Correct FLA Battery Size for Lithium Hybrid Configuration

[rgleason]

Considering this Franz Veldman "Lithium Hybrid Configuration" where a FLA battery is used in parallel with the LiFepo4 Bank with BMS, which allows the charging alternator to discharge to the FLA when the BMS cuts out the LiFePo4 batteries.


How should we size the FLA to prevent alternator rectifiers from frying and bad voltage spikes and transients when the BMS cuts out and shuts down the LiFepo?


For a hot alternator continuously charging at 100 amp output, what size FLA should be used to prevent alternator damage? Smallest appropriate size?


Formula?

[Bill O]

Rick,
Was hoping someone would help out w/some pointers to answer your question as this seems like a viable option to add some lithium w/o the major investment to change all the systems.

[Dsanduril]

I feel slightly responsible for possibly having sent you down this rabbit hole I don’t have an answer but am posting to see what answers you get.

Over at Nordkyn Design Eric just says a “starting battery” is sufficient to absorb transients. In his hybrid diagrams he discusses a sealed lead acid battery, from context “small”, but I haven’t seen details.

For more than you ever wanted to know about transients Littlefuse’s paper is quite interesting. In a car they estimate disconnect spikes at 10J. From looking around at various sources even at high charging currents I suspect 10-20J (over 250-400ms) is what would be expected.

1Wh is 3600J, so one Ah (at 12V) is 43200J. Putting 20J into a 50Ah battery, even if it is close to full is a relatively small event.

Another consideration is having enough useful capacity in the LA to run for some period of time while you sort out any possible problems. For me that would be about 12 hour capacity, which leads to 120Ah battery (at 50% DoD). I think that leads to transient suppression being a minor consideration in comparison.

[rgleason]

Thank you. Very helpful. If I were to go with FLA 120ah in parallel with LFP/BMS that would surely protect the BMS when starting from the house battery wouldn't it?
(3ym30 - 30hp)

Also would I even need the second battery bank? -just thinking.

I suppose another way to look at it is to be sure the FLA in parallel will start the engine, because those spikes will be greater than the alternator in all likelihood (?).

I've been looking at Littlefield diodes to help protect. Mitiempo says that is too complicated and you have to always check them to be sure of protection. It's a good website.

[rgleason]

From Littelfuse's paper that Dsanduril's post referred to.


Load Dump

The load dump over voltage is the most for midable transient encountered in the automotive environment. It is an exponentially decaying positive voltage which occurs in the event of a battery disconnect while the alternator is still generating charging current with other loads remaining on the alternator circuit at the time of battery disconnect.

The load dump amplitude depends on the alternator speed and the level of the alternator field excitation at the moment of battery disconnection.

Independent studies by the Society of Automotive Engineers (SAE) ha ve shown that voltage spikes from 25V to 125V can easily be generated [1], and they may last anywhere from 40ms to 400ms.


The internal resistance of an alternator is mainly a function of the alternator rotational speed and excitation current. This resistance is typically between 0.5Ω and 4Ω (Figure 2).


Which I think is within Dsanduril's calcs.

[Dsanduril]

That, along with their table, are where I started. I suspect, however, that SAE's tests didn't include a load dump from an over-sized alternator as might be found on a boat (with lithium...). Thus my thought on something closer to 20J. Most of the big (but maybe not the giant ones - haven't really looked at them) alternators still have a 6A maximum field current, so the size of the collapsing magnetic field is similar even with high output. If the winding resistance goes down and the field current goes up then I suspect things get a bit worse.

One other point, Nordkyn says transients to 100V, but probably from a similar data source.

[rgleason]

For example, I have a 3ym30 30hp diesel engine which will certainly start with a DieHard AGM Battery

• Amp Hour:70.0
• Group Size:24F
• Cold Cranking Amperage:710
• Cranking Amperage:885
• Height:8.68" Length:10.937"

This is smaller than the 120ah bigger Group 31 battery suggested by Dsanduril, however I hope it would be adequate for BMS Cutoff of the Alternator and reasonable storage to dump the charge and suppress voltage transients. The current batteries are Trojan T-105 6vdc GC2 in series which are just a little over Group 24 for 215ah. When these are done, I would like to simply have a start battery in parallel with a 200ah lithium-hybrid setup. Since it is over 50ah, it should be fine.

I am considering eliminating the second bank.

PS: Just saw this below:
"Thus my thought on something closer to 20J. Most of the big (but maybe not the giant ones - haven't really looked at them) alternators still have a 6A maximum field current, so the size of the collapsing magnetic field is similar even with high output."

"...various sources even at high charging currents I suspect 10-20J (over 250-400ms) is what would be expected.
1Wh is 3600J, so one Ah (at 12V) is 43200J. Putting 20J into a 50Ah battery, even if it is close to full is a relatively small event.

I wonder if we should have a field cutoff when the BMS relay cuts off? (maybe not needed!) Would that be so hard to do? Wouldn't that help a lot? How could I accomplish that?