Charging LiFePO4 batt from outboard motor - Page 1
Forum section: electronics / projects

author: frogblender
author_rank: Regular Contributor
timestamp: April 20, 2025, 04:13:43 am
content: I have a 30HP johnson/evinrude 2-stroke. I recently discarded the 20kg lead-acid starting battery, and replaced it with a 1.5kg LiFePO4 battery.The LiFePO4:- has no trouble whatsoever cranking the starter;- at $50USD from amazon, is cheaper than a lead-acid replacement- yes, weighs 1.5kg, compared to 20kg lead-acid- has only 3AHr... which is NOT alot. But it has no trouble cranking, and cranking more, and even more, until I realize the gascan vent is closed.- has an internal BMS.The problem: The outboard motor has magneto-style charging (magnets glued to the flywheel, which whiz past three coils spaced 120° apart). The three coils feed a 3-phase rectifier (andonlya rectifier). There is noregulator.... which means under normal conditions a lead-acid battery will be overcharged... except the coils are weak, and can't deliver much current to a fully-charged battery, and thus can't wreck the lead-acid.However: on the LiFePO4: as soon as the LiFePO4 is charged (14.6v), the coils keep trying to raise the voltage, at which time the BMS entirelydisconnects, and with very little load, the charging voltage soars to 25v or so. Which is not good for the LED bow lights. Nor anything else.Has anyone come up with a solution for charging a LiFePo4 battery from an outboard motor?

author: rteodor
author_rank: Frequent Contributor
timestamp: April 20, 2025, 06:21:53 am
content: Quote from: frogblender on April 20, 2025, 04:13:43 amWith a peak of 25V the simplest solution would be an MPPT charge controller. May be one for wind turbines but a solar MPPT should work too.It might benefit of some filtering after rectification.On a more thought a cheaper PWM charge controller could work too.

author: pcprogrammer
author_rank: Super Contributor
timestamp: April 20, 2025, 07:05:10 am
content: Switch on some extra load when the battery is fully charged. This will keep the voltage at a lower level. Another way to handle this is to use a zener based solution that maintains a somewhat constant current flowing and keeps the voltage in check.To come up with a circuit more information is needed though. How much current can the coils deliver, what is the nominal voltage you need, what current do the LED bow lights draw and what supply voltage do they take. What more is connected to this setup.

author: David Hess
author_rank: Super Contributor
timestamp: April 20, 2025, 12:13:40 pm
content: Quote from: pcprogrammer on April 20, 2025, 07:05:10 amHuge stud mount zener diodes used to be readily available for use as power shunt regulators, but that is decades in the past. A bunch of silicon rectifiers could be placed in series to achieve the needed voltage and power handling capability.A simple zener and transistor power shunt regulator can clamp the voltage, with power limited only by the transistor or transistors.An adjustable series regulator like a 317, or more likely the 338 5 amp version, can be configured as a shunt regulator by shorting the output.

author: bdunham7
author_rank: Super Contributor
timestamp: April 20, 2025, 06:41:00 pm
content: Your best bet would be to use a filter capacitor and voltage regulator configured as a battery charger with a current limit. How much is the maximum current output of the system?

author: mtwieg
author_rank: Frequent Contributor
timestamp: April 20, 2025, 08:05:47 pm
content: Quote from: rteodor on April 20, 2025, 06:21:53 amA charge controller (with or without MPPT) might solve the issue of overvoltage on the battery. But in order to make us of the battery to power everything, you'd have have to power all those systems from the output of that charge controller. I'd be concerned that isolating the alternator like that might have unintended consequences... also you would likely need load dump protection on the charge controller.Simply putting a voltage clamp (effectively a big fat zener) across the battery sounds like a better approach. So long as it's properly rated for the max output the alternator could provide.

author: mikerj
author_rank: Super Contributor
timestamp: April 21, 2025, 04:55:27 pm
content: Replace the rectifier with the 3 phase rectifier/regulator from a motorcycle, almost anything above ~250cc or so made in the last 20 years will have one. Do not buy the unbranded Amazon/eBay crap, these are notorious for badly made counterfeits.

author: rteodor
author_rank: Frequent Contributor
timestamp: April 22, 2025, 05:49:49 am
content: Quote from: mtwieg on April 20, 2025, 08:05:47 pmSorry but I do not see a reason why the alternator could not be separated. May this be related to its wiring from construction ?Also why not set the controller in constant float voltage and connect the all loads directly on the battery ? Chinesium charge controllers (and not only them) have current shunt and/or load switch on the low side of the load output so that output shall of course have nothing connected to it.Anyway, any solution will work as long as it does not exceed float voltage for significant amount of time. LFP is sooo much easy to overcharge, or in this case, trip the BMS overcharge protection.