Trying to charge a Li battery with boat motor.

[MtnMan]

Hi, I new to the forum.
I have recently setup my boat with a 100Ah Lithium iron phosphate battery which powers the nav lights, sounder and the electric trolling motor.
It is setup in a box with a Projecta IDC25X DCDC charger as I can also use this battery when camping to power a camp fridge and lighting. It has alternator and solar inputs for charging.

My hope was that I could use the stator output from my boat motor to charge the battery while motoring along but simply connecting the output from the rectifier regulator to the DCDC alternator input doesn't work. The voltage of the rectifier regulator output is within spec for the DCDC but I suspect that the voltage ripple is not acceptable for the DCDC.

I have tried placing a 12Ah AGM battery between the rectifier regulator and the DCDC and this enables it to charge the Lithium. However the DCDC seems to want to dump 24amps into the Li battery while the rectifier regulator provides 14.5v 8A to the AGM battery. The DCDC will switch on and off as the voltage of the AGM fluctuates. I'll be motoring along, the AGM voltage will build up to 13.4v then the DCDC starts charging the Li battery, it dumps 24A until the AGM battery voltage reads 12.4v from memory. Then the rectifier will charge the AGM up go 13.4v then the cycle repeats. This heavy discharge current and high charge current from the rectifier makes me think the AGM will fail in short order. Using a larger AGM negates the weight savings of getting the Li battery in the first place.

Now my questions-
Can I replace the rectifier regulator with just a full wave rectifier and something to smooth out the ripple?
Both the alternator and the solar inputs of the DCDC accept up to 32v DC. The output of the stator is 28-30v AC 100w.

How do I go about smoothing out the ripple?

Thanks for hanging in there this long with me trying to explain my problem.

 

[Diver300]

I don't think that the ripple is a problem in itself. I think that the problem is that the DCDC charger is expecting to be able to draw lots of current from the 12 V supply. In effect, it is expecting a battery or a much more powerful supply.

Have you tried connecting the alternator output to the solar input on the DCDC charger? The voltage / current curve of a solar panel will be similar to that of the alternator so the DCDC charger may work out that it can only use the 8A from the alternator. Are there settings on the DCDC converter that can change how fast it tries to charge the Li battery?

Just as background information, a solar panel will produce a certain amount of current, and altering the load voltage won't change the current much until the maximum voltage is reached. An alternator is similar, in that the current it produces won't change much if the load changes, until the regulator limits the voltage.

Batteries are completely different, and the current they produce varies a lot as the load changes.

 

[MtnMan]

Thanks for the reply.

I haven't yet tried connecting the boat motor output to the solar input of the DCDC, but I will.

The DCDC current output isn't user selectable.