Re: SM Batteries

Dean Gillies

Yes. In 2020 I designed a directly connected AGM+LFP hybrid system almost exactly as you describe and proceeded to build a 12V version at home during Covid. I tested it thoroughly for 9 months.

Earlier this year, I installed the full 24V version on Stella and it has been running successfully for 6 months. I am extremely happy with the results.    

My design tilts the balance in favour of LFP capacity, using 115Ah of AGM and 540Ah of LFP.  This is simply because lithium likes to be cycled and lead likes to be floated.  In normal cruising, my AGM component is almost always on float at 100% SOC and my LFP component is almost always cycling between 30% and 90%.SOC. 

So why bother with the AGM's at all ?

1. If my BMS decides to disconnect my LFP component for any of the myriad reasons that can happen, then the AGM takes over the load seamlessly. The lights stay on, the instruments keep running and the coffee machine is still available (for a wee while anyway!).

2. If my alternator inhibit system was to fail while motoring and the LFP's were disconnected due to overcharging, the AGM batteries are still online and prevent any over-voltage damage to the 24V system components.

3. When laid up for long periods I can take the LFP component offline (at ~60%SOC) and simply operate the AGM in float either from the mains charger or shallow cycling from solar. I'm home again in Australia and Stella is in Spain so this is what I did before I left her.

Please feel free to email me if you want to talk turkey about the detailed design.
My battery box now likes the attached pics.

Cheers, Dean


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