Basic volts question for battery use

[skipmac]

Lots of good and some not so good information here. If I may summarize some of the good points and information.

1. It isn't volts that start an engine, it's how many amps it can put out. For starting batteries that would be rated in CCA (cold cranking amps). Look at it this way. You could string a whole bunch of AA batteries in series (series voltage adds, parallel voltage stays the same but amp hour capacity adds) so you have a 24 V supply or even more. That isn't going to crank a diesel. But if you have a great big battery like an 8D and a small engine, that battery will crank that engine quite well even at a fairly low state of charge.

2. Voltage is a very rough indication of the state of charge of the battery. BUT, if you are drawing from the battery for a fridge for example, the voltage will read lower. The bigger the load the lower the voltage reading. Turn off the fridge, wait a couple of minutes and the battery voltage will increase. If you have been running the engine and charging the batteries, immediately after you turn off the engine the volts will read high. Wait a couple of hours (with no loads drawing from the battery) to see the true voltage. This is called the resting voltage. A fully charged, liquid filled lead/acid battery will read about 12.6 V at rest.

3. If you are often in a situation where you have no way to charge the batteries except by running the engine then it is, as a friend of mine used to say, the height of folly, not to have a separate charging battery. It is just too easy to run down both batteries and end up dead in the water (speaking from experience having done so more than once). The cheap, manual method is the 1-2-Both-Off switch. Set to both when running the engine to charge all, then switch to one battery to run the boat, leaving a fully charged battery for starting the engine. A more foolproof method is to isolate the starting battery and charge it with an automatic, voltage controlled battery combiner. That way you can't forget to switch from Both to one and run down both batteries (yes I've done that as well).

 

[lw395]

Running a fridge and radar, a decent size house bank seems like a good place to spend money.
This can be achieved reasonably well by buying a separate, smaller engine start battery.
Some solar power would be a good idea.
If you use the engine a fair amount and hence it keeps the batteries well charged, a modest solar panel will keep it topped up when the boat is not in use. Even a 10W panel will be a great asset.
If you don't use the engine much and don't have shore power, then it's time to work out how much solar you need to keep up with your use.
More is obviously better, but maybe there is a happy medium which means you don't have to run the motor just for charging too often?
Everybody's pattern of use tends to be different, so it's probably best to work out for yourself what you will use.
The fridge is the obvious big drain, but later in the season, more lighting in the evenings, more use of PC or TV, and heaters become significant.

 

[PaulRainbow]

Lots of good and some not so good information here.

<snip>

The cheap, manual method is the 1-2-Both-Off switch. Set to both when running the engine to charge all, then switch to one battery to run the boat, leaving a fully charged battery for starting the engine.

IMO this is bad advice.

If he forgets to change the switch over he can end up with no way of starting the engine. Also, catastrophic failure of one battery will take the other one down, i've seen this many times for real with vehicles that have twin batteries.

 

[sailaboutvic]

IMO this is bad advice.

If he forgets to change the switch over he can end up with no way of starting the engine. Also, catastrophic failure of one battery will take the other one down, i've seen this many times for real with vehicles that have twin batteries.

I have to say paul , quite a lot of people had these 1/2 switchs , me for one and most never had. A problem with them .
Once you had one you remember that once the engine is switched off you change it to bat 1 or 2 .
It's only in the last 20 years plus there VSR and other ways of contro in battery's .
Anyway the OP knows he need to sort out his wiring and for now he just need his question answered to get him over the next few days .

 

[Dutch01527]

This is incorrect. The engine only needs to be running at a fast idle, absolutely not high revs.

The alternator does indeed run from the engine, but it is not good practice to run the engine without it being under some load, hence why it should be in gear.

Disagree. There is a misunderstanding here about " under load" and " up to operating temperature". A simple Diesel engine run at medium/high revs is up to operating temperature and is fine. Even at tick over with no load it will last for a very long time, think of a dumper truck on a building site, the same engine - most of it's time is in idle.

The faster the engine turns, the more power the alternator generates and the quicker the battery charges. That is why you raise revs when jump starting a car.

Unless a professional diesel engineer corrects me I will continue to think that "under load" is a yacht club bar urban myth.

 

[skipmac]

IMO this is bad advice.

If he forgets to change the switch over he can end up with no way of starting the engine. Also, catastrophic failure of one battery will take the other one down, i've seen this many times for real with vehicles that have twin batteries.

Was not recommending this solution but offering information. If I understand the OP he already has a 1-2-Both switch and can implement this immediately. If you read the whole post I added "A more foolproof method is to isolate the starting battery and charge it with an automatic, voltage controlled battery combiner. That way you can't forget to switch from Both to one and run down both batteries (yes I've done that as well)."

And yes, although not common, a totally dead or worse, a shorted battery can drain all batteries in the system if all are connected. In the case of a totally shorted battery it can do so dramatically as in setting the boat on fire.

 

[PaulRainbow]

I have to say paul , quite a lot of people had these 1/2 switchs , me for one and most never had. A problem with them .
Once you had one you remember that once the engine is switched off you change it to bat 1 or 2 .
It's only in the last 20 years plus there VSR and other ways of contro in battery's .
Anyway the OP knows he need to sort out his wiring and for now he just need his question answered to get him over the next few days .

Was not recommending this solution but offering information. If I understand the OP he already has a 1-2-Both switch and can implement this immediately. If you read the whole post I added "A more foolproof method is to isolate the starting battery and charge it with an automatic, voltage controlled battery combiner. That way you can't forget to switch from Both to one and run down both batteries (yes I've done that as well)."

And yes, although not common, a totally dead or worse, a shorted battery can drain all batteries in the system if all are connected. In the case of a totally shorted battery it can do so dramatically as in setting the boat on fire.

The thing that i believe to be a bad idea is setting the switch to "both" with the engine running (or any other time apart from emergency starting). I think he should charge one battery at a time, as Vics described.

 

[PaulRainbow]

Disagree. There is a misunderstanding here about " under load" and " up to operating temperature". A simple Diesel engine run at medium/high revs is up to operating temperature and is fine. Even at tick over with no load it will last for a very long time, think of a dumper truck on a building site, the same engine - most of it's time is in idle.

You may disagree, but you'll still be incorrect

Mr Cox explains the issue well https://coxengineering.sharepoint.com/Pages/Boreglazing.aspx

The faster the engine turns, the more power the alternator generates and the quicker the battery charges. That is why you raise revs when jump starting a car.

A modern alternator will produce most of it's power at around 2000 engine RPM. Running the engine at med/high RPM is pointless. When jump starting a car from a donor vehicle that is fitted with an alternator, there is no need to rev the engine. Back in the Olde days of dynamos, things were different.

Unless a professional diesel engineer corrects me I will continue to think that "under load" is a yacht club bar urban myth.

I've been working with engines and all things automotive in a professional capacity since the early 70's, does that count ?

 

[Spyro]

(The electrics are in a pretty shocking state, disintegrating insulation, some bare wires and shorts everywhere apparently, so is a priority to be totally replaced this winter. )

I don't think you will have shorts everywhere. You would have a lot more problems.

 

[William_H]

Regarding charging from the engine. The alternator hopefully at your selected engine speed idle or a bit more your alternator is capble of producing full power. Now a battery only takes current (charge) according to the difference between the charge voltage and the inherent voltage of the battery. ie state of charge. So a battery at 12v will take a lot more charge current than one at 12.5 volts. However the battery voltage will rise quite quickly when under charge so current falls off quite quickly. So often you will find a short engine run will put a fair bit in but a longer engine run will not put a lot more in. You need an amp meter measuring charge current so that if you are going to charge the battery from an engine run you can see when the charge current falls to a point where there is little value in continuing charge considering noise etc. The next point is that a bigger battery will accept more current so 2 batteries in parallel on charge should take twice as much charge current so I would always charge in parallel rather than one at a time.
Regarding the min voltage for engine start. I suspect that 2 large batteries in parallel will provide enou8gh current for engine start even when quite far discharged. (compared to a small engine start type car battery). But that does not mean you should allow batteries to discharge so far. It is bad for them.
So yes charge in parallel and if necessary start engine parallel but remember you have dual battery system so that one battery can be discharged while the other is quarantined for engine start. good luck olewill

 

[skipmac]

The thing that i believe to be a bad idea is setting the switch to "both" with the engine running (or any other time apart from emergency starting). I think he should charge one battery at a time, as Vics described.

I guess I missed that post. Charge one battery at a time? Why??? If you're running the engine why not take maximum advantage of the run time? The only reason I can think of to charge only one battery is because you have a grossly undersized alternator that can't handle the load.

On the other hand, I agree that you only need to run the engine at a fast idle to get most of the alternator output.

 

[VicS]

The thing that i believe to be a bad idea is setting the switch to "both" with the engine running (or any other time apart from emergency starting). I think he should charge one battery at a time, as Vics described.

I guess I missed that post. Charge one battery at a time? Why??? If you're running the engine why not take maximum advantage of the run time? The only reason I can think of to charge only one battery is because you have a grossly undersized alternator that can't handle the load.

On the other hand, I agree that you only need to run the engine at a fast idle to get most of the alternator output.

Paul Rainbow refers to my "preferred procedure" for managing a system comprising two batteries linked by a 1,2,both, off switch, which I described in #11.

Not so much intended to restrict charging to one battery at a time, more as a way of only having one battery in service at a time, The designated engine start battery for engine starting only and the domestic services battery for all the other loads. Done properly this will ensure that the engine start battery is always recharged after use but can never be discharged by inadvertently running the domestics from it.

I am quite happy for the banks to be parallelled, by switching to "both" after the engine has been started and for them to be charged together but then one has to remember to switch to the domestics battery only when the engine is stopped.

 

[PaulRainbow]

I guess I missed that post. Charge one battery at a time? Why??? If you're running the engine why not take maximum advantage of the run time? The only reason I can think of to charge only one battery is because you have a grossly undersized alternator that can't handle the load.

On the other hand, I agree that you only need to run the engine at a fast idle to get most of the alternator output.

Pretty much what Vics said. My reasoning for not charging using the boat setting is that ;

a) If you forget to set it back to the domestic batteries when you stop you can end up with no way to start the engine.

b) If on battery fails it can discharge to good one and, again, no way to start the engine.

That's why i'd support Vics "prefered method". I'd also highly recommend getting rid of 1-2-both switches and have permanently separate circuits where both battery banks get charged via a VSR or similar.

 

[Dutch01527]

Mr Cox explains the issue well https://coxengineering.sharepoint.com/Pages/Boreglazing.aspx

Very good article - it says that bore glazing can be an issue with prolonged idling early in an engines life. Load does not mean in gear it means working harder e.g. at higher revs. Quote from article "Cummins, Perkins and MAN all have issued directives limiting the run time at idle to prevent bore glazing." Hence why I suggested higher rpm.

A modern alternator will produce most of it's power at around 2000 engine RPM. Running the engine at med/high RPM is pointless. When jump starting a car from a donor vehicle that is fitted with an alternator, there is no need to rev the engine. Back in the Olde days of dynamos, things were different.

2000 rpm is med/high on most small marine engines. I think we are in agreement that the quickest way to charge the low batteries with the engine running at about that speed. I would just prefer to do that out of gear instead of in gear straining against the mooring ropes.

 

[lw395]

Most modern alternators seem to produce perhaps 80% of their rated current at quite low rpm, what would be fast idle on a car.
say 1200 rpm where max rpm is 6000 or something.


Scale the revs for a diesel where max rpm is under 4000 and you'll find 800 rpm is usually plenty.
This obviously depends on the pulley ratios being designed around the rated rpm of the alternator.
Small engines may spin the alt slower to limit the load at low rpm.
See if you can find data for your exact alternator.

So after a couple of minutes there may be enough current available at idle on a marine diesel.
At these rpm, the alt is actually putting a significant torque load on the engine, which is better for avoiding bore glazing.

If you have issues with belt slippage, use more rpm until the current drops.

If you let the start battery take all the charge for a minute or two, then switch to 'both', you should see the volts quickly come up to full charging voltage (typically 14.4 or so), at this point the alternator is giving both batteries as much as they will take.
So long as the alternator is giving its regulated voltage, changing the rpm won't change the charging current.

Revving an engine in neutral is NOT running it under load.

 

[Heckler]

Most modern alternators seem to produce perhaps 80% of their rated current at quite low rpm, what would be fast idle on a car.
say 1200 rpm where max rpm is 6000 or something.


Scale the revs for a diesel where max rpm is under 4000 and you'll find 800 rpm is usually plenty.
This obviously depends on the pulley ratios being designed around the rated rpm of the alternator.
Small engines may spin the alt slower to limit the load at low rpm.
See if you can find data for your exact alternator.

So after a couple of minutes there may be enough current available at idle on a marine diesel.
At these rpm, the alt is actually putting a significant torque load on the engine, which is better for avoiding bore glazing.

If you have issues with belt slippage, use more rpm until the current drops.

If you let the start battery take all the charge for a minute or two, then switch to 'both', you should see the volts quickly come up to full charging voltage (typically 14.4 or so), at this point the alternator is giving both batteries as much as they will take.
So long as the alternator is giving its regulated voltage, changing the rpm won't change the charging current.

Revving an engine in neutral is NOT running it under load.

Just picking up on your last point, you are quite correct. People dont understand the way that diesels work. Some diesel manufacturers refer to the engine speeds as hi idle and lo idle. With hi idle being the maximum governed speed and lo idle the minimum. At hi idle with no load the engine is consuming little fuel, producing relatively little power and not producing much heat relatvely speaking either. However, put it in gear and put a load on it and it is a different story. People also struggle with the concept of, say, setting the revs at 1800 and putting a variable load on the engine and getting full power out of it with the governor opening the fuel rack up to full power. My CAT 3306 diesel gennies in Angola were set at 1800 rpm but developed up to 135kva depending on how much power we demanded off them. They would sit there humming away at 1800 rpm, a few ACs would kick in, the camp boss would switch on the massive kitchen range and the only way you could tell from the gennie was a deepening of the exhaust note, some more black smoke as the governor opened the fuel rack but the rev counter would stay at 1800rpm.
Going back to our situation, do some basic math and we can see what the alternator load can put on a main engine. Say 13.8v output, say flat batteries and 30 amps as the alternator starts charging, gives us 13.8 times 30 which is 514 watts which is 2/3 hp very roughly, so not much of a load in the grand scheme of things. So if you want to charge your batteries, put the engine in to gear, give it a few revs, not to get the alternator giving out more charge BUT to put a load on the engine.
Stu

 

[lw395]

Just picking up on your last point, you are quite correct. People dont understand the way that diesels work. Some diesel manufacturers refer to the engine speeds as hi idle and lo idle. With hi idle being the maximum governed speed and lo idle the minimum. At hi idle with no load the engine is consuming little fuel, producing relatively little power and not producing much heat relatvely speaking either. However, put it in gear and put a load on it and it is a different story. People also struggle with the concept of, say, setting the revs at 1800 and putting a variable load on the engine and getting full power out of it with the governor opening the fuel rack up to full power. My CAT 3306 diesel gennies in Angola were set at 1800 rpm but developed up to 135kva depending on how much power we demanded off them. They would sit there humming away at 1800 rpm, a few ACs would kick in, the camp boss would switch on the massive kitchen range and the only way you could tell from the gennie was a deepening of the exhaust note, some more black smoke as the governor opened the fuel rack but the rev counter would stay at 1800rpm.
Going back to our situation, do some basic math and we can see what the alternator load can put on a main engine. Say 13.8v output, say flat batteries and 30 amps as the alternator starts charging, gives us 13.8 times 30 which is 514 watts which is 2/3 hp very roughly, so not much of a load in the grand scheme of things. So if you want to charge your batteries, put the engine in to gear, give it a few revs, not to get the alternator giving out more charge BUT to put a load on the engine.
Stu

I agree, maybe I wasn't clear enough.
Unless you have a big alternator on something like a 1GM, the load is pretty small in relation to the torque the engine is capable of.
You might initially be looking at 80A at 15V at <80% efficiency, which is a few HP. A lot for a poor old v-belt.
A yacht engine ticking over at idle will often take a very long time to warm up. Better to slip the mooring and exercise it for 10 minutes.
Or stick it in reverse on the mooring.
Also gets some hot water in the calorifier.
I find our engine will maintain its coolant and oil temperature idling at 1000 rpm charging the batteries once it's warm.
But it's not something we aim to be doing very often.
Lots of diesel engines run idle a lot with no problems. My car sits in traffic idling for too many hours a year, you rarely hear of engine problems these days. But cars normally get up to temperature every time they are started.

 

[steve yates]

Thanks all, I ran the engine on battery one in idle at around 1000rpm for about 20 mins, then did the same for battery 2, that brought them up to 13v, from around 12.1. I will adopt vics suggested use of the 1/2 switch until the electrics are redone and will then add a Sep engine battery, and solar power. Plenty of space for panels on her. There is always some load as the new depth sounder is wired permanently in, it was the only way they could do it.
Ran the engine again briefly this morning to put charge back into batt 2, and am keeping batt 1 for engine start.

One more question, can I turn the selector switch from 1 to both once the engine has started and running? I thought touching that with the engine on was a big no?

 

[lw395]

Every 1-both-2 switch I've seen has been OK to switch while the engine is running, provided you don't go through 'off'
The main point of them is to start on one battery, then charge both.

 

[PaulRainbow]

Mr Cox explains the issue well https://coxengineering.sharepoint.com/Pages/Boreglazing.aspx

Very good article - it says that bore glazing can be an issue with prolonged idling early in an engines life. Load does not mean in gear it means working harder e.g. at higher revs. Quote from article "Cummins, Perkins and MAN all have issued directives limiting the run time at idle to prevent bore glazing." Hence why I suggested higher rpm.

Very selective quoting, did you selectively read too ?

Bore glazing ; Raw-water cooled engines are likely to be more affected by the problem, both generally and in particular when battery charging.

Avoiding problems ; However, bore polishing is always a possibility with light loading. The solution is always to load the engine when battery charging, by driving the propeller. An engine that is driving the boat will never suffer from either glazing or polishing, even if it is at anchor or on a mooring.

"Load does not mean in gear it means working harder e.g. at higher revs." That is backwards, increasing RPM does not equate to being under load, it just means everything is going faster. Putting it in gear and driving the propeller is under load, making it drive a large alternator/generator is putting it under load.

2000 rpm is med/high on most small marine engines. I think we are in agreement that the quickest way to charge the low batteries with the engine running at about that speed. I would just prefer to do that out of gear instead of in gear straining against the mooring ropes.

I'm not suggesting that you run it at 2000 RPM, i'm saying fast idle. In other words, just a few more RPM than no throttle.