WIth regards to installing a hybrid-lithium bank with a single alternator engine.

[peterboat]

16 hours ago, Tasemu said:

 

Sort of, all i'm really worried about is that the VSR is not disconnecting on engine shutdown, so the next time I start the engine it would be cranking off the lithium bank. And I've heard that's a no-no.

The main lead is to the starter battery, as you crank thats where the power will come from I cant see the LifePo4s supplying anything unless the engine fails to start then they will charge the starter battery

[Tony Brooks]

24 minutes ago, peterboat said:

The main lead is to the starter battery, as you crank thats where the power will come from I cant see the LifePo4s supplying anything unless the engine fails to start then they will charge the starter battery

 

If the VSR fails to disconnect as the engine stops the engine and domestic banks will remain paralleled up, so when the starter is operated the lithium bank will try to supply some starting current via the still closed VSR contacts. How much is open to question, but as lithiums seem (from what I can gather) to have a lower "internal resistance" under load than lead acids I suspect it could be a lot. Tsaemu seems to be saying that the VSR does not disconnect when the engine stops.

[Jen-in-Wellies]

Is there a way in this system to restrict the current the Lithium batts are taking from the alternator? They will take all that they can until disconnected and most alternator don't like running at full power output for long periods, as they are designed on the assumption that the power draw will decline gracefully as lead acids charge up. Overheating and damage to the alternator could result.

[Tony Brooks]

3 minutes ago, Jen-in-Wellies said:

Is there a way in this system to restrict the current the Lithium batts are taking from the alternator? They will take all that they can until disconnected and most alternator don't like running at full power output for long periods, as they are designed on the assumption that the power draw will decline gracefully as lead acids charge up. Overheating and damage to the alternator could result.

 

That is one thing I was alluding to in my first reply, I suspect he may be using the "long cable" method. If that is what he is doing, then things may not be as bas as he thinks because the long cable will also limit the starting current drawn from the lithium bank.

[Tasemu]

16 minutes ago, Jen-in-Wellies said:

Is there a way in this system to restrict the current the Lithium batts are taking from the alternator? They will take all that they can until disconnected and most alternator don't like running at full power output for long periods, as they are designed on the assumption that the power draw will decline gracefully as lead acids charge up. Overheating and damage to the alternator could result.

 

9 minutes ago, Tony Brooks said:

 

That is one thing I was alluding to in my first reply, I suspect he may be using the "long cable" method. If that is what he is doing, then things may not be as bas as he thinks because the long cable will also limit the starting current drawn from the lithium bank.

 

Yeah I am using the long cable method.

 

I will admit i didn't think about it also limiting cranking current. Correct me if i'm wrong, but doesn't the long cable method only work because the alternator can be 'limited' by it? I imagine the starter motor will pull what it needs regardless and a longer cable would just result in more heat but the same amperage drawn?

[Tony Brooks]

10 minutes ago, Tasemu said:

 

 

Yeah I am using the long cable method.

 

I will admit i didn't think about it also limiting cranking current. Correct me if i'm wrong, but doesn't the long cable method only work because the alternator can be 'limited' by it? I imagine the starter motor will pull what it needs regardless and a longer cable would just result in more heat but the same amperage drawn?

 

No, it works because the cable has resistance so the longer the cable the greater the volt drop along it, a lower voltage at the batteries gives a lower charging current. Also, the higher the current, the higher the volt drop for a given size of cable. The same thing happens in reverse for starting loads, the lower the voltage at the starter end of the cable, the lower the current flow.. This method is a bodge because volt drop = heat, so it is all too easy to overheat the cable.

Edited February 10, 2023 by Tony Brooks

[nicknorman]

12 minutes ago, Tasemu said:

 

 

Yeah I am using the long cable method.

 

I will admit i didn't think about it also limiting cranking current. Correct me if i'm wrong, but doesn't the long cable method only work because the alternator can be 'limited' by it? I imagine the starter motor will pull what it needs regardless and a longer cable would just result in more heat but the same amperage drawn?

The long cable introduces resistance which introduces voltage drop when current flows. So there will be a balance between current drawn from the starter battery and current drawn from the Li - more current from Li drops more voltage which means the voltage at the LA battery dips and makes the LA supply more current until equilibrium is reached. The proportion of current supplied by the Li will therefore depend on the resistance of the long cable and the other infrastructure between the Li and the LA.

 

I don’t have any practical experience of commercial BMSs but if this one is the type with a separate charge and discharge connection, does it in fact allow discharge via the charge connections (or charge through the load connection)? It could be that the charge input is protected by a “perfect diode” bit of electronics that only allows current to flow one way - into the battery. And if that is the case, there is no possibility for the Li to discharge into the starter battery.

Edited February 10, 2023 by nicknorman

[Tasemu]

34 minutes ago, nicknorman said:

The long cable introduces resistance which introduces voltage drop when current flows. So there will be a balance between current drawn from the starter battery and current drawn from the Li - more current from Li drops more voltage which means the voltage at the LA battery dips and makes the LA supply more current until equilibrium is reached. The proportion of current supplied by the Li will therefore depend on the resistance of the long cable and the other infrastructure between the Li and the LA.

 

I don’t have any practical experience of commercial BMSs but if this one is the type with a separate charge and discharge connection, does it in fact allow discharge via the charge connections (or charge through the load connection)? It could be that the charge input is protected by a “perfect diode” bit of electronics that only allows current to flow one way - into the battery. And if that is the case, there is no possibility for the Li to discharge into the starter battery.

 

Unfortunately the BMS does not have separate charge and discharge connections. This might sound silly, but is there like a big ass diode you can just put in line with the VSR to make sure charge can only go one way? lol

[Tony Brooks]

13 minutes ago, Tasemu said:

 

Unfortunately the BMS does not have separate charge and discharge connections. This might sound silly, but is there like a big ass diode you can just put in line with the VSR to make sure charge can only go one way? lol

 

That was my first thought but you then get into the volt drop across the diode, however in this case it might mean you just need a shorter "long cable". To a degree, it will depend upon alternator maximum output. Using one half of a conventional split charge diode may well work but as lithiums pull high charging currents for long periods could well mean the heat sink is not up to the job. A large diode on a heat sink might even need fan cooling. This is a bit out of my area so perhaps one of our electronics experts like @nicknorman will comment.

[peterboat]

2 hours ago, Tony Brooks said:

 

If the VSR fails to disconnect as the engine stops the engine and domestic banks will remain paralleled up, so when the starter is operated the lithium bank will try to supply some starting current via the still closed VSR contacts. How much is open to question, but as lithiums seem (from what I can gather) to have a lower "internal resistance" under load than lead acids I suspect it could be a lot. Tsaemu seems to be saying that the VSR does not disconnect when the engine stops.

It doesn't I have tried it just as the starter operates the voltage drops below 12.8 and the VSR disconnects for about 3-5 seconds. I have solar on the campervan so it charges  both batteries by default, its currently charging at 14.55 as we speak

[nicknorman]

 

3 minutes ago, peterboat said:

It doesn't I have tried it just as the starter operates the voltage drops below 12.8 and the VSR disconnects for about 3-5 seconds. I have solar on the campervan so it charges  both batteries by default, its currently charging at 14.55 as we speak

 

However the disconnection happens AFTER the peak current is taken by the starter activating. So there will be a short period when the Li is supplying a lot of current. This might only be milliseconds but that could be enough to damage the BMS, depending on how much current and what the BMS rating is

[Tasemu]

4 minutes ago, peterboat said:

It doesn't I have tried it just as the starter operates the voltage drops below 12.8 and the VSR disconnects for about 3-5 seconds. I have solar on the campervan so it charges  both batteries by default, its currently charging at 14.55 as we speak

 

So you're saying the act of actually cranking the engine will drop the voltage enough to disengage the VSR and splitting the banks before there is a chance to draw any real power from the lithiums?

[Tony Brooks]

Just now, Tasemu said:

 

So you're saying the act of actually cranking the engine will drop the voltage enough to disengage the VSR and splitting the banks before there is a chance to draw any real power from the lithiums?

 

Unless you have a fast oscilloscope and shunt, I doubt that question can be answered, because most meters in use now just sample the electricity over small time periods, so any current surge could be over before the next sampling takes place.

 

If a diode works, then I think you may not need a  VRS.

[nicknorman]

1 hour ago, Tasemu said:

 

Unfortunately the BMS does not have separate charge and discharge connections. This might sound silly, but is there like a big ass diode you can just put in line with the VSR to make sure charge can only go one way? lol

 

Yes why not? Schottky diodes drop less voltage, but they will still drop around 0.6 or 0.7v at high currents. You don't say what the alternator max. output is (or rather, the input to the Li you get with the long cable) but let's say it is 100A, that means the diode is disspating 60 or 70 watts and so good heat sinking would be essential. You then have to factor in the max ambient temperature (engine bay temperature) etc in order to work out how much heat sinking is needed. But it certainly could be done.

If the alternator regulatates to 14.4v then losing 0.6v will significantly decrease the charge current - but that can be compensated for by reducing the length of the "long cable". A Schottky diode at low currents will drop around 0.3 to 0.4v so that does limit the max charge voltage to around 14 to 14.1v but perhaps that is no bad thing.

5 minutes ago, Tasemu said:

 

So you're saying the act of actually cranking the engine will drop the voltage enough to disengage the VSR and splitting the banks before there is a chance to draw any real power from the lithiums?

It will prevent any significant amount of charge being removed from the Li, but that is not the issue. Starting an engine probably uses less than 1Ah anyway. It is the momentary peak current that might be the issue, damaging the MOSFETs in the BMS. But that depends on the BMS and how well designed it is.

[Tony Brooks]

8 minutes ago, nicknorman said:

 

Yes why not? Schottky diodes drop less voltage, but they will still drop around 0.6 or 0.7v at high currents. You don't say what the alternator max. output is (or rather, the input to the Li you get with the long cable) but let's say it is 100A, that means the diode is disspating 60 or 70 watts and so good heat sinking would be essential. You then have to factor in the max ambient temperature (engine bay temperature) etc in order to work out how much heat sinking is needed. But it certainly could be done.

If the alternator regulatates to 14.4v then losing 0.6v will significantly decrease the charge current - but that can be compensated for by reducing the length of the "long cable". A Schottky diode at low currents will drop around 0.3 to 0.4v so that does limit the max charge voltage to around 14 to 14.1v but perhaps that is no bad thing.

It will prevent any significant amount of charge being removed from the Li, but that is not the issue. Starting an engine probably uses less than 1Ah anyway. It is the momentary peak current that might be the issue, damaging the MOSFETs in the BMS. But that depends on the BMS and how well designed it is.

 

Thanks Nick.

[peterboat]

23 minutes ago, Tasemu said:

 

So you're saying the act of actually cranking the engine will drop the voltage enough to disengage the VSR and splitting the banks before there is a chance to draw any real power from the lithiums?

The draw from the heater plugs was enough to disconnect the VSR as it turns out

[Tasemu]

1 hour ago, peterboat said:

The draw from the heater plugs was enough to disconnect the VSR as it turns out

 

Ah Excellent! This is really good to know. I guess it could just work then. 

[Tony Brooks]

13 minutes ago, Tasemu said:

 

Ah Excellent! This is really good to know. I guess it could just work then. 

 Probably, but test it, just in case.

[peterboat]

After reading the thread I have had a good play with the VSR, I fitted it because it is bi directional, the van has a tachograph fitted which left to it's own devices would flatten the starter battery. The 300 watt solar panel is big enough to keep both batteries fully charged I winter and big enough to keep the 12 volts compressor fridge (inlander) running in summer. We have a warm air heater website or Eibersplutter which will keep the van warm on low as I found out today. The upshot is the VSR worked as the destructions said it would disconnecting at 12.8 reconnecting at 13.3 volts so fingers crossed for the op

[170968]

This thread about the Victron Cyrix Li ct maybe be helpful:

 

https://community.victronenergy.com/questions/42660/how-does-cyrix-li-ct-work.html

Edited February 12, 2023 by 170968

[Tasemu]

On 10/02/2023 at 18:29, peterboat said:

After reading the thread I have had a good play with the VSR, I fitted it because it is bi directional, the van has a tachograph fitted which left to it's own devices would flatten the starter battery. The 300 watt solar panel is big enough to keep both batteries fully charged I winter and big enough to keep the 12 volts compressor fridge (inlander) running in summer. We have a warm air heater website or Eibersplutter which will keep the van warm on low as I found out today. The upshot is the VSR worked as the destructions said it would disconnecting at 12.8 reconnecting at 13.3 volts so fingers crossed for the op

 

Seems to be working decently for him. Though i think this is probably not a silver bullet solution as I could imagine more solar and/or a bigger lithium bank could keep the voltage drop dropping enough when engaging the glow-plugs. Food for thought.