If that is 26V at 65C you still get current at 27V a bit less and not optimum but you do.
What does slow spontaneous discharge means ?
The two months for my system seems more than reasonable since I will never be away from the system for more than two months. And the chance of PV array to fail is extremely low.
Those voltages are not adequate to fully charge a nominal 24V lead acid or LiFePO4 (8s) battery bank.
The discharge that occurs in any battery when it just sits, unconnected to any load or charging source.
They are more than adequate at 3.4V under 0.3C charge rate a battery is around 90% SOC
And looking at that IV graph again you can see about 32V open circuit voltage with cell temperature at 65C
So even at 28V battery voltage you still get a decent charge current with cell temperature at 65 even 75C
That self discharge on any good LifePO4 is below 2% capacity per month so as long as you stop at about 10% SOC (3V very low discharge rate OffGrid or 2.8V high discharge rate eBike) you have enough time to recharge as I mentioned before.
That self discharge on any good LifePO4 is below 2% capacity per month so as long as you stop at about 10% SOC (3V very low discharge rate OffGrid or 2.8V high discharge rate eBike) you have enough time to recharge as I mentioned before.
You will not fully charge a LiFePO4 cell with 3.4V or below.
There's a reason LiFePO4 chargers use a voltage of 3.6-3.65 per cell for the constant voltage portion of the charge cycle.
What voltage does your charger use?
Sure, but that has nothing with the point I was making about your incorrect statement that you can't damage LiFePO4 batteries by failing to charge them or maintain them.
Look - I'm a big fan of LiFePO4. If I was designing an off grid solar system today I would seriously consider using them instead of LA batteries. But this is an EE forum and hyperbolic, incorrect statements such as "MPPT is uneccessary due to cheap PV prices" and " you can't damage LiFePO4 like you can LA by not charging or properly maintaining them" are not going to go unchallenged
QuoteThat self discharge on any good LifePO4 is below 2% capacity per month so as long as you stop at about 10% SOC (3V very low discharge rate OffGrid or 2.8V high discharge rate eBike) you have enough time to recharge as I mentioned before.Any specs on this?
So if you left some LiFePO4 at 10% SOC. would it be 8% next month, 6% after 2 months, 0% after 5 months?
Roughly at what point do they start getting damaged? Or what SOC should they never go below?
Just noticed, you got funded. Congrats.
Roughly at what point do they start getting damaged? Or what SOC should they never go below?
constant voltage portion of the charge only adds at most 5% more capacity while reducing the life of the battery substantially.
I was referring to a battery protected by a BMS where the discharge is terminated by the BMS at proper voltage level based on battery and application requirement. If that low cut of voltage is properly set for that particular application so that at least 10% capacity is still available then that 10% is enough for that battery and protection circuit to survive a few weeks or months without a charge.
In an OffGrid application you will notice immediately if the load is disconnected and that should never happen in a properly dimensioned installation.
So yes all you need to do to protect a LifePO4 is to keep it between 2.8V and 3.6V and not charge when battery temp below 0C seems way simpler than all the conditions that affect Lead Acid life and can be done by BMS.
Yes, congrats on getting your funding!.
Any references on that? I have seen that stated for other Lithium battery chemistries but not for LiFePO4.
It seems you're claiming something different than all the other LiFePO4 charger manufacturers I've seen. All the ones I've researched have a constant current stage followed by a constant voltage stage at 3.6-3.65V. This is true for the charges used by home build EVs with LiFePO4 banks and Ebike users. Also, the people who I know of using large LiFePO4 banks for off grid homes are using one of the currently available programmable MPPT controllers with CC and CV stages.
Ok thanks for clarifying but that's entirely different than your previous statement and in fact no different than Lead Acid batteries in that as long as you properly maintain them they do no suffer premature damage.
In a solar setup using normal MPPT Lead Acid charger with CC and CV charging you will keep the battery for many hour every day forced at that higher than 3.4V level.
LiFePO4 at rest is below that 3.4V so keeping it above that creates stress and encourages dendrite growth.
The dendrite formation on LiFePO4 is less problematic than LiCo variants but will still exist and affect the life of the battery even if is harder to see in real life do to superior cycle life of the LiFePO4.
Ok - sounds reasonable enough but it seems that you're making a pretty remarkable claim - that is that you know something the other LiFePO4 charger manufacturers don't - some actual references to support your theory would be nice. I mean there are many battery chemists, physicists, and engineers who have spent a lot of time developing LiFePO4 technology. How is it that you know something they don't - or if they do - where have they published it?
Ok - sounds reasonable enough but it seems that you're making a pretty remarkable claim - that is that you know something the other LiFePO4 charger manufacturers don't - some actual references to support your theory would be nice. I mean there are many battery chemists, physicists, and engineers who have spent a lot of time developing LiFePO4 technology. How is it that you know something they don't - or if they do - where have they published it?
What other LiFePO4 chargers ?
During the conventional lithium ion charging process, a conventional Li-ion Battery containing lithium iron phosphate (LiFePO4) needs two steps to be fully charged: step 1 uses constant current (CC) to reach about 60% State of Charge (SOC); step 2 takes place when charge voltage reaches 3.65V per cell, which is the upper limit of effective charging voltage.
You probably know this graph from battery university about the cycle life versus just small cell over voltage on LiCoO2 the LiFePO4 has better tolerance to over voltage but the life cycle will still be affected in a similar way.
Many. Here's two companies to start:
Manzanita Micro
Powerstream
From Powerstream (the second link):QuoteDuring the conventional lithium ion charging process, a conventional Li-ion Battery containing lithium iron phosphate (LiFePO4) needs two steps to be fully charged: step 1 uses constant current (CC) to reach about 60% State of Charge (SOC); step 2 takes place when charge voltage reaches 3.65V per cell, which is the upper limit of effective charging voltage.
Nice graph but not relevant to the question.
BTW - I'm not disputing that you can charge LiFePO4 cells at 3.4V - it's just that I've never seen any data supporting what you say about a CC stage followed by a CV stage at 3.6V shortens battery life.
LiFePO4 do have the advantage over LA in that the don't sulphate or otherwise degrade by chronic mild undercharging. But with LiFePO4 batteries relatively expensive - I don't think anyone will want to pay for more capacity than necessary/
Many. Here's two companies to start:
Manzanita Micro
Powerstream
From Powerstream (the second link):QuoteDuring the conventional lithium ion charging process, a conventional Li-ion Battery containing lithium iron phosphate (LiFePO4) needs two steps to be fully charged: step 1 uses constant current (CC) to reach about 60% State of Charge (SOC); step 2 takes place when charge voltage reaches 3.65V per cell, which is the upper limit of effective charging voltage.
They have absolutely no idea of what they talking about.
How will a LiFePO4 with CC charging to 3.65V be just 60% SOC it will be around 95% and even their own graph shows that.
Like I mentioned before Battery manufacturers give spec based on dual stage CC and CV up to 3.6V most mention that 3.65V is acceptable in a sting of series connected cells for the highest cell.
The Solar BMS does not charge the battery at 3.4V but at 3.55V (highest cell not average pack voltage) then as soon as it gets there it stops charging so that battery is as much as possible above 3.4V and charging. With this do to the nature of the solar PV panels I can achieve 100% SOC wile only using constant current charging and not exceeding 3.55V on any cell.
I think you're misreading what they wrote. I read it to mean it is the second, CV stage with a charging voltage of 3.65 that completes the charging. The 60% is what they cite for the 1st, CC stage.
You realize that a charge voltage of 3.65 during the CV stage does not mean you actually charge the cell to that voltage, right? As soon as it is disconnected from the charger the voltage will drop - just as it at the end of the absorption stage in traditional LA 3 stage charging algorithms.
I think you're misreading what they wrote. I read it to mean it is the second, CV stage with a charging voltage of 3.65 that completes the charging. The 60% is what they cite for the 1st, CC stage.
Yes I read correctly no misunderstanding with the first CC charge to 3.65V you get well over 90% even at 0.5C charge rate and not 60%.
They have absolutely no idea of what they talking about.
How will a LiFePO4 with CC charging to 3.65V be just 60% SOC it will be around 95% and even their own graph shows that.
All DIY solar installations use a car BMS in combination with a Lead Acid charger in combination with LiFePO4 and that is because there is no real alternative and because there is not enough eduction about Lithium in stationary energy storage.
In any case, based on your (non) answers - I guess your charging algorithm is based on your own ideas. Nothing wrong with that. It will be interesting to see the evolution of LiFePO4 charging as they become more widely used in home RE systems.
Good luck with your Kickstarter product.
I agree. It's early in the LiFePO4 adoption phase. Lots of room for experimentation.
LiFePO4 prices still need to come down a bit and BMS systems to be more robust.
One thing LA still has on LiFePO4. One bad mistake will generally not kill a LA battery as it will a LiFePO4 cell.
Sorry, but you lose some credibility with a table like that which is very skewed in its numbers and assumptions.
The Trojan L16RE-2V batteries are a top of the line LA battery with a 7 year warranty and can be expected to routinely last 10 yrs or more from a company with a long history of real world experience and your comparing them to a no name Chinese brand with no warranty and no proven track record with an optimistic guess of 20-25 yr lifespan.
The L16RE-2V have a (20hr) Ah rating of 1110 not 1021.
Charging efficiency of these is typically going to be much better than 75% - more like 85%,
Again - I'm a big fan of LiFePO4 based on their real merits. No reason to overhype them. It just makes knowledgable consumers skeptical of what you're selling.
Have you retested the capacity of your system now that it has passed the two year mark?
Is there any need to balance or equalize LiFePO4 cells? If so, how is this done? Would periodic CV charging do this?