Dedicated starter battery 'architecture'

[skyflyer]

Sub-title: should battery banks be 'balanced' in capacity?

[this is similar but not quite the same as an earlier question here, by me, in case you get deja due!]

The manufacturer's set up is two 180Ah batteries through a 1/2/Both/Off switch.

I dont like this because if I keep one battery as a dedicated start battery I am wasting usable Ah (as it is way way over spec for engine starting) but if I risk using it as well as the other one for services onboard, there is a risk I over deplete it and cannot then start the engine.

So the obvious solution is to parallel the two 180Ah to make a 360Ah bank and fit a new small start battery.

The question that's confusing me is how to arrange the charging!

At the moment I have an Adverc advanced charging system and the alternator output goes via split charge diode to supply each battery - 2 cables from SCD, one to each battery. The voltage drop in the SCD is not an issue as the Adverc reads the SCD output volts (on one terminal only) and compensates.

So if I just 'transfer' this arrangement to the new starter battery, isn't there a risk, because (hopefully) a start battery never gets badly discharged, that it will 'overcharge' and gas-off as the engine runs because the depleted service battery pulls down the voltage so the Adverc sensor ups the alternator output accordingly.

If I switch the Adverc sensor to the starter battery side of the SCD then conversely as soon as the starter battery is charged and the voltage rises the Adverc will reduce voltage output and the leisure bank wont get fully charged!

So then i thought, OK, ditch the SCD and install a VSR between the starter and leisure battery. This could be a one way or two way VSR. If one way I guess the best thing would be to have the alternator output go to the leisure battery and the VSR kick in to connect the starter battery to it when it's voltage has risen?

I know the theory of the VSR. It makes a connection when the first battery has charged up. But the practice (looking at my voltmeter anyway) is that as soon as the engine starts the alternator drives the system voltage up to at least 14v. Thus a VSR will immediately close (connect) on engine start and both batteries are being charged and so we are back to the original problem - the starter battery will be getting 'more volts' than it really needs?

I have s suspicion I am misunderstanding this in a big way. After all my car battery (presumably) is quite happy to be at alternator output voltage of 14v all day long without damaging it!

The only way i can see to do this without the above issues is to have a DC-DC charger between leisure bank and starter battery. Expensive solution and possibly unnecessary?

Its just that mentally I cant come to terms with one battery bank being 360Ah and the other being (say) 50Ah - it doesnt feel right.

Does the team think I am making problems where none exist ?

 

[lw395]

I think you are unlikely t oshorten the life of your start battery significantly by overcharging, if the only source is the alternator.
Add in some solar and shore power and it might become significant.
How many engine hours do you do in a year?
How does that compare with a typical car averaging 35mph for 15k miles a year with a 5 year battery life?

 

[lpdsn]

Its just that mentally I cant come to terms with one battery bank being 360Ah and the other being (say) 50Ah - it doesnt feel right.

I have 360Ah and, IIRC, 55Ah. It feels very right to me. As built the alternator charged both banks through a regulator with two outputs but that was very slow at recharging the battery banks. I fitted a Sterling A2B which gives 14V into the starter battery and 14.8V into the domestic bank. That avoids overcharging the starter battery the way a VSR could do.

PS one idea to consider is to maybe have a second diode in series for the starter battery to give it a greater voltage drop than for the domestics. Haven't though it through yet, just an off the top of my head idea.

 

[jac]

In a word yes.

I'm not sure about your system though. It sounds like you have 2 separate regimes to split the charge - the adverc advanced charging system AND a split charge diode??

The "usual" basic system is a VSR - lots of issues with voltage drop and relay chatter - Split charge diodes are better. Finally you get the "sophisticated" charge spliters thar manage charge electronically.

You only need one.

I'm not familiar with the adverc system but the usual design of smart chargers is that the charger measures both batteries ( or the engine start as a minimum) - on first start it switches all charge output to the engine start - once fully charged it switches the output to the domestic bank. Size is irrelevant.

The key thing is that they are not permanently paralleled - Instead you have a means of switching the charge output from one to the other

 

[skyflyer]

No the Adverc doesnt do any switching it just boost the voltage out of the alternator. The SCD doesnt switch anything either it just prevents one battery discharging into the other.

So as the system stands, whatever voltage is coming out the alternator goes to both battery banks. This is independent of the 1/2/Both/Off switch, which solely directs how battery power reaches the boat services not the other way round.

Ditto with shore power. Its a Stirling Pro clone (US Pro Nautic) with two outputs but i don't believe the voltage differs on each. There is one LED display of voltage on the charger panel and no way of switching so i believe that although each battery is wired independently, the same voltage goes to each. Obviously the current flowing will then depend on the resistance/state of charge of the individual battery.

However, I like the idea of adding some extra resistance to the starter battery side of the SCD to further reduce the voltage to that battery.

It is the addition of solar that has precipitated this. I want extra capacity to utilise what is being generated by that when we are off the boat (generally about two to three weeks off for every week aboard) so we have a "full load" to sail away with when we return!

 

[lpdsn]

However, I like the idea of adding some extra resistance to the starter battery side of the SCD to further reduce the voltage to that battery.

It is the addition of solar that has precipitated this. I want extra capacity to utilise what is being generated by that when we are off the boat (generally about two to three weeks off for every week aboard) so we have a "full load" to sail away with when we return!

Sounds like the Adverc booster doesn't distinguish the Starter & Domestics as different types of output. The Sterling equivalent does.

The idea was to use a diode rather than a resistor, so the extra voltage drop would be the Vf of that diode. A resistor wouldn't work as the voltage drop would be current dependent, whereas for the diode it more or less isn't.

What I did with the solar output was to tweak the output voltage (programmable) down to 14V. That stops it competing with other charging sources. If I'm going to be off grid for a few days I can up it to 14.8V to get the extra boost into the batteries - it only feeds the domestics. It's not a flash controller just one of Amazon's cheapies so any decent one should have a programmable output feature of some sort.

Edit: on further recollection I may even have set the solar output as low as 13.8V.

 

[Stemar]

I use a solar controller that manages two banks. It's set to charge the starter battery first and only when that's full, does it charge the domestic battery. WHen the domestic battery burps, it turns the panels off. No need to worry. The system's been trouble free for several years apart from when I do something stupid like leave a big drain turned on. Split charging from the alternator is handled by a standard relay that was change from a fiver from Maplin. There's a small risk that a discharged domestic battery could pull charge from the stater battery, in practice, I've never had a problem. There's also a theoretical risk of an imbalance in charging, between domestic and starter batteries but the solar controller sorts it out if it ever does happen.

I have a little boat and my needs are modest, so a simple system works. A bigger boat with bigger needs may be better off with something more sophisticated, but I'm unconvinced.

 

[lpdsn]

I use a solar controller that manages two banks. It's set to charge the starter battery first and only when that's full, does it charge the domestic battery. WHen the domestic battery burps, it turns the panels off. No need to worry. The system's been trouble free for several years apart from when I do something stupid like leave a big drain turned on. Split charging from the alternator is handled by a standard relay that was change from a fiver from Maplin. There's a small risk that a discharged domestic battery could pull charge from the stater battery, in practice, I've never had a problem. There's also a theoretical risk of an imbalance in charging, between domestic and starter batteries but the solar controller sorts it out if it ever does happen.

I have a little boat and my needs are modest, so a simple system works. A bigger boat with bigger needs may be better off with something more sophisticated, but I'm unconvinced.

The OP's challenge is to get a lot of oomph into a big domestic bank, for which he'll need a high voltage, typically 14.8V, for quite a period, without boiling his starter battery at the same time. It's not a risk of pulling charge from the starter as avoiding seriously overcharging it.

 

[MonniotC]

What you need is to route the alternator output through a 1-2-Both-Off switch with a digital voltmeter in the panel showing the voltage of the selected battery. Then when you start the engine, you select the starter battery. When it comes up to voltage you switch to the house bank. I know that this is a very modern and radical approach, but it might work.
Fx: Sound of rapidly disappearing running footsteps followed by slamming of door.

 

[skyflyer]

What you need is to route the alternator output through a 1-2-Both-Off switch with a digital voltmeter in the panel showing the voltage of the selected battery. Then when you start the engine, you select the starter battery. When it comes up to voltage you switch to the house bank. I know that this is a very modern and radical approach, but it might work.
Fx: Sound of rapidly disappearing running footsteps followed by slamming of door.

I realise you jest but it’s actually no good because the voltage will come up immediately the engine starts. Voltage is only any use in determining the state of charge when the battery has been at rest with no current in or out for at least 15minutes!

I’ve cone to the conclusion that the cost of a small starter battery is not great so if it needs regular replacement because it gets a bit boiled from time to time - so be it!

 

[BabaYaga]

At the moment I have an Adverc advanced charging system and the alternator output goes via split charge diode to supply each battery - 2 cables from SCD, one to each battery. The voltage drop in the SCD is not an issue as the Adverc reads the SCD output volts (on one terminal only) and compensates.

So if I just 'transfer' this arrangement to the new starter battery, isn't there a risk, because (hopefully) a start battery never gets badly discharged, that it will 'overcharge' and gas-off as the engine runs because the depleted service battery pulls down the voltage so the Adverc sensor ups the alternator output accordingly.

I don't think that risk is so great. The voltage drop over over the diode will depend on the current to the service batteries. When these are low, a big current will flow and the Adverc will raise the voltage considerably to compensate. But at this point the system voltage will still be low, so even with the compensation, the starter battery is unlikely to see unhealthy voltage levels.
As the service batteries get fuller, the voltage drop diminishes and so the compensation.
Adverc says somewhere in their literature that there is a slight risk of over charging the starter battery on very long engine runs (if the service bank is big). I think their recommendation is to simply turn the engine battery off on these occasions.

 

[PaulRainbow]

In a word yes.

I'm not sure about your system though. It sounds like you have 2 separate regimes to split the charge - the adverc advanced charging system AND a split charge diode??

No. The Adverc is a regulator, not a split charger.

The "usual" basic system is a VSR - lots of issues with voltage drop and relay chatter - Split charge diodes are better. Finally you get the "sophisticated" charge spliters thar manage charge electronically.

Totally wrong. Modern (decent) VSRs do not chatter and they do not not suffer from voltage loss, never did. The split charge diode does suffer from a voltage loss and is the worst of all of the options mentioned. Part of the point of the Adverc is to overcome the voltage losses of the diode.

 

[PaulRainbow]

Does the team think I am making problems where none exist ?

Yes.

Parallel the current batteries and fit a dedicated starter battery.

Keep the Adverc and the diode, ensuring that the Adverc is set to overcome the voltage loss of the diode.

Bin the 1-2-Both switch, so your new battery banks can be separately switched and kept isolated, by two separate om/off switches. This is made even easier because you already have split charging. Add a 3rd switch to parallel the two circuits for emergency use.

 

[skyflyer]

Sounds good but what’s the advantage of ditching the 1/2/both and fitting individual switches? Surely the I/2/both does exactly what your three new switches will do?

 

[PaulRainbow]

Sounds good but what’s the advantage of ditching the 1/2/both and fitting individual switches? Surely the I/2/both does exactly what your three new switches will do?

No it does not. The 1-2-B will select one bank to run everything, so when you select "1" everything will run from the engine battery, or "2" will run everything from the domestic bank. Having 2 switches allows you to have the engine battery starting the engine and the domestic bank running the domestic circuits, with both circuits being isolated from one another. Your diode allows both banks to be charged from the alternator, again keeping them isolated from one another.

 

[VicS]

Sounds good but what’s the advantage of ditching the 1/2/both and fitting individual switches? Surely the I/2/both does exactly what your three new switches will do?

What PR says!

Your Adverc and diode splitter system has already taken you half way to having fully separated engine and domestic systems. All that remains is to replace the 1,2,B switch with separate switches for the two systems ( and, of course, a third "combining" switch to link the two in an emergency)

However

If you like the idea of just one switch consider a Bluesea "Dual Circuit Plus" switch. It combines the functions of the three separate switches into one unit.

https://www.bluesea.com/products/6011200/m-Series_Mini_Dual_Circuit_Plus_Battery_Switch_-_Black

Switch to "On" and both circuits are separately powered up. In an emergency switching to "combine batteries" does what your 1,2,B switch does in the B position.

The only downside is that it does not allow a totally defective battery to be switched off.... but in the unlikely situation that is necessary it can be physically disconnected or its fuse pulled. You'd need to do that anyway to disconnect it from the charging circuit with your present system.

 

[skyflyer]

Excellent. Thanks

 

[lpdsn]

What PR says!

Your Adverc and diode splitter system has already taken you half way to having fully separated engine and domestic systems. All that remains is to replace the 1,2,B switch with separate switches for the two systems ( and, of course, a third "combining" switch to link the two in an emergency)

However

If you like the idea of just one switch consider a Bluesea "Dual Circuit Plus" switch. It combines the functions of the three separate switches into one unit.

https://www.bluesea.com/products/6011200/m-Series_Mini_Dual_Circuit_Plus_Battery_Switch_-_Black

Switch to "On" and both circuits are separately powered up. In an emergency switching to "combine batteries" does what your 1,2,B switch does in the B position.

The only downside is that it does not allow a totally defective battery to be switched off.... but in the unlikely situation that is necessary it can be physically disconnected or its fuse pulled. You'd need to do that anyway to disconnect it from the charging circuit with your present system.

I have to admit I still don't understand why anyone would want to turn an engine battery on when they aren't going to run the engine. My three switches are each turned on when they are needed. Domestics when I arrive at the boat. Engine when I'm going to cast off. Link switch last used about 9 years ago before I'd sorted out the PO's bodges.

I even have a little LED at the chart table that reminds me to turn off the engine battery when I tie up - OK it is a drain but in testing it took about seven days to drain a phone battery that the phone thought was flat so I reckon a lead acid battery will support it for a while.

 

[VicS]

I have to admit I still don't understand why anyone would want to turn an engine battery on when they aren't going to run the engine. My three switches are each turned on when they are needed. Domestics when I arrive at the boat. Engine when I'm going to cast off. Link switch last used about 9 years ago before I'd sorted out the PO's bodges.

I even have a little LED at the chart table that reminds me to turn off the engine battery when I tie up - OK it is a drain but in testing it took about seven days to drain a phone battery that the phone thought was flat so I reckon a lead acid battery will support it for a while.

Likewise and it could be a nuisance if you need to do la little work on the engine that needs the engine electrics turned off while you might still want some of the domestic circuits energized.

Some people, at least there is one forumite, like the idea that all you have to do when arriving on board is turn one switch on and only turn one off before leaving.

Other times the lack of space available may make a single switch an attractive option.

 

[skyflyer]

Fortunately each battery already has an isolation switch so for me the simplest solution is to replace my 1/2/both/Off switch with the Bluesea. All cables already in place and it avoids a redundant switch or hole in the panel