1 OCT 2018:
Let’s just agree that it is early days for LFP, some people already have them on board for different reasons, others argue that LFP is still too expensive and more difficult to replace if they break in a far away place.

I believe LA and LFP can actually coexist peacefully even on the same boat!

Outrageous, some might say. Heresy, others will cry and start to pick up a stone, or at least pick up a keyboard to hurl in my direction.

It might even be possible that the combination can give you the best of both worlds?
As an intro: What really happens if you connect LFP and LA together?

Not too long ago a gentleman on this forum had a stern warning for us to never, ever directly connect a fully charged batt to an empty batt as the current will be huge and the cables might melt or worse and the batts will get damaged beyond repair! Well I’m proud to say I just topped that and dared to connect an LFP sitting at 13.2V in my lab to a discharged LA batt measuring 12.1V open circuit. And I survived the stunt to tell the story to the world: nothing happened! Nada. Zilch. The Amp meter jumped to 1.5A and is now slowly dropping off. The “bus voltage” dropped to 12.9V. The battery controller is documenting this as we speak, so to speak. Tomorrow I can show the graph of this “disaster waiting to happen”, if someone is interested.

Looking at the different voltage thresholds and recommended charge regimes for LA and LFP I can even see signs of a symbiosis here: the LFPs can provide the bulk of the power until they are down to 10 or 20% SOC, then we automatically take them off the bus and the LAs would provide the “reserve” power on those rainy days.

How can that ever work, you might ask? Say we set our charge controllers to simply bulk charge to 14.3V then drop to some float voltage around the LFPs resting voltage (say 13.3V?). Now once our charging source kicks in the LFP will likely absorb more current due to their lower impedance (internal resistance). Charging might continue until the threshold is reached, which means the LFPs are full, the LAs are maybe sitting at 80-90% SOC.

The charge source (if it’s one we can control, e.g. alternator, generator, land power) now gets disconnected and the LFPs are topping off the LAs until they’re full. LAs at 100% SOC is what we want anyway, right? Brilliant! The LFPs are getting slowly discharged immediately after reaching 100% which is also what’s recommended to maximise their life span. Even better!

All the consumers are now driven by the LFPs higher voltage relative to the LAs. If, however, we can’t fully charge the LFPs because we rely on wind and solar, nothing bad is going to happen either. The only thing we need to make sure is removing the LFPs from the bus when they’re empty, nothing more (unless I missed something crucial here).

WDYT?

The Cruisersforum post is here.


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