Epever boost cut-in difference between 2 models

MarkNewYork

I had been using the Epever Tracer A2210 20amp mppt controller (the one with the LCD display on the front) with my 2 100 watt panels (hooked in series) and 100AH sealed battery (vmaxtanks).  I used that controller for a few months - but 2210 is positive ground and I realized I needed a negative ground so I got the Epever negative ground 20amp mppt unit (no LCD and big heat sink). I just swapped it out - I had the serial PC monitor hooked up with both units.

The question I have is: With the first LCD unit - as long as the battery voltage stayed above the Boost Reconnect Voltage the controller never went into boost mode, just stayed at the float voltage. But with the other unit, boost mode always came back on in the morning even though the Boost reconnect voltage hasn't been reached. The actual overnight battery voltage dips the same with each unit - and I'm putting no load on the battery. I've tried both volt comp and soc mode on the 2nd unit. Is this daily boost mode putting more wear on the battery since it takes the voltage much higher each day than just leaving it in float? Which scenario is better for the battery?

So wondering why the 2nd unit goes into boost mode without hitting the boost cut-in voltage and the first one doesn't. Any ideas appreciated!

Estragon

I'm not familiar with that controller, so general comment only.

Some controllers have dip switches or other means of changing charging profiles, which may explain the different behaviors. Depending on boost(absorb?) voltage, it can be hard on a battery to force it to do a full charge cycle daily. With no load, the battery will likely be better off just floating most of the time, with a full cycle done only weekly or so, IMHO.

I have my controllers set to do a full bulk/absorb cycle only after 4 days of no loads. I may extend that to a longer period this winter.

MarkNewYork

Thanks, that's very helpful. I don't think there are dip switches on this unit - I use the PC interface to set voltages and modes - there are 2 "charging modes" to select from: SOC & Volt.Comp. The SOC reading on this brand always seems wacky - it's 100 when it's reached its "charging limit" voltage (set to 15) then it stays at 100 until it goes back into "boost" voltage - at which it drops to 30. Weird.

Estragon

As a matter of interest, have you used (discharged) the battery to any significant extent recently, and if so, did it appear to behave normally (voltage dropping steadily in proportion to the load applied)?

15v strikes me as a bit high for AGM, especially if it's warm (>~25°C).

MarkNewYork

I haven't really discharged this battery at all - it's meant as a standby source. It's been online for about 6 months. It's probably gone to about 12v once or twice but not below that - and not recently. The 15v "Charging Limit Voltage" is the default setting for a "sealed" battery in this particular charge controller so I left it there.

Estragon

If the battery is in a warm spot, I'd recommend a bit lower, say 14.6-14.7. Assuming the controller doesn't have a remote temp sensor, and the battery can get warm, too high voltage can make the battery vent gas and/or break down recombination catalyst.

Purely a guess, but if this has happened voltage might drop pretty rapidly absent float current, causing the SOC to reset lower and cause a charge cycle. The damaged battery would also recover voltage quickly in the charge cycle, resetting SOC to 100%? You could rule this out by load testing the battery using a known load like an automotive filament bulb. If the battery behaves normally, this isn't the issue.

MarkNewYork

Good ideas, thanks. The ambient temp this time of year is about 77f (25c). I have a temp sensor taped to the side of the battery and it reads about 80f (26.6). I'm going to change the voltage to 14.6. Thanks for the comment! Do you have a link handy on how to test the battery with a known load?. Thanks again.

mike95490

Some controllers sense nightime, and at sunrise, start a charge cycle.

Estragon

Basically just hook up any 12v load with a known wattage. A 12w bulb, for example, is ~1a at 12vdc. Your battery is 100ah, so it should run the bulb load for ~100 hours to a 0% SOC. You don't need to (and shouldn't) drain it that far though, just far enough to get an estimated capacity. If it takes 100hrs to drain, it should take ~10hrs to get to 90% with a voltage ~12.5v. If it gets there in an hour, the battery has lost most of its capacity.

If available, something that draws ~5% of capacity (so 5a or 60w in you case) would be ideal. A larger load can be used, but voltage can sag under load then recover to a better SOC indicative voltage.

Chart of battery voltage vs SOC (and other useful info:

https://www.solar-electric.com/learning-center/batteries-and-charging/deep-cycle-battery-faq.html#Battery Voltages

You can also get automotive load testers like
http://www.canadiantire.ca/en/pdp/motomaster-battery-load-tester-100a-0113003p.html

These testers may be misleading for deep cycle batteries though, as they're designed to measure instant power available for engine starting applications (cranking amps), rather than capacity (amp hours). If you happen to have one around or can borrow one, you could try it as a rough indicator of battery health.

MarkNewYork

Thanks again! Very helpful. I think the SOC on this controller is whack.  At the evening solar cutoff battery voltage read 13.5 and SOC 100%. During the night voltage went down to 13.1 with SOC staying at 100%. (no load at all) Then when solar kicked back in SOC snapped to 78%, but voltage stayed at 13.1 - "boost" charge mode started and took it gradually to 15v - with SOC gradually rising back to 100%. So I don't know why this controller would report a SOC of 78% when the voltage was 13.1. Thanks for your thoughts, I'm learning.

Estragon

I suspect you may be right about the SOC being out of whack. If voltage stayed above 12.8ish overnight with no charging sources at all, that suggests the battery may be ok.

SOC can be tricky. It's likely estimated based on current and/or pwm duty cycle of the controller. Once it starts charging, it takes account of how much it takes to raise the voltage over time. There are lots of variables, so we generally treat them as rough estimates only. Accurate state of charge requires either measuring specific gravity of electrolyte (most accurate, but not possible for AGMs), or resting voltage (still pretty accurate, but battery needs to be left with no loads or charging sources for several hours.

Where are the voltages coming from (ie measuring with a multimeter at battery terminals, charge controller output terminals, or?). It might also be worth checking these
connections. A wonky connection could account for some wierdness in the controller.

MarkNewYork

I'm getting the voltage readings from my PC interface hooked to the controller. Each morning at sunrise the panels have been at cut-out for at least 7 hours so I'd think the battery voltage reading at that time is accurate. But I will double check next time with a multimeter. I wonder since I've only had the CC connected for a week, you're saying the CC might be noting the charge cycles and may "getting to know" the battery over a longer period of time? So at some point its behavior might change? I think I might deliberately discharge it to maybe 70% so the CC can really get an idea of how the battery is doing/charging. I've found nothing in the manual about this - but it's all in broken english anyway. Thanks for your very helpful comments!

Estragon

It's probably good to run the battery down to about that level periodically anyway, if only to be confident it'll be fit for purpose if/when needed for an outage.

I guess there might be a sort of "learning" algorithm in the controller, but I'm doubtful. Did you enter any of the variables like bank size (ah) or charge efficiency (%)? To learn meaningfully, the controller would need to be able to differentiate between current going to charge vs current used by loads, which would require a DC shunt (typically on the battery negative connection). Some controllers do use shunts, but I'm guessing yours doesn't. Even controllers that do this (mine do, but mainly to control transition to float) don't have particularly accurate SOC.

As a lead acid battery gets full, the current required to hold it a given voltage drops off, regardless of current available. Depending on depth of discharge and charge rate, this starts at about 80% full. It continues to drop off on a curve until it gets to ~1-2% of capacity, at which point the battery would be considered full. In this stage of charging, the controller "pulses" pv current at pv voltage to the battery as often as needed to hold the battery at a given voltage (15v in absorb/boost in your case). As the battery gets fuller, the time between pulses needed to hold the battery voltage gets longer. My guess is the controller is estimating state of charge during this stage of charging based on the timing of these pulses.

When solar kicks in, these pulses would be longish as voltage is raised from overnight voltage to 15v, and the controller is likely reporting SOC on that basis.

MarkNewYork

Very interesting. It does have me enter the ah of the battery. Going to let it run for a few weeks longer and see what happens. Thanks for all your help!

mcgivor

A friend of mine has 2 Epever 40A PWM controllers in parallel charging a single battery bank each on its own array, the LCD state of charge and voltage readings differ widely from each other and both read voltage inaccurately compared to a quality digital multimeter. Checking with an accurate meter may be a good idea.

MarkNewYork

Followup - after a couple weeks the new controller did indeed settle down and started staying in float mode like the old one. Thank to all who chimed in with help and discussion!