LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

[T1 Terry]

Originally Posted by Maine Sail

I have a BMS with HVC relay for the regulator. I also have a Junsi ordered but more for visual and tracking.. Alternator is charging direct to the house bank but the HVC relay can cut it. I don't want to get to the HVE point though and would prefer to keep the voltages out of HVC cut range.

It depends on how you plan to run the charging alternator. if you only want to run it to refill the batteries, set max voltage to 14v and turn the charging system off when that is reached, the absorption could be set at 13.6v and float at the same, these will only come into play if you forgot to turn the charging system off, the active BMS units will do a bit of balancing in needed.
If you will need the engine running for a long period so battery charging is only a by product, set max voltage to 13.8v, absoption to 13.6v and float to 13.4v, the alternator will power any loads while it's running leaving the batteries fully charged or close to it when the charging is turned off.

T1 Terry

[ebaugh]

Originally Posted by deckofficer

I agree. On the last draw that I got the 165 a-hr figure when cell voltages were 3.14, 3.02, 3.12, 3.16, so I stopped the draw down due to the 3.02 volt cell.

What do you think it should be at a 3.7 amp draw? Maybe 180 a-hr?

I don't know exactly. See the attached chart. Even these show >100% at large current draws. But even at your 3.7 amp draw, I would not expect more than 130% of the 3C discharge rate.

[deckofficer]

Originally Posted by ebaugh

I don't know exactly. See the attached chart. Even these show >100% at large current draws. But even at your 3.7 amp draw, I would not expect more than 130% of the 3C discharge rate.

These charts for a person like me who is so accustomed to how lead acid batteries get their a-hr rating at 10 hour rate (0.1 C) and 20 hour rate (0.05 C), is just amazing.

I've just cruised the top golf cart battery sites and they all rate their batteries at the 20 hour rate and the best is 234 a-hr at this rate. At just 0.3C rate this lead acid can only deliver 155 a-hr, what a rip off compared to how our cells perform under load. Our cells will deliver at least their rating in a 1.0 C load, while LA can't even muster 1/2 their rating under this same load.

Odyssey AGM, the best I've used because it could handle very high discharge and could be charged at 1.0C instead of the normal 0.1 C, if hit with a 17C load, the capacity of a 100 a-hr battery drops to 9 a-hr. Good thing it depletes in 20 seconds at that load because at those amps I wouldn't think the internal cell connects would last, just melt.

Lead is Dead.

[T1 Terry]

Keep in mind when viewing any of these battery capacity charts, where the end voltage is. If you want to stop the discharge at 3v per cell you need to draw the line across at that point. Looking at the GBS chart, a 3C discharge will only produce 50% of it's capacity before the cell voltage drops below the 3v mark, if you drop the cut off point to 2.8v, the rest of the capacity is available.
Here is a similar chart for the Winston cells. The different cell chemistry appears to produce different discharge curves

T1 Terry

[ebaugh]

Originally Posted by T1 Terry

Keep in mind when viewing any of these battery capacity charts, where the end voltage is. If you want to stop the discharge at 3v per cell you need to draw the line across at that point. Looking at the GBS chart, a 3C discharge will only produce 50% of it's capacity before the cell voltage drops below the 3v mark, if you drop the cut off point to 2.8v, the rest of the capacity is available.
Here is a similar chart for the Winston cells. The different cell chemistry appears to produce different discharge curves

T1 Terry

No argument at 3C rate voltage comparisons, but we don't see that in this application. And the difference moderates at lower rates.

Both charts show somewhat similar capacity differences between 3C and .5C rates, all of which are higher than our normal discharge rates. But it look like there is a point of diminishing returns where bank capacity stops increasing dramatically at lower discharge rates.

[Maine Sail]

Being that it is winter I am really getting a good chance to do some simulations of our on-water use. We only use about 17-30 Ah's per day (engine driven refrigeration) but I often supplement with the DC refrigeration when I don't want to run the motor in a quiet anchorage. This extends our silent time.

In those scenarios we can use 40-50Ah's per day (excluding the 140W panel which contributes about 20-25Ah's).

Out of curiosity I ran a 10.6A load for the last 5 hours and removed about 53.5 Ah's from the bank. At the end of 5 hours at 10.6A the bank was still at 13.25V and the cells perfectly balanced. Essentially at these tiny loads the battery voltage simply does not move.. This is a huge adjustment from LA.....

[Lagoon4us]

400 Amps of Lithium = How many Amps equivalent in Lead Acid?

Are LA's 60%?

[T1 Terry]

Originally Posted by Lagoon4us

400 Amps of Lithium = How many Amps equivalent in Lead Acid?

Are LA's 60%?

That one depends on where you set your max discharge point. If you use 12v unload, double the useable capacity to what you would expect to get from any lead acid batteries. The other side of the coin is recharging, the lithium ferrous battery will accept 100% of the charging current till 95% SOC, if the charge current is less that 0.1C (40amps for a 400Ag battery) right up to 99% SOC. A lead acid battery will only accept around 0.25C max without the terminal voltage climbing and the charge rate must be tapered after 80% SOC to stop overvoltage and thermal run away in AGM batteries.
With 400Ah of lithium you are unlikely to exceed 0.5C discharge rate, but if you did, the difference becomes even greater as the full 400Ah is available if needed at much higher discharge rates, something Bob was mentioning earlier, you can't get the full 50% capacity from lead acid batteries if you exceed the C20 rate (capacity divided by 20 expressed as amps) 20 amps for a 400Ah lead acid battery pack.
It's not until you actually start using them do you realise just how different and advanced these batteries are compared to lead acids batteries. The real bonus is, no sulphating if not fully recharged, short generator runs while performinf some other task is all that is often needed to top up the battery a bit, it doesn't need to be fully recharged each time, it's happy if it stays above 20% SOC, but it will keep on giving all the way down to 0% SOC if you really need it.

T1 Terry

[Lagoon4us]

Am i correct in thinking then that there's quite substantial and ongoing savings in the energy spent to charge a bank of Lithium and the time involved compared to LA's?

So savings are not just in the longevity of the bank?

[diugo]

The general consensus is that LFPs are 95% efficient, so if you take 100Ah out of a cell, you need only put 105Ah back to restore it to the same SoC. Subject to Peukert adjustment, of course.

Lead acids, by comparison, are only 80% efficient. Plus they require an often lengthy absorption phase, whereas LFPs do not.

[deckofficer]

Originally Posted by Lagoon4us

Am i correct in thinking then that there's quite substantial and ongoing savings in the energy spent to charge a bank of Lithium and the time involved compared to LA's?

So savings are not just in the longevity of the bank?

Terry put it quite well. We all knew going in that the cycle life made these cells attractive over lead acid. All the other benefits combine to make it even a better bargain. First savings in usable a-hr for both DOD (50% vs 80%) and the realized fact that at the same discharge rate, a 400 a-hr LA might match the output of a 200 a-hr LiFePO4. Then add to the mix when putting electrons back into our cells, a higher % of energy harvested (solar, wind, gen set) flows into them than LA. The savings come in many forms, and their light weight also.

[senormechanico]

Originally Posted by deckofficer

These charts for a person like me who is so accustomed to how lead acid batteries get their a-hr rating at 10 hour rate (0.1 C) and 20 hour rate (0.05 C), is just amazing.

I've just cruised the top golf cart battery sites and they all rate their batteries at the 20 hour rate and the best is 234 a-hr at this rate. At just 0.3C rate this lead acid can only deliver 155 a-hr, what a rip off compared to how our cells perform under load. Our cells will deliver at least their rating in a 1.0 C load, while LA can't even muster 1/2 their rating under this same load.

Odyssey AGM, the best I've used because it could handle very high discharge and could be charged at 1.0C instead of the normal 0.1 C, if hit with a 17C load, the capacity of a 100 a-hr battery drops to 9 a-hr. Good thing it depletes in 20 seconds at that load because at those amps I wouldn't think the internal cell connects would last, just melt.

Lead is Dead.

A true believer ! Welcome to the club !
Not to make this about mono vs. multi, but LEAD IS DEAD in other ways too.

[deckofficer]

Originally Posted by senormechanico

A true believer ! Welcome to the club !
Not to make this about mono vs. multi, but LEAD IS DEAD in other ways too.

Do we have a secret club handshake? Or just more coin left in the cruising kitty as time marches on?

[Lagoon4us]

Below is the charge recommendation, 4volts is a bit scary when you look at the graphs and consider what Terry says about the speed that last .25 volt comes on.

FAQ: 3.2V LFP cell initial charging
Question: How should new cells 3.2V 40AH, 60AH, 90AH, 100 AH, etc. be formed before first usage?
Answer: The new 3.2V LFP cells delivered from the warehouse are partially charged. However before the first use, it is essential to charge each cell to full capacity. The initial charging should be done with the charging current set to less than 1C (typically 0.5C), till the voltage level of 4.0 V is reached.
There is no other need for LFP 3.2V cells to be formatted or otherwise specially prepared for the use. After the first charge to full, the cell is ready to be used for use.
The above charging rule is also applied to the initial charge after the cell has been unused or stored for a long time. If not used more than 1 month, it is recommended to make again an initial charge to full.
Important: if you plan not to use the cells for a long time, before storing you must charge cells to full. In the case of storage longer than 1 year, it is required to recharge each cell after 12 months to full.

[T1 Terry]

Originally Posted by Lagoon4us

Below is the charge recommendation, 4volts is a bit scary when you look at the graphs and consider what Terry says about the speed that last .25 volt comes on.

FAQ: 3.2V LFP cell initial charging
Question: How should new cells 3.2V 40AH, 60AH, 90AH, 100 AH, etc. be formed before first usage?
Answer: The new 3.2V LFP cells delivered from the warehouse are partially charged. However before the first use, it is essential to charge each cell to full capacity. The initial charging should be done with the charging current set to less than 1C (typically 0.5C), till the voltage level of 4.0 V is reached.
There is no other need for LFP 3.2V cells to be formatted or otherwise specially prepared for the use. After the first charge to full, the cell is ready to be used for use.
The above charging rule is also applied to the initial charge after the cell has been unused or stored for a long time. If not used more than 1 month, it is recommended to make again an initial charge to full.
Important: if you plan not to use the cells for a long time, before storing you must charge cells to full. In the case of storage longer than 1 year,
it is required to recharge each cell after 12 months to full.

Where did that come from? It's completely against all I've learnt from cell manufacturers and testing so far.
As far as the 4v per cell, this is known as a conditioning charge, not suitable for traction batteries where high current demains are required as it will result is serious voltage drop at high discharge rates, but it makes the cells last a lot longer as it forms an even coating on the cathode and resists dendrite build up as it stops isolated attachment, there just isn't enough materials in the plates to build an even layer of dendrite large enough to short through the separator to the anode. The coating is a result of partial breakdown of the electrolyte but it traps lithium ions within the coating, this loss of free lithium ions from the electrolyte is part of the reason for voltage drop at high discharge rates, the part is because the ion can't escape from the intercalation bonds of the graphite cathode as fast as the lithium ferrous anode can absorb them, resulting in a reduction of ion flow, the driver that creates current and therefore voltage..... is anyone still awake out there :lol: you really don't want to know the rest of it do you
It's one of those very fine line things, the slightest loss of concentration and irreversible cell damage will result because the voltage will go over 4v in the blink of an eye, you certainly don't want to be doing it on a regular basis.
This is where the basis for the "staying in the middle of the charging curve" theory comes from, 4v is the top of the curve, the bottom is somewhere south of 2.4v, so 2.8v and 3.6v is safely inside this envelope, it fits well with all the information from my personal testing that of others on various other forums and others who have carried out cell destruction testing, some of unintentionally

T1 Terry