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Charging 12V LiFePO4 from 12VSLA?

Category: Hardware

Assuming a system in an RV or boat where the starter battery is SLA and the battery for consumers is LiFePO4. Further is the main vehicle for charging the LiFePO4 a solar panel with an MPPT regulator. It would be useful though to be able to charge the LiFePO4 battery at times when the engine is running and I am looking at ways to do that.

You cannot safely charge LiFePO4 from a regular alternator because the internal resistance is so low, but when charging the SLA and the SLA reaches near full, then the battery voltage will rise to 14.4V. The LiFePO4 battery is normally charged to 13.6 or 13.7V conservatively, ie no more than 3.42V/cell.

One way to charge would be to have a current-limited buck regulator set to kick in when the SLA voltage rises above a certain point and then deliver say 5 or 10A to the LIFePO4 battery until you reach 13.6 or 13.7V. or the engine stops charging. The kick in point would be set so high that the converter would not start, unless the alternator is running.

Does this sound workable or just daft? Suggestions on a suitable buck IC?