LFP vs LA/AGM bank size

[TBill36]

Reading the AGM battery charger thread got me thinking about battery bank size and differences between LFP and AGMs. Maybe this is common knowledge and I'm just behind the times.

Since the allowable DODs of LFPs is up to 95% and AGMs is 50% then the bank of LFPs can be significantly smaller and still deliver the same allowable amp hr consumption.

A 600 amp hr AGM bank give you 300 amp hrs, where as a 316 amp hr LFP bank gives you the same 300 usable.

Does it really work like that and is that real world experience?

 

[Don L]

That is common thinking. But it doesn't seem people really reduce their capacity. I went from 481ah of lead acid to 400ah LFP because that was same physical size. I still mostly operate in the 70-100% SOC range.

The thing that gets lost in that "useable" discussion is that I still had the 20-70% SOC range on the lead acid, I just didn't use it much. Nothing says you can not use more that 50% of an acid battery, just that doing so reduces it's life (I wonder if this is really true as I bet it is minor as long as you truly fully recharge).

So I feel the change for bank size still involves the same thinking: 1- planning around your normal useage 2 -planning for the abnormal times like 3 days of clouds when on solar

 

[mvweebles]

Also depends how you use the energy. Slow draw of 12vdc would get you the full 300ah of LFP capacity. But if you're using an inverter to power a microwave of something, you are likely to get an under-voltage shut down long before the 300AH theoretical DoD capacity.

If you're using AGMs and it's working for you, there's no reason to change. But most folks add a bunch of power consumers and thereby don't really extend their time between charges, just increase the usage and charge replishment.

I have 800w of solar and 600ah LFP. I thought I would be net-zero. Ha! Shelve that fantasy alongside losing weight.....

Peter

EDIT - Don L (post previous to this one) is one of the few people on TF who have long-term off-grid usage data on LFP (sadly, we lost another datapoing with passing of Simi60). I agree with his analysis, including skepticism of the often cited 50% DoD floor for AGM/FLA. Off grid cruising is really hard on batteries because they can go long periods without a complete charge which is also hard on batteries, LFP included. How hard? I don't know - rarely see anything empirical. Might not be optimal, but not end of world either.

 

[rslifkin]

The thing that gets lost in that "useable" discussion is that I still had the 20-70% SOC range on the lead acid, I just didn't use it much. Nothing says you can not use more that 50% of an acid battery, just that doing so reduces it's life (I wonder if this is really true as I bet it is minor as long as you truly fully recharge).

With lead, drawing it down gets you less cycles, but unless you go really low, not significantly less total power over the battery's lifespan. 50% is just a good compromise point in many applications, and going too low can lead to issues with excessive voltage sag under load depending on how big your loads are relative to the battery bank.

 

[DavidM]

Expanding on the post above, occasionally drawing down a FLA battery doesn’t significantly affect their life, ie 3-4 times a year down to 80% won’t result in appreciably lower life, but 50 times a year will.

So it all depends on your usage pattern, whether you need more amp hours or not and whether they need to be Li.

David

 

[SteveK]

Yes I went from 800+ FLA to 800 LFP. I started with 400 LFP, 1/2 the amount of usuable 800+ FLA and did not like that LFP was being asked to go to near 100% DOD with no wiggle room.

I had 860Ah FLA and it did not meet my needs discharging to 30% DOD, below long life threshold. The 400 LFP was being cycled down to below 100A, leaving not enough buffer.

Adding 400 accomplished a side benefit.
FLA needs to be fully recharged, LFP does not.
Now I can cycle daily needs of 400A anywhere within the 800Ah avialable. ie: use 400 but onlt recharge 200 to 600Ah and then use 400Ah etc etc

With FLA you need to fully recharge every time and that means 80% is full, use no more than down to 50% a window of 30%. Rinse and repeat.

LFP has benefits.

 

[guy with a boat]

DonL makes a good point, that AGM has the ability to use beyond the 50% SOC if needed. If you routinely use 90-100% of your LFP, you have almost no reserve capacity. Most of us that have converted to LFP have added more capacity for the reasons mentioned.

The biggest advantage to LFP for us (in addition to increased capacity) is that you can charge them very fast and whenever you want. We typically run our gen once or twice a day for other reasons (some 240v devices) and all the charging we need happens at the same times. Gen run times are usually about an hour unless we need/want AC. We have lots of charging capacity and can pour the amps into the bank when we have a gen running. We also have some solar that carries some of the load during daylight hours. Between solar and large alternators, we rarely “need” to run the gen just to charge batteries on our power-hungry boat.

For AGM/lead acid batts, having a high-output alternator and some solar panels can help tremendously with charging to 100% SOC, while avoiding the 5-hr gen run with minimal load.

We have large alternators with external regulators that limit the output to 84% of max and we typically arrive at our destination each day with the house bank fully charged, with no gen run time.

 

[Don L]

You know that this "only discharge to 50%" has been around since the days of operating batteries on boats between 50-75% because it was so hard before solar to charge higher. Lead acid battery manufacturers publish life "cycles" data for depth of discharge, but don't explain it really. You never really know if it mean they discharged to 50% then recharged to 75% and repeated, or if they recharged to 100%? You don't know what the rate of each discharge/charge was and how long each was held. You don't even know if when they say 3000 cycles if they in fact do 3000 or just 200 and expended out the line.

Now we have lithium batteries were they say the batteries are good for 10,000 plus cycles. Has anyone done a 10,000 cycle test?

Don't believe marketing claims at face value.

 

[Don L]

Also depends how you use the energy. Slow draw of 12vdc would get you the full 300ah of LFP capacity.

I am not really even sure this applies to lithium or if it is a hold over from acid. In acid you can get more AHs out of a battery than the labeled capacity if you do it slow. It is a proven scientific fact with a factor (PE) to go along with it that we plug into our battery monitors.

I have a PE of 1.05 plugged into my battery monitor for my LFPs. I recently let it discharge (slow use of mainly the freezer/refrigerator) down to 19.5% reading on the BM. But the voltage after suggested the math capacity of 15% that would have been a PE = 1.00 as correct.

I don't know. I feel it5 is a moving target and varies with a lot it "depends" just like acid batteries

 

[rslifkin]

I am not really even sure this applies to lithium or if it is a hold over from acid. In acid you can get more AHs out of a battery than the labeled capacity if you do it slow. It is a proven scientific fact with a factor (PE) to go along with it that we plug into our battery monitors.

I have a PE of 1.05 plugged into my battery monitor for my LFPs. I recently let it discharge (slow use of mainly the freezer/refrigerator) down to 19.5% reading on the BM. But the voltage after suggested the math capacity of 15% that would have been a PE = 1.00 as correct.

I don't know. I feel it5 is a moving target and varies with a lot it "depends" just like acid batteries

I agree, LFP has a Peukert exponent close to 1, so the rate of discharge has only a very small effect on the total power you get out before end of discharge. It's much more significant with lead-acid. My AGMs have a PE of 1.18, for example.

 

[tiltrider1]

I went from 1400 amp/h LA to 1000 amp/h LFP. I felt like this was comparable bank sizes.

There was not much science in my decision nor any past experience, ask me again in 10 years.

The decision was based off of cost, foot print size and the fact that LFP voltage has a much less significant drop over capacity consumed. In other words the last 25% of LFP is still at a reasonable voltage where the last 25% of LA is at an extremely low voltage.

 

[DDW]

Lead acid battery manufacturers publish life "cycles" data for depth of discharge, but don't explain it really. You never really know if it mean they discharged to 50% then recharged to 75% and repeated, or if they recharged to 100%?

For AGM at least, they mean recharged to 100%. Running AGMs between 50 and 75 will reduce their capacity pretty quickly in just a few cycles. Lots of data on this, and in manufacturer's literature. Lithium on the other hand will last longer if not recharged to 100% routinely, they like to be operated in the middle of the envelop, not at the corners. Again lots of literature on this, but the degradation in going to the corners is far slower than not recharging an AGM to 100%.

 

[Don L]

I have to wonder why we even still ponder these agm/fla/sla verse LFP questions anymore

A G31 Lifeline AGM on Amazon is $489, and Renegoy is $188, and meantime a LI-Time 100Ah "G31" LFP is $239

 

[tiltrider1]

Let’s be fair. LFP pricing only recently dropped so low and only recently has availability been so strong. Just 3 years ago pricing was very high and availability very low.

Five years from now we will be wondering why any one would buy LFP over Lithium-Sulfer.

 

[Delfin]

Reading the AGM battery charger thread got me thinking about battery bank size and differences between LFP and AGMs. Maybe this is common knowledge and I'm just behind the times.

Since the allowable DODs of LFPs is up to 95% and AGMs is 50% then the bank of LFPs can be significantly smaller and still deliver the same allowable amp hr consumption.

A 600 amp hr AGM bank give you 300 amp hrs, where as a 316 amp hr LFP bank gives you the same 300 usable.

Does it really work like that and is that real world experience?

I replaced 1280 Ah LA with 600 Ah LFP years ago. No problem at all.

 

[Bmarler]

Also depends how you use the energy. Slow draw of 12vdc would get you the full 300ah of LFP capacity. But if you're using an inverter to power a microwave of something, you are likely to get an under-voltage shut down long before the 300AH theoretical DoD capacity.

If you're using AGMs and it's working for you, there's no reason to change. But most folks add a bunch of power consumers and thereby don't really extend their time between charges, just increase the usage and charge replishment.

I have 800w of solar and 600ah LFP. I thought I would be net-zero. Ha! Shelve that fantasy alongside losing weight.....

Peter

EDIT - Don L (post previous to this one) is one of the few people on TF who have long-term off-grid usage data on LFP (sadly, we lost another datapoing with passing of Simi60). I agree with his analysis, including skepticism of the often cited 50% DoD floor for AGM/FLA. Off grid cruising is really hard on batteries because they can go long periods without a complete charge which is also hard on batteries, LFP included. How hard? I don't know - rarely see anything empirical. Might not be optimal, but not end of world either.

Peter, I’m surprised that 800 watts isn’t making your daily needs. Are you using more than expected or not producing as much as you thought?
For me, in the summer, when the sun is more overhead, I will be floating by 1:00 or so with .75 kw of solar. In winter, solar produces much less. Maybe 25% of what I need.

 

[rslifkin]

In decent weather, 820 watts keeps up with our needs, but with our current systems we still run the generator for hot water and stove use. If we get any significant cloudy or rainy weather (not a partly cloudy day, but a dark overcast day) then we're falling short on power. On good days, I've seen our panels harvest anywhere from 3 - 3.7 kwh of power.

Next time we're up for replacing house batteries I'm planning to swap the 415ah lead bank for something significantly bigger (and probably LFP) and also up-size the inverter a bit (either from 2kva to 3, or parallel a second 2kva). At that point I'll be able to handle stove and water heating from the inverter, but we'll need more solar. If for some reason I stay with a lead bank, it'll probably be in the ~850ah range due to weight and also what'll fit in the space I want to move the house batteries to. That same space will happily hold 3 of the 300ah Epoch LFPs, which are the current likely candidate if I go LFP (and nothing more attractive comes up between now and whenever I upgrade).

I figure if we want to comfortably keep up power-wise (including cooking and hot water) and have a bit of surplus (for less of a shortfall on poor solar days and more margin for recharging after) then I'll be looking to get us up to about 2kw of solar (which will require adding a hard top and should increase our daily production in good weather into the 7 - 9 kwh range). That should be enough that the generator will only be needed if the weather is bad for more than a day and we're not moving the boat, or if we need A/C or something.

 

[DavidM]

As noted in this thread, water heating is one need that isn’t served very well by solar electric power. It takes a lot of energy to heat 6 gallons of water (the capacity of most medium size boats water heater) and the solar panel’s efficiency in converting solar energy to electric energy is low, about 20%.

But why not heat your water directly by solar? A good friend who cruised full time for years on a limited budget installed one of these systems: https://heliatos-solar.myshopify.com/products/boat-solar-water-heating-kit

It had a solar panels with a water coil built in and a solar powered electric pump to circulate the hot water through an existing marine water heater. When the sun was up high enough the electric solar panel would start the pump circulating warm water and turn off when the sun got down too low.

It worked pretty well. The panel would heat the hot water up by 20-30 degrees by the end of the day. He would take a quick Navy shower before bed and the water temp would drop 10+ degrees and when the sun started heating the next day the water heater temp was still up by 10 degrees.

When this cycle stabilized after a few days, the water temp was about 100 F at the end of the day. The Heliatose single panel worked well for just him, but a couple would probably need at least two.

The existing electric element on the water heater would continue to work with his generator on cloudy days.

David

 

[ksanders]

As noted in this thread, water heating is one need that isn’t served very well by solar electric power. It takes a lot of energy to heat 6 gallons of water (the capacity of most medium size boats water heater) and the solar panel’s efficiency in converting solar energy to electric energy is low, about 20%.

But why not heat your water directly by solar? A good friend who cruised full time for years on a limited budget installed one of these systems: https://heliatos-solar.myshopify.com/products/boat-solar-water-heating-kit

It had a solar panels with a water coil built in and a solar powered electric pump to circulate the hot water through an existing marine water heater. When the sun was up high enough the electric solar panel would start the pump circulating warm water and turn off when the sun got down too low.

It worked pretty well. The panel would heat the hot water up by 20-30 degrees by the end of the day. He would take a quick Navy shower before bed and the water temp would drop 10+ degrees and when the sun started heating the next day the water heater temp was still up by 10 degrees.

When this cycle stabilized after a few days, the water temp was about 100 F at the end of the day. The Heliatose single panel worked well for just him, but a couple would probably need at least two.

The existing electric element on the water heater would continue to work with his generator on cloudy days.

David

Heating hot water is one thing, and very important.

Desalinating sea water and washing clothes are two others, and these are very real needs for a boat that is cruised long term vs a boat that returns to the dock every few days.

These needs are every day and it's almost impossible to have enough solar on board to meet them.

So... circling back to the original subject of this thread...

I need to run my generator every day anyway. My water needs require two hours of run time. Laundry can happen about the same time, as can heating water.

so, for my boat on the hook I need the batteries to last overnight, because I'll be running the generator in the morning anyway. After an initial recharge the solar will keep up with the loads until sunset, and that starts the discharge cycle.

If I do not run the generator in the evening my boat will use around 500 amp hours from sunset to sunrise. I have currently a 840 amp hour bank of FLA batteries.

At my next battery changeout cycle I'll probably switch to LiFePo. Based on my real loads I'll go with a minimum of 600AH and preferably a bit more depending on the actual batteries I choose, their footprint, and cost.

 

[SteveK]

If I do not run the generator in the evening my boat will use around 500 amp hours from sunset to sunrise. I have currently a 840 amp hour bank of FLA batteries.

What is your use sunrise to sunset? Another 500Ah? Do you need to run gen twice a day or does your solar cover daytime use needs.

 

[Don L]

Wow this is eye opening to a sailboater. I use around 150-200 ah/day, which is almost twice what it was 7 years ago.

As another data point, in the Bahamas or southern waters, where the water in our tanks is probably in the 70s, it takes 15 minutes to warm up our 5-gal tank enough for 2 showers. I will use the generator if need, but will sometimes use the inverter. I had to rewire my AC panel to allow the inverter to power the water heater, it takes about 25ah to do 2 showers.

 

[mvweebles]

Wow this is eye opening to a sailboater. I use around 150-200 ah/day, which is almost twice what it was 7 years ago.

As another data point, in the Bahamas or southern waters, where the water in our tanks is probably in the 70s, it takes 15 minutes to warm up our 5-gal tank enough for 2 showers. I will use the generator if need, but will sometimes use the inverter. I had to rewire my AC panel to allow the inverter to power the water heater, it takes about 25ah to do 2 showers.

I now have 4 weeks away from a dock or marina, though only recently got my alternator system running correctly. I have 600ah of LFP plus 800w solar. My energy hogs are two small fridges and a Domestic 45 quart freezer. Right now, my energy budget is around 400 ah per day at anchor, but that includes shutting down inverter at night or when we're away from the boat. Seems inverter standby and Starlink run around 3 amps. It's also pretty warm down here - mid 80s during the day, mid/high 60s at night.

My solar has been generating around 2.5 - 3.0 kwh/day which is a bit less than I expected, maybe 60% of my usage. Not sure if it's because it's winter or maybe the panels are t great to be fine with. So they generate around half my at-anchor consumption. Every 2 - 3 days or so either we move or run the generator. Also run watermaker.

Water in the tanks remains fairly warm. I need to replace the heater element in my water heater but hasn't been high on the list since it gets dang hot with engine running.

But I'm with you Don. I don't understand how most powerboats manage or even bother with much solar. Our stateroom is a V-berth so ventilation at anchor is excellent (plus I added opening ports and installed a larger overhead hatch). I think KSanders post where he just wants to get through the night is probably as much as can be hoped for on a cruising powerboat with ice maker, multiple fridges, chest freezer, entertainment center, etc.

Peter

 

[rslifkin]

To some extent power budgets are based on what you can power. If you can fit a ton of solar, your power usage will likely be higher, as the power is there and you don't feel as much need to skimp and save.

On my boat, the current 2x 410 watt panels sit on the cabin top forward of the windshields. There's very little shading concern and the space isn't otherwise useful. Once I hard top the aft deck where the helm is, fitting another 1200 watts or so will be easy. So space for solar isn't really limiting in my case.

For perspective on what we're powering, we have a 9 cubic foot fridge/freezer, an ice maker, cellular internet, lights, etc. Inverter runs 24/7. No water maker or laundry in our case, but we're still not a particularly low power boat. At anchor I also leave a chartplotter on 24/7 (in standby at night) and the VHFs are on when we're awake. I'd put our total daily power usage including what comes from the generator at about 6 - 7 kwh / day.

Adding more equipment for extended off grid cruising would increase power demands further, but realistically on this boat we'll start running out of places for additional stuff before we run out of room for added solar to power it.

 

[ksanders]

What is your use sunrise to sunset? Another 500Ah? Do you need to run gen twice a day or does your solar cover daytime use needs.

It's about the same day and night.

Solar pretty well keeps up during the day. Some days it's a bit behind, some days it's a bit more.

I have four Remology 175W flexi panels on the pilothouse roof, connected to two MPPT, one port, one starboard.

 

[SteveK]

I need to replace the heater element in my water heater but hasn't been high on the list since it gets dang hot with engine running.

Peter, I have thought about changing element to the 12V ones to save that bit of conversion power. But not before one fails, enogh to do stuff without making work.

 

[Bmarler]

Wow this is eye opening to a sailboater. I use around 150-200 ah/day, which is almost twice what it was 7 years ago.

As another data point, in the Bahamas or southern waters, where the water in our tanks is probably in the 70s, it takes 15 minutes to warm up our 5-gal tank enough for 2 showers. I will use the generator if need, but will sometimes use the inverter. I had to rewire my AC panel to allow the inverter to power the water heater, it takes about 25ah to do 2 showers.

Kevin’s usage represents the high end of power draw for boats in our size category. He doesn’t appear to try to conserve power at all, just make sure he’s got enough to supply his needs and be replenished with equipment on hand.
I use probably half of that, maybe less. If I use conservation techniques I can really stretch it out. I typically leave the inverter on24/7, and have a fairly large fridge and medium sized chest freezer. My parasitic load is around 9-11 amps. Before I added solar I could sit at anchor for two full days before needing to recharge.

Peter, I’m also surprised at your solar output. Sun angle makes a huge difference though. Let’s compare when the season changes and it’s more overhead.
Is you output inline with other boats in your area?

 

[mvweebles]

Kevin’s usage represents the high end of power draw for boats in our size category. He doesn’t appear to try to conserve power at all, just make sure he’s got enough to supply his needs and be replenished with equipment on hand.
I use probably half of that, maybe less. If I use conservation techniques I can really stretch it out. I typically leave the inverter on24/7, and have a fairly large fridge and medium sized chest freezer. My parasitic load is around 9-11 amps. Before I added solar I could sit at anchor for two full days before needing to recharge.

Peter, I’m also surprised at your solar output. Sun angle makes a huge difference though. Let’s compare when the season changes and it’s more overhead.
Is you output inline with other boats in your area?

My panels are skewed a bit due to arch of hardtop. One side puts our more than the other depending on how I'm anchored or moored. Other installations vary widely, mostly sailboats with shading issues. And frankly, I suspect solar output and energy conservation is fudged as much as MPG claims. My take on Ksanders statements was he was being honest about his usage, but I also think his use profile is much more common for powerboaters, even cruising boats. In warmer climates, vast majority of powerboaters run a generator at least a few hours a day if for no other reason than the need AC because their stateroom is in the back of the boat.

For me, there has been a big difference between being off-grid for a few days vs several weeks (or perpetually as Don L often is on his sailboat). My wife asked me the other day "how come nothing broke when we sea-trialed to San Diego and back?" (12 hours each way). In my opinion, we just didn't stress the systems enough. Full time usage coupled with warmer climate of southern Baja against boats designed for tepid California and PNW climate, engine room temperatures well past 100 degrees, drawing down battery banks to 20% or lower, and suddenly inadequacies that were masked by relatively light or short duration usage are penetrated.

Peter

 

[Don L]

Wow when on anchorage that is my active load when everything cycles on. That works out to only 5ah/hour on average for the freezer, refrigeration, some LED interior lights, some fans, the stereo, and the anchor light.

When I turn on my Starlink on it's own 500W inverter that uses 3-5 AH/hour and when I turn on the TV and sound bar on the same it uses about 5ah/hr.

It isn't completion. I think people with big generator and boats with enough fuel tankage to run it a month, that has electric stoves etc. just don't really think about power use ............................. till the generator breaks down.

Meanwhile if I just turn my main inverter on it is 5 amp draw.

 

[rslifkin]

Meanwhile if I just turn my main inverter on it is 5 amp draw.

That's a big parasitic load. The idle draw for my 2kva Victron is just under 1 amp. Low idle draw was one of my selection criteria when I put the inverter in, as I knew I wanted to run it 24/7 when away from shore power.

 

[Bmarler]

Wow when on anchorage that is my active load when everything cycles on. That works out to only 5ah/hour on average for the freezer, refrigeration, some LED interior lights, some fans, the stereo, and the anchor light.

When I turn on my Starlink on it's own 500W inverter that uses 3-5 AH/hour and when I turn on the TV and sound bar on the same it uses about 5ah/hr.

It isn't completion. I think people with big generator and boats with enough fuel tankage to run it a month, that has electric stoves etc. just don't really think about power use ............................. till the generator breaks down.

Meanwhile if I just turn my main inverter on it is 5 amp draw.

Yeah, it’s a far cry from my last sailboat. I used about 45 amp hours per 24 hour period. I was constantly monitoring the amp draw for every .05 amp draw. Took me a while to come to grips with it after so many years of power conservation.