Curious about Lithum batteries

[mcarling]

Originally Posted by Pete O Static

I will be looking at a battery replacement for my boat in about 24 months and seriously considering Lithium.

24 months from now, you will probably have the option of being either a conservative adopter of LiFePO4 or an early adopter of Lithium-Silicon. 2016 will probably be the first year of commercial availability for Lithium-Silicon batteries, though they've been used for years in experimental applications, like electric aircraft, satellites, etc. While Lithium-Silicon batteries will offer a lot of advantages, one disadvantage is that the voltage-discharge curve is steep (1.5V to 2.7V per cell), so a DC-DC converter must be placed between the battery and the load bus to provide a stable voltage over the charge/discharge cycle (unless all your loads will accept 12V to 24V, which is unlikely).

For a marine house bank of Lithium-Silicon cells, I would put eight (or maybe nine) in series, then use one DC-DC converter to step down to 12V for the load bus and another DC-DC converter either to step up from nominal 12V charging systems or to step down from nominal 24V charging systems to whatever is the optimal charging voltage (probably somewhere between 21V and 22V) for eight Lithium-Silicon cells.

[ramblinrod]

Originally Posted by Pete O Static

Currently, I have FLA and don't see the advantage for the added cost of AGM.

Depends. Are your current standard FLA in the cabin? Then you would benefit from switching to AGM.

Is your charger or alternator capable of putting out more than 25% of your existing bank capacity? Then you would benefit from switching to AGM.

Deciding to leapfrog AGM technological advancement for the next, may be a legitimate plan under certain circumstances, and one that I will likely consider as well, BUT there are legitimate reasons to switch from standard FLA to AGM now.

[john61ct]

I was reviewing my notes and found this, which goes against many authoritative sources

Originally Posted by mcarling

3. Leaving LiFePO4 cells at a state of charge below 20% for long periods will reduce the lifespan of the cells from several thousand cycles to just a few thousand cycles, where lifespan is defined as capacity reduced to 80% of spec.

My understanding is the above is true for lead, but not LFP.

In a regular cycling context, it makes no difference, there is no reason to go out of your way fully charge the bank, and in fact the danger to longevity comes when people try to get close to theoretical 100% at voltages over 14/3.5V.

When stored or infrequently needed, best longevity comes from DISCHARGING down to 40-60% SoC, the bank's longevity is HARMED sitting at full, or anywhere near full like 80+%.

Confirmation of this would be appreciated.

And as an aside, the other major factor is heat, cooler part of the operating range will extend life, storing very cold is best as long as no usage or (especially charging.

And not discharging too low.

Anything else?

[pacific_voyager]

Agreed. Misinformation is everywhere and the trick is to filter for the reasonable options, if any, to manage and charge and protect these LiFePO4 batteries used as house banks.

It is a waste of space to insist LiFePO4 are actually "Lithium Ion". Of course they are, but one thing is clear is that they are the only type of Lithium battery today that is useful for DIY battery banks on a cruising boat. This use is what this forum is about. Although motor homes and vans can make use of this technology, it is still a different application on a boat.

1. By now we all know the benefits of LiFePO4. No disagreement there, so lets stop talking about it.
2. The "trick" is to devise a method to START and STOP charging sources safely which is also as immune to human error factors as possible.
3. It is NOT clear from this thread and others that voltage sensing alone is adequate to this task.
4. It is NOT clear that existing LA charging sources (AC chargers, alternators, solar, wind) are adequate to this task without external control, which can be complex and subject to error.
4. There is disagreement whether cells-in-parallel vs strings-in-parallel are better.
5. There is disagreement whether large cells (>200AH) are safe.
6. It does not seem clear that safety net LVP and HVP devices will be adequate to save a bank. They should be in place but the system has to be designed so they are actually never needed.
7. By "adequate" I mean that the system will not accidentally over or under charge (and thereby destroy) the batteries, leaving the cruiser in a potentially dangerous situation without electrical power, not to mention spending another $4K or more an replacement batteries. One of the arguments for LiFePO4 batteries is that low cost per AH over their as-yet-to-be-determined lifetime. One bank failure wipes out that advantage.

There are many voices on this subject that express different and sometimes diametrically opposite viewpoints. I want to see the use cases. Maybe they are all true? Perhaps your method worked 33 times and so did the opposite method? Are we hearing about the failures? Is "n" large enough yet?

FWIW I am at the end of 6 years of AGM use with 1218 total cycles of about 100AH each and 117,793 total measured amp-hours. This is still short of the factory cycle life, but better than most. The bank is 900AH now degraded to about 1/2 that. The construction is six 2V 900AH AGM lifelines. Parallel strings of LA batteries invariable lead to very short lifetimes. I also now know how to better charge these AGM batteries, largely from studying a potential move to LiFePO4. This is not because they are comparable in any way (except they both store energy) but because I gained a lot of understanding on what works and doesn't work to monitor and control a charging regime.

For me, the jury is still out and whether to simply (and I do mean SIMPLY) replace the 6x AGM bank, good for another 5 years, or undertake the very large and expensive effort to control a LiFePO4 bank. (Yes, I crave the light weight, consistent voltage, and long life but it's going to be a LOT of work). I suspect the KISS group of simple voltage-based charging with LVP and HVP relays may find their systems too prone to HEF (human error factor) and difficult to work with when the inevitable failures happen.

I know I am dreaming, but I would like to see a definite answer to each question or issue, with research and use cases behind the arguments. I know the issues are interlinked but I would like to see some of them separated into separate threads (i.e. cells-in-parallel vs. strings-in-parallel) or whether cell monitors/balancers are useful). I hesitate to do this as it might be beyond the scope of what is usually a modest forum and it would require input from the major contributors.