Fitting a Sterling Battery to Battery Charger

[Ivory55]

How do you know what size fuse to put between the batteries, is it to do with the size of the batteries ? Cheers

 

[hilldweller]

How do you know what size fuse to put between the batteries, is it to do with the size of the batteries ? Cheers

Purely the size of the cables, should one short to earth you need the fuse to blow before a fire starts.

 

[DBK]

I've got it all working now but I still can't measure the charging current! I think the return current bypassed the shunt somehow and went directly back to the vehicle battery. However, as Geoff suggested the only thing that matters is the voltage and that is staying high so it must be doing its job. I'm going to leave things as they are and see how we get on.

 

[Stealaway]

I've just daisy chained them. The one from the vehicle battery goes to the leisure batteries and from there on to the B2B. There is only one common negative on the B2B.

Firstly thanks for spurring me on to eventually get around to buying one of these.

The instructions specifically state to maximise efficiency to take the black to the starter battery. In my van I would then have to take a further one to the domestics as per the wiring diagram. You didn't think it necessary? and I can't see what difference it would make.

Do you think it necessary to monitor the unit? I can fit either side of the battery box - inside or outside.

 

[DBK]

Firstly thanks for spurring me on to eventually get around to buying one of these.

The instructions specifically state to maximise efficiency to take the black to the starter battery. In my van I would then have to take a further one to the domestics as per the wiring diagram. You didn't think it necessary? and I can't see what difference it would make.

Do you think it necessary to monitor the unit? I can fit either side of the battery box - inside or outside.

I haven't run a dedicated black cable from the B2B to the vehicle battery. I've just connected it to the leisure batteries mostly using the existing 35mm cable I fitted for the inverter plus a short 25mm link at the B2B end before it joins the 35mm cable. From the leisure batteries I then added a 25mm cable to connect them to the negative on the vehicle battery. Both sets of batteries were already connected to the chassis so I reckoned with the chassis link and my cables the connection should be good enough. We shall just have to see how it works in practice.

You don't need to monitor the unit and it could be mounted out of sight as mine is. You can get remote monitors but they are not needed. Just make sure there is a reasonable airspace around it as they have a cooling fan fitted. You will know when it is working as the voltage will jump up.

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[Misterg]

I still can't measure the charging current!

This is why:

I haven't run a dedicated black cable from the B2B to the vehicle battery. I've just connected it to the leisure batteries

The current into the B-B charger and the current out of B-B charger are both going through your shunt but in opposite directions and (near enough) cancelling each other out.

Run a single negative from the B-B charger back to the vehicle battery or a convenient local chassis connection and it will work fine. You don't need a negative connection from the B-B charger to the leisure batteries (assuming that there is an existing connection to chassis / vehicle negative via your shunt).

 

[DBK]

This is why:



The current into the B-B charger and the current out of B-B charger are both going through your shunt but in opposite directions and (near enough) cancelling each other out.

Run a single negative from the B-B charger back to the vehicle battery or a convenient local chassis connection and it will work fine. You don't need a negative connection from the B-B charger to the leisure batteries (assuming that there is an existing connection to chassis / vehicle negative via your shunt).

Thanks but that doesn't make sense. In electrical terms I've connected it exactly as Sterling recommend. All the negatives of the three batteries are linked together as per the wiring diagram they supply. The only difference is which end of this link is connected to the B2B.

 

[Pausim]

Thanks but that doesn't make sense. In electrical terms I've connected it exactly as Sterling recommend. All the negatives of the three batteries are linked together as per the wiring diagram they supply. The only difference is which end of this link is connected to the B2B.

Which negative is the shunt connected to?

 

[tonyidle]

I'm not certain that your description of alternator operation is correct. Up to quite recently a vehicle alternator output was pegged at approx. 14.2v. Since then smart alternators have appeared but control by the engine ECU has, as far as I'm aware, more to do with emissions control and engine power maximisation than any attempt to improve their charging regime. Your are completely correct in that ECU control of the alternator renders it less effective at charging the leisure battery.

"The smart alternator system allows the vehicle to control the output voltage from the alternator based on vehicle operating conditions to reduce electrical load and in turn mechanical load on the engine by the alternator, this renders it unsuccessful at charging a secondary battery system to a usable level."

 

[tonyidle]

Thanks but that doesn't make sense. In electrical terms I've connected it exactly as Sterling recommend. All the negatives of the three batteries are linked together as per the wiring diagram they supply. The only difference is which end of this link is connected to the B2B.

To measure the all current going into and out of the leisure battery the shunt needs to be the first connection to the leisure battery negative post. All connections that would normally attach to the leisure battery negative post should be made to the other end of the shunt.

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[Misterg]

Is this the Sterling diagram you're working to?

If so, your shunt should be here:

How have you wired it?

And this:

Which negative is the shunt connected to?

[Edit to add 'other stuff' to 2nd sketch]

 

[Two on Tour]

One side of my Victron shunt connects to the negative of my leisure battery and the other side of the shunt goes to a bus bar where all negatives from the hab circuits, my B2B and chassis earth connect to the bus bar.
The Victron battery monitor needs to know every thing that charges and discharges to be able to correctly calculate the batteries remaining capacity percentage as well as giving a reading of the real time charge and discharge watts/amps.

 

[DBK]

Thanks for all the input. I won't respond individually as I've just had a productive dog walk because while wandering through a wood I focussed my limited grey matter on the question that has been bothering me for a while. What is the current in the common negative measured at the B2B terminal? What follows is my analysis of what I think is going on and I would welcome comments on it.

Here is the recommended wiring diagram. I have wired it the same except the black cable from the B2B connects at the leisure battery end of the cable linking the vehicle and the leisure batteries.

I think the key to understanding what is happening is to appreciate that the current flows in the two red cables are in the opposite directions. Ignoring losses and assuming (for now) both sets of batteries are at the same voltage then you might typically see 30A flowing out of the vehicle battery and 30A flowing into the leisure batteries. So what then is the current in the back cable at the B2B end? I initially thought it might be 60A but if this was correct and this cable took the combined currents then it would need to be a thicker cable and the terminal on the B2B would also need to be larger.

Then this afternoon while dodging showers I realised that the current at the B2B common return terminal in this example must be zero. Now consider the batteries, amps in must equal amps out - it doesn't get lost. So if 30A is flowing out of the vehicle battery and 30A is flowing into the leisure batteries then 30A must be flowing along the linking cable.

In practice because the voltage across the leisure batteries will usually be higher than the vehicle battery voltage then a small current will flow in the black cable going into the B2B. So if 35A was coming out of the vehicle battery and 30 going into leisure batteries then the missing 5A will be in the common return and there would be 35A in the linking cable. It is this small current which I was measuring and why the readings seemed weird.

So how can I measure the charging current? Not easily without as suggested fitting a new shunt directly on the first leisure battery negative terminal with the linking cable and the return from the B2B on the other side of the shunt. My existing shunt is some distance away from the leisure batteries and to use this I would need to rewire it and I would need additional cables.

You can get cheap shunt monitors on eBay but I'm reluctant to fit one because if it started a fire I suspect the insurance might be a bit reluctant to pay up if it was due to a modification by me and fitting a Chinese knock-off copy. The alternative might be a Hall effect current sensor like this:

https://ebay.us/M9Qrdo

As an aside, the above came to me after I had been trying to work out why the fuse in the split charge system was 100A, which seems very high. To understand why a big fuse is needed you need to consider what happens if you start the engine with a fully charged vehicle battery and well discharged leisure batteries. Yes, some current will flow from the alternator to the leisure batteries but a whole lot more current will flow out of the vehicle battery into the leisure batteries. Basically, when the split charge relay closes it is like connecting a pair of jump leads from the vehicle battery to the flat leisure batteries. It was realising the current flows out of the vehicle battery into the leisure batteries which created the lightbulb moment to work out the current flows in the B2B cabling.

The above is my guess at what may be going on but happy to hear alternatives.

For the moment I'm just going to see how we get on and see what the impact of confusing the battery monitor means.

If I was doing this again I would connect the B2B black cable to the vehicle battery negative then run the linking cable not to the leisure batteries but to the shunt. This should then give a correct charging reading. I may still have to do this is if I run into problems with the monitor not knowing the charge state but it will cost £60+ in new cables!

 

[Pausim]

Sounds like you have it sussed. Could you divert the negative from the vehicle battery to go to the shunt instead of to the B2B? If so, and if I have understood your wiring, then the problem should be solved by connecting the vehicle battery negative to the other end of the shunt from the leisure battery. The common negative would be maintained but only leisure amps would flow through the shunt.

If it were me I would want to separate the two battery banks with some form of relay to stop the leisure battery back feeding the vehicle battery as you described. In my system the internal EBL relay still performs this function. Sterling make a Pro Connect IFR 12 80 device which appears to do this job. Perhaps worth a look https://www.sp-shop.co.uk/search.php?mode=search&page=1&keep_https=yes

 

[Misterg]

what then is the current in the back cable at the B2B end?

It's the *difference* between the currents into and out of the B2B: I1 - I2 in my 2nd picture above

if 35A was coming out of the vehicle battery and 30 going into leisure batteries then the missing 5A will be in the common return ... which I was measuring and why the readings seemed weird.

You've got it - that's the point I was trying to make (perhaps badly) in my original reply.

If I was doing this again I would connect the B2B black cable to the vehicle battery negative then run the linking cable not to the leisure batteries but to the shunt.

You can run the B-B negative to a convenient local chassis point with very little impact. I don't understand why you would need to add a linking cable if your existing split charge system worked - the leisure battery negatives must already be connected to the van battery negative or chassis (via the shunt) & so acting as the link: this just needs to stay in place...

Good luck, anyway

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[Pausim]

Not sure the second paragraph of my previous post is the right answer, it probably depends on exactly how you have wired up your B2B.

 

[DBK]

Sounds like you have it sussed. Could you divert the negative from the vehicle battery to go to the shunt instead of to the B2B? If so, and if I have understood your wiring, then the problem should be solved by connecting the vehicle battery negative to the other end of the shunt from the leisure battery. The common negative would be maintained but only leisure amps would flow through the shunt.

If it were me I would want to separate the two battery banks with some form of relay to stop the leisure battery back feeding the vehicle battery as you described. In my system the internal EBL relay still performs this function. Sterling make a Pro Connect IFR 12 80 device which appears to do this job. Perhaps worth a look https://www.sp-shop.co.uk/search.php?mode=search&page=1&keep_https=yes

There are other ways to do it but they all need additional cables over what I've got.

The split charge system has been disabled by me so what you suggest in your second paragraph can't happen. I hope!

 

[Misterg]

Not sure the second paragraph of my previous post is the right answer, it probably depends on exactly how you have wired up your B2B

The B2B itself stops current flowing back to the vehicle battery.

 

[DBK]

It's the *difference* between the currents into and out of the B2B: I1 - I2 in my 2nd picture above



You've got it - that's the point I was trying to make (perhaps badly) in my original reply.



You can run the B-B negative to a convenient local chassis point with very little impact. I don't understand why you would need to add a linking cable if your existing split charge system worked - the leisure battery negatives must already be connected to the van battery negative or chassis (via the shunt) & so acting as the link: this just needs to stay in place...

Good luck, anyway

Thanks. As you say you don't have to use a linking cable but Sterling recommend it so that is what I've done.

 

[Deneb]

You can run the B-B negative to a convenient local chassis point with very little impact. I don't understand why you would need to add a linking cable if your existing split charge system worked - the leisure battery negatives must already be connected to the van battery negative or chassis (via the shunt) & so acting as the link: this just needs to stay in place...

Yes, it does seem to be over-complicating things. FWIW the instructions for my Votronic B2B only require an earth cable between the leisure and vehicle batteries if their respective earths are otherwise isolated (e.g. not to a common ground through the vehicle chassis or body). I only have a single earth cable from the B2B which runs to the busbar connection after the shunt on my leisure battery earth connection. The battery monitor reports all current in and out including charging current from the B2B when it is running.

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[autorouter]

The B2B unit has a circuit board that requires 12 volts positive and negative to give it power to do its job, checking the voltage and switching the main current on and off. It will use a small amount of power, maybe an amp or do. It can get 12V positive from the battery positives, but it needs a small separate negative wire. That's what the black wire to the B2B is for.

The main current passes between the battery negative terminals. If they're both grounded to the chassis, the main current can pass through the chassis. If that current path is suspect, then a separate black wire linking wire between the battery negative terminals is a good idea.

 

[DBK]

The B2B unit has a circuit board that requires 12 volts positive and negative to give it power to do its job, checking the voltage and switching the main current on and off. It will use a small amount of power, maybe an amp or do. It can get 12V positive from the battery positives, but it needs a small separate negative wire. That's what the black wire to the B2B is for.

The main current passes between the battery negative terminals. If they're both grounded to the chassis, the main current can pass through the chassis. If that current path is suspect, then a separate black wire linking wire between the battery negative terminals is a good idea.

That's a good point about the negative cable from the B2B not needing to be as thick as the positives. However, for safety just in case a fault occurred in the B2B it might be wise to add a suitable fuse in it, rated at the safe current for the thinner cable. I've fitted 60A midi-fuses in the positive cables but these only blow at twice this current.

 

[Ivory55]

Open heart surgery seems a piece of cake compared to 12v electric in a motorhome. Haha

 

[DBK]

Open heart surgery seems a piece of cake compared to 12v electric in a motorhome. Haha

I first heard about B2Bs when I joined this Forum. It then took me more than a few years to understand their benefits. This project of fitting one has highlighted I still didn't really understand when I bought it how they actually worked!

 

[DBK]

After nearly a year of use and in the last few weeks giving the B2B a good test when we stayed on a few aires in a wet Brittany I'm happy to say the B2B has performed well.

We've used several aires without electricity recently and didn't drive very far between them. Typically the batteries would be showing about 70% capacity just before we drove off in the morning. This is on a total of 180Ah capacity so we had used in round numbers about 60Ah of power. This is mainly down to the Webasto diesel heater which with single figure temperatures outside would be running on and off all night. We also run a compressor coolbox during the day but switch it off at night to save power. These are the main users of electricity as we haven't watched television for the last eleven weeks. All lights are LED.

Our experience was an hour of driving was more enough to replenition the batteries and even thirty minutes had them looking much more healthy.

The caveat is the NASA battery monitor needs time to work out the battery state and it only does this if there are no heavy currents flowing in or out. For example, after the hair dryer is used in the morning the battery state usually drops to 60% but within a very short time the meter shows it has recovered to 70%. Then when we are driving the battery state may say 80% when we arrive but within a few minutes it is saying 100%. I suspect the meter needs to see what the battery voltage is at rest. It also calculates the state from the knowledge of the battery capacity and measuring the current going in and out but this is only an approximation as it can only estimate how much of a charge going in is converted into stored energy so it improves on the estimate when things have settled down. The batteries also recover through a chemical process of course.

Ours is a 60A B2B which for 180Ah of battery is probably too much but I'm not going to change it and will wait to see how long the batteries last. The B2B is out of easy access but it does get hot as I can hear the fan on it running away when we stop but the cables to the B2B didn't get hot or even warm when it was running at full tilt which was reassuring and no sound of fizzing batteries either but we probably wouldn't hear those.

I haven't done anything about the shunt and how to accurately measure the current going into the leisure batteries, which is probably why the NASA monitor gets confused but I have finally worked out how to wire things so they do. If you look at the diagram in post #43 above the cable linking the two batteries should not go directly to the leisure negative but to the shunt and to the side of the shunt opposite where the leisure battery is connected. Similarly the negative from the B2B should go to the same place. Wiring it this way ensure that all current coming from the leisure batteries has to go through the shunt.

I'm very pleased with the B2B and recommend them, especially if you have a diesel heater and a compressor fridge or cool box and don't plan to drive far each day at times of the year when solar isn't going to do much. But the currents involved are large so I would advise a professional fit unless your DIY and electrical skills are up to it. If you set fire to your MH with a poor installation the insurance company might be loathe to pay up.

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[Ingwe]

With only one leisure battery and a 120w solar panel, I think we should now have a B2B installed. No way could I do this myself.
Will investigate this when back in the UK.

 

[G7UXG]

A very informative thread DBK
Interested to know whether, when the solar input raises the vehicle battery voltage above the switching threshold, does this cause the B2B charger to switch on? If so, does that cause a problem?

 

[Lenny HB]

A very informative thread DBK
Interested to know whether, when the solar input raises the vehicle battery voltage above the switching threshold, does this cause the B2B charger to switch on? If so, does that cause a problem?

The latest ones can be wired for ignition or D+ triggering. If it is a problem just fit a relay in the output switched by ignition or D+.

 

[DBK]

A very informative thread DBK
Interested to know whether, when the solar input raises the vehicle battery voltage above the switching threshold, does this cause the B2B charger to switch on? If so, does that cause a problem?

This is mentioned in the first post on this thread where I've quoted Sterling as saying "this shouldn't be a problem" and so it seems to have turned out. I am not sure why it isn't a problem but there is no evidence of the B2B kicking in every time the sun shines. I suspect it is because the Battery Master only provides a trickle current and this may not be enough to raise the vehicle battery voltage above the threshold which activates the unit. In other words, the leisure batteries might be seeing 14.4 volts from the solar but there isn't the current going into the cab battery to raise its voltage to the same level. I must check some (sunny) day.

As Lenny suggests I think it is better to set them up so they only work when the engine is running but I had problems getting the B2B to work like this for unknown and still unresolved reasons - but it works fine on automatic.

 

[Leakylunar]

Well that’s put the cat amongst the pigeons!
put some £ aside for solar for our camper van that has 12v fridge and diesel heater thinking would I be better investing in a b2b charger as I’m only allowed 130ah battery max with Sargent ec325 !!!

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