Trying to charge a Li battery with boat motor.

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.

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.

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.

So if I wanted to try a rectifier with a smoothing capacitor how do I work out what components to get?
I understand the formula- C=I/2f x y. It's getting the input values for the formula that I'm not sure about.
The boat motor output is 28-30v AC 100w. It revs to about 5000rpm so not sure of the frequency. Is it as easy as 5000÷60= Hz. ?
Also not sure what ripple factor to input.
What rectifier specs would be acceptable? Does it need to be excactly 30v 4A?

The capacitor value is worked out from the ripple current, the lowest frequency and the allowable voltage ripple.

The capacitance you need is the

ripple current / ( freq * voltage ripple)

The frequency will be a multiple of 5000/60, but without knowing the construction of the alternator it's not possible to know what multiple. However, the frequency that you need is lowest frequency, not the highest.

However, I don't think that the problem is ripple in the conventional sense. If the charging runs at 24 A, and the generator runs at 8 A, then the current needed from the capacitor is 16 A. If you want that to run for 5 seconds at a time, between turning on at 13.4 V and turning off at 12.4 V, then the frequency is 0.2 Hz and the "ripple" would be 16 A, and the voltage ripple would be 1 V (between 12.4 V and 13.4 V)

That gives a capacitance of about 80 Farads. As most supercapacitor work at about 3 V, you need at least 5 in series, so they need to be 400 Farads each or more.

You could go for something like this:- https://uk.farnell.com/vinatech/vec3r0-507qg/cap-500f-3v-super-cap-radial/dp/2663723

That would be lighter than a lead-acid battery, and far more suited to being charged and discharged repeatedly.

I suggest adding some balancing resistors. Here is a link which says what needs to be done:-https://www.analog.com/en/resources...uper-capacitor-connection-for-max3888689.html

The engine revs at idle would be less than 1000rpm so that would be the lowest frequency.

If I try this with a rectifier only I'll connect it to the solar input of the DCDC. It doesn't seem to demand as much current. Running on a solar panel the most I've seen it draw is around 15A.

The 5 second cycle you mention would be too short for the DCDC. It takes up to 30 seconds to start charging after receiving input current.

Not sure of the stator configuration.
It simply has 2 wires coming out of the stator for AC output.
Does that mean 2 poles? Does that mean 1 sine wave/rev or 2 sine waves/rev?

The number of wires has nothing to do with the number of poles. You might be able to work out the number of poles with magnetic compass or some other way of finding where the magnetic poles are.

If you assume a 2 pole magnet and 1000 rpm, that is 17 Hz. If you full-wave rectify, that is 34 Hz ripple. For 8 A, and 1 V of ripple that would need around 220,000 μF. However I am not sure if that is a good way of doing it as that is very large for a standard capacitor (not a supercapacitor). Most battery chargers just let the ripple continue into the charging current because it doesn't matter if the current varies.

Do you think something like this is suitable?

MtnMan said:

Do you think something like this is suitable?

https://www.aliexpress.com/item/1005004567161382.html?src=google

Click to expand...

That is just for the circuit board. There are no capacitors fitted, just space for two 25 mm diameter capacitors or one 35 mm diameter capacitor.

It's actually a pack of 10 boards.

Have you tried connecting to the solar input of the battery charger yet?

Oh, looked like it was with the capacitors.
Haven't had a chance to try it in the solar input yet.

I tried it with the rectifier regulator that's on it now connected to the solar input of the DCDC charger and it didn't work.

Have you asked Projecta? It seems to me that if you could reduce the charging rate it could work fine, maybe with some capacitor to take out the ripple.

Asked them all about it before I bought it and never got a reply.

I'm sorry to say that I don't have any other ideas. I can't tell what the DCDC charger is expecting or what settings can be changed on that.

MtnMan said:

I tried it with the rectifier regulator that's on it now connected to the solar input of the DCDC charger and it didn't work.

Click to expand...

Try it to the solar input with the AGM battery added to the regulator output.

Ok so I got a full wave rectifier and a capacitor and tested again.

The rectifier is 50v 15a and the capacitor is 50v 22000uf.

The DC output is now 70v! Why so high now? The AC output of the motor is 32v.

Now I need a buck convertor just to drop the DC volts down to a maximum of 32v before I can see if the charger will work since that is the maximum that the DCDC charger will accept.