and probably staying at float charge all the time! Or that with 450W panels and full sun, pwm charger put only 100 watts of energy in the battery over 5 hours!

The result is that I ended up stressing my whole system and especially the inverter by connecting a very small car battery just to run 2 x 10amps (120watt / 220V - Europe) ongrid battery chargers to charge separately each of the batteries of my solar system through inverter!

Should I increase the float voltage setting in my PWM charger at 14.5V or it's just a problematic controller ?

BB. · March 2020

A couple of general statements... Charging is (mostly) a function of battery chemistry (and some design).

In general, a GEL/AGM/Sealed battery should charge at ~14.0-14.4 volts at 25C (increase charging voltage when batteries are cold, decrease charging voltage when hot). Some AGMs have a charging limit of C/20 or 5% of capacity charging current. AGMs generally support upwards of 10-13% rate of charge.

Flooded cell lead acid batteries typically are charged around 14.5 to 14.8 volts or so (Absorb charging). C/10 or 10% is usually the minimum rage of charge, and higher rates of charge are doable, but could limit battery life a bit (age faster).

Float is just that, it is the voltage that keeps the battery charged (keeps up with self discharge).

Of course, always try to get the charging information direct from the Manufacturer.

In solar power systems, most of the batteries die from under charging... Very few seem to be killed by over charging (assuming you keep within the voltage limits). Given that with solar you can only "charge" around 4-8 hours per day (when the sun is up), a daily cycling battery bank is not likely to ever "overcharge"--And given the choice, you are much better off setting your "float voltage" on the solar charge controller to the 14.X volts above (FLA, AGM/GEL/Sealed, etc.), especially if you cannot get the charger's Absorb Voltage and Time on ch (arge to work correctly (Get battery to Absorb voltage, and hold ~2-6 hours--More hours the deeper the discharge).

With sealed type batteries, you cannot refill with distilled water--So, you want to make sure that you do not overvoltage, over charge, overheat the batteries while on charge. If they vent, you are losing water and shortening the battery's life.

And there can also be questions about the charge controller. You want short/heavy leads from the controller to the battery bank. With a 12 volt battery bank, you probably want a maximum of 0.05 to 0.10 volts drop in the wiring from the controller to the battery bank. You also want the controller in the same area as the battery bank (same temperature as bank, for thermal compensation of battery charging voltage-Avoiding the electrolyte fumes--if any).

Since you have a cell battery bank, hopefully use distilled water (or similar), and keep the cell plates covered (don't run plates dry). And get a hydrometer to measure specific gravity (always rinse with distilled water before putting hydrometer away):

https://www.solar-electric.com/search/?q=hydrometer

Having an accurate volt meter to confirm solar array and battery bus voltages is always nice... And these days, getting a DC Current Clamp DMM (digital multimeter) is great for debugging/understanding your system... They are pretty reasonably priced these days (note, a DC current clamp meter is really an AC/DC clamp--does both. There are AC only clamp meter which are more common--But we want the DC Amperage measuring capabilities here:

https://www.amazon.com/gp/product/B019CY4FB4 (mid range/price AC/DC Current Clamp meter)
https://www.amazon.com/UNI-T-UT210E-Capacitance-Multimeter-Resolution/dp/B075ZHDQFP (lower cost, "good enough" for our needs)

Watch the prices... Some vendors are increasing prices (decreasing supplies from China/epidemic supply chain issues?).

Looking at your system (assuming for the moment the solar charge controller is working correctly otherwise). 3x 150 Watt panels, should expect a maximum charge current (sunny day, noontime, panels facing directly at sun, no shading):

3 * 150 Watt array * 0.77 panel+charge controller derating * 1/14.5 volts charging = ~24 amps maximum

A 140 AH @ 12 volt battery bank, 28 Amps charging would be around a 20% rate of charge (0.20*140=28amps). That is a pretty healthy rate of charge, and I would expect anything between ~14 to 28 amps to be within the range of "full charging current" for your bank+solar array.

I would double check the charging voltage at Vbatt terminals of the charge controller and at the Vbatt terminals. And confirm that the Meter display is close to your DMM measurements. And that Vbatt charger is not much more than ~0.10 volts higher than Vbattery of the battery bank.

At this point, 13.6 volts charging is not telling you much about your system and battery state (sulfated?). The fact that you get more capacity from your alternate charger is probably indicating your battery bank is not sulfated (yet).

A good/quick test is to connect your Vpanel (solar array) directly to the battery bank (watch polarity, reverse polarity on the solar panels can ruin them instantly). See what charging current you get without the charge controller, and you can measure the current from each solar panel (clamp DMM, or only connect 1 panel at a time to the battery bank), and make sure the rest of the wiring is OK.

The fact that you are not even hitting 13.6 volts (lots of cloudy weather?)--Could be simply weather, or a bad panel(wiring), or you need to get a different/better charge controller.

Need to do some more debugging to figure out...

-Bill

teopap · March 2020

Thank you for your answer, very informative. I'll set the float to 14.4 and I'll see how it goes. All of my batteries have caps therefore I can add water when needed. Until now no water was needed, and was also confirmed from the store I bought them when I brought them for a water check and a possible desulfation.

What I realized now is that charge controllers have such a low float because they are designed for cottages, or remote stations that don't need cycling every day therefore maintance must be in lower voltage. But cycling every day needs a much higher float voltage. The worst case senario is that they'll start to vent. Maybe I should test it at 14.6v with a cheap old car battery first and see how it goes. The smart charger I use reaches 14.4v on led display when near at 80% of charge and 14.6 after 85-90%. So I guess the same way works the pwm controller, but never reaches 14v as it stays at 13.6v in float mode, thus battery will never charge more than 60% (at least in a day or 2)

BB. · March 2020

Car batteries do not like to be deep cycled... So getting them up and fully charged is job #1.
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Once you get them charged, you can look at playing with setpoints and monitor water usage... You don't want the batteries hot and "boiling" all the time, that will kill them too.

As long as you need to add a bit of water every 1-6 months, that is good place to be. If you are filling an "empty" cell every month, then crank back on charging voltage/absorb time. (and never let the plates get exposed).

Yes, for weekend cottage use, reaching 14.4 volts charging every day is less than ideal... Higher end controllers will "skp" absorb charging until the the battery is actually discharged (below 12.6 or 12.5 volts or so, at some point)... Then they will kick into absorb cycle.

--Bill

fratermus · March 2020

As much as I love those cheap little bastages, they do not have staged charging in the normal sense.* They have one setpoint. I agree that for lead banks cycled daily the setpoint should be set to Vabs and tweaked manually seasonally to account for temp differences.

* I'd argue that they aren't PWM either, no matter what the label says. They are on/off chargers, sometimes called "shunts" (descriptive but technically inaccurate).

teopap · March 2020

Ok so I set the float charge voltage at 14.6v. It was a fully sunny day. Within 3 hours it was charged from 30% to 85! I was using a desktop computer for 4 hours now (about 160watt with its lcd display), a charged laptop plugged in with a 32'' led tv and the battery is near to 50%. It lasted incredibly long compared to even when used an on grid smart charger. Inverter started displaying voltage 12.8v, compared to 12.3v I got when I charged 100% with an on grid smart charger. Maybe the battery is overcharged and lasted that long ? I would say that 12.8v (13.8v on PWM display) is not a surface charge value, as I put heavy load to test it and then came back at 12.7v.

I also checked the fluids and no acid was lost (so as a sign of overcharge). The same water levels just like the first day I purchased the battery. I'll keep checking next days.

Apparently the float charge voltage on the PWM is not the actual voltage that floats the battery, but the voltage that the battery will get into float mode. Maybe setting it higher than 14.6v to avoid altogether the float stage? Again it reached 14.6v relatively quick when the panels were in full sun.

BB. · March 2020

For most battery chemistries, you cannot get higher than normal energy/amphour storage. You are hopefully seeing your real battery capacity now.

Generally flooded cell lead acid batteries are pretty forgiving with minor to moderate over charging.

Undercharging and over discharging can kill FLA batteries in months or sooner.

If you are using the system daily, keep the higher charging voltage.

If it is going to sit for weeks or longer unused (out of season storage), fully charge them drop to float voltage.

Bill

teopap · March 2020

Thank you. So do you think maybe setting the float stage to enter at 14.8v ? Is that safe or should I leave it as is at 14.6v ?

I am using the system daily. I am not connected to grid power.

BB. · March 2020

Watch water usage and bubbling action inside the battery (some fizzing is about right, actual bubbles/rolling boil, not good). Also check battery temperature--They can overheat with even 2.5% to 5% rate of charge into a full battery--Basically equalization (which is usually 15.0 or higher charging voltage.

Adjusting by 0.1 volts or so at a time (for 12 volt system) is normal. Deeper/frequency cycling, a bit higher voltage. Shallow cycling and less frequent, drop the voltage a bit.

If the system will be in storage (more than a week or so of non-usage, dial back the charging voltage towards float.

You are looking for reaching 90%+ state of charge at least once or twice a week. Trying for 100% charge every day--That is a bit hard on FLA batteries. Excessive gassing raises battery temperature (not great), sheds plate material, and the oxygen formed (Oxygen and Hydrogen generated from gassing), corrodes the positive plate/plate grid over time.

You are looking for the "happy medium" for your battery bank/system. For less expensive batteries, 3-5 years of life is typical (if in a cold climate, you can get a year or so more life). And "murdering your first battery bank or two" is very common as you learn how everything works and dial in your charging+loads.

And car batteries are just not designed for deep cycling. They are designed for discharging to ~85% State of Charge--Not for discharging to 75% to 50% or low SoC... If you get a few years of usage, you are probably doing good.

-Bill

teopap · March 2020

Ok. I'm using 3 deep cycle batteries and one car battery in my setup. The car battery is fizzing very obviously, when you are close to it in a quite environment you can hear it bubbling, whether it is on pwm charger or on the on grid charger. The other 3 deep cycle batteries, you have to stick your ear on the battery and the fizzing seems to be way less than the car battery. It's barely heard. This was spotted at 14.5v of charging.

In cloudy weather, I put the batteries in a close neighbor's house to charge on grid. So definitely they get a 100% charge every week.

PWM Float stage problem

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