Exploiting a large alternator via battery-to-battery chargers?

[john61ct]

those who can build their own lithium cell/BMS combo

Which is anyone really.

Note I am not one to insist "a BMS" as such is absolutely required either.

That is really just a collection of separate functionalities, different use cases may require redundancy, more or less automation. . .

Plenty banks are just bare cells closely monitored by a watchful owner, maybe even manually regulated for over or under voltage, fuses to handle over-current or an accidental short

cells only balanced when needed, can maybe go years without.

Overly trusting in a single all-in-one device may actually be less safe.

Too much complexity introduces more points of failure.

The key is understanding the fundamentals, selecting protections that fit your particular use case.

 

[DiploStrat]

Tanx! I have several friends who have built their own LiFePO4 banks, but they are younger, smarter, and more handsome than me. A friend suggested that I do just what you proposed and another does much the same thing using an Electrodacus as opposed to a Sterling B2B. I can think of two advantages, beyond price - you balance all of your cells at once, no danger of dueling BMS's, and a single dead cell can be replaced easily. Suspect that you could probably save some space as well.

What you are saying makes a lot of sense, even if you only do it "logically" as opposed to physically.

Tanx again!

 

[MTVR]

Okay, so John, are you saying that the 24V B2B should be considered part of the house battery bank, and that all electrical sources should be routed through it, and that nothing else should be connected to the house bank directly?

So for example, shore power could be as simple as connecting through a 30A connector feeding a dumb power converter which in turn feeds the B2B, and the B2B is thus responsible for administering the correct charging profile? That would be simple.

And generator power would be as above, plugged into the 30A connector after unplugging shore power?

And would 24V solar panels feed the 24V B2B directly, without a charge controller?

And the 24V alternator would obviously be connected to the 24V B2B.

How would you switch source inputs and how would you protect them from each other, or do you even need to? I realize that shore power and generator can be "switched" by unplugging one and plugging in the other, to the 30A connector.

Would you use a battery monitor as your sole source of information to monitor what's going on?

 

[john61ct]

Okay, so John, are you saying that the 24V B2B should be considered part of the house battery bank, and that all electrical sources should be routed through it, and that nothing else should be connected to the house bank directly?

Can be, not necessarily should be.

> So for example, shore power could be as simple as connecting through a 30A connector feeding a dumb power converter which in turn feeds the B2B, and the B2B is thus responsible for administering the correct charging profile? That would be simple.

Yes, great example "power converter" in this case being a PSU/rectifier doing the AC-to-DC.

That 30A bit of course depends on which DCDC unit and how many you choose.

> And generator power would be as above, plugged into the 30A connector after unplugging shore power?

Yes, that same PSU could be fed by AC from either shore mains or the genset. Ideally accepting either 110 or 240Vac for flexibility.

However make sure the genset is sized to comfortably carry the load you present to it; going too overkill high would also be wasteful.

______
> And would 24V solar panels feed the 24V B2B directly, without a charge controller?

In theory maybe but. . .

If you aren't making use of existing gear, IMO better to take advantage of MPPT's increased efficiency and get 40+V panels.

Maybe save a little money on the controller, but to be honest the Victron SmartSolar line are already such good value, and very adjustable, I would hook those up directly to the bank, no harm in parallel with other sources, running concurrently.

> And the 24V alternator would obviously be connected to the 24V B2B.

Yes.

> How would you switch source inputs and how would you protect them from each other, or do you even need to?

No need.

> Would you use a battery monitor as your sole source of information to monitor what's going on?

Depends on the monitor.

Bank voltage needs to be monitored for extremes.

Ah in and out from a shunt is very useful, should be easily resettable when you know reaching your definition of 100% Full, usually via acceptance rate / endAmps.

Estimating SoC% is fantastic **if** the meter is accurate, but closer than 4-8% is pretty challenging, need to cross-reference over time with info from the above, get a feel for just how close it gets, how often needs resetting.

Go here https://marinehowto.com and CTRL-F for

battery monitor

and read closely those articles.

Especially this one https://marinehowto.com/programming-a-battery-monitor

Everything at his site (MaineSail) regarding electrickery is worth close parsing

great intro to LFP care as well

 

[john61ct]

Yes, heard and read great things about Electrodacus, is he still active developing / supporting those units?

no danger of dueling BMS's

That should never be an issue.

Redundancy, layers of failsafe protection can be a good thing if a bank is worth many thousands

but that does not mean multiple "all in one BMSs", way too complex.

KISS a better approach.

 

[MTVR]

That 30A bit of course depends on which DCDC unit and how many you choose.

Just to clarify, I was talking about a 30A RV or marine type 120VAC input connector- I wasn't talking about only 30 amps of DC.

Plugged into that, would be either a household extension cord (with an adapter for the 30A connection) or a small 2kW(ish) generator. Either way, I don't see us regularly having access to more than 15-18 amps of 120VAC.

I figure (not counting conversion losses) that 15A of 120VAC is going to be worth roughly 150A of 12VDC or about 75A of 24VDC, which will hopefully get us close to that 0.4C charge rate, to minimize our charging time by taking advantage of how fast lithium batteries can be recharged.

If you aren't making use of existing gear, IMO better to take advantage of MPPT's increased efficiency and get 40+V panels.

Maybe save a little money on the controller, but to be honest the Victron SmartSolar line are already such good value, and very adjustable...

Okay- I can do the Victron SmartSolar charge controllers.

...I would hook those up directly to the bank, no harm in parallel with other sources, running concurrently.

So I could use generator AND solar at the same time, without risking going over 0.4C charge rate?

And I could use solar AND alternator at the same time, without going over 0.4C charge rate?

And regarding SoC, "my 100%" should actuay be a little bit less than the theoretical 100%SoC, for better battery life, right?

 

[john61ct]

Aha, yes.

Again, that 0.4C is a guideline rough as guts not a magic number.

If you are hooked up overnight, lower would be better for longevity

but really, might be a 50 cycle difference out of many thousands.

But in very cold weather, going down to 0.2C could be very important.

If you really have a strong reason to refill quickly sometime then a higher rate is fine, especially in hot weather.

Solar input is usually a small fraction of what you get from shore or ICE sources.

My point is, no need to worry about them interfering with each other

And yes, LFP should by default be kept at low SoC unless going to Full is actually needed at the time.

And Full should not mean going anywhere near the vendor spec for maximum voltage. I use 3.45Vpc myself when the charge rate is low.

Some cheap / stupid BMS require you to do so in order to balance, avoid those, adjustable is better, best of all let you balance at any SoC point you like.

 

[MTVR]

So 3.45 volts per cell is like 99% SoC?

How low can I discharge them on a daily basis, safely, if I want to keep them healthy for a long time?

 

[john61ct]

There is no magic hard line.

Best for longevity is cycling around the midpoint and lower % of capacity the better.

But of course they are expensive, and might die from something else over the decades.

So long as you stop well before hitting 3.1V, that might be triple the life cycles compared to 3.0V.

And that's an average, so occasional exigencies going low are OK, but actually approaching the rated lowest V is definitely unhealthy

and dead flat can render the bank worthless scrap.

So LVC can be worth redundant protections.

 

[MTVR]

So are you saying that an additional free-standing Low Voltage Cutout, redundant to the batteries own internal BMS LVC function, is a good idea? Victron makes bluetooth and non-bluetooth LVC units, although I don't understand Victron's disclaimer regarding them not being designed to handle charging current. Are they saying that their LVC needs an additional diode in series with it, to protect it, or what?

 

[john61ct]

I dunno Victron cutouts.

Yes in a larger system where extra automation is worth the investment

BMS is just last ditch protection, that LVC should never even be approached if you want good longevity.

You want adjustable setpoints on your various "userspace" circuits, grouped by how essential they are.

"Battery Protect" is a good search term.

Many choose to just switch loads manually

others custom program MCU to base off SoC rather than voltage.

Obviously each rig is different, and solar-only vs ICE energy available on demand

disaster prevention / safety / security
comms / navigation
lighting
ventilation / heating / cooling controls
refrigeration
screens / stereo / entertainment

The lower functions taken offline sooner, so the more mission critical ones can last longer.