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What I've got

5x, 200Ah LiFepo4 batteries in a 12V, parallel bank. I'm looking at putting in a Sinister Diesel 320A alternator to supply loads of juice to the battery bank. If this is even possible, without damaging the batteries and circuit, then I figure I don't really need to go the standard roof, solar panel array nonsense.

How I understand it

Low-powered alternators can get overloaded due to the very low internal resistance of lithium batteries drawing too much current from the alternator, causing it to overheat and burnout (if there's no dc-dc charger). My batteries have a 100A max input charge and 35mΩ resistance.

Questions

Q1 - do I have this right? - the input current will divide evenly among the batteries so each battery will "see" roughly 320A / 5 = 64A?

Q2 - these batteries are equipped with an internal BMS. Does this control the battery charging such that it "shuts off" the battery from the circuit when it's fully charged? That's wishful thinking, probably, the BMS is more about internal cell balancing and not preventing harmful currents?

Q3 - overall, is what I want to do possible? I'm not going to hand-build any necessary parts, but I'm happy to purchase the proper hardware. However, as far as dc-dc chargers go, I wasn't really seeing anything above 60A. I guess that'd be OK if I put a separate dc-dc charger on each battery? ... although that'd be one charger per battery which is annoying and costly.

Thanks for any guidance!

Update: After considering everyones' feedback (thanks!) and ruminating on it for a while, I've decided just to go ahead and get a DC-DC charger. The output voltage of an alternator isn't quite high enough to bring LiFepo's up to a full charge. It took some hunting but I finally found a single maker that has a high-amperage charger that should do the trick: Sterling Alternator-to-battery Charger. So I'll put this between the beefy alternator and the battery bank. However, I'm still unclear on how to protect each, individual battery: there will be, say, 300A feeding from the charger into the parallel bank. Each battery can't get more than 100A applied to it. Is there some hardware I can install on each battery to limit the incoming current? Is there hardware that can fully open and shut off current when the full-charge voltage has been achieved?

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    \$\begingroup\$ The BMS is a backup in case your charger goes haywire. You should not rely on it for charge termination regularly. That is like crashing into a wall because you are wearing a safety belt. You really need to talk to the alternator people to see if the alternator can be configured to charge your batteries properly. I wouldn't think you would need anything between the alternator and the batteries other than fuses and disconnect switches. And very, very thick cables with flawless termination. Fire is a real possibility unless everything is perfect. 320 Amps is MORE than your starter uses. \$\endgroup\$
    – mkeith
    Oct 24, 2020 at 1:10
  • \$\begingroup\$ Is the alternator going to go in its normal place next to the engine, and also be connected to the vehicle's lead acid battery? If this is the case, you have two batteries to deal with, which may be at different voltages, and this could be a problem. \$\endgroup\$
    – bobflux
    Oct 24, 2020 at 13:31
  • \$\begingroup\$ Yep, it will. And yes it'll be charging both types. I'll be using an automatic battery isolator switch that only closes when the voltage is at the correct level. It's mostly just to disallow the accessories from discharging the starter battery; the output voltage of the alternator is made for this application. Here's a good video showing its performance (note the voltage maintaining around 13.7V throughout the test). youtube.com/watch?v=VTjAS-tVYCQ \$\endgroup\$
    – xanadont
    Oct 24, 2020 at 17:38
  • \$\begingroup\$ OK so you don't need a voltage booster or anything since it outputs the proper regulated voltage, right? Is the alternator's regulator aware that it is charging the LiIon? I mean, the regulator senses battery voltage via an extra wire to compensate for voltage drop in the high current wire... but if it is charging the LiIon, will the sense wire sense the voltage at the correct place? \$\endgroup\$
    – bobflux
    Oct 24, 2020 at 18:26

1 Answer 1

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Q3- So you need just a little more complex system: Alternator -> Boost convertor -> bulk capacitor -> Charging circuit -> battery.

The battery charging current must be controlled- otherwise, you risk damaging the battery, and basically, there is no reason not to do it. Also, this will allow adjusting the charger to various battery setups.

The boost converter will take whatever the alternator provides up to a certain voltage, allowing a stable input voltage for the charger. The reason not to try combining booster and charger is this- if you do, the input of the charger may not be stable enough to sustain the normal operation. This is why some bulk capacitor is required in between.

So yes, the currents are going to be pretty wild, and controls require some minimal EE experience, but all of it is totally doable. Actually, I am now working on a system capable of performing in this range.

Q1- no, not exactly. It greatly depends on what is allowed on the batteries and what does the charger support. Whent he battery is empty (voltage significantly lower than the charger input voltage) you may be able to push more current- it's a thing about energy conservation, you take I1 from the voltage V1, put it into V2 with the current I2, I1 * V1 = I2 * V2 (neglecting losses), but since V2 < V1, I2 > I1. But as a rule of thumb what you write is fine.

Q2- i wouldn't count on it, as you say, it's probably all about balancing and shut off on overcurrent/overvoltage, but charging current should be controlled externally.

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