LiFePo4 addition to a SLA House battery?

[eabyrd]

Are you saying your SLA is a Sealed Flooded Lead Acid. A 'sloshy'?

I am certain that acid stratification is a real thing. How do you stir the acid solution?

The other definition for equalization is when you believe the cells of your lead have become unbalanced. This is when you rely on a longer timed eq period at extra high Voltage. Some cells are deliberately overcharged in order to restore the weaker cells.

SLA is a catchall term for AGM (absorbent Glass Mat) or Gel batteries. Neither require equalization in the way of a “wet cell” batterie. And yes lithium cells are often equalized as they approach top (or bottom) of their voltage ranges

 

[gelmjw]

SLA is a catchall term for AGM (absorbent Glass Mat) or Gel batteries. Neither require equalization in the way of a “wet cell” batterie. And yes lithium cells are often equalized as they approach top (or bottom) of their voltage ranges

I'm glad you cleared that up. I thought SLA meant maintenance free, where you seal up the acid in a sloshy.

So what do you have?

I have Eaton Powerware AGM. Eaton is a NYSE(symbol ETN) listed worldwide turn-key power management company. The only specs printed on the case are 'Float 13.5 - 13.8V'. Interesting! It may be safe to say these are for UPS service.

There are two banks of 100Ah 12V; a single in the Trailer, and 4 in 2s2p for 24V 4800Whs in the van.

I am trying out LFP+AGM in the trailer. I want to abuse the LFP in a way that slow charges the AGM. This will be called tuition.

If this works out, I will add a 100Ah 24V string to the van giving 7200Whs. The van is my shore power source when dry camping. Even now, I can run the A/C overnight. This will be a 2:1 ratio of AGM:LFP.

 

[Horsefly]

SLA is a catchall term for AGM (absorbent Glass Mat) or Gel batteries. Neither require equalization in the way of a “wet cell” batterie. And yes lithium cells are often equalized as they approach top (or bottom) of their voltage ranges

Actually, the world of lead acid batteries is a bit more complicated than that. SLA also includes "Maintenance Free" or "Valve Regulated" batteries, that are actually just flooded lead acid batteries (yes, they slosh) that don't have caps for you to open and add water. I believe that most places I've seen refer to SLA as only the sealed version of the flooded, and AGM and GEL are separate.

 

[wattmatters]

Well the manufacturer recommends the charging voltage of 2.40V per cell, or 57.6V for your 48V bank (at 20°C). That would be 3.6V / cell for LFP.

The recommended range is 2.35-2.40V/cell @20°C. Given I'm located in a warm to hot climate, I'm erring on the lower side of the voltage range.

Here's the relevant page from the operations guide for off-grid hybrid applications:

I've read about those batteries before, and got the impression they were really intended for backup (float) applications, rather than off-grid (cyclical) applications. However, I see they do claim it can be used in cyclical applications. However, they emphasize that it has to get back to full charge between discharges. If you are only charging to 2.35V per cell, you (strictly speaking) are not getting the batteries to full charge.

The primary purpose of my batteries is for grid outage backup. They also serve as ballast during the day for my off-grid solar PV which supplies the energy to run my pool pump as well as a few sundry items.

The batteries get a brief boost charge in the morning when the sun comes up, spend most of the day on float charge and then sit overnight with a small idle current draw from connected devices and the inverter, <1% of capacity drawn, not enough for coulomb counter to register even that much most nights. Here's the SOC chart for past couple of weeks:


I did a grid outage simulation test overnight and that took SOC down to ~80%. We had an actual grid outage 5 days later in the afternoon for 1.25 hours and that dropped SOC to ~96-97%. Else a couple of minor blips in SOC during cloudy days following the daily pump cycle start up.

Adding a bit of LiFePO4 would mean I could potentially repurpose the storage bank to do more cyclic work powering home loads off-grid while the SLA banks stays pretty much fully charged ready for grid outage backup duties.

 

[eabyrd]

My "48V" AGMs bulk charge voltage is 56.4V (2.35V/cell). That's a lithium cell voltage of 3.525V/cell.

Float voltage is 54.0V (2.25V/cell) = lithium equivalent of 3.375V/cell.

This sounds like a match made in heaven.

I have 380Ah of 48V SLA. Adding 100Ah of LiFePO4 would be pretty neat. I won't be doing it unless 100Ah of LiFePO4 was available for SFA but a neat trick to keep up one's sleeve.

I'm glad you cleared that up. I thought SLA meant maintenance free, where you seal up the acid in a sloshy.

So what do you have?

I have Eaton Powerware AGM. Eaton is a NYSE(symbol ETN) listed worldwide turn-key power management company. The only specs printed on the case are 'Float 13.5 - 13.8V'. Interesting! It may be safe to say these are for UPS service.

There are two banks of 100Ah 12V; a single in the Trailer, and 4 in 2s2p for 24V 4800Whs in the van.

I am trying out LFP+AGM in the trailer. I want to abuse the LFP in a way that slow charges the AGM. This will be called tuition.

If this works out, I will add a 100Ah 24V string to the van giving 7200Whs. The van is my shore power source when dry camping. Even now, I can run the A/C overnight. This will be a 2:1 ratio of AGM:LFP.

My AGMs are inexpensive mightymax 110AH in a 4S2P configuration. For cycle use they recommend charging to 13.6. For standby 13.2. My LifePO4 are 1x big battery husky, with a 4s chins plus string too just to mix things up. I charge the whole mishmash to 54.4 then float it at 53.6

 

[LVLAaron]

Youtube suggested this for me, and I scrubbed through it. Guy has built a special device that:


Keeps batteries connected
Charges to where the lithium cells need to be
disconnects the lead from the lithium
continues to charge the lead to it's high point
waits for the lead to drop back down to the lifepo4 voltage
connects everything.

Pretty clever

 

[Horsefly]

Youtube suggested this for me, and I scrubbed through it. Guy has built a special device that:


Keeps batteries connected
Charges to where the lithium cells need to be
disconnects the lead from the lithium
continues to charge the lead to it's high point
waits for the lead to drop back down to the lifepo4 voltage
connects everything.

Pretty clever

Well, I will say that is kind of interesting. I'd have to think about it some more.

I think there will still be the mismatch in discharge profile when they are connected together, but this approach would at least allow you to get your LFP bank charged (mostly, but will no absorption time), disconnect, and finish charging the lead acid bank.

It sounds like this device isn't a commercial thing. I'm not sure I would want to buy something from a guy on YouTube with a boat.

 

[LVLAaron]

He only showed it for a second, but I'm guessing it's Arduino and some contactors.

 

[eabyrd]

He only showed it for a second, but I'm guessing it's Arduino and some contactors.

Appears he will be selling this solution. It’s interesting that he disconnects the LI given the charge voltage of lithium at 14.6 volts is well above 13.6 volt suggested voltage of my AGM. using tail current to control charging cycles is available on victron chargers today too, but sadly not on my MPP

 

[LVLAaron]

Update from the bench.


I really like having LiFePo4 and Lead connected for discharging. Seems to work totally fine, when heavy loads start I can see some power coming out of the lead cell, and then the system will quickly equalize the two batteries.


Charging is a problem however.

My LifePo4 pack is discharged at 12.74 volts. (I get 400AH between 14.1 and 12.74) - I started my AIMS 75a charger which is set to stop at 14.1 and started watching the batteries. 2-3 amps going to the lead cell, 45-50 going to the lithium.

The charger is throttling back amps because, I assume, it's seeing the 13.8 volts on the lead battery and starting to go into that constant voltage stage.

So that's kind of a bummer. In my situation, I'd be charging with a generator if my batteries are low, so I want them to charge as fast as possible...

Thoughts welcome.

 

[grizzzman]

Update from the bench.


I really like having LiFePo4 and Lead connected for discharging. Seems to work totally fine, when heavy loads start I can see some power coming out of the lead cell, and then the system will quickly equalize the two batteries.


Charging is a problem however.

My LifePo4 pack is discharged at 12.74 volts. (I get 400AH between 14.1 and 12.74) - I started my AIMS 75a charger which is set to stop at 14.1 and started watching the batteries. 2-3 amps going to the lead cell, 45-50 going to the lithium.

The charger is throttling back amps because, I assume, it's seeing the 13.8 volts on the lead battery and starting to go into that constant voltage stage.

So that's kind of a bummer. In my situation, I'd be charging with a generator if my batteries are low, so I want them to charge as fast as possible...

Thoughts welcome.

Your Aims charger is NOT throttling back the amps to your batteries as your batteries get near full the resistance increases in the batteries causing less amps at the voltage you have it set at.

 

[LVLAaron]

Your Aims charger is NOT throttling back the amps to your batteries as your batteries get near full the resistance increases in the batteries causing less amps at the voltage you have it set at.

Fair, and technically correct. Net-net is the same... Lead makes the charging slower.

 

[eabyrd]

Fair, and technically correct. Net-net is the same... Lead makes the charging slower.

They aren’t hindering the flow into you LI batteries, are they? I’ve never noticed that phenomena in mine. Yes, the pct of power going into each shifts as you climb up above 53 volts and the lead “fills”, but the total power supplied doesn’t drop off till you get to your absorb limit, at least mine doesn’t.

 

[LVLAaron]

They aren’t hindering the flow into you LI batteries, are they? I’ve never noticed that phenomena in mine. Yes, the pct of power going into each shifts as you climb up above 53 volts and the lead “fills”, but the total power supplied doesn’t drop off till you get to your absorb limit, at least mine doesn’t.

When both are connected, the Lead battery was pulling 2-3 amps, and the lifepo pulled 45-50. If I connect only the lifepo, I get the full 75 amps of the charger.

So maybe the lead is wasting 20 amps? I honestly think it's because the lead battery was close to top voltage so the charger slows down.

 

[eabyrd]

When both are connected, the Lead battery was pulling 2-3 amps, and the lifepo pulled 45-50. If I connect only the lifepo, I get the full 75 amps of the charger.

So maybe the lead is wasting 20 amps? I honestly think it's because the lead battery was close to top voltage so the charger slows down.

That is interesting, may I ask what you’re charging them with? I am using a MPP solar LVX 6048 right now, but I’ve also use a Victron 150/35. I didn’t see similar behavior in in either

 

[LVLAaron]

That is interesting, may I ask what you’re charging them with? I am using a MPP solar LVX 6048 right now, but I’ve also use a Victron 150/35. I didn’t see similar behavior in in either

AIMS 110v mains charger. Excellent unit that will uses and recommends.

 

[eabyrd]

AIMS 110v mains charger. Excellent unit that will uses and recommends.

No beef, just curious about the disparity in our banks’ behavior

 

[LVLAaron]

No beef was assumed.

 

[Marinepower]

I would like to start a new thread "LTO start battery added to a LFP bank." To me, these are two chemistries that are compatible as the LTO likes being in a partial state of charge. I recognize this is a new topic. No need for responses. Just musing....
'

 

[eabyrd]

No beef was assumed.

I am going to experiment with my setup to see if I can duplicate your limited flow issue. Can you give me a summary of how your batteries are wired? My 200ah AGM bank is 4s2p of 110 batteries wired into the 1 pole of a 0, 1, 2, 1+ 2 selector. My Lithium battery is currently a 100ah big battery husky, but I am prepping a 4s Chins 100ah bank to join it. They will share the 2 pole on the selector. i usually just keep the selector on 1 + 2 while charging