Some quick small system LiFePo4 questions

[Rednecktek]

So I need to extend the run time between brighter* days on my driveway lights and I'm looking at the idea of swapping out my AGM batteries for some LiFe's to extend that battery time. I'm hoping to do this on a budget because I work for a living and have other projects for the house. So here's what I've got:

2x 15Ah AGM batteries
100w Renology panel
PWM controller
2x 3w LED lights
1w solar day/night eye

What I'm thinking of doing is swapping those 15Ah AGM's for a pair of 20Ah LiFe's (because the case only has so much space inside), but I've got some questions:

What happens if I try to save the PWM controller built for AGM's and feed the LiFe's? I know they don't put out the voltage that LiFe's want, but I've read that keeping the batteries between 20%-80% for best lifespan. What kind of losses/problems am I looking at?

What happens if the batteries try to charge during those few days a year that it's below freezing out? The batteries SHOULD have low temp shutoff, but what if?

Leaving an acid battery deeply discharged for any amount of time damages the batteries and they stop holding full capacity (so far only a day between the time the lights are dim and I can get down the driveway with the Jackery and jumper cables so far) but what happens to the LiFe's if they're left deeply discharged for days on end?

Thanks gang!

 

[sunshine_eggo]

So I need to extend the run time between brighter* days on my driveway lights and I'm looking at the idea of swapping out my AGM batteries for some LiFe's to extend that battery time. I'm hoping to do this on a budget because I work for a living and have other projects for the house.

Adding batteries only extends run time between charges. Given the paltry solar you collect this time of year, I don't know that it's really much of an improvement.

What happens if I try to save the PWM controller built for AGM's and feed the LiFe's?

Nothing. Most AGM battery programs in charge controllers work well for LFP.

I know they don't put out the voltage that LiFe's want, but I've read that keeping the batteries between 20%-80% for best lifespan.

Charge controller set to anything higher than 13.6V on bulk/absorption is going to charge the battery to 95% given enough time.

What kind of losses/problems am I looking at?

None.

What happens if the batteries try to charge during those few days a year that it's below freezing out? The batteries SHOULD have low temp shutoff, but what if?

The batteries will be damaged and permanently lose capacity.

Why you cannot charge LiFePO4 below 0 degrees Celsius

This study summed it up nicely: With the complex material system used in LIBs, the performance degradation at low temperatures can be attributed to several different sources. First, the low temperature will affect the property of electrolyte. With the decrease of temperature, the viscosity of...

diysolarforum.com

what happens to the LiFe's if they're left deeply discharged for days on end?

If you're talking completely tapped out, it's really not good for them, but it's less damaging than for AGM.

Do you have the option of topping off the battery with an AC powered charger? Seems like the easy answer for your depressing skies.

 

[Samsonite801]

Does your PWM give a spec in the manual on what it floats at?

On charging while below freezing, 'what if', I guess it will damage them is what I always understood. (DO NOT WANT TO EVER CHARGE BELOW FREEZING)

And on the last question, I've heard it isn't really bad for LiFePO4 to sit discharged, but that if you plan to store them for long periods, they say optimal for longest life would be to store them around 50-60% or something.

Perhaps you can just find a cheap MPPT charger that supports LiFePO4 battery bank (or custom settings option on the charging profile)... MPPT will do better at charging efficiency anyways.

Perhaps could get a cheap heater and thermostat, and just build the bank large enough to handle a little heater pad (and put some insulation around the pack)?

 

[Rednecktek]

Do you have the option of topping off the battery with an AC powered charger? Seems like the easy answer for your depressing skies.

I'd have to run a 500' extension cord down the hill to do that, hence building this all on solar in the first place.

Adding batteries only extends run time between charges. Given the paltry solar you collect this time of year, I don't know that it's really much of an improvement.

It takes about a week and a half right now to kill the batteries, I'm thinking that if I can get 3 weeks out of a charge I'd have better odds of having a sunny hour or two in there to feed them.

Charge controller set to anything higher than 13.6V on bulk/absorption is going to charge the battery to 95% given enough time.

Does your PWM give a spec in the manual on what it floats at?

This is the charge controller I'm using down there. The last one krapped out on me about the beginning of November. It says it floats at 13.5, but no mention of a bulk setting. Any recommendations on a better, cheap-ish controller?

Perhaps you can just find a cheap MPPT charger that supports LiFePO4 battery bank (or custom settings option on the charging profile)... MPPT will do better at charging efficiency anyways.

The biggest issues I have with that idea (and it has crossed my mind) is that there is only so much space in the Pelican case and MPPT's are quite large in addition to the batteries already inside, plus it's an air and water tight case so 115F summers would be horrible for the poor thing. It's in the shade but still needs some ventilation. Plus I don't think the 10-20% gain would really buy me enough. I'm going to scope it out today and see if I can get another 100w panel down there, but topography is a PITA out here.

Perhaps could get a cheap heater and thermostat, and just build the bank large enough to handle a little heater pad (and put some insulation around the pack)?

LOL! I can't even store enough power to keep it going now and adding another load of any kind is stepping backwards. I had a similar thought until I did the math and it was bad. I'm just going to have to make sure it's a brand/model that has low temp.

 

[Samsonite801]

Maybe it would just be simpler to put 4 AGM in that box instead of two, and not worry about going lithium? It looks like you could fit another 2x in that box?

 

[sunshine_eggo]

I'd have to run a 500' extension cord down the hill to do that, hence building this all on solar in the first place.

Ouch

It takes about a week and a half right now to kill the batteries, I'm thinking that if I can get 3 weeks out of a charge I'd have better odds of having a sunny hour or two in there to feed them.

Better than I thought.

This is the charge controller I'm using down there. The last one krapped out on me about the beginning of November. It says it floats at 13.5, but no mention of a bulk setting. Any recommendations on a better, cheap-ish controller?

Yuck. If it only charges to 13.5V, it might be part of your problem.

If you're thinking about adding a 3rd panel, you'll need 30A.

4 stars, 278 ratings:

https://www.amazon.com/dp/B074WZB5XY

14.4V "equalization" and 13.7V (adjustable) float would seem to work.

If it doesn't, you're only out $15.

Recognizable brand that might work as well:

Amazon.com

LOL! I can't even store enough power to keep it going now and adding another load of any kind is stepping backwards. I had a similar thought until I did the math and it was bad. I'm just going to have to make sure it's a brand/model that has low temp.

I'm not aware of any PWM that offer low-temp charge protection. I would only get a battery that protects itself.

 

[Rednecktek]

Update: Well I just went down and connected up the Jackery, and I took a tape measure. It looks like I can fit my old PowMr MPPT controller in there, and I have some used 200w panels out back, so I think I'll run into town today and grab some lumber. The panel in service and the 200w one are too dissimilar to connect together so I think I'll just wire the MPPT output to the batteries in parallel as a stand-alone circuit. Z-brackets will be here tomorrow unfortunately as there's nowhere local that carries them.

Maybe it would just be simpler to put 4 AGM in that box instead of two, and not worry about going lithium? It looks like you could fit another 2x in that box?

That would be about the same price as getting 2 of the 20ah LiFe's, but it had crossed my mind.

 

[sunshine_eggo]

Update: Well I just went down and connected up the Jackery, and I took a tape measure. It looks like I can fit my old PowMr MPPT controller in there, and I have some used 200w panels out back, so I think I'll run into town today and grab some lumber. The panel in service and the 200w one are too dissimilar to connect together so I think I'll just wire the MPPT output to the batteries in parallel as a stand-alone circuit. Z-brackets will be here tomorrow unfortunately as there's nowhere local that carries them.

YAY MPPT!

Have we already discussed the 200W vs. other panels? How dissimilar?

 

[Samsonite801]

Update: Well I just went down and connected up the Jackery, and I took a tape measure. It looks like I can fit my old PowMr MPPT controller in there, and I have some used 200w panels out back, so I think I'll run into town today and grab some lumber. The panel in service and the 200w one are too dissimilar to connect together so I think I'll just wire the MPPT output to the batteries in parallel as a stand-alone circuit. Z-brackets will be here tomorrow unfortunately as there's nowhere local that carries them.

That would be about the same price as getting 2 of the 20ah LiFe's, but it had crossed my mind.

I love LiFePO4 too, but sometimes simplicity wins hehe...

 

[Rednecktek]

The 100w is about 20v @ 5a, the used panels I got recently (for other projects but I have extras) are about 36v @ 7a. Not close enough to strap together.

I also have the option of adding in 2 more batteries as mentioned, but due to the space in the box I can do EITHER batteries (for about $130) OR another panel and MPPT controller (for $cost of lumber, parts on hand).

Also, charge controller ordered, should be here tomorrow. Probably the first time a PowMr product has EVER been recommended.

 

[sunshine_eggo]

The 100w is about 20v @ 5a, the used panels I got recently (for other projects but I have extras) are about 36v @ 7a. Not close enough to strap together.

I also have the option of adding in 2 more batteries as mentioned, but due to the space in the box I can do EITHER batteries (for about $130) OR another panel and MPPT controller (for free, parts on hand).

IMHO, you need to replace the existing controller unless you can document that it will bulk/absorp above 14.0V. It's going to act like a tourniquet preventing juice from going to the battery if it's stuck at 13.5V.