MPPSolar LV2424 Newbie questions..

[Sventvkg]

So the question I have is I’m only running two 80 amp hr RV and Marine lead acid batteries right now in 24 V configuration. That’s all I have. I was under the impression that you could discharge his batteries about halfway and I thought that these could power my refrigerator all night, but apparently even watching my power requirements. I get about two hours before the inverter starts chirping low DC voltage. Apparently low DC is set somewhere around 21v.

Well, my question is what use is that? If I can barely discharge my batteries before the thing starts going off how am I supposed to get through the night? Right now I’m in a bus and I have to basically shut off the inverter. Keep the refrigerator iced or fire up my generator if I’m Boondocking. This is not acceptable. Obviously I missing something here so I’m trying to figure out what it is. I’m going to get some type of lithium battery as soon as I can. It’s fine when I’m running down the highway during the day and the solar is charging but I need to be able to get to the night. Any tips?

 

[Rednecktek]

A couple thoughts to throw out there and some rough napkin math;

80Ah @ 24v = 1920Wh * 50% (for lead acid) = 960Wh
The LV2424 uses about 25w just to exist and run, so 25w * 24hr = 600Wh of battery just for the inverter

That only leaves about 300Wh or so for all your loads, and if your fridge needs 100w (as an example number) then it's only got about 3 hours of run time per day before the batteries need a recharging. If you're getting low voltage hits after about 2 hours, then you've probably got somewhere in the neighborhood of 300w of load between lights, fridge, computer, etc.

So, either the batteries are old and not up to snuff and you're not getting the full 40Ah worth of capacity out of them, and/or your loads are just more than the capacity can handle. You're the perfect candidate for moving to a 24v LFP battery, preferably in the 200Ah range if you can fit it (which considering what 2x FLA batteries takes up should be pretty close physically) and afford it.

 

[Sventvkg]

A couple thoughts to throw out there and some rough napkin math;

80Ah @ 24v = 1920Wh * 50% (for lead acid) = 960Wh
The LV2424 uses about 25w just to exist and run, so 25w * 24hr = 600Wh of battery just for the inverter

That only leaves about 300Wh or so for all your loads, and if your fridge needs 100w (as an example number) then it's only got about 3 hours of run time per day before the batteries need a recharging. If you're getting low voltage hits after about 2 hours, then you've probably got somewhere in the neighborhood of 300w of load between lights, fridge, computer, etc.

So, either the batteries are old and not up to snuff and you're not getting the full 40Ah worth of capacity out of them, and/or your loads are just more than the capacity can handle. You're the perfect candidate for moving to a 24v LFP battery, preferably in the 200Ah range if you can fit it (which considering what 2x FLA batteries takes up should be pretty close physically) and afford it.

Thank you. Looks like I need to learn these formulas to calculate my energy needs. When the PV is charging the batteries all day it works fine. However, at night when the batteries have to carry the load is where it all falls apart.

I was aware the inverter has the idle consumption and my fridge used 75 watts. I still don’t understand why I get low DC when my batteries are 21v. It’s a 24v system and I thought it could discharge down to 12…confused.

 

[Rednecktek]

I still don’t understand why I get low DC when my batteries are 21v. It’s a 24v system and I thought it could discharge down to 12…confused.

Aaahhh, you're mixing up voltage with capacity. The inverter needs a voltage range between 21-ish volts and 27ish volts to run, but your capacity is how many AMPS the battery holds. Think of a battery like a bucket of water. The AMPS is how much water the bucket holds, the VOLTS are the water pressure from the hole in the side that lets the water out. The higher the volts, the larger the hole.

So, lets say you had a pair of 10Ah 12v batteries in series. That adds the voltage together for 24v, but the amps are still 10, so you have a 24v, 10ah battery. If you use 5 of those amps, you now have a 24v 5ah battery left over. The voltage stays the same, the capacity drops. The hole in the bucket is the same size, it's just a milk jug so not much in the tank.

Now lets say you replace those krappy 10ah batteries with 100ah batteries. Well, they're still 12v batteries so you're still getting 24 volts out of the bank (the hole in the side is the same size) it just stays that way longer because you've got more gas in the fuel tank (5 gallons instead of 1 gallon). As batteries get depleted they have a harder and harder time maintaining that 24v when they're running out of amps to play with so the voltage drops off as the battery gets weaker (the water pressure pushing out the hole has less Oomph!), until it's just got no more oomph to maintain that 24v your inverter wants and you get the low voltage alarm (out of water).

What you're thinking of with the discharge of 50% is the AMPS in the lead acid batteries. The label on the side of your batteries says they're 12v and 80Ah each, right? So if you put two in series you get 24v at 80Ah. Lead acid batteries (flooded, sealed, gel, AGM) start damaging the plates inside if they get too low which is why you never want to drain them more than half way. That half way we talk about is the AMPS of the battery, not the VOLTS. Remember, your batteries are always trying to keep up that 24v that your inverter is asking for, it just doesn't have it in the tank because you're drawing off the AMPS.

So somewhere around 40 amps worth of discharge the batteries are hitting that half-dead mark and really working it to try to keep up that 24v that the inverter keeps asking for and eventually just cries out "I canna do it captain!" and you get the low voltage alarm.

The other issue with lead acids is that by the time you're getting to the low voltage alarms, not only have you dropped below that 50% threshold but you've gone beyond that and probably done a little damage to the plates inside. The next day you don't get it enough charging to fully absorb up and drain it again doing a little more damage, and again the next day, and so on. Now your 80Ah batteries are only good for 75Ah, or 68Ah, or 50Ah as the plates inside get more and more beaten up. LFP doesn't have this problem nearly as much as lead acids do, they have their own quirks to work around but have many MANY benefits over lead acid.

Does that help make sense?

 

[SamDeleted]

@Sventvkg

You need more batteries!

 

[Sventvkg]

Aaahhh, you're mixing up voltage with capacity. The inverter needs a voltage range between 21-ish volts and 27ish volts to run, but your capacity is how many AMPS the battery holds. Think of a battery like a bucket of water. The AMPS is how much water the bucket holds, the VOLTS are the water pressure from the hole in the side that lets the water out. The higher the volts, the larger the hole.

So, lets say you had a pair of 10Ah 12v batteries in series. That adds the voltage together for 24v, but the amps are still 10, so you have a 24v, 10ah battery. If you use 5 of those amps, you now have a 24v 5ah battery left over. The voltage stays the same, the capacity drops. The hole in the bucket is the same size, it's just a milk jug so not much in the tank.

Now lets say you replace those krappy 10ah batteries with 100ah batteries. Well, they're still 12v batteries so you're still getting 24 volts out of the bank (the hole in the side is the same size) it just stays that way longer because you've got more gas in the fuel tank (5 gallons instead of 1 gallon). As batteries get depleted they have a harder and harder time maintaining that 24v when they're running out of amps to play with so the voltage drops off as the battery gets weaker (the water pressure pushing out the hole has less Oomph!), until it's just got no more oomph to maintain that 24v your inverter wants and you get the low voltage alarm (out of water).

What you're thinking of with the discharge of 50% is the AMPS in the lead acid batteries. The label on the side of your batteries says they're 12v and 80Ah each, right? So if you put two in series you get 24v at 80Ah. Lead acid batteries (flooded, sealed, gel, AGM) start damaging the plates inside if they get too low which is why you never want to drain them more than half way. That half way we talk about is the AMPS of the battery, not the VOLTS. Remember, your batteries are always trying to keep up that 24v that your inverter is asking for, it just doesn't have it in the tank because you're drawing off the AMPS.

So somewhere around 40 amps worth of discharge the batteries are hitting that half-dead mark and really working it to try to keep up that 24v that the inverter keeps asking for and eventually just cries out "I canna do it captain!" and you get the low voltage alarm.

The other issue with lead acids is that by the time you're getting to the low voltage alarms, not only have you dropped below that 50% threshold but you've gone beyond that and probably done a little damage to the plates inside. The next day you don't get it enough charging to fully absorb up and drain it again doing a little more damage, and again the next day, and so on. Now your 80Ah batteries are only good for 75Ah, or 68Ah, or 50Ah as the plates inside get more and more beaten up. LFP doesn't have this problem nearly as much as lead acids do, they have their own quirks to work around but have many MANY benefits over lead acid.

Does that help make sense?

Yes it does thank you. I will get PFP batteries as soon as I can swing it.

 

[Sventvkg]

The weirdness continues. Even running my gennie at night to power my AC and keep the inverter/batteries charged the inverter will start beeping and it looks to me on the screen like it’s hunting for PV input when none exists. It won’t charge the batteries from AC power source even though there is blatant power showing coming in from my gennie. Just keeps beeping and cycling and showing low voltage. Ended up having to shut it off and on before it saw the AC power input and began charging the batteries again but was still beeping and cycling looking for PV power. I’m at a loss!

It’s 3AM. No AC power is running besides a couple phones plugged in and my magic chef 10.1cf AC fridge that is efficient. Effectively shore power via my generator keeping everything running and I’m still having to shut down this inverter. This makes no sense. I have it set right. Nothing like this has ever happened during the day when the PV panels (1472 watts) on my roof are providing power. Only at night. I get that I need more batteries but why is this happening when I’m on power?

 

[Rednecktek]

Sounds like it's time to post all your settings, there's got to be something off in there somewhere or the firmware is flakey.

 

[cisumma]

Sounds to me like a universal problem ( same in my world of vmax lead acid batteries).
Some further feed back would be good.
I divided a 12v system ( 300 watt harbor freight and Marine 12 v that has been very consistent with lights and charging for the last 3 year only if a larger 24v mppt regulates into the 12 volt system at night. Never gets dark and always have charging , but no appliances at night ; of course.
I don't see lithium BMS in my future; but, I am considering a smaller 48 volt system to charge during the day and regulating it into the 24volts system at night.
Does this sound okay? I will just have to manually watch the voltage levels unless there is an inline device to protect each battery bank from depleting.
Just a thought.