B2B/A2B charger with decent regulation

[Mrkva]

Hi,
I'm currently designing a LiFePo4 system for my boat. Planned setup is: internally regulated 60A alternator, 400W wind gen, some solar (which is not important here), and 260Ah house bank size. Both wind gen and solar must not be open circuited when running.



The problem is the 260Ah bank will most certainly draw a significant current when significantly discharged. So, I need to limit the charging current.
I could wire windgen and alternator to AGM starter batt. Then get a 60A B2B charger. Except - the alternator can die even at rated output, when running in slow speed. And when I'll be capable of generating 90A, I want these 90A to go to the house bank, not being limited by the B2B charger.
And the big question here is whether the 60A B2B will work if the alternator is outputting a lower current, because of lower RPM. I'm a bit worried it will turn itself on, charge for 2 minutes, voltage will drop, B2B will go to standby for some time, turn on for 2 minutes... Sterling have not replied yet.



Or I can connect alt and windgen using Sterling A2B charger. There would be at least a temperature protection of the alternator - how much it'll like being overloaded is a open question.


Isn't there some decent B2B charger, that would monitor the voltage on the source system, and based on that regulate the charging current?

[john61ct]

Any solar controller or mains charger (really any device) designed for charging a storage bank will have current limiting built in.

Dunno much about controllers designed for wind, but have to believe the same.

All above should be wired directly to House.

That really only leaves your alternator.

Choice 1 is a single B2B charger, limited to under 60A, which is likely about the maximum continuous output from a 90A stock alt in warm conditions anyway.

To (try to) get higher current, either

1. convert to an external regulator, or

2. parallel a second B2B

The A2B unit is only for alt output.

Those are not IRL problems you're speculating about otherwise, B2B handles all that just fine as a general regulator, but simply not needed for solar or wind.

[Mrkva]

Well, it is my undestanding that nothing must be connected directly to the LiFePo4 battery - everything needs to be connected to a "charging" or "load" bus, both of those disconnectable from the battery by BMS in case of too low or too high voltage.


The manual of (at least) D400 and couple other units says it should be connected to battery at all times, so here's that.


If those other possible issues I've been worried about that are in fact non-issue, that's great! Thanks!

[Justin Cook]

Originally Posted by Mrkva

Hi,
And when I'll be capable of generating 90A, I want these 90A to go to the house bank, not being limited by the B2B charger.
And the big question here is whether the 60A B2B will work if the alternator is outputting a lower current, because of lower RPM. I'm a bit worried it will turn itself on, charge for 2 minutes, voltage will drop, B2B will go to standby for some time, turn on for 2 minutes... Sterling have not replied yet.


Isn't there some decent B2B charger, that would monitor the voltage on the source system, and based on that regulate the charging current?

I will note that the Sterling B2B chargers will turn themselves on/off based on voltage, not amperage, so as long as your alternator is sending charging voltage to your start battery (Sterling B2B default is 13.6v, but this is adjustable), the Sterling will be on and charging your house bank.

That being said, Victron's B2B chargers are almost into the US as well (we should have them in stock by mid-February at latest), and although they're lower-A (30A max for a 12/12 model), you can of course parallel as many of them as you like (though that goes for the Sterlings as well). Besides possessing a much smaller form factor, a primary selling point of the Victrons will be the bluetooth interface, which makes setting up your voltage triggers and charge profiles much easier than the Sterling units.
In both cases, however, whether the B2B charger is on and charging is determined by voltage at the start battery rather than particular amperage of your alternator.

[YANDINA]

The Combiner 100 limits alternator overload by introducing cooling cycles if the alternator cannot maintain voltage but once the destination battery accumulates sufficient charge that it is no longer an overload it will stay on 100% and handle the output of alternators rated up to 100 amps.
The Combiner 200 and the Combiner 600 have the same features and can carry alternators up to 200 and 600 amps.

[john61ct]

Overcurrent **protection** is certainly a good thing, both to protect the alternator from overloading / overheating, and

to avoid reducing LFP bank longevity with too high a C-rate.

But most owners would want to keep charging the whole time, which a **current limiting** B2B or external VR can provide,

rather than hiccup mode OCP.

Obviously less important a distinction if you're running your engine many hours every day anyway.

[YANDINA]

Originally Posted by john61ct

Overcurrent **protection** is certainly a good thing, both to protect the alternator from overloading / overheating, and

to avoid reducing LFP bank longevity with too high a C-rate.

But most owners would want to keep charging the whole time, which a **current limiting** B2B or external VR can provide,

rather than hiccup mode OCP.

Obviously less important a distinction if you're running your engine many hours every day anyway.

Not really the case, in actual use the overload current includes the alternator output plus battery to battery current due to the low target battery voltage. This can be well over a 100 amps. When it "hiccups" the amp hour or two that was transferred in battery to battery current is then replaced in the starting battery until the next cycle repeats so the result is all "available" alternator output is ending up in the target. Typically about 1 to 2 amp-hours transfers in each cycle until the additional load is no longer drawing excessive current and cycling ceases.

[john61ct]

But if a LFP target bank may be trying to pull 500A from that 100A source,

wouldn't that make the isolate periods longer than the combine ones, for an extended time?

If I were specifically running the engine for charging purposes I'd want to minimize runtime by using a current-limiting VR that keeps delivering the maximum rate possible, rather than an on/off duty cycle.

[YANDINA]

true but it will cost a lot more than $55.

A current limiting transfer (other than a resistance) would use PWM switching at high speed. This is doing the same thing but at higher speed, fully ON for x% of the time and OFF for 100-x%, transferring smaller packets at a higher rate.

[john61ct]

If someone's stressed about a few hundred bucks, they shouldn't be dreaming LFP in the first place.

Reducing engine runtime over the years would usually be worth the difference.

[Mrkva]

Why's that? LiFePO4 can easily be the cheapest option in a long run.
My 260Ah setup will cost me around 1700 USD (around 350 in it is a balancer/BMS).
For that, I'll get around 5000 cycles at 50% discharge - a total of 650kAh will go to battery and back.
Deep cycle AGM lead batteries can be discharged to 50% as well, and will last around 800 cycles. That means I have to replace lead battery over 6 times. hat's around 4k USD total.
Flooded batteries will be around the same price as lithium as well...

[john61ct]

Quality FLA deep cycling costs under $1 / Ah in the US, and no changes to VSRs, chargers, your alt setup

And the infrastructure to care for LFP so you (maybe) get the decades' lifespan needed for ROI on the difference can cost as much as the cells.

I think LFP is fantastic, just need to justify it functional advantages not economically.

My point was cash on hand needed anyway, not amortized lifetime expense