diy solar

diy solar

MT50 Phantom Load

good sydus

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Apr 8, 2021
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Hi all,

Now that the days are getting short and cloudy in the northeast, every amp of solar I can get counts. I’m finding that the voltage on my BigBattery Hawk (24v, 170ah) is dropping a lot over night with no significant load on the battery end (one small exhaust fan and a small diesel heater).

I’ve noticed that the light bulb symbol on my MT50 turns on periodically, even though I keep turning it off in the settings. It’s on the manual setting, so there shouldn’t be any timers or anything turning it on. But what’s even stranger is that it shows a small load even though there is nothing plugged in to the load terminals on the MPPT. Most of the time it just shows voltage but no amps (no actual load?), but every once and a while it will even show some amperage (usually just 0.1). I keep trying to get a picture to share, but the phantom load knows how to disappear whenever I pull out my phone…

Is this normal? Is there any way I can prevent this, assuming it’s actually somehow dumping power to nothingness? This seems to violate my very limited understanding of how electricity works, but I’m probably missing something.

I’m really hoping this, and maybe programming the MT50 for a LifePo4 battery will help me get and retain more power. Otherwise, I’m going to have to start disconnecting the DC breakers to see if one of my devices is drawing more power than I think which sounds very tedious.
 
24V LFP left sitting (no loads at all) may eventually settle to as low as 26.4V.

  1. What is your voltage after overnight loads?
  2. What is your absorption voltage?
  3. What is your float voltage?
  4. Are you getting your battery fully charged every day (NOT based on MPPT display)?
 
It varies from night to night, but this morning I woke up to 25.6v. It was pretty low last night (26.2v) since it’s been so cloudy lately, but this seems like a big drop to me.

I’m not sure of the absorption voltage, but the MT50 says the float voltage is currently at 27.6v with the sealed battery preset. According to the battery manufacturer, neither voltage is applicable to lithium batteries. I don’t understand enough about lithium batteries to know why this would be the case, but I do think my multiplus regulates voltage when plugged into shore power (the lights for absorption and float come on). Obviously my multiplus is not involved here though, and the MPPT settings don’t say anything about an absorption voltage.

And no, most days lately the battery is not charging fully. We’re down to just 9 hours of daylight and rarely get full sun (or really anything but overcast days) this time of year. But even based off the limited power I’m getting each day, I think I should still have enough of a surplus that the battery should be charging/staying charged, not draining.
 
25.6V is a bit concerning as that's about 3.2V/cell.

IMHO, here's your problem: "been so cloudy lately" = you aren't routinely fully charging the battery, and you may see reduced voltage in the morning, i.e., you aren't replenishing your daily usage from the battery, so the battery level is continually dropping, and you're seeing weird behavior as a result.

How much solar?

That datasheet is just wrong. Their float voltage will damage their batteries. I doubt anyone with any real knowledge of LFP developed that datasheet, especially considering that solar power systems are a big part of their market. Bigbattery specifications are notoriously inaccurate for this reason:

LFP batteries do not require a float voltage to keep them charged.

HOWEVER, in a solar power system, you NEED a suitable float voltage to act as a "floor" to ensure your solar charger pulls loads from the array and not the battery.

You charge LFP to 3.55-3.65V for a reasonably fast charge rate to 100% with a very short absorption phase.
You charge LFP to 3.45V for a long absorption charge to ~98%.
You float LFP at 3.40V to ensure your solar will support your load.

The sealed program is probably a good compromise as long as you 1) don't have a temperature sensor and 2) it doesn't provide lead-acid temperature compensation.

If the charger has a USER option, then that's your best bet.

To wrap it up, if you're not seeing the battery reach > 28.0V and then floating at 27.6V, you're not getting your battery fully charged due to your poor solar.
 
Thank you for the detailed response! I figured the amount of solar I was bringing in would be enough to power a few small things, but just barely. Maybe I’m not pulling in enough though—I’ll have to try and do a better energy audit.

I’m actually trying to figure out how best to program the USER option on the controller. I reached out to the manufacturer for some help, and they just sent me the same data sheet again. Sigh.

To be clear, I do have a temperature sensor, and there is a -3mv/C/2V temperature compensation coefficient (not sure if this is the same as lead-acid temperature compensation).

Could you tell me which parameter I would have to change to set that float “floor”? The logic makes sense to me (sort of), but doesn’t float happen once there’s enough voltage in the batteries that you have to slow down the amount added by the charger? So wouldn’t the float charge need to be closer to 27v, which my battery hasn’t made it to in some while?

Here are the current parameters that I can control:

Temp comp coeff: -3mv/C/2v

Rated voltage: auto

Over voltage disc: 32v

Charge limit: 30v

Over voltage rec: 30v

Equal charge: 29.2v

Boost charge: 28.8v

Float charge: 27.6v

Boost rec: 26.4v

Low voltage rec: 25.2v

Under voltage rect: 24.4v

Under voltage warn: 24v

Low voltage disc: 22.2v

Discharge limit: 21.2v

Equalize time: 120min

Boost time:120min
 
Thank you for the detailed response! I figured the amount of solar I was bringing in would be enough to power a few small things, but just barely. Maybe I’m not pulling in enough though—I’ll have to try and do a better energy audit.

I’m actually trying to figure out how best to program the USER option on the controller. I reached out to the manufacturer for some help, and they just sent me the same data sheet again. Sigh.

To be clear, I do have a temperature sensor, and there is a -3mv/C/2V temperature compensation coefficient (not sure if this is the same as lead-acid temperature compensation).

LFP do not use temp compensation, and it can damage them.

Could you tell me which parameter I would have to change to set that float “floor”? The logic makes sense to me (sort of), but doesn’t float happen once there’s enough voltage in the batteries that you have to slow down the amount added by the charger? So wouldn’t the float charge need to be closer to 27v, which my battery hasn’t made it to in some while?

Here are the current parameters that I can control:

Temp comp coeff: -3mv/C/2v ZERO
Rated voltage: auto
Over voltage disc: 32v
Charge limit: 30v
Over voltage rec: 30v
Equal charge: 29.2v
Boost charge: 28.8v 28.4V
Float charge: 27.6v
Boost rec: 26.4v
Low voltage rec: 25.2v
Under voltage rect: 24.4v
Under voltage warn: 24v
Low voltage disc: 22.2v
Discharge limit: 21.2v
Equalize time: 120min ZERO
Boost time:120min 30 min

Please expand the above to see my comments.

Boost 28.8V pushes cells to 3.6V/cell. I prefer 3.55 (28.4V) unless generator charging.

Float at 27.6V (3.4V/cell) is good.

AGAIN, how much solar do you have?

And, what is your geographic location?
 
This is extremely helpful and good to know. I’ve made all the changes you suggested. I’m glad you told me about the temperature compensation—I didn’t know that about LFP.

I (supposedly) have 780 watts (4 x 195 watt panels, 2S2P). I rarely see much more than maybe 200 watts of real PV from the monitor though. I never tested the panels directly to verify since I know the rated output is always going to be a lot higher than real world conditions.

I’m outside of Albany, New York. Definitely not an area known for sunny weather, especially this time of year…
 
780W/24V = 32.5A. 32.5A is a low charge rate for 170Ah of LFP, so your absorption period should be brief, 30 minutes is good.

1639174252664.png

If your panels are facing due South and at a 47° tilt, you get 2.73 * 780W = 2168Wh/day average.

This does take weather into account. If your panels are not at this ideal orientation, it will be less. However, you may have several consecutive days of poor performance.

25.6V * 170Ah = 4352Wh

So you can recharge about half of your battery per day - assuming you use NO power.

If you use an average of more than 2168Wh/day, you will drain your battery bit by bit.
 
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