LiFePo4 addition to a SLA House battery?
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LVLAaron
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My two batteries were connected and charged to 14.1 - Charger disconnected.
I saw ~1.5 amps going to the lead battery for maybe an hour... and then .5 amps until the lifepo4 pack reached 13.3. No power movement at this time. I will let it sit for 24 hours and monitor the BMS to see how much paracsitic draw the lead has on the lifepo4 pack.
I saw ~1.5 amps going to the lead battery for maybe an hour... and then .5 amps until the lifepo4 pack reached 13.3. No power movement at this time. I will let it sit for 24 hours and monitor the BMS to see how much paracsitic draw the lead has on the lifepo4 pack.
Horsefly
Solar Wizard
I'm still mostly convinced that putting lead acid directly in parallel with LFP is a really bad idea.
Rule B: The best way to get a good life out of LFP is to treat it right: Stay between the knees. Charge to only 3.45V-3.5V per cell bulk / absorption, and the tail current will drop to near 0 very quickly. Discharge only to 3.0V. If the BMS cuts out, you've got something set wrong. Don't do any temperature compensation in charging.
You cannot do both rule A and rule B, ergo if you put lead acid in parallel with LFP you are choosing to sacrifice one, the other, or both.
- My AGM batteries should to be charged (bulk / absorption) at 14.6V @ 25°C. That's a full 3.65V per cell of LFP. It is virtually certain that the BMS will cut out due to high voltage on at least one cell every time. My desire is to charge my LFP to only about 3.5V per cell.
- AGM charging has to be temperature compensated, so if the temperature is below 25°C, the bulk / absorption voltage will be higher than 14.6V (i.e., more than 3.65V per cell).
- AGM batteries like to stay in absorption for a long time, until the tail current gets down to 1%-2% of the 20hr C rating. Per #1 above, the LFP BMS will have cut out with a HVD several times during that period.
- If you choose to not charge as high as the 14.6V, your AGM battery will sulfate over time and will be a doorstop. A person who sells thousands of AGM batteries every year has told me that the #1 cause of AGMs dying an early death is that people don't fully charge them by staying at absorption voltage until the tail current drops to 1% of C.
- The lead acid discharge curve is somewhat steep, while the LFP discharge curve is very flat. So as current is pulled from both batteries, eventually the LFP is mostly sending current over to the AGM. That's lots of energy moving around not doing anything useful for you.
Rule B: The best way to get a good life out of LFP is to treat it right: Stay between the knees. Charge to only 3.45V-3.5V per cell bulk / absorption, and the tail current will drop to near 0 very quickly. Discharge only to 3.0V. If the BMS cuts out, you've got something set wrong. Don't do any temperature compensation in charging.
You cannot do both rule A and rule B, ergo if you put lead acid in parallel with LFP you are choosing to sacrifice one, the other, or both.
LVLAaron
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Good info, and there's two scenarios I see people doing here.
1. Someone who has a bunch of lead that's still got some life left, but wants to use lithium. This is the camp I'm in. I have a camper with 2x 12v batteries, but they're not enough. I also don't want to throw them away or deal with trying to sell them for a few bucks.
2. Someone who's charging from an alternator, and wants to keep the lead in place to prevent damaging the alt. In that case, a cheap crap motorcycle battery is adequate.
1. Someone who has a bunch of lead that's still got some life left, but wants to use lithium. This is the camp I'm in. I have a camper with 2x 12v batteries, but they're not enough. I also don't want to throw them away or deal with trying to sell them for a few bucks.
2. Someone who's charging from an alternator, and wants to keep the lead in place to prevent damaging the alt. In that case, a cheap crap motorcycle battery is adequate.
Horsefly
Solar Wizard
Yeah, I get that. I'm against putting LFP in parallel with LA, but I am also sympathetic.
I'm currently working a 42' sailboat transition to LFP for the house battery. We've decided to keep AGMs for a separate bank on the bow thruster and windlass, with a DC-to-DC charger between the two. The DC-to-DC charger acts as a load in the event that the LFP BMS shuts down, so the alternator (connected to the LFP) will not freewheel without a load. So there's a good use of LA in conjunction with LFP: A separate bank!
As for your camp, I think that is probably OK. Just be aware and watch your BMS carefully. It would be good to watch (via clamp meter) the current between the LFP and LA, but that may be overkill.
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shadowsteve
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So this is all about using up the last bit of life in an existing LA battery bank?
I chose to pull out my 4x 6v AGMs in my 5th wheel and replace them with LFP as they were 6 years old and down to about 80% capacity. I sold them and recouped about 20% of my initial cost on them. Saved about 170# of weight too
I chose to pull out my 4x 6v AGMs in my 5th wheel and replace them with LFP as they were 6 years old and down to about 80% capacity. I sold them and recouped about 20% of my initial cost on them. Saved about 170# of weight too
wattmatters
Solar Wizard
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.
Horsefly
Solar Wizard
If that's really what the AGM manufacturer recommends, then maybe you are right. Mine said 29.2V for my 24V bank, so 3.65V per cell LFP. 56.4V is only 14.1V @ 12V, which sounds closer to float for an AGM than bulk / absorption. Trojan says 57.6V (3.6V/cell LFP equiv), which is about as low as I can recall seeing. What brand are your AGMs?
You still have to deal with the flat LFP vs steep AGM discharge curve. As long as you know what you are getting in to, go for it. It is your choice.
wattmatters
Solar Wizard
Enersys Powersafe SBS190F
wattmatters
Solar Wizard
Well I'm not doing anything but the way the voltages work it would seem to be quite viable option to consider at some stage.
I was suggesting that the BMS could use High Voltage Disconnect(HVD) during the eq cycle of the lead. No extra charge profile needed.
12VoltInstalls
life passes by too quickly to not live in freedom
My thoughts exactly.
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.
Horsefly
Solar Wizard
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.
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.
Horsefly
Solar Wizard
First off, if you are talking about equalization you must be talking about flooded lead acid, not AGM. You do not want to equalize AGM batteries.
Second, I would never recommend planning for an HVD or LVD from a BMS as a normal course of using the LFP bank. The LVD and HVD are meant as a last line of defense for your LFP cells. But, to each his own.
eabyrd
Solar Enthusiast
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
Solar Wizard
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
Solar Wizard
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:
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
Solar Enthusiast
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
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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
Solar Wizard
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.
eabyrd
Solar Enthusiast
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
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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.
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.
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