Turbo-charging Motorhome Electrics

[wildebus]

A couple of hours with the charger limited to 20A, and then changed so it could run upto its full 70A potential and we get an idea of both the Discharge and the Charge patterns on the Hybrid PbC/LiFePO4 Lead-Lithium Battery Bank.

The Results are pretty well as the Mathematics and the Logic would suggest.

The Lithium does the great majority of the work upto around 80% or so of its capacity (80Ah in this case), and the Lead Acid takes over after that point. In this example the AHs drawn is over 160Ah.
Going down to that 160Ah using the 300Ah Lead Bank would have "used up" a charge cycle by going below 50%. In this case, the Lead Bank only dropped to around 75% which should be much healthier for it in terms of Service Life.
The Lithium in going down to 80Ah used up one of its Charge Cycles, but as a key benefit of Lithium is supposed to be its much greater cycle count (and which is what makes it cheaper per Ah once that is taken into account), taking out a cycle from a Lithium Battery instead of a cycle from a Lead Acid battery is economically better.

On the Charge side, the Lithium recharged much faster meaning it will be ready for action for the next usage pattern, again reducing the Leads used and prolonging its service life Further.
From this data, even though it is just one day, it looks like this Hybrid Battery Bank should work as Clarks video suggests
Why would you not just get 2 100Ah Lithiums? Well that would take you down to the 80% DoD on both in this example and leave you with 40Ah of available capacity. The 300Ah Lead Bank here still has 75Ah before you even touch 50% DoD - and if need be could carry on to draw another 75Ah without a problem (that would draw into the longer term Service Life but having the reserve capacity there is a pretty good feeling )
Also the 300Ah Lead Carbon Battery bank costs 75% of the cost of the 100Ah Lithium Battery, so not only are you having greater capacity with many of the advantages of Lithium, you are saving a couple of hundred pounds at the same time.

Time will tell, but I believe this Hybrid Battery Setup should work out very well.



(You don't need the monitoring kit to the level I have but I think it is important to have the charging devices that can be programmed precisely (As well as being able to program the Voltage levels of Victron Mains Charger and Victron MPPT Charger, I can do the same with the Ablemail B2B Charger) for this setup to work however)

 

[xsilvergs]

I'm sat here thinking "why"? Just chuck out the lead based batteries and go with lithium. It's like the guy on the boat has realised he's bought into outdated technology and is now trying to convince others he hasn't.

Lithium charges faster, weighs less, has a better charge efficiency, and on and on and on.

 

[wildebus]

I'm sat here thinking "why"? Just chuck out the lead based batteries and go with lithium. It's like the guy on the boat has realised he's bought into outdated technology and is now trying to convince others he hasn't.

Lithium charges faster, weighs less, has a better charge efficiency, and on and on and on.

Why?
Why throw money at something when there is an alternative way that is cheaper, using proven technology and works well?


Let's take the guy on the boat .... How much money would it cost to replace his 1500Ah lead bank with lithium? Oh... I wonder....
TBH, I dont think you really watched the video with an open mind to make those comments, but each to their own.

Why do you make your own VE.direct cables instead of buying ones from Victron? Are your own better? No, but they are cheaper, and you maybe get some satisfaction in making them yourself as well?

 

[mb2tv]

I totally get the idea! If you have already the batteries there is no good reason to chuck them out, unless money and waste is not an issue. The boat does not have a weight problem and changing all batteries is very expensive. Also note the Lithium is nit recommended for starter batteries. If you start from scratch, yes, i would go completely Lithium. If you can lengthen the life of current batteries, why not?

 

[PeteS]

Its interesting and I'm pleased its working as you intended, time will tell if it has any major benefit.
I think if I had just bought 300Ah of Lead Carbon batteries then I would have been tempted to try this but I along with a lot of others dont have that weight capacity so moving to Lithium gave me some weight advantage, I bought 150A lithium battery instead of two AGMs.

Hoefully you will give regular updates as to how it progresses

 

[xsilvergs]

I have probably watched the video about 3 times, it seems so long-winded. After the first viewing I had to watch it again as I thought I'd missed the point.

Also I do use Victron USB-VE.Direct leads in the van. The latest are an improvement as they have a wire lead from the RPi to the FTDI part.

I agree with PeteS's choice and reason for going Lithium, having just gone Lithium but why would I want to keep an AGM? It's sat on the garage floor right now and was earmarked for my home solar but I think it will go to scrap merchant when I get home.

 

[wildebus]

....As added extra monitoring for the new battery, decided to also move the cabling for the original habitation electrics from the midi fuse block to the MPPT 100/20 Load Output. This means it is limited to 20A maximum output, but shouldn't be a problem as the hab electrics are fused at 20A as standard anyway. The reason I wanted to move it is for better granularity in the info.... With the Hab electrics on the Load Output, I can see precisely how much power is being drawn at any time which can be handy when you have a charger that Victron does not log data for (e.g. the Ablemail B2B in my setup).
View attachment 3799

Think I will move the Hab Electrics back to the Midi Fuse Block
You can see the current being pulled when you look at the MPPT Load Current data - which is good . But it takes it away from the Solar Yield number - which is not so good and makes the PV data as shown incorrect. You can end up with negative PV Yield.

I guess it would tell you up front if your Solar was keeping up with your system demands, but you would loose the info on just how much solar you were getting - and that is interesting to know, I reckon.

 

[Markd]

One of the key points I draw from this is that the hybrid system really comes into it's own when you have significant usage in no charge periods.
In the video we hear that aircon overnight was the main driver.
Probably not a big issue in the uk but might be if we ever get back to summers in Spain-Portugal

For heavy users with payload capacity and a significant investment in lead batteries already I think this is really worth thinking about.

I haven't watched the 'add lead to lithium' video but he is obviously convinced that hybrid is the way forward.

 

[Nabsim]

I haven’t watched the video yet but will do. I doubt I will change back to carrying lead acid but as others have said it sounds a neat way of extending what you already have.
Modern vans and Motorhome’s May struggle with weight for lead acid whereas boats and houses will not have a problem

 

[wildebus]

One of the key points I draw from this is that the hybrid system really comes into it's own when you have significant usage in no charge periods.
In the video we hear that aircon overnight was the main driver.
Probably not a big issue in the uk but might be if we ever get back to summers in Spain-Portugal

For heavy users with payload capacity and a significant investment in lead batteries already I think this is really worth thinking about.

I haven't watched the 'add lead to lithium' video but he is obviously convinced that hybrid is the way forward.

He hasn't posted the detailed video yet, but this guy has also developed what sounds like a very low-power Air Con system (I don't know his background but he knows a lot about refrigeration . I doubt it is stuff I would ever do myself, but his video on how the Boat Fridge works and his repair video was (to me, anyway ) very interesting ).

Not watched his "add lead to lithium" video in any detail either but will do. I have seen lots of general comments on the web on how a little semi-sacrificial lead battery on a Lithium bank can be a good idea (can't remember the exact details though). Will be interesting what Clark says about it. I am guessing it might be more theroretical then the "lithium to lead" setup - unless he did this on a friends boat?


Weight is a key factor in the selection of Lithium for sure. I don't think I would have installed the bank I have if I was remaining limited to 3500Kg for that reason. But once I decided to do an weight uprate for general reasons and got on a weighbridge to see what's what in normal running, decided I could cope with the ~60Kg overhead of the Lead over Lithium within the extra 350Kg the uprate was giving me.

 

[Markd]

I saw on a you tube thread on bus conversion RVs someone ditching a couple of Dometic traditional on roof ac units for a much more modern remote unit (pump etc in locker) that could run off batteries or alternator via inverter.

 

[wildebus]

Ok, Final few Discharge Graphs showing the "Day in the Life of a Hybrid Battery Bank".
These I think are interesting as they show a full 24 Hour period on a day where there is some significant heavy usage. As well as the Fridge set to AC (but not running that much as was quite a cool day) plus the Water Heater electrics enabled - so an initial heat up then sporadic reheats to maintain the temp (and it really was nice and hot water!). I would think this would be as much off-grid electric as I would be likely to use.
Like before, the supply to the Fridge (and Water Heater) disconnected when the battery bank dropped to 60% SOC.

Firstly the overview of the Day

The "noise" power is pretty much delivered by the LiFePO4 battery. There is a big jump in usage between 08 and 10 hours (that is when I switched the water heater on).

The next graph shows what current is being drawn and the share from which type of battery (The Water Heater stayed on a fair time because the source water was so cold and it had to raise it to 60C).

As the LiFePO4 Battery starts to discharge, the AGM starts to come in to contribute. It is not a dramatic change but very gradual and you can see from the Amperage lines how the Lithium current slowly starts to drop and the Lead current increases, but the LiFePO4 battery is still providing most of the power despite being 1/3rd of the size of the AGM Bank.

Going to look at hour 16 now. The fridge was running for all this hour - so around 20A draw there, but reduced as we had some solar harvesting coming in. This hour is significant/interesting as we also have the Water Heater kicking in on a maintenance mode - so the inverter was delivering a fair bit of power and we were pulling over 100A out the battery even after whatever the solar was contributing

With this higher load the gap between the Lithium and the Lead in terms of current delivery was a lot less. I think this is as the high current demand causes voltage sag, the Lithiums voltage sags a little more than before due partly to the increased DoD and this causes the AGMs to provide more.

The graph above becomes particularly interesting in terms of the relationship between Lithium and Lead when compared to the next graph. A near identical power demand on the battery for virtually the same duration - but compare what battery type is providing what current this time.

As the LiFePO4 Battery reaches a high DoD approaching 90% it can still deliver a decent current of between 20 & 40A to contribute, but it is the AGM this time that is providing the main energy demand.
And then when the Water Heater turns off thermostatically and it is just the Fridge running, the LiFePO4 battery does not do anything and it is the AGM that is powering the Fridge.

And final chart ... There is this thought of the LiFePO4 working as a bit of a 'trickle charger' to the AGM batteries due to the voltage mismatch and eventually discharging itself. Well, the discharge bit won't happen but it is true to say there is some (very small) current flow due to the voltage differences of the batteries, but it actually works both ways depending on the batteries SOC (and therefore voltage).
This chart drills down the first and final hours of the day....

In hour 00, the LiFePO4 line is above the AGM line showing it is providing more current. In fact, the AGM provided a net 0.0Ah in that hour and the LiFePO4 provided 0.8Ah. There was some current recorded INTO the AGMs which mut have been from the LiFePO4 but it was insignificant.
In hour 23, the picture is reversed and the Red AGM line is above the Green LiFePO4 line. In this hour, the AGM provided a net 1.0Ah and the LiFePO4 0.0Ah.
I don't see any danger of one battery killing the other prematurely. What will, I think, likely happen is that the Lithium Battery might never go into shut-down mode due to its own self-discharge as the Lead Battery bank will end up keeping a trickle into it until they are dead - but as we are talking large battery banks and 10ths of an Amp, in practical terms it ain't gonna happen.

So that is what I would say an extreme example of what might be used in a day in the life of a Motorhome and what impact it had on the 300Ah Lead + 100Ah Lithium Hybrid Battery. A typical user is unlikely to draw 160Ah in a single day (actually the large majority of that 160Ah was drawn within a 12 hour period!)


To conclude this analysis I would say that having a Hybrid setup like this has some significant advantages over a pure Lead or a Pure Lithium Battery Bank. Some of those advantages are Performance (power delivery and recharge ability), some of them are Financial. It is a matter of deciding and balancing how much money you would want to spend and how much energy storage you want available.
If you want to investigate a Hybrid Setup for yourself, make sure you chose the right kind of batteries though. A key factor to consider is if you have a high-current inverter, remember the Lithium battery might try and deliver all that current, and many are fairly current-limited.

 

[Markd]

Thanks for the comprehensive report.
It's amazing how well it can work and proves that going 'all electric' is now a viable option without enormous expenditure or massive weight penalty.

 

[wildebus]

Yesterday was the first day I went out in the Motorhome after setting up the Battery Bank in the Hybrid manner, so I thought I would post a video of how it worked when being charged by the B2B.
Preamble: Went out around 11:30m driving for around an hour to a Garden Centre. Spent a while there, then repeated the drive back home, but with a slight detour for Fuel near home.
This shows the SOC for the Overall bank (note that it is not as accurate as a normal setup - I need to find the right blended settings to get it as accurate as possible) as well as the LiFePO4 bank.

You can see at the start of the day the Lithium Battery is right at the bottom but the overall SOC for the bank is still healthy at over 60%. Lithiums cope way better as we know at very low discharge, so it is the that battery that has been doing the work, saving the Lead.
As the B2B starts to work at around 11:30 and again just before 15:00 you see the Lithium Battery getting a good charge and with the approximate 2 1/2 hours drive the overall SOC went from 64% to 88%, but the Lithium surged from 5% to 75% - so clearly the bulk of the charge went into the LiFePO4 battery.


I made one other change to the setup in the evening after I got home... Bit the bullet and did what I knew would inevitably happen and installed a dedicated Battery Monitor for the Lead Portion of the Hybrid Bank in the same manner I have one on the Lithium. This will allow each part to be monitored more accuratly in terms of SOC as the monitors can be setup more precisely (and after a while the results will let me tune the overall Hybrid monitor better).
This screenshot is from today and shows all three monitors since around Noon today, with all batteries starting off at 100%. The load on the system is the 200W fridge on an approximate 50% duty cycle.

This allows a good view of the discharge pattern for each battery direct from the VRM. The 400Ah Hybrid Monitor is not a simple addition of the 300Ah and 100Ah ones as mentioned. It is certainly very close and within a reasonable tolerance but I am sure I can improve it after a while.

 

[wildebus]

This bit of video might be interesting - it is a capture of the SOC (State of Charge) of the Overall Bank (400Ah Hybrid), plus the Lead Acid (300Ah PbC) and the Lithium (100Ah LiFePO4) batteries that make up the bank.

The timeline for all three Graphs are identical and you can see how one technology does the Lions share of the work until it is close to depletion and the other Technology essentially takes over. This is not by design or configuration, but is a consequence of how those two different battery technologies inherently work.
Some key time points:
200W Intermittant load until 15 May 19:00
General 'noise' load until 16 May 08:00
Increase in SOC due to Solar Harvesting until 16 May 12:00
200W Load back and constant until 17 May 01:30 when overall SOC reaches 50%, disconnecting 200W load

(you will need to make it full screen (and on a 'proper' screen) to be able to read the numbers I think)

 

[wildebus]

Little view of the Hybrid Battery over the last 4 days ...
Bear in mind that the LiFePO4 Battery and the Lead Carbons are connected in parallel to present to the van a single Battery Bank, this is a State of Charge view of the Overall bank, the Lithium and then the Lead.

The Lithium is like the striker in a football team, running around doing all the work. The Lead is like the reliable old Subsitute, ready to come on and take over when the Strikers got tired.

 

[wildebus]

This might be of interest to those curious to see a "day in the life" of a Hybrid Battery Bank....

Preamble:
Went out yesterday, but forgot to turn the fridge earlier, so rather than having switch to 12V operation on engine start, I just set it for 240V Operation so it would continue to cool further (12V is not efficient for that).
I have the Battery Bank SOC control set at 75%. i.e. When the Battery Bank drops down to 75%, the AC (Inverter) supply to the fridge is cut.

This is what the 24 Hours looked like:

Some Translation ...
VE.Bus State [276]: Device 276 is the Multiplus.

Starts off on 'Low Power' as plugged into EHU (so Mains providing 240V);​

Switches to 'Inverting' when EHU lead pulled in preparation to leaving the house;​

Goes back to 'Low Power' later on (due to low SOC condition met).​

​

System - Relay 1 State: System is the Cerbo GX. Relay 1 is the programmable Relay

The Relay is Open all the time the Battery Bank SOC starts at 100% and stays above 75%​

The Relay Closes when the SOC drops down to 75% and will not Open again until the SOC goes back up to 90%​

When the Relay is Open, the Inverter 240V supply to the fridge is available; When the Relay is Closed, the Inverter supply is cut​

Battery SOC - 100Ah LiFePO4 [288]: This is the Lithium (LiFePO4) Battery

You see it initially at 100% - this is when it is doing nothing whilst the EHU is plugged in powering the fridge and the solar is ticking over.​

It then starts to drop down as the EHU is pulled and the Inverter is now supplying the fridge - using the Battery Bank​

It climbs up again and hits 100% and stays for a while at that level - this is when we are driving and the B2B is doing its thing using the alternator power to recharge the battery as well as make up the power the fridge is using (note: the fridge is still on 240V power while driving - it is NOT switching to 12V)​

It then drops again as we are parked up for a few hours, then SOC recovers a bit as we are driving home and then starts to drop again once home but fridge still running, eventually dropping down to around 5% SOC​

Battery SOC - 300Ah PbC [279]: This is the bank of 3 100AH Lead Carbon (PbC) Batteries

Here you see the State of Charge staying around 100% for a long time (until around 11PM in fact). The PbC batteries don't start to come into play until the Lithium Battery has dropped to a low SOC and then start to provide the power​

​

Battery SOC - 400AH Hybrid [278]: This is a view of the overall bank of Batteries, both Lithium and Lead.

The SOC here shows the overall picture of the State of Charge of the overall 400Ah worth of batteries.​

Essentially, for each 4% the Lithium Battery drops in SOC, the Hybrid Bank drops 1% SOC (as the Lithium makes up a 1/4 of the bank). So what you see in the Hybrid Battery between 100% and 75% SOC is pretty well the Lithium Battery working, and below 75%, you see the Lead Acid Batteries working (where each 4% drop in the Lead Bank will show as a 3% Hybrid Drop as the Lead makes up 3/4 of the bank).​

​

Because the Relay Control is set to 75% and the first 25% of power delivery is from the Lithium (over-simplifying a bit but that is the principle), it is really only the Lithium the fridge is taking power from even though the Lithium and Lead Batteries are all connected in parallel physically and there is nothing installed or fitted that is preventing power from the Lead - it is just the batteries operational characteristics that make it work like this.
If I set the Relay Control to say 50% instead of 75%, then the next 25% would come virtually exclusively from the Lead Carbon side (you can see this from the graphs as there is still power, albeit at a lower level, being drawn after the Relay Opens for other devices and it is the PbC [279] SOC that is dropping, not the LiFePO4 [288] SOC.


So what does this mean?
Well, the Lead Batteries were not doing much on this 24 hour period out but the Lithium worked hard.
Were the Lead Batteries needed? The maximum discharge taken was 107Ah - that is more than a single 100Ah Lithium battery could provide, so if I didn't have the Lead Carbon bank I would have had to have a second Lithium Battery. That would have given me an extra 100AH (at full DOD) - but at a greater cost than the 300Ah of Lead Carbon batteries. And the Lead Carbon bank also gave me an extra 150AH - 50% more than a second Lithium - extra capacity at a 50% DOD; and would give me 200Ah - twice the extra Lithiums capacity - at a 67% DOD, which is pefectly fine for these batteries.

Given the Lithium has many thousands of charge cycles available to it, and they are getting used - but the working life of it is still many years into the future. The Lead Batteries on the other hand have a much lower cycle count, but we are not using those up at all (end of the 24 hours, we are down to just 95% SOC for the Lead battery bank) and so their service life is significantly extended whilst being there and available for 'excess' power requirements.

It is now 2 months since I first set about installing the Lithium/Lead Hybrid Battery Bank and I have to say it is actually working exactly as both the mathematics would have predicted and how I was hoping it would work.

 

[wildebus]

Thought I would do a little update whilst sat in my Motorhome on the 12th night away and most of time spent off-grid. Firstly, the setup is slightly different as I have added a 2nd Lithium, so the bank is now 200Ah Lithium + 300Ah Lead-Carbon. I did this as I knew I would be off grid for at least 7 days straight, Solar not expected to be that mega given time of year and also now running a Electric-only (240V Compressor) Fridge, so no option to divert to gas to save electric.

This is the last 7 days so the start of the timeline includes the time driving to the Overland Show in Stratford from a Campsite where I had hookup, and at the end it includes the time driving from Stratford to a Campsite with Hookup.

You can see the bumps just around 12:00 each day - that is the solar harvesting exceeding the power draw in that period. Apart from that time, the solar was not keeping up with my daytime consumption. I did not limit my power use and used things as I would want to generally (so TV, Laptop, fan on heater running, bit of Microwave use, etc) so it would be quite possible to reduce this consumption for sure, but I wanted to see what was possible. Ultimately I took the battery down to 100Ah remaining capacity (overall a 20% SOC on the Hybrid Bank).

The Lithiums went down to around 10% at which point the Leads kind of take over automatically (down to respective voltage levels). 10% is around the kind of discharge level it is fine to take Lithiums down to and is generally regarded as around an optimum level to balance using the capacity and getting a good design cycle life.

The Leads dropped down to around 43% at their lowest point just about at the time I was starting to pack up to leave the site. 50% is often quoted as the point to never go below for Lead Acid batteries. Lead-Carbon batteries are a different kind of animal... You don't damage a Lead-Carbon battery by taking it much lower and you can take it just as low as Lithium if you want (although the voltage it would be at is pretty low at that point). As you take it lower and lower, the design charge cycle count reduces (just like all batteries, including Lithium) so it is not something you would do as a matter of course. For my Lead-Carbon batteries, I have no concerns for them to hit 40% SOC in general use and if need be go down to 30% if need be on occasion, so the 43% here is perfectly fine.

So the 10% for Lithium and ~40% for Lead I have shows I am ok and in "design parameters" for 7 days of off-grid in Autumn generally, and if solar was poor, I could drop further if need be, or cut back on optional draws so something like Microwave use.
Also, bear in mind this was 7 days of off-grid in a static location. If I was to have been driving to different overnight spots, the B2B would have put in a good charge during the drive.

To get an idea of costs, the regular retail price of a Hybrid Battery Bank of the same capacity of mine would be around £1,750 (Lithiums £1,300, Lead-Carbon £450). The Lithiums are admittedly at the upper-end of the price of Lithiums nowadays as they are high-spec with 150A BMSes with Bluetooth monitoring, plus integral heaters, so cheaper could be bought quite easily. Similarly, you can get basic Lead Acid for cheaper.

If I wanted to use the same amount of power in the 7 days but had a pure Lithium Setup, I would need 5 100Ah Lithiums. This would give 450Ah of usable capacity at an SOC no lower than 10%
5 Lithiums would actually be physically the same number of batteries as my hybrid setup taking up the same space in the van, but you would of course have a weight saving of around 40-50Kg in total.
The cost of 5 Lithiums would be £3,250 - so cost an extra £1,500 - a fair chunk of change there. That £1,500 would pay for my Victron Multiplus, Ablemail B2B and Victron MPPT Controller!

The cost of a regular Lead-Carbon bank delivering 400Ah and the Batteries not going below 40% SOC would mean a total quoted capacity of 666Ah - so 7 x 100Ah Batteries would be required, giving 420Ah at 40% SOC.
The cost of that battery bank would be around £1,050, saving £700 over the Lithium/Lead Hybrid, but would mean adding an extra 100Kg of weight and finding room for an additional 2 batteries. You would also not benefit from the key Lithium Benefits of fast recharge and maximum conversion of Solar Energy into battery recharging (bearing in mind Lithium Batteries not only charge much more rapidly, but waste less input energy in the process).
I can illustrate this by a graph showing the recharge of the batteries after leaving the Stratford location to a campsite around 80 miles away and then plugging in to hookup. Now the Lead Bank is 50% larger of course, but the amount taken out of each bank was much closer so they needed a similar amount of energy put back in

So after this 7 day-off-grid session, all worked as it should I would say and allows for a comfortable extended off-grid camping experience
In the summertime in the UK, or for longer periods in Southern Europe or similar weather locations, would be able to use more more - electric cooking etc, without any problems and at a much more economical way as all-Lithium.

 

[SquirrellCook]

Now that you’ve had time to reflect on your big adventure David, are you happy with the way it works? Or are you planning on reworking things.

 

[wildebus]

Now that you’ve had time to reflect on your big adventure David, are you happy with the way it works? Or are you planning on reworking things.

I will be posting my findings shortly as will be (and have already) making some tweaks

As far as the Electrics so and the Battery Setup specifically, worked very well. 19 nights away, of which 4 had EHU but never at any time in a position where I could not use battery power when I wanted to.
There is a final component that is due to be fitted once it has been made - the VSDR Lithium Controller which will disconnect the Lithium Section once charged so the Lithiums are not sitting at a Absorption level voltage for potentially hours after they are fully charged. This VSDR will be standalone, but while I am waiting for this device I decided today to fit a standard high-power relay controlled by a BMV to replicate the functionality and check it will work as I wanted. I also upped the Charger voltage settings to standard Lead (14.4V Absorption) to give it more of a test

Operation:
The Lithium Power Relay is controlled by the BMV monitoring the Lead side (this could also just as easily be a BMV monitoring the overall Hybrid Bank).
Relay is activated (closed) until the Hybrid Bank (Lithium +Lead) rises to 14.3V - at which time the Lithium is fully charged.
It then opens and the Lithium is disconnected from the Bank until the Voltage (of the Lead alone now) drops to 13.4V, at which point the Relay activates again and we are back to having a Lithium + Lead Hybrid Battery Bank.

Works really well. I am not using the "right" relay really for this test, just a standard off-the-shelf 200A relay which means there is a bit of a voltage drop over it, and it is fairly inefficient when active (which will actually likely be a significant majority of the time). The proper VSDR is a latching relay which means virtually no power loss in either on or off state and should be no noticable voltage loss (and won't need the BMV to control it as it is a microprocessor self-controlled device).
The VSDR is quite a bit overdue currently but is scheduled to be on the bench for code re-write on Monday so fingers crossed!

Oh ... I did also fit the Interconnect cable between the two Lithium Batteries. This keeps the battery use of the two lithiums balanced by allowing each batteries BMS to talk to each other and keeps the State of Charge sychronised between the pair (this just uses a standard RJ12 cable).