LiFePO4 reference diagram

[s/v Jedi]

Originally Posted by Kit_L

@ s/v Jedi (love the Yoda quote),

Thinking aloud here; Eric Bretcher and others claim that LiFePO batteries last the best when discharged from full to 10% charge or so, then recharged. Many systems recharge these batteries long before the 10% level is reached. I am wondering whether your two-bank system can allow for this, if this claim is accurate. Is this any kind of problem in your view?

Then I was wondering if having a fully charged, but modestly sized FLA system might provide the emergency backup that will be needed if the Lithium BMS detects a fault and disconnects the system. I know you have mentioned an alarm scenario, in advance of this happening, but I wonder if an alarm plus automatic switching over to the backup FLA system might obviate the need for the second LiFePO bank?

The charge - deep discharge regime is not the best for lifespan in calendar years, but for total kWh delivered by the battery during that life span. Here you can see the typical use case for electric vehicles.

For house power as we have on boats, with solar power generation in particular, things change: as all consumed power must first be generated, it is crucial to optimize solar power generation as much as possible. This is done in two ways:

1 - most importantly, use available solar power directly to power systems like an inverter. By doing this, you skip inefficiencies around storing energy. A good example is to run the watermaker during hours of big solar production rather than when part, or all of the energy required comes from battery.

2 - store all excess energy in batteries. Unlike grid connected systems that can sell the energy, for off grid applications we have to store it or lose it.

These considerations mean that we have to fully utilize our solar/wind power generation and store all excess power. This means cycling not as deep as an electric vehicle does. The disadvantage of not getting the most kWh from the battery is offset by getting more calendar life out of them and this is very important to cruisers as well.

I do not see a LA bank as a good replacement for a 2nd LFP battery. Currently, my second bank is LA and exists of 4x Lifeline AGM L16 400Ah batteries in series for a 24V 10kWh bank which is very similar to our 24V 10.5kWh LFP bank which has 8x Winston 400Ah cells in series. The difference in performance is stunning, I must force myself to keep the LA bank longer to earn back the investment and create an age gap between the two LFP house batteries.

The other LA bank is the start battery bank. For this application AGM is superior to LFP because we need very little energy overall, but at high power for very short periods. This means you want a full battery and you want a start battery, with more and thinner plates for a large surface area.

LFP can do the high power burst very well but it does not want to stay fully charged at all, this will greatly reduce it’s lifespan. Setups to keep LFP start batteries half charged are doomed to fail because it is sheer impossible to know the SOC for a battery that never goes empty and never goes full. AGM on the other hand loves being fully charged and on float charge. With the thin plate versions like Odyssey you get power hot rods that can do 6kA Isc which is amazing. They are also compatible with standard alternators and do not get HVC events that can destroy the rectifier diodes in the alternator. While a flooded LA start battery delivers better price/performance, it can not deal with any secondary tasks like powering loads like an autopilot, navigation systems etc. Start batteries are designed to start the engine only and the alternator powering everything else.
The Odyssey batteries are more expensive, but they last longer and they can do the secondary tasks very well. This is where throwing some $$$ at it makes a big difference.

So, back to my diagram: even though all 12V loads are powered from the LFP house banks via a dc-dc converter, the converter is programmed to charge AGM batteries and those are the start batteries. This means they quickly go to float charge and stay there. The dc-dc converter does the job of the alternator in the typical car scenario. The difference with a boat is that high power devices can use this 12V. Examples are a windlass, an electric winch, an electric toilet etc. For a 12V boat that converts to 24V, everything needs to be powered from this 12V system initially. I recommend to change the windlass motor to a 24V version, but for other systems it makes no economical sense to replace before their lifespan has been completed.
The task of the dc-dc converter is to keep up with average power consumption, while the AGM batteries take care of high power peak requirements. There is also a redundancy aspect: these batteries help when the converter breaks down, but I still recommend to carry (or install) a spare.

[Baronkrak]

Originally Posted by Kit_L

Then I was wondering if having a fully charged, but modestly sized FLA system might provide the emergency backup that will be needed if the Lithium BMS detects a fault and disconnects the system.

may be what you are looking for?

Originally Posted by T1 Terry

The thing to keep in mind, …

T1 Terry