Engine, Alternators and Battery Banks

[Col_T]

The engine is a Beta BV 1505, with two alternators - 70A connected to the domestic bank, and 45A connected to the engine start battery. At present, the domestics and engine start batteries are connected by a voltage sensitive relay and, because I don’t completely understand electrics, I am considering removing the VSR.

 

My understanding is that the VSR will switch the output from one alternator to the battery being charged by the other alternator when the relevant voltage is reached. In my case, the output from the engine start alternator will be directed to the domestic battery bank. However, what I think will happen is that, once both alternators are supplying the same battery bank, one of them will drop out - stop charging - because a voltage limit in it’s regulator has been reached. If this is correct, I’m not sure that I see any benefit in having the VSR fitted.

 

Any comment explaining where I have this wrong would be much appreciated.

 

 

[Tracy D'arth]

If you are correct, why do they make VSRs and why do a lot of us fit them?

 

They allow both alternators to charge both battery banks.

Edited July 28, 2022 by Tracy D'arth

[BEngo]

Depending on where the VSR is sensed it may enable the start  alternator to parallel the domestic alternator once the start battery is recharged.  If the domestic battery is low then this will be beneficial.  If the domestic battery is past the bulk charge stage it will do no harm.

 

If, with VSR wired as above,  you ever have a domestic alternator failure the VSR will be useful as it will automatically start charging  ( albeit slowly) the domestic battery from the start alternator, once the engine battery is refilled.

I would check how it is wired, and leave it, if it is sensed off the engine battery, or swap the sense wire so it is.

N

[Col_T]

15 minutes ago, Tracy D'arth said:

If you are correct, why do they make VSRs and why do a lot of us fit them?

To avoid the use of split charge diode, with attendant voltage drop, on a single alternator engine?

 

Thanks for the info @BEngo - it’ll stay fitted.

[Tony Brooks]

1 hour ago, Col_T said:

My understanding is that the VSR will switch the output from one alternator to the battery being charged by the other alternator when the relevant voltage is reached. In my case, the output from the engine start alternator will be directed to the domestic battery bank. However, what I think will happen is that, once both alternators are supplying the same battery bank, one of them will drop out - stop charging - because a voltage limit in it’s regulator has been reached. If this is correct, I’m not sure that I see any benefit in having the VSR fitted.

 

To try to explain some more. VSRs do not switch output, they only connect things together. There are two types, duel sensing that reacts to a given voltage on EITHER main terminal and single sensing that closes when the voltage on just on main terminal reaches the threshold. That threshold is typically about 13.6 volts. When the voltage falls below a threshold of perhaps 13.4V they open. Nowadays, I would only use a dual sensing one.

 

VSRs do not care  what is producing the charging voltage, only that it is above or below the thresholds, so wiring a VSR as yours seems to be  wired whenever the voltage on either battery (assuming duel sensing) rises above 13.6ishV the contact will close so whatever the charging source it will see the batteries as one big bank.

 

It is the battery that determine how much currant it will accept providing the that current is less than the charger source's maximum output. If the battery wants more current that the source can supply the voltage will fall so during the early stages of charging the charging voltage will be such that the VSR can't close, but as the batteries charge the currant starts to fall and the voltage rises until it is high enough to close the VSR. At that point the battery looks like just one big bank to the charge source. At this point each battery in the new big bank (engine plus domestic batteries) will accept a charge that is in proportion to its degree of discharge. This probably means the domestics take the greater proportion and the engine battery far less.

 

This will continue with each battery taking the proportion of the charge it needs and as the batteries charge the currant falls allowing the voltage to rise. This will go on for probably several hours until the voltage reaches the ALTERNATORS' regulated voltage, but by then all batteries will be well charged and the current they want is likely to be well below either alternator's maximum output.

 

What happens at that point depends upon what each regulator is set to. If both are identical both will stay charging but if one is set  slightly higher than the other the lower one may shut down. This can happen with solar and no VSR. If that happens a warning lamp may glow and the sounder beep, but the other alternator will do all the charging required.

 

By using a VSR you allow solar or a single output charger to charge both banks.

 

 

 

 

[David Mack]

With twin alternators the VSR is unnecessary for engine charging, but does no harm, and does mean you benefit from the combined alternator output if one bank is particularly discharged . It also means that if one alternator fails you continue to charge both battery banks - but you may well be unaware that the alternator has failed!

If you have solar (connected to one bank) the VSR allows the solar to charge both banks.

[Sea Dog]

2 hours ago, BEngo said:

Depending on where the VSR is sensed it may enable the start  alternator to parallel the domestic alternator once the start battery is recharged.  If the domestic battery is low then this will be beneficial.  If the domestic battery is past the bulk charge stage it will do no harm.

 

If, with VSR wired as above,  you ever have a domestic alternator failure the VSR will be useful as it will automatically start charging  ( albeit slowly) the domestic battery from the start alternator, once the engine battery is refilled.

I would check how it is wired, and leave it, if it is sensed off the engine battery, or swap the sense wire so it is.

N

Eminently sensible stuff above and from others. There is merit in having both alternators able to supply the more demanding domestics once the starter battery is recharged (very quickly in normal use) when the domestic alternator alone might struggle.

 

However, if you have an ammeter, just have a look at how quickly your domestic bank charge current drops to below the 70 amps your domestic alternator alone can already supply - I suspect it's minutes, and after that there's little point in the VSR. If that's not the case and the VSR is maintaining a higher charging current for much longer, then the VSR may be useful.

 

As you say you have limited knowledge (although since you've identified the VSR and know what it's doing you may be selling yourself a little short!) I'd be tempted to disconnect the VSR if you judge it unnecessary as it might confuse or mask the symptoms in the event of a charging system fault (including VSR!).

 

You could always leave the VSR disconnected but ready to reconnect if needed. Rather more simply, I carry a jump lead in case one of my pair fail, but then my 150a domestic alternator means I definitely have no need of a VSR to boost domestic charging current.

 

The idea of sending solar current to both banks sounds attractive but, unless you're leaving your boat for many months, a fully charged starter battery won't really need it. Several months of not being able to go to my boat during COVID verified that theory for me! 

[Tony Brooks]

11 minutes ago, Sea Dog said:

Eminently sensible stuff above and from others. There is merit in having both alternators able to supply the more demanding domestics once the starter battery is recharged (very quickly in normal use) when the domestic alternator alone might struggle.

 

However, if you have an ammeter, just have a look at how quickly your domestic bank charge current drops to below the 70 amps your domestic alternator alone can already supply - I suspect it's minutes, and after that there's little point in the VSR. If that's not the case and the VSR is maintaining a higher charging current for much longer, then the VSR may be useful.

 

As you say you have limited knowledge (although since you've identified the VSR and know what it's doing you may be selling yourself a little short!) I'd be tempted to disconnect the VSR if you judge it unnecessary as it might confuse or mask the symptoms in the event of a charging system fault (including VSR!).

 

You could always leave the VSR disconnected but ready to reconnect if needed. Rather more simply, I carry a jump lead in case one of my pair fail, but then my 150a domestic alternator means I definitely have no need of a VSR to boost domestic charging current.

 

The idea of sending solar current to both banks sounds attractive but, unless you're leaving your boat for many months, a fully charged starter battery won't really need it. Several months of not being able to go to my boat during COVID verified that theory for me! 

 

Just to point out that when a 70 amp alternator charging current dropped below 70 amps the voltage probably has some way to rise until the regulator starts to work. For simplicity, you need to see when the charging voltage stops rising, not when the current drops off the alternator's rated output. Then we get into the "soft" regulation curves of automotive regulators.

[nicknorman]

What Tony says above is key. This forum, along with a number of other sources, is guilty of spreading misinformation. They talk about “Bulk” and “Absorb” phases of charging as if these were discrete things, the charging is in Bulk (constant current) then suddenly switches to Absorb (constant voltage). The reality is nothing like that, in most cases neither state exists, it is nearly all some sort of blend of the two concepts in varying ratios.

 

So by combining the two alternators the result is a slightly higher charging voltage and thus increased current and thus a reduced charge time, certainly during the early and mid parts of charging.

[Col_T]

Many thanks for the replies, especially @Tony Brooks, whose first comments put the whole thing in context.

 

15 hours ago, Sea Dog said:

However, if you have an ammeter, just have a look at how quickly your domestic bank charge current drops to below the 70 amps your domestic alternator alone can already supply

 

In all honesty, I’ve never seen anything close to 70 amps on the ammeter, even at first start-up, but then we are very light electrical users, have a reasonable size battery bank (4 * 110Ah), almost always cruise for about 4 hours and have solar for the beginning and end of each day.

 

[Tony Brooks]

36 minutes ago, Col_T said:

Many thanks for the replies, especially @Tony Brooks, whose first comments put the whole thing in context.

 

 

In all honesty, I’ve never seen anything close to 70 amps on the ammeter, even at first start-up, but then we are very light electrical users, have a reasonable size battery bank (4 * 110Ah), almost always cruise for about 4 hours and have solar for the beginning and end of each day.

 

 

At this time of year the Solar has had several hours to recharge the batteries before you start so I am not surprised you do not get maximum alternator output. However, be aware that the alternator is actually 3 alternators in one case with a few common parts so if one fails everything will appear to work normally but with a reduced voltage and currant. If you fear this may be the case post the charging voltage after your four hours cruise @ about 1500 rpm and then the current at 1500 rpm when  first start together with the alternator's rated output. It very useful to understand and get use to taking rested voltage readings at the end of engine running and early in the day but solar tends to make that difficult unless you can isolate the panels and discharge the batteries a bit. Taking rested voltage readings regularly will often give an early warning of developing battery/charging problems before they are noticable.

[BEngo]

9 hours ago, nicknorman said:

What Tony says above is key. This forum, along with a number of other sources, is guilty of spreading misinformation. They talk about “Bulk” and “Absorb” phases of charging as if these were discrete things, the charging is in Bulk (constant current) then suddenly switches to Absorb (constant voltage). The reality is nothing like that, in most cases neither state exists, it is nearly all some sort of blend of the two concepts in varying ratios.

 

So by combining the two alternators the result is a slightly higher charging voltage and thus increased current and thus a reduced charge time, certainly during the early and mid parts of charging.

While this is true, the separation of  the charge cycle into discrete stages is a convenient model.  It makes it a lot easier for the non-technical to grasp and therefore add to their understanding.

 

Most boaters don't care if their alternator or solar regulator is soft or not.  They should know that sometimes the regulator has nothing to do because the battery is controlling  the activity by the charging kit.  

 

N

[Tony Brooks]

3 minutes ago, BEngo said:

While this is true, the separation of  the charge cycle into discrete stages is a convenient model.  It makes it a lot easier for the non-technical to grasp and therefore add to their understanding.

 

That is also my view and what I tend to work to when I am trying to explain something. I only added the soft regulation bit because I felt sure Nick would probably jump on it if I did not.

Edited July 29, 2022 by Tony Brooks

[nicknorman]

8 minutes ago, BEngo said:

While this is true, the separation of  the charge cycle into discrete stages is a convenient model.  It makes it a lot easier for the non-technical to grasp and therefore add to their understanding.

 

Most boaters don't care if their alternator or solar regulator is soft or not.  They should know that sometimes the regulator has nothing to do because the battery is controlling  the activity by the charging kit.  

 

N

But this is where the oversimplification does damage. The model is not fit for purpose. You repeat the mantra “the battery is controlling the activity” but that is only a half truth. The other half of the truth is that the alternator regulator is also controlling the activity. All conventional regulators are “soft” and this means that the charge voltage remains below the nominal regulated voltage until very late in the process when the charge current is very low. This slows down the charge process.

 

Put simply, the unfit model indicates that there is no point in having an advanced PID regulator, whereas the reality is that it makes a massive difference. But only people who have actually tried it know that, the majority who haven’t tried it just self-perpetuate by repeating the mantra.

Edited July 29, 2022 by nicknorman

[eightacre]

20 hours ago, Col_T said:

The engine is a Beta BV 1505, with two alternators - 70A connected to the domestic bank, and 45A connected to the engine start battery. At present, the domestics and engine start batteries are connected by a voltage sensitive relay and, because I don’t completely understand electrics, I am considering removing the VSR.

 

 

We have the same engine and twin alternator arrangement that you have. The VSR has never given any trouble in the 11 years we have owned the boat, so I see no reason to change anything.

[Sea Dog]

On 29/07/2022 at 08:40, Col_T said:

In all honesty, I’ve never seen anything close to 70 amps on the ammeter, even at first start-up, but then we are very light electrical users, have a reasonable size battery bank (4 * 110Ah), almost always cruise for about 4 hours and have solar for the beginning and end of each day.

Seems to me that you have a solid charging regime, that you understand your system better than you give yourself credit for and that your present setup is serving you (and @eightacre) very well. If it ain't broke... 

[Tony Brooks]

1 hour ago, Sea Dog said:

Seems to me that you have a solid charging regime, that you understand your system better than you give yourself credit for and that your present setup is serving you (and @eightacre) very well. If it ain't broke... 

 

Only as long the low charge rate is in response to well charged batteries and not alternator, battery or circuit faults.

[Sea Dog]

On 01/08/2022 at 20:37, Tony Brooks said:

 

Only as long the low charge rate is in response to well charged batteries and not alternator, battery or circuit faults.

Well yes, but there's nothing in the OP to suggest there was anything wrong. He was just looking at whether to remove the VSR for simplicity's sake as I read it.

[Col_T]

@Sea Dog has it right.

 

The batteries were new July 15th this year, as the previous set had been on the boat since late 2014, and were replaced because a couple of voltage related issues with our 12v fridge caused me to lose confidence in them. No known problems with either alternator, except the engine alternator needed it’s drive belt tightening a tad, or circuit faults.

 

As a genuine question, is installing a VSR a good solution to 

On 28/07/2022 at 14:24, Col_T said:

avoid the use of split charge diode, with attendant voltage drop, on a single alternator engine?

 

 

[Tracy D'arth]

22 minutes ago, Col_T said:

@Sea Dog has it right.

 

The batteries were new July 15th this year, as the previous set had been on the boat since late 2014, and were replaced because a couple of voltage related issues with our 12v fridge caused me to lose confidence in them. No known problems with either alternator, except the engine alternator needed it’s drive belt tightening a tad, or circuit faults.

 

As a genuine question, is installing a VSR a good solution to 

 

 

Yes, the diode splitter robs you of 0.7v when charging on both sets of batteries.

[Col_T]

Thanks for confirming that. 

[Tony Brooks]

However, the OP stated that he has a twin alternator engine so I can't see why one would fit a passive split charge diode system. If you wanted to use one in that scenario both alternator outputs would need connecting to the input terminal. The VSR seem to me to be a far simpler solution and if needs must simply by disconnecting its negative cable puts it out of operation.

Still does not later the fact that the produce up to about 0.7V of volt drop.