LiFePo4 addition to a SLA House battery?

plaidcabin

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Stumbled upon this gentlemans approach to mixing battery technologies, and gotta say, I like the sound of this.


I am running 100% Lithium right now, but my brother has a large Lead Acid system and wants to add some Lithium Iron Phosphates to his, using this method. He would be building his batteries using 80AH cells and equipping each with a BMS as well.

Any of the intelligent people here see any reason this wouldn't work, or care to comment on why it is a bad idea?
 

eabyrd

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I saw those videos and figured it couldn’t hurt since my SLA weren’t at end of life. Right now I have 200AH SLA and 100 AH LifePO4 cycling between 56.4 and 49 volts. It has only been a few months, but they both seem happy, and play nicely. Most of the time it’s more like 54 to 49 since clouds get in my way. I am going to double the Lithium shortly

yes I know I’m not utilizing the full curve, but eveything I hear says 90 /10 or even 80 / 20 is preferred
 

plaidcabin

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Absolutely, the closer your range remains to 50% on these batteries, the longer they last. But Lithiums doing the daily grind while the SLA's trickle charge and sit ready for stormy weather is a pretty awesome combination, I'm tempted to put some into mine as well...
 

nonoceb

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there is absolutely no problem with mixing SLA and lifepo4 but you will need some smart logic to cycle both battery in their needs.
You can setup bms to disconnect lifepo4 at 14V to charge SLA up to 14.4V
For the discharge periode you can start to discharge first SLA battery to a 40-60% Soc then connect lifepo4 battery, his voltage at 13.3V will charge slowly your SLA battery and finish the night.
I think for this smart logic you will need arduino and cheating with one cell voltage sensing of your bms to connect disconnect lifepo4 battery
 

Pim57

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Me too was inspired by this video. The video made me confident it could be done!

I have 200 AH FLA and 100 AH LiFePO4 on my boat since last April. I have the 4S 100A smart Daly bms that will disconnect charging at 3.400 Volts per cell so 13.6 V in total .

Because the Daly bms has separate MOSFET’s for charging and discharging, there is no problem with reconnecting both FLA and LFP when the overall voltage drops under 4 x 3.400 = 13.6 Volt.

The 200 AH FLA trickle charge creates a constant discharge of 0.5 amps or 12 AH per day. You need solar on your boat to compensate for this.

My fridge (biggest consumer) takes 40-50 AH per day.
My solar generates 80 AH per day.
During sunshine the fridge runs on solar and during nighttime or clouds on LFP.

This makes the LFP cycle between 30-80 % SOC when we’re on the boat.
When LFP is full and charging from solar or alternator is available, the voltage rises and the FLA gets charged to 14.4 Volt
FLA stays on trickle charge the rest of time.
FLA gets only discharged when the LFP gets depleted during multiple clouded days and no alternator running.

To protect the alternator from overheating, I put a resistor of 0.033 ohms (3 parallel 0.1 ohm 25 Watt on a heatsink) between the FLA and LFP.
This limits the current from the alternator to (14.4-13.2)/0.033=36 amps when the LFP is depleted and 24 amps when the LFP is close to 13.6 Volts.

The resistor limits the current to (13.6-12.8)/0.033=24 amps before the FLA kicks in when discharging.
My fridge takes 5 amps and works therefore okay(fully on LFP or solar).

Later I lowered the resistor to 0.020 ohms, because there was already some resistance in the cabling that made the total resistance of 0.033 ohms.
 
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eabyrd

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there is absolutely no problem with mixing SLA and lifepo4 but you will need some smart logic to cycle both battery in their needs.
You can setup bms to disconnect lifepo4 at 14V to charge SLA up to 14.4V
For the discharge periode you can start to discharge first SLA battery to a 40-60% Soc then connect lifepo4 battery, his voltage at 13.3V will charge slowly your SLA battery and finish the night.
I think for this smart logic you will need arduino and cheating with one cell voltage sensing of your bms to connect disconnect lifepo4 battery
From my observations it isn’t that complex. Charging the whole bank to 56.4 once in a while, generally to somewhere between 54 and 55, and then letting it settle and maintain a trickle (about 30 watts) on the SLA seems to be keeping them perfectly happy.
 

nonoceb

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From my observations it isn’t that complex. Charging the whole bank to 56.4 once in a while, generally to somewhere between 54 and 55, and then letting it settle and maintain a trickle (about 30 watts) on the SLA seems to be keeping them perfectly happy.
offcourse but you will never cycle your sla battery without discharging a lot your lifepo4 battery and the goal is to use sla battery up to his end of life to remplace it by lifepo4 efficient battery. trickle charge is a loss for something you don't use everyday, non sense for me
 

eabyrd

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Not at all. Goal is to hold SLA in reserve only cycling it when conditions overrun the capacity of the LifePO4. Let’s you scale the expensive LifePO4 to an 80% need while keeping the SLA in service and out of the landfill. The SLA draws .25 amp or so. It’s not a material parasite
 

grizzzman

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I have been running a hybrid FLA-LFP for several years now. The system is simply tied together while in use. In my system the FLA bank "load assists" during higher loads. As an example in the morning drawing 80 amps for coffee, The FLA starts out supplying 20 amps and over 12 minute brew time she surface charge depletes, depending on the LFP's SOC the LFP supply's the full 80 amps. For the most part the FLA bank just gets trickle charged. And acts as a UPS.
 

nonoceb

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By experience, i often see sla battery died or capacity fade after long floating period (many month) without cycling just on ac charger at 13.3V.
I don't know exactly why but I can't recommend using sla battery without cycling once a week.
 

eabyrd

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By experience, i often see sla battery died or capacity fade after long floating period (many month) without cycling just on ac charger at 13.3V.
I don't know exactly why but I can't recommend using sla battery without cycling once a week.
Which is where the 20% part of the 80 / 20 rule comes in. On occasion a draw down to 75%, or even 65% occurs while the LFP hovers at 2 or 3%.
 

grizzzman

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By experience, i often see sla battery died or capacity fade after long floating period (many month) without cycling just on ac charger at 13.3V.
I don't know exactly why but I can't recommend using sla battery without cycling once a week.
I get around this by disconnecting and allowing the FLA to charge on the factory converter.
 

eabyrd

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I get around this by disconnecting and allowing the FLA to charge on the factory converter.
You have more energy than I do. probably in all the ways that could be taken, but specifically my current LifePO4 isn’t up to the task fairly often so I regularly dip my SLA somewhat
 

LVLAaron

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That's super interesting. I have a new camper with some 12v lead batteries. I have a lithium pack that's much higher capacity and want to at least get some life out of the lead batteries. I think I'm going to try this!
 

Substrate

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In the wheelchair world, this is known as an "add on" pack of LFP.

Usually mounted on the back of the chair, or in a large backpack. The LFP bank gets worked hard first and is not designed to hold all the capacity needed, but give either extended range to an existing lead-acid or even larger custom LFP bank install underneath.

Depending on your setup, it may be advantageous to charge each chemistry with it's own charging infrastructure, but seems possible to hang it all on one system if you line the specs up.
 

gelmjw

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That's super interesting. I have a new camper with some 12v lead batteries. I have a lithium pack that's much higher capacity and want to at least get some life out of the lead batteries. I think I'm going to try this!
I am new at this but here is my understanding. You will not likely touch your lead until you use up your days of autonomy on the lithium. You will be hauling lead!

OTOH, I am wanting to double my Ahs by adding a matching Ah of LFP to an existing AGM bank. I believe I will be extending the life of the AGM because I will not be so far down in DOD. I need the extra Ahs anyway because my days of autonomy are in the genny's gas tank.

Could you split out a smaller bank of the lithium to add to the lead? There must be some rule of thumb ratio that would let you 'at least get some life out of the lead batteries' without having them last forever.
 

eabyrd

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That’s what I did, I added 100ah of L to 200ah SLA. Don’t know any rule of thumb however. I follow 80 / 20 so the SLA doesn’t always sit fully charged, but is never deeply discharged. I started with 2 year old SLA batteries, but they still seemed quite solid. Couldn’t bear to toss em
 

gelmjw

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Which is where the 20% part of the 80 / 20 rule comes in. On occasion a draw down to 75%, or even 65% occurs while the LFP hovers at 2 or 3%.
Equalization might be the solution. In this case I am meaning to stir the acid to remove acid stratification. This type of equalizing raises the Voltage high enough to excite bubbling at the plates. The rising bubbles stir the solution. The process can be completed in 15 minutes. The LFP could enter HVD and then HVR automatically.

There is no need to draw down the lead battery at all.

IMHO, and YMMV.
 

Substrate

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Aye, there's the rub. Do not use the LFP addition as a band-aid for not keeping your lead-acid fully charged!

That's an expensive band-aid. :) Make sure your charging regimen is proper for your SLA, otherwise consider total replacement with LFP.
 

eabyrd

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Equalization might be the solution. In this case I am meaning to stir the acid to remove acid stratification. This type of equalizing raises the Voltage high enough to excite bubbling at the plates. The rising bubbles stir the solution. The process can be completed in 15 minutes. The LFP could enter HVD and then HVR automatically.

There is no need to draw down the lead battery at all.

IMHO, and YMMV.
Yeah, but then you need a charge profile that includes an eq cycle which means separating the SLA from the LI. Just using them lightly one in a while is plenty
 
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