Is a relay based split-charge good enough for Lithium?

[wildebus]

or do you need to provide some kind of current limiter to protect the alternator from potential overload?

I have a new 140A Alternator in my van, but I still wouldn't want to connect it to my Battery Bank with a relay.

My Battery Bank was pretty low this morning, so I decided to see how much I could hit it with...

Charging Battery by David, on Flickr
And that 180A is net current into the battery - the actual total current from the chargers will be higher.

This is of course a combination of Alternator and EHU charger - but if I didn't have limits on the alternator side in the form of B2Bs, it would have been happy to suck as much as it could from the Alternator, risking possible damage in the process.

That's what I would advocate a B2B rather than a relay. Just sayin'.

 

[Landyseverywhere]

It's also unlikely your relay.is capable of handling the full output of your alternator for long periods even if the wiring is ?

 

[trevskoda]

It's also unlikely your relay.is capable of handling the full output of your alternator for long periods even if the wiring is ?

adaepends on the relay, mine is 200amp rated, but im charging 2 90ah pld lead acids.

 

[wildebus]

It's also unlikely your relay.is capable of handling the full output of your alternator for long periods even if the wiring is ?

And the relay does not have a safety limit, so exceeding the current rating of the relay will not cap the current at that draw, it will just cause the relay to cut out or more likely fail.

Not sure how much current I could push into the battery - I was limited by the total available charger output I had. If I did this test in summer, could maybe get another 30A from the Solar Controller?

 

[nabsim]

Lots of strange info given about charging lifepo4. In this belo how does a b2b manage to charge quicker than a split charger? A B2B would be limited to the output of the alternator (if a big enough B2B) also. This is Fogstar info as well

However, it’s important to note that a split relay is NOT a battery charger, and it can only provide a maximum charging rate that is limited by the output of the alternator. This means that if the battery bank is deeply discharged, it will take time to fully recharge using the split relay, and additional charging may be required via solar or mains.

One way to maintain a consistent output to the battery is to use a DC-DC or B2B charger. These devices regulate the current from the alternator and provides an optimal charging rate - ensuring your leisure battery is always topped up.

Full Fogstar Article

 

[Exwindsurfer]

I ask Phil if I need a b to b he said no hence the split charger and never had a prod in 5 years so if its not broken don't fix it lol

 

[Molly 3]

I dunno , ask wildebus he's the man ,

 

[wildebus]

Personally, I would not take the risk of having a big battery bank connected to a standard alternator without any current protection. You saw how much current my battery bank was taking in inthe first post (>180A) and nothing to say it would not have tried to take more if it was given the chance. A direct (via Relay) connection to the alternator could well have overloaded it.

A Lithium battery does not need a B2B in terms of the sort of charging profile that a Lead Acid benefits from, but what key feature a B2B does give is current limiting, and that can be invaluable to protect the charging source.

 

[Okta]

Particularly important to have a B2B not a split charge relay with a smart alternator. As I understand it with a split charge relay there is a risk of current flowing out of the higher voltage Li leisure battery back through the relay and into the lower voltage engine battery when the alternator switches to a lower voltage. Other more knowledgeable people such as @ wildebus might know reasons why this should not happen but it is what appeared to be happening on my 2015 Euro 6 Mercedes Hymer.

 

[Exwindsurfer]

2009 citroen relay no smart alternator so doesn't apply.

 

[wildebus]

Particularly important to have a B2B not a split charge relay with a smart alternator. As I understand it with a split charge relay there is a risk of current flowing out of the higher voltage Li leisure battery back through the relay and into the lower voltage engine battery when the alternator switches to a lower voltage.

Now a relay is just a electric switch. So when enabled, the two sides - Engine Battery and Leisure Battery are connected. And when you have two batteries connected, there will be a natural inclination to equalize the voltages of the two. So if you had a Lead (engine) Battery sitting at say 12.4V and a Leisure (Lithium) Battery sitting at 13.1V, any current flow will always be Lithium to Lead.

It depends how the Relay is activated to if this occurs and is an issue. In the 'old days', a split-charge relay would be activated by the D+ signal. This comes from the Alternator when it is running and when the alternator is running, the voltage goes up and so the engine battery voltage would be higher.
But some installers use the ignition signal rather than D+ to activate the relay - Not always a good idea depending on how the van is used.
In self-builds, the VSR (voltage sensing relay) became popular and they turn on and off based on voltage levels. Problem here is if say you have Lithium, the VSR will turn on when the engine starts (goes over 13.8V) but it will then never turn off as the off-threshold is below the voltage of a Lithium Battery (VSRs are almost all bi-directional and use both sides to sense voltage) so it stays on after the engine is turned off. Not really what is wanted.
The D+ on a new van with a Smart Alternator seems unpredicatable. Some makers 'spoof' a D+ signal based on Voltage (bit like a VSR) to then enable a B2B but this does not always seem to work with the way the B2B is configured and I've come across factory-built campers which have Compressor Fridges that cause the Leisure Batteries to drain because the B2B is not working as it should either
A B2B will not allow any reverse current to occur like a Relay could, but still needs correct installation (for example, the instructions that Victron provide for their Orion B2Bs with Smart Alternators don't work - at least with the Fiat vans).

Other more knowledgeable people such as @ wildebus might know reasons why this should not happen but it is what appeared to be happening on my 2015 Euro 6 Mercedes Hymer.

 

[merl]

or do you need to provide some kind of current limiter to protect the alternator from potential overload?

I have a new 140A Alternator in my van, but I still wouldn't want to connect it to my Battery Bank with a relay.

My Battery Bank was pretty low this morning, so I decided to see how much I could hit it with...

Charging Battery by David, on Flickr
And that 180A is net current into the battery - the actual total current from the chargers will be higher.

This is of course a combination of Alternator and EHU charger - but if I didn't have limits on the alternator side in the form of B2Bs, it would have been happy to suck as much as it could from the Alternator, risking possible damage in the process.

That's what I would advocate a B2B rather than a relay. Just sayin'.

Like I've posted many times the wiring resistance will affect and ultimately limit the current.
So when you ask "or do you need to provide some kind of current limiter " then YES you do and the wiring will do that if it's sized correctly and thus preventing the batteries ability to "suck as much as it could from the Alternator."
If you repeat your experiment a second time with the lifepo4 nearly fully charged you'll see that the alternator current has fallen considerably so the charge current isn't stable and linear like you get from a B2B but rather increases as the battery becomes more discharged, this could be viewed as either good or bad, bad because charge current isn't "steady state", alternatively it could be seen as actually desirable because the battery receives more charge when it really needs it most and the current tails off as the battery becomes full. Personally I like the way I'm able to control and toggle the alternator current (zero/20A/40A) with my B2B but that's my personal choice.
With the correct wiring and switching a SCR does make a decent enough job to satisfy many users in the US (plenty of you tube videos) and Exwindsurfer for one over here.

 

[wildebus]

Like I've posted many times the wiring resistance will affect and ultimately limit the current.
So when you ask "or do you need to provide some kind of current limiter " then YES you do and the wiring will do that if it's sized correctly and thus preventing the batteries ability to "suck as much as it could from the Alternator."
If you repeat your experiment a second time with the lifepo4 nearly fully charged you'll see that the alternator current has fallen considerably so the charge current isn't stable and linear like you get from a B2B but rather increases as the battery becomes more discharged, this could be viewed as either good or bad, bad because charge current isn't "steady state", alternatively it could be seen as actually desirable because the battery receives more charge when it really needs it most and the current tails off as the battery becomes full. Personally I like the way I'm able to control and toggle the alternator current (zero/20A/40A) with my B2B but that's my personal choice.
With the correct wiring and switching a SCR does make a decent enough job to satisfy many users in the US (plenty of you tube videos) and Exwindsurfer for one over here.

Not going to agree.
Yes, you would likely get a lower current if the batteries were much closer to fully charged BUT you have to plan for the worst case scenario.

Cable size ... so if you fitted smaller cable, that increases resistance and will lower the current. I would argue that that is not the definition of being correctly sized, but undersized, but I am not going to argue the point with you.

Each to their own and I will stick with my logic.

 

[merl]

Not going to agree.
Yes, you would likely get a lower current if the batteries were much closer to fully charged BUT you have to plan for the worst case scenario.

Cable size ... so if you fitted smaller cable, that increases resistance and will lower the current. I would argue that that is not the definition of being correctly sized, but undersized, but I am not going to argue the point with you.

Each to their own and I will stick with my logic.

Yes, you would likely get a lower current if the batteries were much closer

Not just likely David, it's inevitable. Ohms law dictates it.

BUT you have to plan for the worst case scenario.

Yes, agreed.and It's possible to plan.

so if you fitted smaller cable, that increases resistance and will lower the current. I would argue that that is not the definition of being correctly sized, but undersized,

You need to fit enough resistance to limit the current to your chosen maximum in the worst case scenario. (When the lifepo4 is at it's lowest state of charge)) Current then decreases there after.
Cable gauge and length is then chosen so as to satisfy the required resistance and gauge to carry the current so not necessarily fitting just thinner cables, it'll depend on cable length too, if this is done correctly the cable obviously won't be undersized.

Not going to agree

I didn't think you would TBF David but you did ask the question