Split Chargers - B2B & Relays: Which one - or Both?

wildebus

Full Member
Two general options for "split-charging" (using the alternator to charge your Lesiure Battery when driving) - a Relay system (such as a VSR) that just connects the Lesiure and Starter Battery together; or an intelligent Battery to Battery (B2B) Charger, similar to the Smart Mains Chargers but using DC-DC instead of AC-DC.

Both have their pros and cons (the main pro of the Relay is price compared to the B2B) and depending on the charge level of the battery, one option can be better than the other (PS. Talking Lead Acid Batteries here - Lithium is a whol different conversation).
The Relay method rarely (most likely never) will fully charge the Leisure Battery as the maximum voltage output of the typical Alternator is lower than the voltage needed to get the final 10% of so of charge into the battery. Add in Voltage losses on the cabling and the situation gets worse.
The B2B method limits the current going into the battery (depending on the size of the B2B) so on a heavily discharged battery this can be a real limitation on how fast you are recharging the battery.

CTEK make the D250SA 20A B2B Charger which is quite a neat product. Unique to CTEK, they also have a "Smart Pass" add-on. This changes the D250SA from a 20A Split-Charge system into a 100A capable Split-Charge system.
However, it does NOT make it a 100A B2B - the "Smart Pass" is really just a relay that the D250SA controls. So what it is is actually a combined B2B and Relay system which could potentially give you the best of both worlds - but at a price (~£600).

Rather than spending £600 to test the CTEK, I thought I would build my own version of their D250SA+SmartPass to see how effective it will be in reality (There are no widely promoted equivalents of the 250SA+SmartPass as CTEK have somehow managed to get a Patent on the concept).

This is my combined 30A B2B +120A Relay (actually a Victron Battery Combiner) - so should beat the CTEK 20A B2B + 100A Relay in performance as well.

B2B+VSR
by David, on Flickr
The CTEK switches between the two using the battery voltage state. I am using a more precise Battery State of Charge to perform the switch.

How did it do?
This graph shows a fairly long journey from 10:30AM to 6PM with a lunch stop of around 30 minutes.
The Leisure Battery Bank was at around 57% SOC at the start of the journey. I was getting charge from both Alternator and Solar.
1568538847129.png

Because of the way the data is sampled, there are apparent dips and a few spikes on the black line - that is an effect of the fridge compressor in-rush and should be ignored.
This shows the battery was getting over 30A effective charge until the battery was at around 78% SOC, so more than the B2B could put out. However it hardly went over 50A for any length of time.
Also bear in mind that Lead Acid Batteries had recommended charge rates that are very rarely over C/4 (i.e. Maximum Charge Current of 25% or 25A into a 100Ah battery) and often only C/5 or even C/10 (10A per 100A of Battery), so you would have to have a very big and well discharged Battery Bank to be even close to the 100A Smart-Pass (or 120A Relay in my case).
The other aspect is the Alternator Performance - can your alternator deliver such high current for a continuous period?

So was it worth it?
The cost of my Combination B2B+Relay would be around £300 retail if if were the 30A B2B/MPPT plus the VSR add-on bits. That is a massive saving over the CTEK D250SA+ SmartPass for essentially the same features (the CTEK has a couple of extra small benefits, but so has mine).
But for the typical Motorhome with less than 250Ah of Lead Acid Batteries, I am not convinced the extra cost of the D250SA+SmartPass, or even the extra £60 or so of my setup to add the Relay to the B2B is worth it for the small additional benefit.
If I get hold of a CTEK D250SA+SmartPass, it would be interesting to repeat the test to see if there is anything that has I missed (but looking at the spec and the patent, I cannot see it).

I'll likely be deconstructing this and putting the B2B and Relay back in the 'lab' and refitting my original Victron 230A Relay (my batteries are unusual in that their full charge voltage can be as low as 14.1V, so I can get to a much higher SOC via the alternator and the Solar has a lot less to do to finish the job off)
For most Motorhomes and Campervans, I would recommend a B2B if the budget allows it.
 

ScoTTyBEEE

Full Member
I have a related question to do with lithium batteries and split charging.

Imagine the case study:
200A alternator on Euro 4 van
200ah lithium battery with 200A separate port Daly BMS which provides 200A discharge protect and 100A charge protect. The lithium battery has a max charge rate of 100A
Connecting the charge input of the BMS directly to the starter battery with vsr to prevent starter battery draining.
Going on the below SOC voltages of a lithium battery, if your alternator only puts out constant 13.5v, which seems to be the case whenever I test them, this will still give 99% SOC.
What is there to limit the current, if your bms has an input current limit of 100A, would putting a 200A alternator onto this overload it, or will it only take what it can? If this could overload the bms, then adding a Victron 12/200 bms into the input, this can limit the current down to whatever you wish it to be. This combination of 2 x bms will allow one to limit the current and the other to handle all the overvoltage/undervoltage/balancing of the batteries.
Can you see any problems with this? As far as I can tell it's a cheap charging solution that can handle 100A, ie charge the equivalent of 400ah of lead acid from flat to full in 2 hours.

1569238765402.png
 
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wildebus

Full Member
I have not used Lithium Batteries, or looked into them in great detail (the price TBH has causes my (and all my customers) level of interest to falter a little!) so the following is not based on any real in-depth research ....

I would expect having a device such as your Daly BMS unit, that has a limit of 100A would take precendent over a 200A Alternator - just as a 30A B2B will output 30A to the battery despite getting fed by a 100A+ Alternator.

An actual output voltage of 13.5V on an alternator does sound rather low? Is this being measured at the Starter Battery or at the Leisure Battery? I am wondering if, especially at high currents, the low voltage is due to the split-charge cable length/gauge resistance? How long is the cable and what gauge thickness is it?
Now going on that chart, a charge voltage of 13.5V wouldn't achieve a 100% charge, but my understanding of Lithium Batteries is that they don't actually want to be sitting at a full charge anyway, so maybe having 13.5V is not actually such a bad thing anyway? (this bit on not wanting to be charging Lithium to 100% is something I have seen from a few sources. It may not be applicable to all Lithium Battery technologies however).
You talk in the end about putting in 200Ah of charge into the battery in 2 hours .... If you are pushing 100A along a 12V DC cable for hours it needs to be a decent gauge cable of course - I would have thought 25mm2 absolute minimum and preferably heavier (I replaced the 16mm2 cable in my van with 35mm2 as 16mm2 is just not enough for any prolonged current of 100A or more. I went for 35mm rather than 25mm as if replacing anyway, may as well go thicker!)
 

RAW

Full Member
Dear @wildebus

Read that with interest. On my Van I have a VSR for split charge. This one https://www.12voltplanet.co.uk/voltage-sensitive-relay-12v-140a.html
Funnily that was left in the Van when purchased and not wired in. I have since wired it in.

I read:
The Relay method rarely (most likely never) will fully charge the Leisure Battery as the maximum voltage output of the typical Alternator is lower than the voltage needed to get the final 10% of so of charge into the battery. Add in Voltage losses on the cabling and the situation gets worse.
Which is good from my point of view as have an isolated +ve Supply between two leisure batteries and this isolates PV supply and I was concerned that if I connected them up and ran both Solar Charge and Alternator charge to leisure side at the same time then potentially I could risk an overcharge on my newer Leoch leisure Battery if travelling for many hours. From reading what you have written I feel that now this is less of a concern.

Cheers
Robert
 

wildebus

Full Member
That PPT VSR is a decent VSR :)
As almost all batteries require a voltage higher than the alternator usually outputs, it is very hard to overcharge via a split-charge VSR system. Also most B2Bs, whilst putting out a higher 'proper' charging voltage, switch into a float mode based on time, not just charge level, to protect the battery. Even with a basic B2B or high-current alternator you would have to be driving for a long long time to that to be a real problem though! (in the chart above I was driving for 7.5 hours with one 30 minute break for lunch and 5 minutes for fuel and still was not fully charged).

BTW, if you wanted to be able to disconnect the VSR for any reason so it will not come on regardless, then a neat way to do that is to put the -ve lead through a basic switch. Without a grond connection the relay cannot turn on and using the ground wire lets you use a low-current little rocker switch instead of a heavy-duty high current mechanical switch on the +ve. (That is the method I used controlling the setup in the kit in the OP)
 

wildebus

Full Member
Got a new B2B in to play with and decided to revisit this thread with a revised setup 🤓

The new B2B is the Victron Orion-Tr Smart Charger aka a B2B
1590750789582.png

Now what is a little different about this unit (apart from Bluetooth- the obvious bit) is that like many of other Victrons products, it has a Remote Control feature. It doesn't mean you can get BBC1 on it, but it does mean you can incorporate a control level that can be harder to do with other makes.

Previously, my B2B-VSR Combo test rig used both high and low power relays to direct the charging +12V to the right device. This worked, but was a bit messy and the extra relays and connections added in extra losses ...
1590751111657.png


With the Orion-Tr, it is possible to control if the unit goes on by applying certain voltage levels to one or other of the remote input plug
1590751302009.png

essentially you use a -Ve signal on the L pin or a +Ve signal on the H pin, so a nice degree of flexibility and choice there.

I used the "L" pin option, which meant that if there was a -Ve signal present, the Charger is allowed to operate, but if there was no -Ve signal (i.e. floating), the Charger is disabled.
Whilst not specifically a control signal, a Relay (any relay) requires a ground connection so it is possible to use the same principle, i.e. to have a relay disabled when the -Ve is floating and allowed to work as normal when the -Ve is present.
So we have both the B2B and the Battery Combiner (aka VSR) both able to be disabled and enabled in the same way - a -Ve control signal.
Step 1 sorted (y)

Next stage - the -Ve control signal
I am using the Relay 1 on the Victron Venus GX to provide this control signal
1590752913080.png

The connections to Relay 1 are NO, COM & NC - NO = Normally Open, COM = Common and NC = Normally Closed.
By connecting the COM to -Ve (i.e. Ground), the Relay will either relay that -Ve to NO when the Relay is on, or to NC when the Relay is off.
I can then connect the NO output to the VSR and the NC output to the B2B, and depending on the state of the Relay, either one or other will be allowed to operate, but never both and never neither at the same time

For my setup, I am using the Venus GX, but the identical setup can be achieved with the Victron BMV-712 Battery Monitor, as it has a relay with the same NC, COM & NO connections - Here is a picture of the rear of the BMV-712 to show this.
1590753492443.png


Word of note: These relays are not present on all Victron Monitoring devices. None of the other Victron BMVs (including the Smart Shunt) have a relay feature. Also the Colour Control GX (CCGX) has a relay, but it only has the NO and COM connections, so to add in the NC function, an additional 5-pin control relay will be required (a simple thing to do, but one to be aware of).
So the wiring was added and Step 2 is complete (y)

Next Stage was setting up the relay. Not going to go into great detail on this but basically you can configure the GX devices and and BMV-712 to have the relay go on and off depending on what the battery voltage is or what the Battery State of Charge is. I use the State of Charge (SOC) and have the relay come on when the SOC is below 80% and off when above 81%
1590754262498.png

So when SOC is below 80%, the relay switches on, the COM and NO get connected together and the connection to the VSR ground terminal is now -Ve. Because the NO Relay output is now not connected and floating, the B2B (connected to NO) is disabled.
As soon as the SOC goes above 81%, the relay turns off, whch means the COM and NC are connected. Now the -Ve to the VSR is floating so that cannot operate and the signal to the B2B "L" remote input is -Ve, whuich means the B2B can operate as normal.

Step 3 is now done and that is actually the last step (y) You have a SOC controlled B2B + VSR combo that will allow as much charge as the battery bank can accept to come into it at lower state of charge levels, only limited by what the alternator will provide (and if it is more than the 120A of the Cyrix -ct 120A, you could use a bigger VSR, such as the Cyrix ct-230A), and when the battery is at a greater state of charge, the system will switch to the B2B Smart Charger to provide a probably higher voltage and a proper charging profile.

This diagram shows how you can wire up this setup. It is not really any different to any cabling of a B2B or a VSR except you would parallel connect those two devices, and you have the addition of a couple of control wires from the relay of the Venus GX, or more likely for most, the BMV-712.
1590755257088.png

_______________________________________________________________________________________________________________

Here are a series of Screenshots showing how this works ...

First, the SOC of the Battery is below 80%. This means the relay is OFF, and therefore the B2B is disabled and the VSR is enabled (remember, it is always one OR other enabled)
1590755440190.png

You will see that the input Voltage is pretty high at 13.6V? That is because the VSR is on as the Solar is active, triggering the VSR to come on, and so the starter battery is getting a charge as well.

I start the engine next
1590755631710.png

Despite the starter battery rising to 14.2V from the Alternator running, the B2B is still disabled as the SOC is too low to enable it (at lower SOCs, we want more current in so want the VSR to be used)

The SOC hits the 81% value, the Relay switches OFF, and this disables the VSR and enables the B2B. As with all smart chargers, they cycle through the modes, albeit briefly sometimes - so starts in Bulk and switches to Absorption
1590755855639.png
1590755947500.png


I then turn the engine off. The B2B does not turn off instantly as it is a Voltage controlled device so it will stay on until the starter battery voltage drops down to whatever has been configured as the "charger off" value.
Going .... Going ... Gone!
1590756112510.png
1590756129102.png
1590756150865.png

So once the input (starter) voltage is below that value, the B2B is disabled due to the engine shutdown but will come on again automatically when the engine starts up again as long as the Leisure Battery SOC is above 81% - if below, then the VSR comes back into play instead.
_____________________________________________________________________________________________________________

Is it worth doing all this?

Well, if you were looking at the CTEK 250 and the Smartpass Setup, this is another option using separate components to achieve a - IMO - better setup. If you look at CTEKs packaged solution - the CTEK 4-257 140AS Off-Road System, that comes in at just under £800 for B2B with MPPT, Smartpass and Battery Monitor (I think a repackaged NASA Marine BM1?)
That gives you 20A B2B, 120A Smartpass, 20A Solar and an SOC Monitor, all with a 2 Year Warranty

The Victron Setup described above will give you a 30A B2B, 120A Battery Combiner (VSR) and a 500A SOC Monitor. Add in a Victron 100/20 MPPT Controller and you have a more capable setup for under £600, with Bluetooth monitoring of B2B, Solar and Battery Monitoring, and all with a 5 year warranty.
And if you already have say a BMV-712, or a solar controller, or you need a solar controller more capable than the one on the CTEK, the price gap becomes much more significant.

Food for thought?
 

RAW

Full Member
Is it worth doing all this?
What are the advantages of adding a B2B in the set-up ?
So with your help and others I now have a charging set-up on Solar that works incorporating all batteries from a Victron non-BT 100/30 and Vehicle battery will only be charged when leisure side hits above 13.2V and vehicle is stationary
When Vehicle Engine is running alternator is outputting 14.2 V and charges all batteries again via Cyrix so the Cyrix VSR is working in both directions effectively. I guess for better battery balancing, perhaps in the case of Lithium Leisure Batteries then it would be an advantage to have a B2B, but in my case I don't think it would be worth the extra cost.
 

wildebus

Full Member
a B2B will give you charging at the correct voltage levels and the right charging profile. For most batteries, a voltage of 14.2V AT THE BATTERY (are you getting 14.2V at the leisure battery?) is to low to be fully charging the battery. My setting could be a bit deceptive as my batteries DO like an absorption voltage of 14.2V (which is why I have it set at that) but usually you would set at 14.4V.

If you have a good quality Solar Controller, like you have, then a B2B can be much less of a key product, but if you are say a wild camper outside summer, you can't rely on Solar being good enough to ever fully charge your batteries, unless you had a big array and had it tilted, so a B2B can be much more important.

The downside of a B2B is that unless you spend big on a high output B2B, you are limited to a much lower current than your batteries could handle (if you have a fair sized battery bank), so you could have spend money on a B2B and still not got the best charging solution.


Which is why, with a biggish battery bank, having both in place can be best of both worlds - and this is why the CTEK Smartpass is fairly popular (and the reason why no one has brought out a similar competing product is that CTEK have managed to patent how the CTEK B2B + Smartpass work, which has disuaded other makes to market a self-contained competitor)
 
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xsilvergs

Full Member
The BMV is a great bit of kit but few talk about the settings. There is a Youtuber that changed from LiFeP04 back to AGM because of repeated flat batteries. Know him?

Peukert Exponent and Charge Efficiency Factor are 2 of the many settings that few talk about. Has anybody tested their batteries efficiency?
 
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wildebus

Full Member
I know of who you speak. I don't believe the issue was anything but user error TBH.

I've checked my battery bank energy use against the reported BMV values and SOC indication and found it to be as accurate as could be expected (so good!).
Another factor people get a bee in their bonnet is about the C rating of batteries (which is about the Peukert exponent of course, which (IMO) is about exaggerated in modern batteries). They see a battery capacity and assume that is what will be delivered ignoring the fact that it depends how it is used to what will be provided (fortunately most users tend to use at below (less than) the C value the battery capacity is rated at and so have a little extra capacity then they realise.
 

RAW

Full Member
Interesting, thanks for the input @wildebus, what you said about charging and the fact that on a standard VSR split charge system then the LB side would not charge fully despite engine being on a long time, as the alternator does not output a charging profile like the MPPT Solar charger I have does.
An Important point

So looking around I have found this

One of the 12-12-9 there could be similarly integrated and I think would suit my set-up and those can be obtained for around £65
I would not have the same ability to monitor charge though as that device is not bluetooth.
However I could integrate the pins on the 12-12-9 to work with the BMV-712 as a relay method when LB side Voltage is low, I think
I could not integrate the Cyrix CT I have though as the Pin-Outs on that are already used for the battery boost effect.
What do you think @wildebus ? Would this be a cheaper way to achieve a similar goal, that is decent charging of LB side when vehicle engine is in use ?
 

wildebus

Full Member
Robert, you need to be careful with the Victron naming standards!
They use the 'Orion-tr' as a generic name for DC-DC converter products.
A B2B is a DC-DC converter, but is an intelligent 'Smart' converter with the multiple charging profiles. A standard DC-DC Converter is just that - a voltage converter, typically from one standard (24V nominal, or 12V nominal) to another (12V or 24V).
Victron decided to call their new B2B an Orion-tr as well as it is of course still a DC-DC converter (engineering dept wins over marketing dept there!). Because of this, it is easy to get the 'smart' and the 'dumb' converters mixed up.
If you did get one of those (you have to get a 12V-12V model of course, you could setup it to have say a constant 14.4V output, but it would be constant - no charging algorithms, just a straight voltage booster (could work fine though). But for a like-for-like output, the actual SMART B2B is not really much more when you compare the cost.
 
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wildebus

Full Member
I posted on the other forum about using an cheap inverter and a smart mains charger together to provide an B2B function. That works well and quite a few people do that as a way to get more use out of their expensive mains charger (and why not indeed!)

Getting off-topic for this thread, but another thing you could to build your own intelligent B2B is get one of those Orion-tr 12V-24V DC-DC converters, connect the output to your existing MPPT Controller and then you would have a B2B setup with little additional expense (you would also. Need to have a relay to switch the MPPT between the PV panels and the Orion-tr, but that is easy enough and can be controlled by the same signal that turns the Orion-tr on and off.
 
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RAW

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I posted on the other forum about using an cheap inverter and a smart mains charger together to provide an B2B function
Have you got a link for that ?

Orion-tr 12V-24V DC-DC converters, connect the output to your existing MPPT Controller and then you would have a B2B setup with little additional expense (you would also. Need to have a relay to switch the MPPT between the PV panels and the Orion-tr, but that is easy enough and can be controlled by the same signal that turns the Orion-tr on and off.
That's an interesting and clever idea. I would need a wiring diagram for that though I think.
 

wildebus

Full Member
Don't think that's just me !! Their naming standards are crazy
that was a "royal" we :)
If you wanted to look at using a DC-DC converter for B2B, this is one way to take advantage of existing kit (but note the disclaimer at the bottom! (again a "royal" note;) )
1590829937802.png
 
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SquirrellCook

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I posted on the other forum about using an cheap inverter and a smart mains charger together to provide an B2B function. That works well and quite a few people do that as a way to get more use out of their expensive mains charger (and why not indeed!)
I guess this is the same setup I have on Murky.

Alternator charges the starter battery. To this is a mains inverter. The mains inverter can power a smart charger connected to the leisure batteries for dull days whilst driving. Mains sockets positioned closely and labelled to make swapping easy.

I don't think this is the most efficient though.
 

RAW

Full Member
@SquirrellCook - think if I was going to do that on the MERC I would somehow get the cheap inverter to power the Victron Mains charger I have, this would need quite some cable run doing and logistics though as the Victron mains charger is in back of Van
I suppose I could put a cheap inverter in the engine bay, wire the output of that back to rear of van and to a seperate socket which I could physically plug the Victron into, but there would be a better way to do it via a relay system I would think. Not much clue on Relays for 240V systems and only just getting my head round the 12v ones.
 

SquirrellCook

Full Member
Not much clue on Relays for 240V systems and only just getting my head round the 12v ones.
The mains inverter is about 1kw. It's historic from it's early conversion. It's mounted close to the starter batteries. The idea was to be able to use power tools around the bus. It's also close to the mains consumer unit. Having plugs allows me to connect what ever I want to different supplies. So if my leisure batteries were at the back, I could just run a mains cable to the back.
 
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wildebus

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I guess this is the same setup I have on Murky.

Alternator charges the starter battery. To this is a mains inverter. The mains inverter can power a smart charger connected to the leisure batteries for dull days whilst driving. Mains sockets positioned closely and labelled to make swapping easy.

I don't think this is the most efficient though.
Rather than swapping over the plugs manually, you could automate it by using a pair of SSRs controlled by the ignition or D+ ? (If I had this setup, I KNOW I would always be forgetting to switch over the inputs :( )

@SquirrellCook - think if I was going to do that on the MERC I would somehow get the cheap inverter to power the Victron Mains charger I have, this would need quite some cable run doing and logistics though as the Victron mains charger is in back of Van
I suppose I could put a cheap inverter in the engine bay, wire the output of that back to rear of van and to a seperate socket which I could physically plug the Victron into, but there would be a better way to do it via a relay system I would think. Not much clue on Relays for 240V systems and only just getting my head round the 12v ones.
ref the SSRs relay I mentioned above - these are Solid State Relays - like the ones in this link - https://amzn.to/2Mfhabo. Around £5 each.
You could have one coming from the Inverter, and one coming from the 240V hookup circuit, and both outputs going to the mains charger.
These use a low-power +12V DC signal (ignition? D+?, switch on dash?) to control the 240V AC relays. Wire it so the ignition or whatever switches on one relay and turns off the other, and signal goes off, it flips the other way.
Or alternatively use a 240V Contactor with auto-switching on the detection of 240V? I fitted one of those on my last van so certain 240V sockets were powered by EHU when plugged in, but would flip to the inverter as a source automatically when no mains was present.
(my own general design goal is to put the effort into the design in place up-front, so when it comes to actually using the stuff, it is seamless and needs no thinking, just enjoying)

If you (or squirrelcook) is interested, this is the contactor I used - https://www.travelvolts.net/product-page/230-volt-auto-switch. Costs around £45.
Works very well, but it is pricer than a SSR solution for sure. I am not using it anymore, as the Victron Multiplus covers that function, so available for sale (used of course) for £20 plus postage (£3.10) and including an ABS box to stick it in.
 
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