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DC DC Charger for alternator and lithium charging

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WaldPinkler:
There was another thread on this forum to ask about a 60A DC DC charger from alternator. Unfortunately no real answer.

The main issue is the price. Like $500 for a 20A charger or $700 for a 40A. It seems excessive. Is there really something special about a boost converter? I mean the lithium charge cycle isn't rocket science.

I believe the basic idea is the alternator charges the lead acid bank, which when running, switches on the DC DC charger to the lithium bank. Many alternators may cook themselves going straight to lithium due to their low resistance and alternator outputting at maximum. Also is the issue of the lithium BMS switching off when full so there's nowhere for alternator power to go.

My alternator puts out 80A so much would be wasted unless multiple chargers are used. It's a dumb alternator but says it has temperature compensation however that works? But how to let it output to lithium bank and have the starter charge also?

The question is, what is the high price for? Are there cheaper alternatives for DC DC charging alternator to 12V (14.6V max) lithium? It seems to be a such a simple operation.

richard.cs:

--- Quote from: WaldPinkler on July 13, 2021, 01:59:26 am ---The question is, what is the high price for? Are there cheaper alternatives for DC DC charging alternator to 12V (14.6V max) lithium? It seems to be a such a simple operation.

--- End quote ---

If the alternator is doing nothing else but charge the lithium battery then I have no idea why you would use a DC-DC for this or have a lead-acid stage. Instead rip out the lead-acid-optimised alternator regulator and replace it with one that produces the output voltage and current limit you want, with whatever temperature compensation you like.

I would start by deciding on the lithium charging algorithm, that then gives you voltages, currents, temperature compensation, time profiles, etc. That can be implemented in a small micro as the top-level behaviour, which provides inputs to two control loops (probably also in the micro, though these loops could also be implemented externally with opamps), one voltage and one current. The hardware has to provide suitably scaled voltage and current measurements, and the control loop adjusts the alternator field current servo the output to whatever it should be according to the charging profile.

One of the advantages of pulling the control loops into the micro is you can better optimise the transition behaviour between CC and CV modes, but it also just means fewer parts and easier to tweak.

WaldPinkler:
There's a lithium house bank and a lead acid engine starter. Makes sense to keep it this way.

Programming a micro with the lithium charge cycle is no problem. I have voltage and current sense devices already that can be attached to battery and alternator. Some temperature monitors on the way actually.

Unfortunately alternator regular is internal. Not sure why kind of rabbit hole it is to replace but I think it's doable. A nice project and possibly not that complicated.

richard.cs:

--- Quote from: WaldPinkler on July 14, 2021, 03:15:02 am ---There's a lithium house bank and a lead acid engine starter. Makes sense to keep it this way.

--- End quote ---
I agree, I would keep a lead acid for the starter, but if all your load is on the lithium it makes sense to optimise the alternator to lithium charging and charge the starter battery either with a small DC-DC from there or with a separate small alternator. Unless you're starting and stopping many times an hour there's not a lot of average load on the starter battery.


--- Quote from: WaldPinkler on July 14, 2021, 03:15:02 am ---Unfortunately alternator regular is internal. Not sure why kind of rabbit hole it is to replace but I think it's doable. A nice project and possibly not that complicated.

--- End quote ---
Looks like you have a combined regulator and brush assembly, that's pretty common, a little harder to mod than when they're separate but not too big a deal. I have modded one like that before, I don't remember the specifics but I think I pretty much just pinged it open, snipped the regulator wires and soldered my own on and clipped/glued it back together. If you trash it they're cheap enough to buy and try again. :) The only annoyance is that you have to do it again if/when you change the brushes.

WaldPinkler:
I thought a big advantage of alternator to lead acid was to avoid a load dump where the lithium BMS disconnects when full then burns out the diodes.

If a modified regulator for lithium is added then it's not really a point. And you're right, 10A buck boost is pretty cheap to go DC-DC for the starter bank. Unfortunately I think sail winch, anchor which and thruster are all on the engine circuit. Nothing is simple.

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