VSR's - surely they are pointless?

[skyflyer]

Unless I misunderstand their function, surely a VSR will click over as soon as the engine is running and the alternator is supplying ~14v+ so that both battery banks are being charged and thus the idea of giving prioritised charging to one battery is defeated?

Have I missed the point?

 

[prv]

surely a VSR will click over as soon as the engine is running and the alternator is supplying ~14v+ so that both battery banks are being charged

Yep, sounds good to me. Why would you not want that to happen?

Pete

 

[sailorman]

it depends, some are time & or voltage switching same with drop out

 

[pvb]

Unless I misunderstand their function, surely a VSR will click over as soon as the engine is running and the alternator is supplying ~14v+ so that both battery banks are being charged and thus the idea of giving prioritised charging to one battery is defeated?

The fact that the alternator theoretically outputs 14+v doesn't change the fact that the actual charge voltage at the battery is lower when the battery starts charging, and rises as the battery recharges. As starting an engine uses very little power, the charge voltage of a dedicated start battery will increase rapidly, until it reaches the VSR switch-on voltage. The start battery has still received prioritised charging.

 

[duncan99210]

I've always understood the idea of the VSR was to prevent the flattening of the engine battery by demands from the domestic side of the boat. Thus it isolates the engine battery automatically from the domestic bank when the engine is off but allows both batteries to be charged when the engine is running..... Or am I missing something?

 

[pvb]

I've always understood the idea of the VSR was to prevent the flattening of the engine battery by demands from the domestic side of the boat. Thus it isolates the engine battery automatically from the domestic bank when the engine is off but allows both batteries to be charged when the engine is running..... Or am I missing something?

You're not missing anything, it does that (always assuming the wiring's correct!)

 

[JumbleDuck]

Unless I misunderstand their function, surely a VSR will click over as soon as the engine is running and the alternator is supplying ~14v+ so that both battery banks are being charged and thus the idea of giving prioritised charging to one battery is defeated?

Have I missed the point?

Yes. If the start battery is very flat it will pull the alternator output well below the trigger voltage and therefore get all the available charge until it's a bit healthier.

I've always understood the idea of the VSR was to prevent the flattening of the engine battery by demands from the domestic side of the boat. Thus it isolates the engine battery automatically from the domestic bank when the engine is off but allows both batteries to be charged when the engine is running..... Or am I missing something?

I think that's the main benefit.

 

[Buck Turgidson]

It's all here: http://www.jgtech.com/pdf/VSR_instructions.pdf

 

[BabaYaga]

It's all here: http://www.jgtech.com/pdf/VSR_instructions.pdf

Although not "pointless", those instructions point out the risk of VSR relay "chatter" and the limitations of house bank size vs alternator capacity that must be observed to avoid it (max 220AH for a 90A alt). In my view one reason good reason why a VSR is not the best method for charge splitting.

 

[snooks]

A VSR isn't for split charging, as it treats the start batteries and house battery as one. Yes there are more complex systems, but let's keep it simple :0)

The primary benefit of a VSR is to disconnect the start battery from the house batteries after the engine is turned off.

Put simply....

When you crank the engine the house and engine batteries are separate, so all the juice comes out of the start battery. Once the engine is running the charge from the engine is put back into the engine battery only. When the voltage of that battery, reaches 14v, it's fully charged and the VSR now connects the engine and house batteries together. Because the engine battery is charged all the charge goes into the house batteries, so you now have one big bank of batteries all charging. Switch the engine off, both engine and house are still connected and you'll use power from both....until the voltage of the bank reaches 13.5v iirc, and the VSR then separates the two banks again. So your engine battery is separate from your house batteries. You can now run your house batteries flat (not something I'd recommend - never go below 50% if you want to look after them) and your engine battery will still remain fully charged and untouched.

It makes the system idiot proof. I have it on my boat, and it worked exactly as it should.

 

[alahol2]

Virtually the same result can be obtained rather more cheaply with a suitably rated 'ordinary' relay activated by the 'ignition' switch. Which is what I have had for the last 7 or 8 years. It has worked faultlessly.

 

[JumbleDuck]

Virtually the same result can be obtained rather more cheaply with a suitably rated 'ordinary' relay activated by the 'ignition' switch. Which is what I have had for the last 7 or 8 years. It has worked faultlessly.

That's fine, as long as you use a very beefy relay, because it will have to pass whatever proportion of starter current comes from the house bank. If the starter battery is knackered, that could be a lot. I combine the two ideas by having a VSR which is only activated when the ignition is on ... to stop it confusing the dual battery solar controller.

 

[alahol2]

That's fine, as long as you use a very beefy relay, because it will have to pass whatever proportion of starter current comes from the house bank. If the starter battery is knackered, that could be a lot.

I have a 40A alternator and have used a 40A relay (£5.70) on a Kubota 13HP. I was a little worried about the situation you mention but it has never proved to be a problem. I have a combining switch in parallel with the relay but have never needed to use it.

 

[JumbleDuck]

I have a 40A alternator and have used a 40A relay (£5.70) on a Kubota 13HP. I was a little worried about the situation you mention but it has never proved to be a problem. I have a combining switch in parallel with the relay but have never needed to use it.

I guess that your starter will take ~80A, same as mine, and a 40A relay ought to be able to handle that for shortish periods, particularly since it's not being asked to make or break it.

 

[Len Ingalls]

Consider what will happen,if the + or - bat.cable corrodes (hi resistance) on the start bat.(or st. bat.is dead < 11V)
The VSR "sense wire" is looking at only the st. bat.,so it will never "combine" the 2 bat.banks,under above conditions.No danger of a heavy rush of current from "house" bank to starter.
With a manually operated relay,activated just by turning key on,& assuming there is enough voltage from the st. bat to operate the low current coil of manual relay-you could have a quick rush of 1-200 amps thru it's contacts.

Just a thot. / Len

 

[BabaYaga]

I guess that your starter will take ~80A, same as mine, and a 40A relay ought to be able to handle that for shortish periods, particularly since it's not being asked to make or break it.

Doesn't sound like a very good idea to allow starter current to pass through a 40A relay (or 80A for that matter), surely the current must be higher than that?
I have a 250A fuse in my starter cable (20hp engine), 200 - 250A is the usual recommendation for smaller engines AFAIK.
One way to avoid that risk altogether when using a simple combining relay instead of a VSR is to wire it so that it is activated by alternator D+ (charging light circuit). With this setup the batteries only get combined when the alternator begins to put out some charge, typically five seconds or so after cranking has ceased.

 

[William_H]

All of the above is correct. However the voltage at the start battery when the alternator comes on will depend on the ability of the alternator to maintain 14v at the charge current and hence the depth of discharge of the starter battery. In practice the alternator can get the starter battery voltage up to 14v fairly quickly if the alternator is given enough revs.
Then much depends on the voltage at which the VSR is set to pull in. This is likely to be set at less than 14v perhaps more like 13.75v so yes the VSR will pull in before the start battery has been fully recharged. Not that it matters at all. So prioritising to a limited and variable degree.
The "chattering" on a good design VSR is stopped by having the drop out voltage a lot lower than the pull in voltage. Also known as hysteresis.

I tend to disregard this impiricle formular of needing 10% of the battery AH rating as an alternator rating. I suppose in a way it might make sense. However in practice an alternator max rating is not often reached. It must be running at rated (max) revs then the batteries must be discharged so much that they plus any loads on the system will reach the rated amps of the alternator. Mostly it is more difficult to get the batteries to take the current rather than the limit of the alternator being reached.
Nevertheless a large alternator will run cooler than a small run working hard.
What I reckon people should do is to observe actual amps out of their existing alternator and try to determine if in most cases the alternator is the limiting factor or if it is the just the batteries not taking the current. Of course a smart alternator controller can up the voltage so up the current into the batteries so conceivable you would end up with the alternator being the limiting factor. Usually if the load on the alternator is too much then the voltage will fall (regardless of Mr Stirling and his friend's efforts) so charge current goes down so the system is self limiting. So I supsect ambny who fit a bigger alternator will be disappointed at no improved charge rate.

 

[sailorman]

A VSR isn't for split charging, as it treats the start batteries and house battery as one. Yes there are more complex systems, but let's keep it simple :0)

The primary benefit of a VSR is to disconnect the start battery from the house batteries after the engine is turned off.

Put simply....

When you crank the engine the house and engine batteries are separate, so all the juice comes out of the start battery. Once the engine is running the charge from the engine is put back into the engine battery only. When the voltage of that battery, reaches 14v, it's fully charged and the VSR now connects the engine and house batteries together. Because the engine battery is charged all the charge goes into the house batteries, so you now have one big bank of batteries all charging. Switch the engine off, both engine and house are still connected and you'll use power from both....until the voltage of the bank reaches 13.5v iirc, and the VSR then separates the two banks again. So your engine battery is separate from your house batteries. You can now run your house batteries flat (not something I'd recommend - never go below 50% if you want to look after them) and your engine battery will still remain fully charged and untouched.

It makes the system idiot proof. I have it on my boat, and it worked exactly as it should.

The correct answer.
my VSR cuts out @ 13.8v + i now have an Adverc fitted & that is just a brilliant Bit-O-Kit

 

[VicS]

The correct answer.
my VSR cuts out @ 13.8v + i now have an Adverc fitted & that is just a brilliant Bit-O-Kit

More commonly, ITYWF,they cut in at 13.8,which is when the priority battery gets to about 80% of "fully" charged, and cut out again at something like 12.8.

 

[sailorman]

More commonly, ITYWF,they cut in at 13.8,which is when the priority battery gets to about 80% of "fully" charged, and cut out again at something like 12.8.

Ah sorry a typo yes cut -out @ 12.8v