Design review for Hybred FLA / LFP system

[iRondo]

So ATM I have 400ah of LFP and 400ah of FLA. What is the optimum ratio of FLA:LFP, 2:1? My old Trojan bank are not horrible but certainly not ideal either. I wonder if 1:1 would be okay, or I could split my LFP bank in half and put half to bed.

 

[grizzzman]

I wouldn't worry about ratio. Use what you have and enjoy.

 

[Steve52]

Just use a charge source on the FLA to bring it up to the rested LFP voltage.

I hope it's OK to chime in here. I'm an avid reader of this forum but registered just today. I'm looking at something similliar. Adding LFP in parallell to my boat that currently has 3*110Ah 12V FLA batteries at 12V. We use the boat max 6 months per year since the other 6 months tends to be quite cold here in Sweden I also want the flexibillity to disconnect the LFPs from the system. In part to be able so sell the boat to someone who doesn't want the complexity of the LFPs but also as a safety meassure if something does go wrong with the pack. Being a dad with 3 children aged 0-7 years I don't have the time to crawl around with a multimeter half summer if something does indeed happen.

So back to the question. How would I go about this in practice? Let's say the LFP bank is resting at 13.4V. I don't have any charger that outputs 13.4V but charging the FLAs fully and then letting them float should mean that they won't accept anymore current and at this stage wouldn't it be quite safe to connect the LFP bank again?

If I may throw in another questions here about floating. My plan is to charge at 14.1V (I can choose between 13.8, 14.1 or 14.8) and then float at 13.2 while at shorepower. This would mean that the LFP bank would settle at around 70% SoC and then stay there. The FLA batteries should be quite happy as well even though the float is a bit low for them. However on this page this is described as something bad. I don't understand what's so bad about it though, the LFP batteries seems to like being stored at a low SoC. (I do have the possibillity to set float at 13.5V as well) My aim here is to prolong the life of the LFP batteries in these situations. While on solarpower I plan to charge at 14.4V and float at 13.6V to have more or less 100% battery availible when the sun settles.

 

[grizzzman]

I have also used the inverter to bring the voltage down on the LFP bank. Once connected its no big deal. Also I use a Kilovac EV200AAANA contactor. This disconnects the LFP bank from the FLA bank. Three Anderson connectors, remove the slides and the LFP bank is out. This leaves the FLA bank including the inverter and everything else on line.
14.4 is for fast charging. I would suggest 13.8 to 14.00 and adjust as needed.

 

[Steve52]

Would you agree with me that my approach to charge the FLAs fully and then connect them with the LFP pack should be quite safe?

 

[danphillips]

The challenge is to ensure the voltage between the FLA and LFP are very close before connecting the two systems for the next outing.

 

[Steve52]

The challenge is to ensure the voltage between the FLA and LFP are very close before connecting the two systems for the next outing.

That is true but could you, or someone else, please elaborate a bit? If the FLAs are full, they have a high internal resistence and will accept very little current. Connecting a LFP pack sitting at 13.4V at this stage should be safe in my opinion but am I missing something here?

To those of you who are actually using hybrid solutions, how do you solve this in practice?

 

[danphillips]

@Steve52 ,
It is common to store your lifePo4 batteries at 50-60% SOC, as long as both batteries are nearly at the same charge, there should not be a problem. If your FLA batteries are low however, you could encounter a large serge.

 

[grizzzman]

I have observed over 100 amps inrush when connecting the FLA to the LFP bank when not matching voltages but.... for a very short time. and yes its due to the high internal resistance. This is what I normally do to hook up. Turn on a load (scrubbing surface charge) turn on FLA converter, It jumps to 13.6 volts Tell the SBMS0 to connect the load contactor. Go have fun

 

[mmtech]

I would really like to see will do a video of him doing such a setup and his results. I have tried researching this on the net.