House and Engine Battery -Update 2-Surge? REGEN threat!

Someone mentioned this thread, so I'm jumping in mid-stream. Pardon if I missed some context.

Regarding "load dumps"... there may be some well known BOAT concerns creeping into some RV discussions. BOATS are famous for having multiple battery banks behind a big "A":"B":":"A+B" red selector switch, in part so one battery bank can sit disconnected in reserve. If the selection switch is used while the alternator is running, the alternator can be toasted by 100VDC+ voltage spikes when a load dump occurs between the switch breaking and making contacts. With the introduction of LiFePO4 batteries with BMS's that can suddenly disconnect, this general class of topic is starting to come up more and more in the RV world.

Without a more modern charger in the mix, spikes towards your alternator and your RV electronics can occur when a BMS disconnects. I don't know if having a lead acid battery in the mix (next to the alternator) partially or fully clamps the spike. Sterling's alternator protector is just a voltage clamp device.

Here is someone's attempt to better explain the load dump problem (~2/3rds down in the article). It is sales pitchy, but some of the points are good.

https://marinehowto.com/understanding-the-sterling-power-pro-batt-ultra-battery-to-battery-charger/

Wrt other merits of a DC-to-DC charger. I posted the following to a nearby forum.

FLA batteries start to accept a charge and pull amps at 12.6V, but LFP batteries start that process at 13.6V, a whole volt higher. Depending on your alternator's voltage output and the voltage drop in your wiring, LFP could charge SLOWER than FLA, and FLP charging could fail to reach 100% SOC where-as FLA could.

With my 45A Progressive Dynamics Inc charger running in FLA mode, my old ~200Ah of FLA would pull 18A over 10ga wiring. Still running in FLA mode, my 200Ah of BB LFP at 30% SOC only pulls 4A over shorter and bigger 8ga wiring. Putting the charger into boost/LFP mode (boost the voltage) increases the pull to 39A. Point being, same charger, the LiFePO4 charge rate is 1/4th the flooded-acid rate.

Sterling (maker of DC-to-DC alternator chargers) has some nice videos on YouTube where they wire LFP direct to an alternator with ~4ga wire, and the charging rate is pathetic. Then they insert a DC-to-DC charger in the line and the charge rate jumps substantially, because the charger can re-boost the voltage.

All in all, current is pulled based on volts being delivered. For 7-pin systems, the Achilles heel is voltage. Solve the voltage problems first, and then you can start to worry about the current problems next.

--tg

Looks like there is enough info now in replies here for folks to decide what to do or not do with their set-up designs. Good work, guys!

Mouse Milk BFL

"Sterling" is as British as "Chatham" or "Fartface At Sea"

I have far too much experience with true A.C. Eddy Currents circulating in alternator stator laminations, perhaps 3x10 as powerful as what a string of batteries can produce. Precision indexing of rotor halves, and uniform coating of silicon on stator laminations can reduce heat as much as 22c Delta T.

But at the scale you describe it is almost a mockery of professional electrical engineering. A 7th Grade Science Project.

A pair of DC/DC boosters with precision voltage regulation and a voltage comparator (meaning an interface) would ensure priority balanced charging. This would be useful if a second bank was remote as with a trailer and a main connector line that could deliver full charging potential when called upon to do so. This simply isn't your Gizmo.

I would not use a Gizmo even as a gift. There are a hundred ways it can fail and a majority of them can damage the batteries. Risk the potential cost of Lithium batteries? I don't think so.

BFL13 wrote:
The term "eddy currents" is used here for parallel Li strings. ISTR seeing the "eddy current" term for ordinary batts too back when, which is where I got mixed up I guess. Whatever! Scroll down to "Eddy currents"

https://www.orionbms.com/manuals/pdf/parallel_strings.pdf

Also, on selling--note that Class C guy got his Sterling gizmo from Battleborn and his understanding of it came from Battleborn. Sterling apparently sells them in the UK for "caravan" towing for the DC-DC, but I don't know anything about their claims if any about surge protection.

There are about 3000 cells in an EV where this might be a consideration. RV with two batteries? Not so much.

Don't need to investigate utility scale generation and transmission to plug into your Honda 2000. The same issues simply do not exist.

Mex, there seems to be at least two different meanings for "eddy current", and yours is the main one. Google "battery eddy current" and see the different topics using the term.

Eddy current = Faraday's Law of Induction. Why oh Why are we here pray tell?

Induction heating? Conflict in paralleling transformers? Leakage to earth? Open wings or sag in a Faraday cage field? Or perhaps it's a conflict between a pair of transformers. One being subtractive polarity the 2nd being additive polarity.
All purely directed at charging a battery.

The term "eddy currents" is used here for parallel Li strings. ISTR seeing the "eddy current" term for ordinary batts too back when, which is where I got mixed up I guess. Whatever! Scroll down to "Eddy currents"

https://www.orionbms.com/manuals/pdf/parallel_strings.pdf

Also, on selling--note that Class C guy got his Sterling gizmo from Battleborn and his understanding of it came from Battleborn. Sterling apparently sells them in the UK for "caravan" towing for the DC-DC, but I don't know anything about their claims if any about surge protection.

BFL13 wrote:
Barre says, "Also, if one battery has a higher voltage than the other, a small circulating current develops between the two parallel batteries...."

No there is no circulating current. There is simply a direct one-way transfer of energy until the batteries are of equal voltage. Then the transfer stops. No eddy, no power loop. Your clamp-on DC ammeter will confirm.

Like most good rumors, there is some kernel of truth. There are some limited situations where a DC-DC charger could be warranted, but in most applications they are not needed. They are certainly not needed to prevent 'surges'.

Sterling and Battleborn are in the business of selling things, if they can convince/scare you into buying more stuff from them, better for them.

Battleborn in particular has some shady marketing practices, so I am not at all surprised that they would try to convince you to buy a $350 DC-DC charger (that they conveniently happen to sell) to 'protect' the $1000 battery you just bought from them.

Kind of reminds me of this:
Fargo - TruCoat

"I would like to hear some more about these 'eddy currents' though"

:) That was me, I think not understanding my battery book on the problems with parallel batteries. Barre says, "Also, if one battery has a higher voltage than the other, a small circulating current develops between the two parallel batteries...."

I got the idea somewhere that this happened even when they are floating together, but have been told on here that does not happen as such, so not an issue between different House and Engine batts being in parallel.

( I am familiar with the joke where the salesman sells a gizmo that prevents elephant attacks in your home. He says after looking around, "See? It works! No elephants here! )

So are you guys just making fun of me :) or are you saying Sterling and/or Battleborn are selling elephant attack prevention?