Outboard Motor charging of LiFePo4 Battery

IdahoFlyGuy

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May 22, 2021
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I have a small 20hp Mercury outboard motor. It has electric start so has a charge coil to charge a start battery. It is a small boat so I don't want to add the weight of a starter battery but I am interested in using the charge coil to charge up a 12V 18ah LiFePo4 battery to run my fish finder. The charge coil is rated at 4.6A at 900 rpm and 11.5 A at 5,000 rpm. I do a lot of putting around at low speed mapping with my fish finder and wanted to use the outboard to keep the battery topped off when I did that. The fishfinder takes 1.8 amps. I haven't been able to find any small DC-DC chargers but wondered about using a small PWM solar charge controller. I see that Renogy has the Wanderer 10A PWM Charge Controller. It says it can be used with lithium and is only 10 amps so seemed like possibly a good match.

So my question is can you use a PWM charge controller with a charge coil and should I be worried about the 10 amp controller being to big for the 18ah LiFePo4 battery. I don't really understand the difference between a DC-DC charge controller for charging off an alternator and a solar charge controller so looking for some education. Then there is also the fact that this is a charge coil and not a true alternator.

Thanks for any insight you can provide.
 

HRTKD

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A solar charge controller will typically handle input voltages higher than 12v and convert down to 12v. A 12-12 DC-DC charger is expecting voltage to be in the 12 volt range.

Before buying any Renogy equipment, I would find the cheapest PWM solar charge controller and try it out first. Renogy's quality and customer service isn't very good right now.

10 amps into an 18 Ah battery could be a problem. Exceeding a .5C charge rate could be tough on the LiFePO4 battery. Check your battery's specification for the maximum allowed charge rate. Playing devil's advocate, that 10 amps would likely be seen at the battery only if the battery was at a very low state of charge. Probably.
 

IdahoFlyGuy

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May 22, 2021
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HRTKD, thanks for the reply, any suggestions on other 10A or lower PWM solar charge controllers that are LiFePo4 compatible. I didn't see any on Northern Arizona Wind and Sun that would work with LiFePo4, they are all just for lead acid. The Renogy one was the only one I found.

Thanks
 

HRTKD

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Here's a 5 amp PWM solar charge controller.

With my MPPT solar charge controller, the incoming voltage has to be +5 over the battery voltage for the MPPT to start charging. I don't recall if PWM has the same limitation or not.
 
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