24v Battery bank on 120v Cabin

[Diemjoe]

I am getting close to pulling the trigger on some 12v batteries for a new 600 sq ft totally off grid cabin I am building. I was working towards a 24v setup by purchasing the MPP Solar 24v charger/inverter with a 24v to 12v stepdown for my shed with lights and a Mini Cosco refridgerator. Works fine.
Previous questions here have me leaning towards running everything on 120v AC/DC. mostly lights, fridge and toaster oven, etc. Can anyone give me some viewpoints on why I should stick with the 24v design other than I already have the inverter.

I have a couple 170 Ah 12v Big Battery Owls that I didnt realize couldnt be wired in series. I would like to use my battery bank to run a 220v well pump to a storage tank but haven't done the draw tests on it yet. There would also need to be another pump to pressurize the house plumbing.

Thanks.

 

[MisterSandals]

Can anyone give me some viewpoints on why I should stick with the 24v design other than I already have the inverter.

How many watts is your inverter and how much power does it consume when turned on (inverting)?

I would like to use my battery bank to run a 220v well pump to a storage tank but haven't done the draw tests on it yet.

I don't think you are anywhere close to being able to run this. What are locked rotor amps? What are your battery (BMS limited) max current limits?

 

[Diemjoe]

I don't have the MPP Solar LV2424 specs in front of mine. Assuming they haven't changed since I bought it a couple years ago, the website says max 2.7kw output at 24vdc. I can't tell from a quick search how much wattage the inverter uses. I am using End Of life Deep cycle Lead Acid so its hard to tell if the Inverter is using much power when the sun isn't shining or the low voltage condition each morning is just the old batteries.
Yes. I realize this is woefully underpowered for the pump but one of the features of MPP Solar is the inverters can be chained to get to the 220v requirement. I have a generator now and will see what the initial draw is. Once I fill the tank, (about an hour of generator time) I should only need to run it once a week to keep it topped off. I do not have batteries yet but I appreciate you calling out the current limits. I'll add it to my list once I can get up and see the pumps power usage documentation.

 

[MichaelK]

I don't have the MPP Solar LV2424 specs in front of mine. Assuming they haven't changed since I bought it a couple years ago, the website says max 2.7kw output at 24vdc.

OK, now this is very, very important. MS questioned you on what the locked-rotor amps are. What that means is what the starting surge of your pump is. You need a special clamp meter that can measure "inrush current". That is, the amount of current the motor will draw in the first 500 milliseconds or so after being switched to on. After 1000 milliseconds or so, the amperage drops down to the running amperage. You MUST know this! A regular meter is not fast enough to catch the inrush.

The MPP in particular is well-known to not have the capability of supporting a starting surge longer than about 16 milliseconds, so depending on the pump size, you might need up to four or more parallel units to be able to handle the starting surge. You might instead want a low-frequency, transformer-based inverter, which typically have far greater surge capacity. Don't just pay attention to the surge watts. Pay careful attention to how many seconds the inverter can support that surge.

I use an Uni-T 216C these days. You can get one on Ebay for ~80$ or so. That will accurately predict your pump's inrush. For my 1hp pump, the running amps is only ~9.5A, whereas the starting amps is ~38A. That's 9100W of start-up demand. My XW+6848 handles that without blinking. Outback is another brand that handles surges very well. Some models put out split-phase 120/240V, whereas other models need two parallel units to make 120/240V split-phase. Do your research here. I don't want to see you spend a bunch of money to create a system that can't power what you want to power here.

 

[Rednecktek]

A small system like yours is going to have a hard time jumping from some lights and a fridge to powering a well pump. There are a few options that all have some serious drawbacks to deal with. Sadly, there are no good options.

1: A second low frequency inverter to feed the well pump.
Pro's: no large standby draw, backup if the MPP goes down
Con's: Expensive, have to manually turn on every time you need to pump up the system, may need lots more battery bank to support the amp draw

2: Split phase inverter,
Pro's: Always ready to run the pump, ability to run 240v loads like aircon in the future
Con's: More expensive, usually reqhires a 48v system, need more batterjes, high standby draw, needs more panels, all your 24v stuff is useless

3: Second inverter synced for 240v split phase.
Pro's: Keeps 24v equipment, only powered on when pumping up water system, cheaper than options 1 or 2
Con's: More battery banks required for amp draw, complicated wiring, have to manually pump up water system.

4: 120v well pump (if possible based on depth)
Pro's: no need to worry about split phase, no complicad wiring, may be cheapest option, retain 24v equipment
Con's: Still need 2nd inverter to provide for amperage draw, more battery banks for amp draw, 2x standby power, have to replace well pump.

5: 2nd complete system.
Pro's: No need to figure out how to integrate with existing system, can be smaller as it only runs intermittently and has more time to recharge, provides emergency backup, can be completely contained in the pump house, cheap FLA batteries work well.
Con's: Cost of a complete system with a low frequency inverter, more panels, space in the pump house may be limited.

Like I said, no good options. I've been looking at option 5 for my camp as it's 320ft away from the cabin but I have space out there for batteries and such. My well pump runs about 5-10 minutes 2-3 times a day so my batteries would have plenty of time to recover.

 

[Diemjoe]

OK, now this is very, very important. MS questioned you on what the locked-rotor amps are. What that means is what the starting surge of your pump is. You need a special clamp meter that can measure "inrush current". That is, the amount of current the motor will draw in the first 500 milliseconds or so after being switched to on. After 1000 milliseconds or so, the amperage drops down to the running amperage. You MUST know this! A regular meter is not fast enough to catch the inrush.

The MPP in particular is well-known to not have the capability of supporting a starting surge longer than about 16 milliseconds, so depending on the pump size, you might need up to four or more parallel units to be able to handle the starting surge. You might instead want a low-frequency, transformer-based inverter, which typically have far greater surge capacity. Don't just pay attention to the surge watts. Pay careful attention to how many seconds the inverter can support that surge.

I use an Uni-T 216C these days. You can get one on Ebay for ~80$ or so. That will accurately predict your pump's inrush. For my 1hp pump, the running amps is only ~9.5A, whereas the starting amps is ~38A. That's 9100W of start-up demand. My XW+6848 handles that without blinking. Outback is another brand that handles surges very well. Some models put out split-phase 120/240V, whereas other models need two parallel units to make 120/240V split-phase. Do your research here. I don't want to see you spend a bunch of money to create a system that can't power what you want to power here.

Thanks, That is great information to have before I get moving on any design changes. the Pump is at about 350 ft so it tkates alot to get going. Right now, I have a gas powered generator that does the job but would like to add to my options someday as a fall back if I cant get into town to fill the gas tanks once the winter weather sets in.

 

[Diemjoe]

A small system like yours is going to have a hard time jumping from some lights and a fridge to powering a well pump. There are a few options that all have some serious drawbacks to deal with. Sadly, there are no good options.

1: A second low frequency inverter to feed the well pump.
Pro's: no large standby draw, backup if the MPP goes down
Con's: Expensive, have to manually turn on every time you need to pump up the system, may need lots more battery bank to support the amp draw

2: Split phase inverter,
Pro's: Always ready to run the pump, ability to run 240v loads like aircon in the future
Con's: More expensive, usually reqhires a 48v system, need more batterjes, high standby draw, needs more panels, all your 24v stuff is useless

3: Second inverter synced for 240v split phase.
Pro's: Keeps 24v equipment, only powered on when pumping up water system, cheaper than options 1 or 2
Con's: More battery banks required for amp draw, complicated wiring, have to manually pump up water system.

4: 120v well pump (if possible based on depth)
Pro's: no need to worry about split phase, no complicad wiring, may be cheapest option, retain 24v equipment
Con's: Still need 2nd inverter to provide for amperage draw, more battery banks for amp draw, 2x standby power, have to replace well pump.

5: 2nd complete system.
Pro's: No need to figure out how to integrate with existing system, can be smaller as it only runs intermittently and has more time to recharge, provides emergency backup, can be completely contained in the pump house, cheap FLA batteries work well.
Con's: Cost of a complete system with a low frequency inverter, more panels, space in the pump house may be limited.

Like I said, no good options. I've been looking at option 5 for my camp as it's 320ft away from the cabin but I have space out there for batteries and such. My well pump runs about 5-10 minutes 2-3 times a day so my batteries would have plenty of time to recover.

Good info. I thik I have access to an amp meter that will check the start up draw.

 

[MichaelK]

Thhe Pump is at about 350 ft so it tkates alot to get going. Right now, I have a gas powered generator that does the job but would like to add to my options someday as a fall back if I cant get into town to fill the gas tanks once the winter weather sets in.

Sounds similar to my own pump. If you can't get a meter that measures inrush, you might take the amp value a regular meter gives you and multiply that reading by 4X. That should be close.

Here are a few more little factoids you can chew on. When my pump is running off my 120/240V generator, it requires 10.0A of power running, and puts out 275 gallons of water per hour. Running on the XW+, the pump draws 9.5A and puts out 305 gallons of water per hour. So, about 18% more water.

That's because the inverter is putting out a cleaner sine-wave than the generator is, so cleaner, less distorted power produces more water per hour. Whether or not the also means increased longevity of the pump, I can't say, but I would assume so.

Sadly, none of the inverter companies mention this in their literature, but for an off-gridder like me, the QUALITY of the power is more important then the Watts/$.

 

[Diemjoe]

Sounds similar to my own pump. If you can't get a meter that measures inrush, you might take the amp value a regular meter gives you and multiply that reading by 4X. That should be close.

Here are a few more little factoids you can chew on. When my pump is running off my 120/240V generator, it requires 10.0A of power running, and puts out 275 gallons of water per hour. Running on the XW+, the pump draws 9.5A and puts out 305 gallons of water per hour. So, about 18% more water.

That's because the inverter is putting out a cleaner sine-wave than the generator is, so cleaner, less distorted power produces more water per hour. Whether or not the also means increased longevity of the pump, I can't say, but I would assume so.

Sadly, none of the inverter companies mention this in their literature, but for an off-gridder like me, the QUALITY of the power is more important then the Watts/$.

That is a good point. My pump is pretty deep so keeping it healthy will save money in the long run.

 

[50ShadesOfDirt]

If you already have a generator, and the gen already drives the pump, have you considered keeping/improving the gen side of things to pump water when needed?

- Upgrade gen to right kind of fuel/power needed to run your pump most efficiently for your site.
- Automate it to turn on and refill storage tank, only when needed; cuts out the incredible resizing effort needed to do it all with solar gear.
- Gen is also available to replenish battery-bank if solar not cooperating that day.

Now size the remaining solar effort to just what is needed for shed, cabin, etc. Control everything from cabin (home automation).

Hope this helps ...

 

[Diemjoe]

If you already have a generator, and the gen already drives the pump, have you considered keeping/improving the gen side of things to pump water when needed?

- Upgrade gen to right kind of fuel/power needed to run your pump most efficiently for your site.
- Automate it to turn on and refill storage tank, only when needed; cuts out the incredible resizing effort needed to do it all with solar gear.
- Gen is also available to replenish battery-bank if solar not cooperating that day.

Now size the remaining solar effort to just what is needed for shed, cabin, etc. Control everything from cabin (home automation).

Hope this helps ...

Thanks, I was at Costco yesterday and was looking at their generators. You are probably right in that getting the pump on solar would be a lot of money for the little amount I use it.

 

[Rednecktek]

Thanks, I was at Costco yesterday and was looking at their generators. You are probably right in that getting the pump on solar would be a lot of money for the little amount I use it.

The only reason I'm seriously looking at getting a solar power backup for my well pump is that I already have a stack of FLA batteries, SCC, and some old panels I can reuse. It's still going to be $1000 for the inverter when I finally pull that trigger so it isn't going to be any time soon.

 

[12VoltInstalls]

small system like yours is going to have a hard time jumping from some lights and a fridge to powering a well pump. There are a few options

5: 2nd complete system.

This is the best option since a $1000 spend was mentioned as a barrier. But read below:

Right now, I have a gas powered generator that does the job but would like to add to my options someday as a fall back if I cant get into town to fill the gas tanks once the winter weather sets in.

have you considered keeping/improving the gen side of things to pump water

I would place a 250 gallon tank/reservoir in the shade and use EPA guidelines to add a bit of bleach to keep it growth free ($50-$120), using the generator every do often to fill. Next, a decent no-surge RV pump is ~$80 and will supply showers, sink etc.
The refrigerator and lights can stay 12V, and it’s a bit ‘bulky’ for watts, but the toastar oven and microwave will run fine on a 2000-3000W pure sine inverter ($250-$300) and 2/0 cables ($60). Maybe not at the same time but they’re going to work.

So for <$500 you get dependably functional with existing equipment and have your full 12V lighting options available. Plus, with a standalone SCC and pure sine inverter, you’ll not have .75kWh/day or more needed production to surpass self-consumption of an AIO. The money saved will buy a tankless hot water water heater ($160) and you’ll be in the lap of luxury.

 

[Diemjoe]

This is the best option since a $1000 spend was mentioned as a barrier. But read below:


I would place a 250 gallon tank/reservoir in the shade and use EPA guidelines to add a bit of bleach to keep it growth free ($50-$120), using the generator every do often to fill. Next, a decent no-surge RV pump is ~$80 and will supply showers, sink etc.
The refrigerator and lights can stay 12V, and it’s a bit ‘bulky’ for watts, but the toastar oven and microwave will run fine on a 2000-3000W pure sine inverter ($250-$300) and 2/0 cables ($60). Maybe not at the same time but they’re going to work.

So for <$500 you get dependably functional with existing equipment and have your full 12V lighting options available. Plus, with a standalone SCC and pure sine inverter, you’ll not have .75kWh/day or more needed production to surpass self-consumption of an AIO. The money saved will buy a tankless hot water water heater ($160) and you’ll be in the lap of luxury.

Thanks. I have a 500 gallon water tank and it does get filled up in about an hour with my Costco Firman. I definitely don't want to be starting and stopping the pump each time I turn the spigot on. The only real reason I want to go to a solar pump is to get off reliance on gasoline. Once the wet season rolls in, there wont be any rolling into town to fill the gasoline tanks. Maybe I should bone up on a rain and snow water filtration system to bypass the well in the winter.