Lithium/ Lead Acid Hybrid System...

capta

.
Jun 4, 2009
4,766
Pearson 530 Admiralty Bay, Bequia SVG
From what I gathered the lithium batteries get fully charged before the LA batteries begin to charge? Doesn't this contradict "Lithium batteries however hate to remain charged but love to cycle and to remain forever in a discharged state"?
 
Apr 5, 2009
2,774
Catalina '88 C30 tr/bs Oak Harbor, WA
I have also wondered about a system like that. My one concern is that it puts the reliability of the BMS at the forefront of protection of the cells rather than as backup. Normally the charging regimen is set to not exceed the limits of the cells and the BMS only cuts in when for some unexpected reason, the max voltage is exceeded. With the system you describe, the BMS must shut down charging every charge cycle.
 
Feb 6, 1998
11,667
Canadian Sailcraft 36T Casco Bay, ME
The article fails to delve into what happens when the lead acid battery fails internally, and they do. A large LFP bank has enough energy to cause a thermal runaway event in an LA battery that has lost a cell.
 
Apr 22, 2011
865
Hunter 27 Pecan Grove, Oriental, NC
The article fails to delve into what happens when the lead acid battery fails internally, and they do. A large LFP bank has enough energy to cause a thermal runaway event in an LA battery that has lost a cell.
Wouldn't a fuse on the lead acid battery blow before a thermal runaway occured?
 
Jan 4, 2010
1,037
Farr 30 San Francisco
I had a lead acid explode once, I think it was a failed cell vs an automotive alternator. Battery blew its top off spraying acid everywhere. I don't think the alternator was putting out super high currents.
 
Apr 5, 2009
2,774
Catalina '88 C30 tr/bs Oak Harbor, WA
Wouldn't a fuse on the lead acid battery blow before a thermal runaway occurred?
I was wondering the same thing. The AGM should have an MRBF fuse connected directly to it that is rated for just a bit more than the max charging current. wouldn't any current leading to a thermal runaway be greater than that and trip the fuse?

On the inverse of this. IF it does not take more current than what the charger can put out to cause a thermal runaway, wouldn't you have the exact same danger from just an AGM battery cell shorting when it is the only battery in the bank and it would make no difference that the LFP was there or not?
 
Apr 5, 2009
2,774
Catalina '88 C30 tr/bs Oak Harbor, WA
The load currents a likely to be much higher than max charging current... Think windlass or electric winch.
If I am understanding the hybrid system correctly, the LFP battery is designed to take the vast majority of the load demands. The AGM will only come into play when the LFP has been dropped to below 13.0V and taken off line by the BMS. The LFP would have a seperate fuse rated for the max loading.
If the LFP is off line, and the windlass is needed the engine would be started (with separate start battery) and the alternator/LFP would supply the power to the windlass.

What am I missing?