AGM to LiFePo4 conversion

[Jonathan123]

I’ve got an RPS Solar water pump with depleted batteries and am replacing them with LiFePo4 and also need to increase capacity. I’m having trouble calculating my energy needs for this due to the variability of the pump draw and am hoping to get some clarity from you all.

Location: Maine
PV panels: 6 @ 100 watts
Outgoing AGM: 6 12v 55 ah in series and parallel for 24v and 330 ah
Pump: 220v ac pulling from the charge controller/inverter. Pump uses ~250 watts at rest and ~700-1,000 watts in use. Rated at 8 amps. Powered 24/7 in standby and for modest off-grid cabin use (toilet, sink, shower).

Questions:

1. I’m looking at 4 12v 100 ah LiFePo4 to replace the AGM to create a 24v 400 ah system. Even with the deeper depth of discharge with these I’m wondering if 400 ah is enough, if I don’t want to run the new ones down too low and accounting for cloudy days. Should I consider more ah? The AGM seemed underpowered and probably died sooner than they should have.
2. I realize at some point I’ll need to add pv panels. If I go with 400 ah I’ll likely wait to see how they do. Is that a fair approach or should I increase panels up front?
3. Would it actually make the most sense to go with 400 ah batteries and spend the extra money on more panels instead of more batteries?

 

[Sennen]

By your stats, the system requires anywhere between 6KWH (no use) and 24KWH (never not running) to operate for 24 hours. Probably averaging 12KWH?

Question 1: That's 4.8KWh of battery (24v 200AH: Series keeps the same AH). Probably not enough with no solar help (cloudy/rainy/snowy days) however, depending on how long your AGM setup lasted, this is an improvement of 840 WH.
Question 2: Your current configuration is 600 watts. Unsure how large they are, but panels are relatively expensive and you can likely use the same area to put more efficient panels. Keep an eye on the Solar Panels for Sale section, someone might be offloading them at $20 a panel.

 

[grizzzman]

I’ve got an RPS Solar water pump with depleted batteries and am replacing them with LiFePo4 and also need to increase capacity. I’m having trouble calculating my energy needs for this due to the variability of the pump draw and am hoping to get some clarity from you all.

Location: Maine
PV panels: 6 @ 100 watts
Outgoing AGM: 6 12v 55 ah in series and parallel for 24v and 330 ah
Pump: 220v ac pulling from the charge controller/inverter. Pump uses ~250 watts at rest and ~700-1,000 watts in use. Rated at 8 amps. Powered 24/7 in standby and for modest off-grid cabin use (toilet, sink, shower).

Questions:

1. I’m looking at 4 12v 100 ah LiFePo4 to replace the AGM to create a 24v 400 ah system. Even with the deeper depth of discharge with these I’m wondering if 400 ah is enough, if I don’t want to run the new ones down too low and accounting for cloudy days. Should I consider more ah? The AGM seemed underpowered and probably died sooner than they should have.

Your 400AH LFP bank will increase your atonamy by a notable amount and your charging speed will be much faster. ( your pump will run at a higher voltage ie less run time per use.)

1. I realize at some point I’ll need to add pv panels. If I go with 400 ah I’ll likely wait to see how they do. Is that a fair approach or should I increase panels up front?

Only testing can answer this but when it comes to small systems more is better and easier on the batteries.

1. Would it actually make the most sense to go with 400 ah batteries and spend the extra money on more panels instead of more batteries?

In my book yes.

 

[Jonathan123]

By your stats, the system requires anywhere between 6KWH (no use) and 24KWH (never not running) to operate for 24 hours. Probably averaging 12KWH?

Question 1: That's 4.8KWh of battery (24v 200AH: Series keeps the same AH). Probably not enough with no solar help (cloudy/rainy/snowy days) however, depending on how long your AGM setup lasted, this is an improvement of 840 WH.
Question 2: Your current configuration is 600 watts. Unsure how large they are, but panels are relatively expensive and you can likely use the same area to put more efficient panels. Keep an eye on the Solar Panels for Sale section, someone might be offloading them at $20 a panel.

Series keeps the same ah but also wiring in parallel should give me 400 ah….

 

[Sennen]

Series keeps the same ah but also wiring in parallel should give me 400 ah….

Agreed. Either way, it's an upgrade from the AGM.

 

[Sennen]

Clarification: 2s2p is 200ah 24v. 4p is 400ah 12v, 4s is 100ah 48v

 

[sunshine_eggo]

I’ve got an RPS Solar water pump with depleted batteries and am replacing them with LiFePo4 and also need to increase capacity. I’m having trouble calculating my energy needs for this due to the variability of the pump draw and am hoping to get some clarity from you all.

Location: Maine
PV panels: 6 @ 100 watts
Outgoing AGM: 6 12v 55 ah in series and parallel for 24v and 330 ah

Nope.

2S3P of 12V 55Ah batteries is only 165Ah.

Ah is voltage dependent. You don't get to add all the Ah up when changing voltage.

When placing in series: voltage adds, capacity unchanged.
When placing in parallel: voltage unchanged, capacity adds.

Pump: 220v ac pulling from the charge controller/inverter. Pump uses ~250 watts at rest and ~700-1,000 watts in use. Rated at 8 amps. Powered 24/7 in standby and for modest off-grid cabin use (toilet, sink, shower).

250W? If this is not a typo, this is your problem 250W needs 250 * 24=6000Wh = 235Ah @ 25.6V.

There's no way a 600W array can provide 6000Wh/day. Period.

If you are having issues with depleting your batteries, you likely do not have a battery problem. You likely have insufficient PV.

PV/solar conditions determines how much energy you can use per day.
Battery capacity determines how long you can go between charging.

 

[Jonathan123]

Nope.

2S3P of 12V 55Ah batteries is only 165Ah.

Ah is voltage dependent. You don't get to add all the Ah up when changing voltage.

When placing in series: voltage adds, capacity unchanged.
When placing in parallel: voltage unchanged, capacity adds.



250W? If this is not a typo, this is your problem 250W needs 250 * 24=6000Wh = 235Ah @ 25.6V.

There's no way a 600W array can provide 6000Wh/day. Period.

If you are having issues with depleting your batteries, you likely do not have a battery problem. You likely have insufficient PV.

PV/solar conditions determines how much energy you can use per day.
Battery capacity determines how long you can go between charging.

Shoot that’s right - and why I’m in the Beginner’s Corner!

So 4 12v at 100 ah is actually 200 ah when in series and parallel. That doesn’t like enough…?

Agree about the array. What would you recommend I upgrade to?

 

[sunshine_eggo]

Shoot that’s right - and why I’m in the Beginner’s Corner!

So 4 12v at 100 ah is actually 200 ah when in series and parallel. That doesn’t like enough…?

I think you're describing 2S2P for 24V

two 12V 100Ah in series (24V 100Ah)
two 12V 100Ah in series (24V 100Ah)
those two 24V strings in parallel.

200Ah

Agree about the array. What would you recommend I upgrade to?

However much PV you need to power your loads in your solar conditions...

In Maine... this may not be a year-round option.

 

[Jonathan123]

However much PV you need to power your loads in your solar conditions...

In Maine... this may not be a year-round option.

This system is dedicated to the pump only and is operative April-October.

 

[sunshine_eggo]

This system is dedicated to the pump only and is operative April-October.

Are you 1000% certain that this thing uses 250W when not pumping? That's pretty outrageous. Based on your actual use description, 600W sounds like plenty.

If 250W is the number, and there is no low-power mode, I would cut the power at night or any significant period of disuse... i.e., turn it on for a few hours in the morning and in the evening and that's it.

 

[Jonathan123]

Are you 1000% certain that this thing uses 250W when not pumping? That's pretty outrageous. Based on your actual use description, 600W sounds like plenty.

If 250W is the number, and there is no low-power mode, I would cut the power at night or any significant period of disuse... i.e., turn it on for a few hours in the morning and in the evening and that's it.

Not certain but the company gave me that figure. Have requested confirmation from them.

 

[sunshine_eggo]

Not certain but the company gave me that figure. Have requested confirmation from them.

Do you have a manual on the unit?

Do you have a pressure tank in the system?

 

[Jonathan123]

Do you have a manual on the unit?

Do you have a pressure tank in the system?

No pressure tank, just straight pumping to use. Manual in my Dropbox here: https://www.dropbox.com/scl/fi/gnq4...nual.pdf?rlkey=4eclbh3am4zzwjxli0jny19sf&dl=0

Page 6 has details.

 

[Jonathan123]

Are you 1000% certain that this thing uses 250W when not pumping? That's pretty outrageous. Based on your actual use description, 600W sounds like plenty.

If 250W is the number, and there is no low-power mode, I would cut the power at night or any significant period of disuse... i.e., turn it on for a few hours in the morning and in the evening and that's it.

Yeah turning it off at least overnight might go a long way toward solving my problem.

 

[Sennen]

Curious if you could put a switch in the house somewhere to make the pump more on demand. Also curious if a mini water tower would save your sundown demand using gravity to pump

 

[sunshine_eggo]

If practical, I think a pressure tank would help a bit. Even with the pump off, you'll have a small reserve of water/pressure for limited use (wash hands, brush teeth, get a glass of water).

It doesn't reduce the total amount of pump runtime, but it reduces the frequency of running and for longer durations while building a reserve.

 

[Jonathan123]

Curious if you could put a switch in the house somewhere to make the pump more on demand. Also curious if a mini water tower would save your sundown demand using gravity to pump

I think a switch would really do the trick. And I’ll have a trencher this spring when I move the panels so I can add a switch pretty easily. I’ll post back when the new set up is done.

 

[Jonathan123]

If practical, I think a pressure tank would help a bit. Even with the pump off, you'll have a small reserve of water/pressure for limited use (wash hands, brush teeth, get a glass of water).

It doesn't reduce the total amount of pump runtime, but it reduces the frequency of running and for longer durations while building a reserve.

Probably the biggest benefit of a tank, it seems, is it could allow us to leave the pump off all night vs just part. Particularly with guests we can’t really not flush the toilet all night so a tank could accommodate that nighttime use.

 

[sunshine_eggo]

Probably the biggest benefit of a tank, it seems, is it could allow us to leave the pump off all night vs just part. Particularly with guests we can’t really not flush the toilet all night so a tank could accommodate that nighttime use.

If it's yellow... let it mellow...