One for the lithium lovers!

[Buck Turgidson]

I leave my yacht in the marina for months on end between sailing. How would one manage the "don't leave lithiums fully charged" when there is a requirement to keep the auto bilge pump active and my IOT connection powered so I can monitor the boat remotely? These are small but in my case necessary parasitic loads which in turn require the boat to be constantly charged by normally solar or occasionally shore power (when solar is not available due to technicalities).
I've been on the cusp of going LI for 6 months but the longer I think about it the less convinced I become that it is the right solution for my usage pattern.

 

[Sea Change]

Set your MPPT to stop charging at around 13.3v?

 

[lustyd]

On mine I’d probably set the battery to not accept charge or discharge. I’d then disconnect the MPPT and set the charger to power supply mode.
Or I’d bridge the batteries so the engine start drives the pump and is charged by the MPPT. The BMS is internal to the lithium battery so it doesn’t matter if it’s connected the battery inside would still be disconnected.

 

[Trident]

It's not difficult - you can set the MPPT to a lower maximum charge to keep the battery at say 50% or set the BMS to cut off charge at a certain level .

But lithium isn't magic and doesn't suit everyone as you well know - if you don't think your usage pattern will give you benefits then stay LA

 

[Sea Change]

It's not difficult - you can set the MPPT to a lower maximum charge to keep the battery at say 50% or set the BMS to cut off charge at a certain level .

Changing the MPPT setting is better than doing it with the BMS, you want to avoid the MPPT being connected to panels but not too the battery.

But lithium isn't magic and doesn't suit everyone as you well know - if you don't think your usage pattern will give you benefits then stay LA

Absolutely. Even though you can now build a lithium battery cheaper than you can buy lead acid, the other system modifications and the different behaviour means sticking with lead acid makes more sense for many people.

 

[Trident]

Changing the MPPT setting is better than doing it with the BMS, you want to avoid the MPPT being connected to panels but not too the battery.



Absolutely. Even though you can now build a lithium battery cheaper than you can buy lead acid, the other system modifications and the different behaviour means sticking with lead acid makes more sense for many people.

I mention the BMS because not all MPPT have customisable settings and I have no idea what the OP has...

 

[Neeves]

And there was me thinking all MPPTs had customisable settings and that only cheap 'drop in' batteries had preset non customisable BMS.

Jonathan

 

[jakew009]

Why are you worrying about leaving it fully charged? It’s still going to perform miles better than lead acid and work out cheaper over the long run even if you do abuse it slightly by leaving it fully charged.

We have thousands of LiFePO4 batteries in standby applications where they are float charged at 13.6V ish continuously with a very occasional trip down to 0%. I think I’ll be dead before I learn whether it makes much difference or not.

 

[Buck Turgidson]

Why are you worrying about leaving it fully charged? It’s still going to perform miles better than lead acid and work out cheaper over the long run even if you do abuse it slightly by leaving it fully charged.

We have thousands of LiFePO4 batteries in standby applications where they are float charged at 13.6V ish continuously with a very occasional trip down to 0%. I think I’ll be dead before I learn whether it makes much difference or not.

Hi Jake,
I raised the point because of a recent thread on lithium where some advocated strongly for compressing the cells and never leaving them fully charged. These were the lithium advocates who have experience with these cells and I therefore am trying to learn from their knowledge rather than just throw something together and then be disappointed when the battery fails prematurely.

I'm not convinced by the idea of setting the MPPT to 13.3v as that goes completely against the guidance from victron which is as follows:
Recommended charger settings
The important charging parameters are absorption voltage, absorption time and float voltage.
• Absorption voltage: 14.2V for a 12.8V lithium battery (28.4V / 56.8V for a 24V or 48V system
• Absorption time: 2 hours. We recommend a minimum absorption time of 2 hours per month for lightly cycled systems, such
as backup or UPS applications and 4 to 8 hours per month for more heavily cycled (off-grid or ESS) systems. This allows the
balancer enough time to properly balance the cells. Please see the Cell balancing [41] chapter for a more detailed explanation
why cell balancing is needed and how cell balancing works.
• Float voltage: 13.5V for a 12.8V lithium battery (27V / 54V for a 24V or 48V system)

So I guess what I would like to understand is how much life will be lost If I use the manufacturers recommended settings rather than the advice from long term users.

I've just ordered my Victron Smart lithium 160Ah and have all the other victron equipment already so all that is missing is knowledge and experience a lack of which could prove costly as I put my system together. Hence the questions.

 

[lustyd]

The concern you have is probably analogous to getting lead on float every day. You can worry about it, and it will damage the battery eventually, but in the real world that damage will usually be minimal.
The guidance Victron are giving there is on charging the batteries, not on long term storage. As such, it will be different than others are saying. Much of the damage to these batteries occurs because more juice is pumped in to a cell* when it's already full. Some of the tiny cells (inside the battery cells) burst and release gas as a result and so are no longer able to store power. The more you lose the worse capacity gets. This is the same as overcooking a lead battery where gas will be produced, it's just more precise in these. If you don't completely fill them all (aka 50% capacity) then they don't burst so you don't lose them. The compression on cells is partly to keep pressure on them which reduces gassing for all the usual reasons.

*lots of artistic licence in here, but hopefully gets the point over about roughly what's happening.

 

[Sea Change]

13.3v, or 3.325v per cell, should ensure that the battery is roughly half full. 13.5v is also not full charge but I would suggest it might be quite close to full.
It's really quite difficult to estimate state of charge from voltage alone, as the voltage barely changes over the 10-90% range.
Personally I would aim for a lower state of charge as this is less likely to lead to problems. But a higher storage voltage will mean you can top up to full much faster before you next use the boat.

Seeing as you have an all Victron system I'd imagine it must be possible to do this in a more sophisticated way, e.g. use your BMV relay to shut off the MPPT?

 

[jakew009]

Hi Jake,
I raised the point because of a recent thread on lithium where some advocated strongly for compressing the cells and never leaving them fully charged. These were the lithium advocates who have experience with these cells and I therefore am trying to learn from their knowledge rather than just throw something together and then be disappointed when the battery fails prematurely.

I'm not convinced by the idea of setting the MPPT to 13.3v as that goes completely against the guidance from victron which is as follows:
Recommended charger settings
The important charging parameters are absorption voltage, absorption time and float voltage.
• Absorption voltage: 14.2V for a 12.8V lithium battery (28.4V / 56.8V for a 24V or 48V system
• Absorption time: 2 hours. We recommend a minimum absorption time of 2 hours per month for lightly cycled systems, such
as backup or UPS applications and 4 to 8 hours per month for more heavily cycled (off-grid or ESS) systems. This allows the
balancer enough time to properly balance the cells. Please see the Cell balancing [41] chapter for a more detailed explanation
why cell balancing is needed and how cell balancing works.
• Float voltage: 13.5V for a 12.8V lithium battery (27V / 54V for a 24V or 48V system)

So I guess what I would like to understand is how much life will be lost If I use the manufacturers recommended settings rather than the advice from long term users.

I've just ordered my Victron Smart lithium 160Ah and have all the other victron equipment already so all that is missing is knowledge and experience a lack of which could prove costly as I put my system together. Hence the questions.

The slightly ironic thing is that the Victron Smart Lithium batteries don't have any compression to speak of. They are just tightly wrapped in strapping tape. Make of that what you will (in context with the thread about lithium requiring compression!)

Victron are suggesting sending the battery to 14.2V occasionally to trigger a balancing of the cells. The reality is that if you don't do that, nothing bad is going to happen. The worst that will happen is the cells are slightly out of balance and you lose a few percent of battery capacity as one of the cells hit it's low voltage cutoff before the others. Just charge them to 14.2V before you use the boat after a long period away if you are worried, but I bet they will stay in almost perfect balance anyway.

Victron are also suggesting that storing the battery at 100% is fine because they refer to standby / UPS applications. They are also recommending a float voltage of 13.5V - that is probably enough to fully charge the battery given enough time.

As to how much capacity you will lose by storing them at 100%? Not a clue, but my own real life experience over thousands of batteries is find something else to worry about.

 

[Sea Change]

I think a lot of the advice about treating lithium batteries with kid gloves stems from the days when they were very expensive.
I'm not suggesting we treat them as disposable, but we should probably re-analyse the cost: benefit ratio of some of the things we do.

For example, it was once quite sensible to have very cautious BMS settings to protect the battery, even if that risked taking it offline at inconvenient times. This is a good way to kill an MPPT, which needs to be connected to a battery as a load dump. These days, I'd be more worried about protecting the MPPT, so I program mine to control charging, and have wider settings on the BMS.

 

[jakew009]

I think a lot of the advice about treating lithium batteries with kid gloves stems from the days when they were very expensive.
I'm not suggesting we treat them as disposable, but we should probably re-analyse the cost: benefit ratio of some of the things we do.

For example, it was once quite sensible to have very cautious BMS settings to protect the battery, even if that risked taking it offline at inconvenient times. This is a good way to kill an MPPT, which needs to be connected to a battery as a load dump. These days, I'd be more worried about protecting the MPPT, so I program mine to control charging, and have wider settings on the BMS.

Couldn't agree with you more.