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I have a power bank consisting of two LiFePO4 cells connected in parallel. They will used to power a range of 12 V electronics e.g. fans, fish finder, USB chargers, L.E.D lights and potentially a coolbox. These will be drawing power both when the engine is running and when it is switched off. I want to charge them using:

  1. The alternator on the outboard motor on my boat. It is a manual-start motor. This means it has no starter battery. It is rated to produce 20 A.

  2. The alternator in my car. The alternator charge current is 70 A.

Both alternators should output 14.5 V max. From what I understand this DC - DC charger would work for the car but not for the boat, as it requires a starter lead-acid battery. Is this correct?

Is there anything that will allow me to charge the battery from both my boat and car using the same charger?

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  • \$\begingroup\$ Hard to tell. What’s the peak unloaded voltage output from the generator? That will determine a lot of things for you. In general, you are on the correct track with a DC/DC charger. A (somewhat) wide range input LiFePO4 charger should be able to do what you need. \$\endgroup\$
    – winny
    Commented Jul 4, 2022 at 15:07
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    \$\begingroup\$ "as it requires a starter lead acid battery. Is this correct"? No? I am not even sure what you mean by this. Just start the engine. You need to make sure the charger specifically supports LiFe, not just lithium which may or may not include LiFe. \$\endgroup\$
    – DKNguyen
    Commented Jul 4, 2022 at 15:12
  • \$\begingroup\$ Oh I think I see what you mean...like using the starter battery as a cap rather than connecting directly to only the alternator. Hmmm...you might be able to just use a cap as long as the alternator output is already rectified. It might not even be necessary if the charger has internal input caps that are large enough. \$\endgroup\$
    – DKNguyen
    Commented Jul 4, 2022 at 15:33
  • \$\begingroup\$ boat alternator capacity V, I is unknown. Desired charge current I chg is unknown. yes there are many solutions after the unknowns are defined along with preferences for solutions. What will the battery be used for? Did you intend to keep it on boat charge when in use? That affects the CC/CV targets or adds more details on the float/recycle algorithm. \$\endgroup\$ Commented Jul 4, 2022 at 17:24
  • \$\begingroup\$ I have updated my question to show these details. Please let me know if there are any other details that would help. \$\endgroup\$ Commented Jul 5, 2022 at 12:40

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Like any lithium-ion battery, you need to put a Battery Management System (BMS) in front of it.

The BMS's job is to make sure the battery isn't over-discharged too low, overcharged, overloaded, or overheated. If you do not do this, you will get a nice battery fire at some point.

Now that Renogy unit you linked seems to be precisely a BMS. I don't see a reason why it wouldn't work with both power sources. The alternator on the boat might not like "driving into nothin'", in which case you could add a large capacitor there.

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  • \$\begingroup\$ The batteries have a built in BMS but from what I understand this is a safety feature and not a substitute for a charger? How do I work out exactly what capacitor I need? I would appreciate if you could point me to some resources that would help me understand the theories used to determine this. \$\endgroup\$ Commented Jul 5, 2022 at 12:32

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