Voltage Sensitive Relay – Problems?

[DBA Forum (B) - Chris Roy]

David,

Not quite right. A VSR is just a mechanism for distributing charge to two battery banks. When one starts to get full, the other shares the charge.
Mine is rated at 140 amps and can handle anything my charging kit can throw at it, but I have not use one before and was surprised that instead of one input terminal and two output terminals, one to each battery bank, the VSR is just a fancy way of connecting the banks together.
My problem is that solar panel voltages fluctuate quite frequently as sunlight comes and goes. Therefore, when the primary bank is nearing full, the relay cuts in and out with every voltage rise or fall.
Apart from the annoying click on, click off noises, the technical support man at Durite tells me that such fluctuations will SHORTEN THE LIFE OF THE BATTERIES by about one season.
Not something I want to contemplate.

Regards
Chris

[DBA Forum (B) - Colin Stone]

Chris,

Post by DBA Forum (B) - Chris Roy
My problem is that solar panel voltages fluctuate quite frequently as sunlight comes and goes.

That surprises me. Once the panels have sufficient light to output something useful and contolled by controller, I would expect current to fluctuate but not volts.
I'll monitor my charging volts vs sunlight and see how much it changes.
Seems to me that the problem is having 2 banks.
I use a relay controlled by Smartguage/bank to connect engine battery to domestic battery when voltages are appropriate. Engine battery now 10 years old.

Colin Stone
KEI
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[DBA Forum (B) - Chris Green]

Post by DBA Forum (B) - Colin Stone
Chris,

Post by DBA Forum (B) - Chris Roy
My problem is that solar panel voltages fluctuate quite frequently as sunlight comes and goes.

That surprises me. Once the panels have sufficient light to output something
useful and contolled by controller, I would expect current to fluctuate
but not volts.
I'll monitor my charging volts vs sunlight and see how much it changes.
Seems to me that the problem is having 2 banks.
I use a relay controlled by Smartguage/bank to connect engine battery to
domestic battery when voltages are appropriate. Engine battery now 10 years
old.

A much more elegant solution is one of the Votronic MPP solar panel
controllers, these have an 'auxiliary' charging output for maintaining
a starter battery. It's quite low current but that hardly matters and
it means there's no need for high current relays/contactors to
parallel your leisure and starter batteries.

I think there are a couple of other MPP charge controllers out there
that have a second output like the Votronic ones.

[DBA Forum (B) - David Beaumont]

Chris your VSR is behaving as it should, connecting when the voltage on the donor battery is > 27.4V and disconnecting when it <25.6V. Your problem is the recipient battery voltage (state of charge) is not considered. Therefore if your recipient battery is flat (low voltage) it takes a large charge from the donor, dragging its voltage below 25.6V, cutting the connection. This will repeat until the recipient battery is charged enough not to drag the donor below 25.6V and trip the VSR.

It's not a good way to charge the recipient battery. Chris G's suggestion is much better. I would send the VSR back, it's not suitable for what you want to do, distribute charge gently between two battery banks. These things are designed to distribute a large charging current such as from an alternator between two batteries, not a small current from a solar panel and not for using one battery to charge another. Especially not if the two batteries have significantly different states of charge.

David

[DBA Forum (B) - Colin Stone]

Chris,

Post by DBA Forum (B) - Chris Green
A much more elegant solution is one of the Votronic MPP solar panel

controllers, these have an 'auxiliary' charging output for maintaining
a starter battery.

It could well be - but I bet the voltage output is the same as the main supply, so fine for same type battery.

As my domestic battery is flooded lead acid and engine start is sealed, the charging and equalisation voltages are different.

Colin Stone
KEI
Sent via BlackBerry® BIS

[DBA Forum (B) - David Beaumont]

Actually I'm wondering if the VSR is wired up correctly. If you just want to distribute the solar charge between two batteries, you don't want to connect the two batteries at all. You just want to switch the solar panel charge from one to another, based on the voltage of one battery. That way one battery will not affect the charge of the other. At no time should your two batteries be connected, but it seems like they are.

David

[DBA Forum (B) - Chris Green]

Post by DBA Forum (B) - Colin Stone
Chris,

Post by DBA Forum (B) - Chris Green
A much more elegant solution is one of the Votronic MPP solar panel

controllers, these have an 'auxiliary' charging output for maintaining
a starter battery.
It could well be - but I bet the voltage output is the same as the main
supply, so fine for same type battery.
As my domestic battery is flooded lead acid and engine start is sealed,
the charging and equalisation voltages are different.

The auxiliary output for maintaining the starter battery is only 1 amp
(or 1.5 amps on some models), the difference between flooded and
sealed isn't going to make a significant difference. The idea is not
to actually charge the starter battery from a heavily discharged
state, just to maintain it when not in use.

[DBA Forum (B) - Colin Stone]

Chris,

My solar controller does an equalising charge every 25 days at 30.2v for flooded and sealed 28.7v.
I don't think 30.2v will do a sealed battery any good at all, even at low amps.
And it does not have engine battery connections.

Colin Stone
KEI



Sent via BlackBerry® BIS

[DBA Forum (B) - Chris Roy]

David

There are only two terminals on most VSRs. So the only way to wire them is across the battery banks, linking them together when the relay energises.
Durite confirmed this when I called them.

Chris

[DBA Forum (B) - David Beaumont]

The Durite VSR says "Allows split charging of second battery bank without discharging the primary/start battery". To do that properly it should use the charge input (solar panel or alternator) to charge the second battery. If it is actually using the primary bank to charge the second bank then it is by definition draining the primary. To then cut the connection when the primary drops voltage (as it will) is just a fudge to prevent complete draining of the primary.

If it really works like that I would send it back, not fit for for purpose. Especially not fit when the input charge is a solar panel, with small amperage.

What you want is a proper split charge relay that isolates the batteries and charges them both.

However I would have thought a decent VSR would be able to monitor the voltage of the primary without ever shorting it to the secondary, and then divert the charge to the secondary when the primary was full, which I think is what you want.

David

[DBA Forum (B) - Pete. Milne]

I think the key words are 'without discharging'. A clear description can be found on the Midsummer Energy web site*, including:

"By sensing the main battery bank voltage, the device will only connect the secondary bank to the source when the first is fully charged and will cut out if the main bank falls below a certain level. This ensures that the secondary bank can never drain the main bank."

This is how all VSRs I've looked at work - and even Smartbank.

"12 Volt: Makes at 13.3V and breaks at 12.8V." So if the main battery is discharged below 12.8 and the solar input fluctuates (above 13.3 and below 12.8) then the relay will switch on and off. If the main battery is charged over 12.8V, the relay should remain closed regardless of the sun. [double the figs for a 24V system]

If it is due to a discharged main battery as above and happens often, I would consider fitting a switch between the relay and main battery, to disconnect whenever switching happens too frequently - although it may well be working as it should.

Pete

* http://tinyurl.com/q82db2c

[DBA Forum (B) - Pete. Milne]

PS On reflection, 12.8V would be a full lead-acid battery - so the Durite VSR is designed to 'overflow' spare charge to the secondary battery as David suggests. Is the main battery perhaps not reaching 12.8V (25.6V) when fully charged?

Pete

[DBA Forum (B) - David Beaumont]

[quote="Pete. Milne" post=57225]I think the key words are 'without discharging'. A clear description can be found on the Midsummer Energy web site*, including:

"By sensing the main battery bank voltage, the device will only connect the secondary bank to the source when the first is fully charged and will cut out if the main bank falls below a certain level. This ensures that the secondary bank can never drain the main bank."[/quote]

I think there's a subtle difference in the marketing blurbs between "dicharging" and "draining". Yes the VSR in question will not drain the primary battery as in 'flatten it'. However it will discharge volts from it in order to charge the secondary. As I said "To then cut the connection when the primary drops voltage (as it will) is just a fudge to prevent complete draining of the primary."

These volts are "overflow volts" as the main battery is over 12.8V, nevertheless the VSR is, however intermittently, shorting a fully charged main battery to a flat battery, not a good practice. It is basically jump starting with an intermittent and automatic switch off/on. I wouldn't put such a device on my batteries.

However if it worked by simply switching the charge based on the voltage of the primary and keeping the batteries isolated from each other, it would be great. An intelligent split charge system.

David

[DBA Forum (B) - Chris Roy]

[quote]
"By sensing the main battery bank voltage, the device will only connect the secondary bank to the source when the first is fully charged and will cut out if the main bank falls below a certain level. This ensures that the secondary bank can never drain the main bank."[/quote]
I think this is misleading. "connect the secondary bank to the source" implies only the source, not the main bank.
The blurb say cut in at 27.4v and cut out at 25.6v. and this means the relay latches open at 27.4v and remains latched until voltage drops below 25.4v, which it does.
However I do find that when, without any load on either bank, the relay has cut in, both banks may be charging simultaneoulsy, or one charging while the other is discharging.
I am awaiting a call from the tech expert at CPC batteries (on holiday over the BH weekend) to get advice on the potential impact on the batteries of using this relay. In the meantime, I have already installed a switch on the earth output from the VSR so I can switch it off if it starts to fluctuate.
However, I could not use this switch over the winter months when we are away from the boat.

Regards
Chris

[DBA Forum (B) - Colin Stone]

I think a basic VSR is only suitable for connecting a starter battery to a domestic bank

Starter batteries by and large remain fairly well charged, so there is not a large current when connected and consequently a large voltage drop, causing disconnection.

The other problem is that if the VSR is voltage sensing through the battery cables and a large current flows, there will be a resistive voltage drop.

So can you adjust and lower the disconnect voltage?

Smartbank allows adjustment of Connect, Disconnect voltages and a Hold duration to maintain connection when voltage drops below the disconnect voltage.
And voltage sensing is via thin wires with no current.

Colin Stone
KEI
Sent via BlackBerry® BIS

[DBA Forum (B) - Chris Green]

Post by DBA Forum (B) - Chris Roy
However, I could not use this switch over the winter months when we are away from the boat.

Over winter just parallel the batteries permanently and the charger
will keep them both happy.

This is what I did before I had separate chargers for my two
batteries. While stop/start on the move the alternator keeps the
starter battery happy, then while away from the boat long term I just
used a battery isolator switch wired between the two battery +ves to
parallel the two batteries and left the solar charger to it.

You need to be reasonably sure that both batteries are at
approximately the same state of charge before parallelling them.

[DBA Forum (B) - Colin Stone]

Been watching my solar charging under variable sunshine and voltage does not vary by more than 0.1v from low amps under cloud to max amps in bright sunshine. Voltage does gently rise as battery SOC increases.

Colin Stone
KEI
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