Battery management

[Hadenough]

I currently have two domestic 110AH and one 85AH, 700A main engine cranking battery through the usual 1/2/BOTH switch. I also have built in 4KVA generator with its own 400A starter battery and a 50A "intelligent" inverter/charger.

I have to change the domestic and main engine batteries and am thinking of (and have space to fit) fitting three 142AH lead acid batteries and while not on passage, leaving the selector switch permanently to BOTH to maximise the domestic bank. We will be living aboard mostly on a swinging mooring or at anchor so the management plan will be to run the generator daily to keep the batteries charged and provide hot water. The generator charges its own battery. When I need to start the main engine I also propose to use the whole bank via BOTH. My logic is that as we will be aboard to monitor and manage the batteries why keep a big chunk of the battery capacity in reserve just for engine starting. Especially as I can still start the generator independantly from its own battery and recharge the main bank.

While on passage under sail I plan to revert to the usual domestic / engine battery split to maintain a reserve for emergency engine starting.

Goes against the grain and I'm not sure yet, what's the view??

 

[jac]

In an emergency

2 questions to ask

1) How quickly can your generator be started and then get enough life into your batteries to start the engine
2) How much safety margin do you leave in situations such as anchoring upwind of obstructions.

If the time for 1 is less than the time for 2 then can't see an Issue. You will be able to start the engine before the brown stuff hits the air recirculator

If you occasionally anchor somewhere a bit closer in, or otherwise put the boat in a situiation where you NEED the engine quicker than you can charge the battery then don;t do it.

Two thoughts though.

1) you say that the Generator has it's own dedicated battery that is charged by the gen. Can that also be used for engine start ?

2) what can you do to improve your generation for other sources, wind, solar etc? IF the output from those is increased then thatwould offset the need for such a large battery bank.

 

[Steve C Cronin]

Can you fit your three new 142s ALL as domestic batteries and still retain an 85 for starting the main engine? This would be MY choice and IS! However, we are in the Med where the fridge dominates everything. I have a link switch to join everything together but have never used it as I don't like the idea of it being possible to inadvertently run everything flat and not be able to get the main engine going in an instant. What do you do for windlass power?

 

[Hadenough]

2 questions to ask

1) How quickly can your generator be started and then get enough life into your batteries to start the engine
2) How much safety margin do you leave in situations such as anchoring upwind of obstructions.

If the time for 1 is less than the time for 2 then can't see an Issue. You will be able to start the engine before the brown stuff hits the air recirculator

If you occasionally anchor somewhere a bit closer in, or otherwise put the boat in a situiation where you NEED the engine quicker than you can charge the battery then don;t do it.

Two thoughts though.

1) you say that the Generator has it's own dedicated battery that is charged by the gen. Can that also be used for engine start ?

2) what can you do to improve your generation for other sources, wind, solar etc? IF the output from those is increased then thatwould offset the need for such a large battery bank.

Very good points thanks. I hadn't really thought of the close in anchoring scenario, we do a lot and and that's just when we'd be most vulnerable. I am going solar too and can accommodate a biggish array so am leaning towards two 142's and an 85 for engine cranking. Thanks again, good food for thought.

 

[Hadenough]

Can you fit your three new 142s ALL as domestic batteries and still retain an 85 for starting the main engine? This would be MY choice and IS! However, we are in the Med where the fridge dominates everything. I have a link switch to join everything together but have never used it as I don't like the idea of it being possible to inadvertently run everything flat and not be able to get the main engine going in an instant. What do you do for windlass power?

I had thought of rewiring to make the genny battery serve the main engine too but it would involve quite a lot of disruption. Coupled with jac's comment about close in anchoring and your reminder that I have the windlass to think about in such a scenario I think I'm going to stick with the traditional set up. Thanks, great to be able to bounce ideas around like this.

 

[NB Willawaw]

If you don't run the bank down below roughly 50% charge while you are still onboard, there should be enough reserve for engine starting. I would possibly go for three 110AH if the space permits. Why have one at only 85?

I would consider something like a Smartgauge to monitor the bank and possibly use its alarm function to indicate when you are down to 50 percent.

 

[Paddingtonbear]

And possibly ask Chris Gibson (Gibbo) of Smartgauge for his thoughts. He is so helpful that you won't be able to shut him up. Absolutely no connection...... no pun intended

 

[Marsupial]

I currently have two domestic 110AH and one 85AH, 700A main engine cranking battery through the usual 1/2/BOTH switch. I also have built in 4KVA generator with its own 400A starter battery and a 50A "intelligent" inverter/charger.

I have to change the domestic and main engine batteries and am thinking of (and have space to fit) fitting three 142AH lead acid batteries and while not on passage, leaving the selector switch permanently to BOTH to maximise the domestic bank. We will be living aboard mostly on a swinging mooring or at anchor so the management plan will be to run the generator daily to keep the batteries charged and provide hot water. The generator charges its own battery. When I need to start the main engine I also propose to use the whole bank via BOTH. My logic is that as we will be aboard to monitor and manage the batteries why keep a big chunk of the battery capacity in reserve just for engine starting. Especially as I can still start the generator independantly from its own battery and recharge the main bank.

While on passage under sail I plan to revert to the usual domestic / engine battery split to maintain a reserve for emergency engine starting.

Goes against the grain and I'm not sure yet, what's the view??

Its a good plan, roughly what I have, but as others have said might be good to use the genny battery as main engine start as well. A large bank is required for this type of usage as your 50A shorepower charger will take a long time to get the batteries to 100% charge, so long in fact that you wont expect to get more than 85% - unless you leave the genny on for a few DAYS (assuming you will be drawing something from the system whilst trying to charge it).

 

[Steve C Cronin]

Its a good plan, roughly what I have, but as others have said might be good to use the genny battery as main engine start as well. A large bank is required for this type of usage as your 50A shorepower charger will take a long time to get the batteries to 100% charge, so long in fact that you wont expect to get more than 85% - unless you leave the genny on for a few DAYS (assuming you will be drawing something from the system whilst trying to charge it).

Can't see why you think that a 50A charger will take a long time to get this system up tp 100%. Mine is only 30amps yet fully charges my batteries over a 24hr period as well as running the fridge. Since my domestic batteries never drop below 21.3volts, it often takes very much less than that. However, having 110watts of permanently installed solar panels charging from sunrise (5.30am) provides a serious advantage.

 

[john_morris_uk]

Can you fit your three new 142s ALL as domestic batteries and still retain an 85 for starting the main engine? This would be MY choice and IS! However, we are in the Med where the fridge dominates everything. I have a link switch to join everything together but have never used it as I don't like the idea of it being possible to inadvertently run everything flat and not be able to get the main engine going in an instant. What do you do for windlass power?

I agree - we keep a completely separate engine start battery that CAN'T be run down by the domestics. Many (most?) smart chargers have the ability to charge sperate battery banks and that is how we have connected ours up.

Either use a VSR or diodes (with battery sensing for the charger and alternator to eliminate the voltage loss through the diodes) and get rid of the 1-2-both switch and this means you have a KISS system that you only have to monitor. The batteries will be dedicated to their task and you can monitor the state of the domestic batteries in the knowledge that the engine start battery is always going to be OK and ready to go.

A hefty jump start cable or some more wiring and an emergency 'combine' switch will cover eventualities such as the engine start battery suddenly losing a cell and failing completely.

I like the idea of leaving the generator battery and charging system completely separate. I wonder whether the battery in the generator is rated suitably in cold cranking terms etc to start the main engine? generators are typically very small diesels and the batteries are sized accordingly.

 

[Marsupial]

Can't see why you think that a 50A charger will take a long time to get this system up tp 100%. Mine is only 30amps yet fully charges my batteries over a 24hr period as well as running the fridge. Since my domestic batteries never drop below 21.3volts, it often takes very much less than that. However, having 110watts of permanently installed solar panels charging from sunrise (5.30am) provides a serious advantage.

its a basic property of lead acid cells and all their derivatives, you cant beat the physics what ever is says on the box your smart charger came in.

as soon as the baulk charging phase is reached usually 30 - 60 minutes the last few percents take ages, which is why the equalising charge takes a day or more when you are on shore power.

many users think that as soon as the battery monitor readout falls to "float" 13.7 (or some other number suggested in a book) volts the battery is fully charged this is not so. To compound the problem as I suggested above we don't rest the battery for 24 hours before taking another voltage reading to confirm the charge state of the battery instead we are using it while we charge it and continue to do so when the charger is off. In this way whatever you do to charge your batteries even constantly connected solar you are never sure EXACTLY what the state of charge is at any given time.

To sleep at night without worrying unduly about it reckon on getting them to 85% of their stated capacity and recharge them when or before the voltage drops to 12.1 in other words before they get to 50% discharge.

Its sound like you are achieving that, I assume a typo 21.3 should be 12.3 which is where I would choose to recharge. The OP needs a large bank because he will find the capacity of his 100AH battery is in practical terms only about 40ah of usable amps if he lets them discharge to 50%.

 

[Stemar]

I'd be inclined to have a separate battery for starting the engine for the time when the batteries run down when you aren't looking - it will happen!

You don't need a huge battery if your engine's a decent starter. I have an aged VP2003 that wants a good long churn before it will deign to splutter into life, but my second hand 35AH car battery has now been on the boat for about eight years and is still going strong (kiss of death?)

I'm fitting solar panels and have a controller from Sunstore that cost <£30 but can send the charge to the engine battery until it burps then top up the domestics, which I reckon will give me the best practicable assurance that I'll get the donk going and keep the domestics topped up.

 

[NB Willawaw]

You can use the Smartgauge to activate a relay at 50% state of charge and isolate the starter battery ensuring you have reserve to start the engine in an emergency.

Like a VSR but brighter.

 

[pvb]

You can use the Smartgauge to activate a relay at 50% state of charge and isolate the starter battery ensuring you have reserve to start the engine in an emergency.

Like a VSR but brighter.

If you have a VSR, which is a fraction of the price, the starter battery isn't going to get discharged anyway!

 

[Steve C Cronin]

its a basic property of lead acid cells and all their derivatives, you cant beat the physics what ever is says on the box your smart charger came in.

as soon as the baulk charging phase is reached usually 30 - 60 minutes the last few percents take ages, which is why the equalising charge takes a day or more when you are on shore power.

many users think that as soon as the battery monitor readout falls to "float" 13.7 (or some other number suggested in a book) volts the battery is fully charged this is not so. To compound the problem as I suggested above we don't rest the battery for 24 hours before taking another voltage reading to confirm the charge state of the battery instead we are using it while we charge it and continue to do so when the charger is off. In this way whatever you do to charge your batteries even constantly connected solar you are never sure EXACTLY what the state of charge is at any given time.

To sleep at night without worrying unduly about it reckon on getting them to 85% of their stated capacity and recharge them when or before the voltage drops to 12.1 in other words before they get to 50% discharge.

Its sound like you are achieving that, I assume a typo 21.3 should be 12.3 which is where I would choose to recharge. The OP needs a large bank because he will find the capacity of his 100AH battery is in practical terms only about 40ah of usable amps if he lets them discharge to 50%.

Well, it WORKS!

What is important is that the fridge keeps down to <5C, that there is sufficient power to start the engine in the mornings and pull up the anchor, all the electronics continue to work, all the lights give us sufficient illumination for our purposes, that nothing blows up and that all my batteries last at least eight years. ALL of the above are achieved and have been so on my last five boats so I must be doing something right! I have all the sophisticated battery management and monitoring kit and know a fair bit about 12volt electrics but sometimes you just have to learn not to worry and get on and enjoy the sailing.

 

[NB Willawaw]

No but its not going to contribute either unless the engine or generator is running so doesn't help the OP.

If you have a VSR, which is a fraction of the price, the starter battery isn't going to get discharged anyway!

 

[Marsupial]

Well, it WORKS!

What is important is that the fridge keeps down to <5C, that there is sufficient power to start the engine in the mornings and pull up the anchor, all the electronics continue to work, all the lights give us sufficient illumination for our purposes, that nothing blows up and that all my batteries last at least eight years. ALL of the above are achieved and have been so on my last five boats so I must be doing something right! I have all the sophisticated battery management and monitoring kit and know a fair bit about 12volt electrics but sometimes you just have to learn not to worry and get on and enjoy the sailing.

Agreed the fridge is everything and as you say forget the sparks and go sailing