Voltage Sensitive Relay – Problems?

[William_H]

Back to the original question. It is not surprising that the VSR is acting strangely with solar charge. The VSR senses when one battery voltage rises to appear to be being charged and connects the second battery to the first battery in parallel. Now at this point of batteries being paralleled you have one battery fairly well charged being connected to an other battery that is low in charge. A large current flows from the charged to discharged battery to equalise the charges. OP reports 20 amps.
This 20 amps will pull down the first battery in voltage especially if it is old and low in capability.
Now in a normal set up where the VSR operates to connect a domestic battery when the engine is charging this drop in voltage caused by the sudden addition of more charge load can be carried by the alternator. (high current capability)
The VSR is usually designed to have a hysteresis where the turn on (paralleling) voltage is much higher than the turn off voltage. So typically 13.0 volts for turn on so detecting engine charging. 12.5 volts for detecting engine stopped charging. The on might be a little higher but should not be so high as to require engine battery to acheive a high voltage before charging the second battery. The off can not be too low or a well charged engine battery might keep it on even with engine stopped.
OK so I imagine the solar charging is not very high current. So it slowly brings the first battery up to on voltage where the relay operates and connects the second battery, however the seconfd battery is so discharged (low in voltage) that the huge rush of equalising current discharges (pulls down the voltage) of the first such that the VSR senses low voltage and opens. So first slowly starts to recharge.
I think OP would be better off with a solar regulator designed for 2 isolated battery banks. (simultaneously) However to swap the primary solar charging to the most depleted battery might solve the problem. But to put it bluntly VSR is not best for solar recharge paralleling. Adjustment of the on and off voltages if adjustable might solve the problem. In any case if the VSR does cycle off and on the solar power is not being wasted and eventually the second battery might rise in voltage enough that in rush current diminishes and it all stops cycling.

Regarding previous argument over battery size and charging efficency (speed). Clearly it is the size of the battery that matters. So 2 small in parallel is the same as one big ie same total AH rating. While the bigger battery (or multiple small ones) will not be discharged so deeply after a given amount of discharge so will not accept recharge so much as if it were deeply discharged, the larger battery will continue to accept more charge even though the voltatge rises with charge. So getting more total AH of recharge witha bigger battery on a limited engine run charge time. good luck olewill

 

[nigelmercier]

Now in a normal set up where the VSR operates to connect a domestic battery when the engine is charging this drop in voltage caused by the sudden addition of more charge load can be carried by the alternator. (high current capability) ...

Quite. Assuming the VSR is the single sensed type, the solution is to wire the solar panel to the non-sensed battery.

 

[pvb]

There may be practical advanatges of having one large battery bank rather than two smaller ones `but a battery is storage device and efficiency can only mean one thing. Namely the fraction, or percentage, of the energy input that is recoverable as output

I have no idea what the percentage efficiency of a lead acid battery is but suppose it is 90%. That would mean for every 10 joules of energy input 9 would be recoverable as output.

If this applies to one battery then it will apply to a second identical one connected in parallel with the result that for every 20 joules input to the pair 18 would be recoverable as outpu,t 9 from each. 18 is still 90% of 20! No increase in efficiency

I think you're overlooking the fact that there are efficiency losses involved in charging two separate banks, rather than one large bank. If the banks are charged via a diode splitter, there are losses in the splitter. If the banks are charged via a VSR, there are losses because not all the batteries will be connected for 100% of the engine running time.

 

[JumbleDuck]

If the banks are charged via a VSR, there are losses because not all the batteries will be connected for 100% of the engine running time.

As long as the charged produced is going somewhere, there are no additional losses from a VSR. Filling a saucepan and then filling a kettle is no less efficient than filling a saucepan and a kettle simultaneously.

 

[pvb]

As long as the charged produced is going somewhere, there are no additional losses from a VSR. Filling a saucepan and then filling a kettle is no less efficient than filling a saucepan and a kettle simultaneously.

If not all of the batteries are connected, the charging current will be less, therefore the engine will need to be run for longer in order to restore a given number of Ah. Therefore, less efficient than one bigger bank.

 

[halcyon]

If not all of the batteries are connected, the charging current will be less, therefore the engine will need to be run for longer in order to restore a given number of Ah. Therefore, less efficient than one bigger bank.

Unless the big bank takes 50 amp charge and you have a 30 amp alternator.

Old horses for courses.

Brian

 

[sailinglegend420]

I've posted this before but it's worth posting again.


One Big Bank Better Than Two Small Banks?

In the past it was often popular to have two service banks and no starter battery. It was thought a smaller bank would charge quicker. Current thinking is that it is much more efficient to have just one large service bank and a much smaller starter battery.

1. Doubling the service bank size means the life cycle is longer as the DoD is unlikely to fall so close to 50% so often. Life cycle at 50% DoD may be 1000 charge and discharge cycles. At a DoD of only 25% the life cycle may be 2500 or more.


2. Doubling the service bank size also means the “apparent capacity” is greater. Peukert’s law says that the apparent Ah size of a bank changes depending on the current draw.


A bank is designed to deliver a capacity with a current discharge that will flatten the battery in 20 hours. (The 20 hour rate)


So with a 100Ah battery, a 5A load will flatten the battery to 10.5v in 20 hours.


When drawing currents higher than 5 amps the "actual" bank size will be much smaller, so the bank will not last as long before it needs re-charging. Conversely when using much less than 5 amps the bank size will be larger and will deliver more Ah.


If a 100 Ah that battery has a Peukert value of 1.25, then higher or lower loads than 5 amps will change the actual capacity of the battery by the following amounts.


With a 10A load for 20 hours there are only 84Ah's in the bank.


With a 1A load for 20 hours there are 150Ah's in the bank.


3. Doubling the service bank size also means it will be more efficient and accept more Ah more quickly from all charging sources during the boost phase up to 80%.


It takes a bit of very over-simplified maths to prove the point, but a 100 Ah battery that is discharged to 50% may accept 20Ah in the first hour during the boosts stage, maybe 10Ah in the second hour during the start of the less efficient absorption phase, and the remaining 20Ah in another 5 hour. Doubling the battery size to 200Ah, with the same charging source of 20 amps, will accept 10Ah into each battery in 1 hour, that’s 20Ah into the bank. In the second hours it will store another 20Ah. That’s 40Ah replaced in two hours, as compared to 30Ah with a single bank. In the 3rd hour it may still accept 20 amps into the bank because a single battery in the start of the absorption phase could accept 10 amps. That’s 60Ah in three hours. The key point is that for two hours it is still in the more efficient boost stage where the battery is taking all the current the charge source can give it. Note that the initial boost charging stage has captured 40Ah in two hours and 60 Ah in three hours. With the smaller bank it could only capture 20Ah in the first hour during boost and 30Ah after the second hour during the start of absorption. The third hour may add another 5 amps. That’s 35Ah with one batteries and 60Ah with two batteries. So a bigger bank will be more efficient and accept more Ah more quickly from all charging sources.


Since a lot of the time we are only charging up to the absorption stage which is about 80-85% then this increase in stored Ah is significant.


4. If you have a larger bank - or many smaller batteries in one large bank, it is easy when they start failing to just disconnect the bad ones and run on the others as long as you can until you can replace the whole bank. This may also allow skippers to search around and find the batteries they really want - not just be forced to buy the local "rubbish" because they are desperate.

 

[pvb]

Good to see you supporting the philosophy I've been promoting here for so long!

 

[JumbleDuck]

3. Doubling the service bank size also means it will be more efficient and accept more Ah more quickly from all charging sources during the boost phase up to 80%.

Please define "efficient".

Your argument has a contradiction: first you say that bigger banks discharge less, then you say that they accept more current because they are discharged more.

 

[halcyon]

I've posted this before but it's worth posting again.


One Big Bank Better Than Two Small Banks?

In the past it was often popular to have two service banks and no starter battery. It was thought a smaller bank would charge quicker. Current thinking is that it is much more efficient to have just one large service bank and a much smaller starter battery.

Two service were often popular

when ? very rare to me, or do you add 1, 2, Both switches into this group ?

Current thinking one large bank and smaller starter battery

that is old thinking, please read ...http://www.kddpowercentre.co.uk/data/P4000_82_lit.pdf this is 32 years old now.

With regard to having a large service bank totally agree, have for years

Brian

 

[ghostlymoron]

That's the way i have mine too.

A VSR is used between the dedicated engine starter battery and a bank of domestic batteries to prevent the discharging of the starter battery whilst the engine is not running.

The solar charge controller output should just be connected directly to the domestic bank without a VSR.

 

[sailinglegend420]

...Your argument has a contradiction: first you say that bigger banks discharge less, then you say that they accept more current because they are discharged more....

You need to read and understand the text more closely and not quote two ideas that I didn't connect. If you discharge a bank overnight and rely on solar or motoring to recharge to go sailing then a bigger bank will be discharged to a (EDIT) HIGHER SoC each day so it's life will be longer. When you can't or don't recharge till it is at 50% then a bigger bank will accept more Ah more quickly, I didn't say it would accept more current.Why do some people just try to pick an argument and spoil threads.

...Current thinking one large bank and smaller starter battery...when ? very rare to me...

Again misquoting me - I said it's more efficient to have one large bank and MUCH smaller starter battery. In a post of yours you talk about mobos and others who may choose to have two large banks.

Maybe I should have expanded that thought even more. My MUCH smaller starter battery is a Red Flash 37Ah AGM that has started my 56 HP Yanmar for 10 years. It has been lying on its side in the bilges which has made more space available for a MUCH larger house bank.

 

[JumbleDuck]

You need to read and understand the text more closely and not quote two ideas that I didn't connect. If you discharge a bank overnight and rely on solar or motoring to recharge to go sailing then a bigger bank will be discharged to a lower SoC each day so it's life will be longer.

A bigger bank will be at a higher state of charge for a given discharge.

When you can't or don't recharge till it is at 50% then a bigger bank will accept more Ah more quickly, I didn't say it would accept more current.

Accepting Ah more quickly means accepting more current, by definition.

Anyone wanting an "efficient" battery setup needs to know precisely what they are trying to achieve. Is it worth adding a second battery to increase charge rate when a better alternator could achieve the same result? Does the increased battery life you get from lower discharges justify adding more batteries? In some circumstances it might, but in some it might not.

 

[sailinglegend420]

A bigger bank will be at a higher state of charge for a given discharge.

Thanks, I've corrected my typo


Accepting Ah more quickly means accepting more current, by definition.


Ah and current are not the same, by definition. Ah are current x time.

I've managed to edit my original post.

 

[pvb]

Is it worth adding a second battery to increase charge rate when a better alternator could achieve the same result?

Don't fall into the classic trap of assuming, as many people do, that a bigger alternator will somehow magically push more amps into a battery. In the vast majority of cases, it won't. The charging current is governed by the charging voltage and the characteristics of the battery - not simply by the theoretical output of the alternator. For most boatowners with just one or two batteries, the standard alternator will produce more than enough current.

 

[JumbleDuck]

Don't fall into the classic trap of assuming, as many people do, that a bigger alternator will somehow magically push more amps into a battery. In the vast majority of cases, it won't. The charging current is governed by the charging voltage and the characteristics of the battery - not simply by the theoretical output of the alternator. For most boatowners with just one or two batteries, the standard alternator will produce more than enough current.

Of course. I was really thinking of sailinglegend's example of a charging system limited to 20A. A bigger battery may, in some circumstances, be able to use that 20A for longer, but it's also worth looking at the options for having more charging available.

 

[halcyon]

Again misquoting me - I said it's more efficient to have one large bank and MUCH smaller starter battery. In a post of yours you talk about mobos and others who may choose to have two large banks.

That's not what I said, the idea of two small service banks is a odd system that is not good, hence my ? at the end, as for the system you are pushing, we introduced back in 1980, so your current thinking is not new but over 30 years old.

With regard to mobo's to expand a little back in 1983 we supplied Sealine with a 3 bank system, engine starter battery for each and a large service battery bank, which worked okay. In 1991 they changed to a 2 bank system for cost reduction and match Princess, this used a starter battery for 1 engine, then a large bank for service loads and starting for second engine. This resulted in a lot of problems with engine starter noise and low voltages. as some posts were talking one large bank, this was a caution to be careful what you do.

Lastly if you read to the end of the post you will see I was agreeing with you regarding a engine starting battery and a separate large service bank system.

Brian