Basic volts question for battery use

[steve yates]

I have had a voltmeter fitted, in one of those little combined USB socket/12v socket/meter combos, and the electric cool box and radar are now working. I realise these can use a lot of power, so what is the meter actually telling me? I have two brand new 120 aha batteries installed, in series, with a battlers selector switch and with a cranking charge of 800 for the engine, to replace the old ones which were apparently goosed and fluctuating like mad.

(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'm currently on a harbour wall, and the meter reads 12.5v, I may be here till sun am, so at what point does the voltGe get critical for the engine to start? Also, once the engine is running, how long till the batteries are recharged? I am assuming the engine needs to be in gear to recharge? Or can I just run the engine in neutral for an hour or so while tied up to boost them?
Thanks.

 

[sailaboutvic]

This is how understand it , I sure someone will come along soon and put you and me right .
12.1V is the limit , that about 50% of your battery uses .
I don't believe running your engine for an hour once you battery's that low is going to do much good , for sure I would t run the engine in neutral, I would run it in gear .
I would consider fixing an solar panel , there not that expenses these days .
Once your battery's that low it going to take quite a long time to charge them back up just by running the engine , plus it's not doing your engine any good .
Ok I await to be corrected now .

 

[lpdsn]

Difficult to say for certain based just on Volts but 12.5V should be OK. Is that the voltage of the domestic or starter battery?

Or is it two 6V batteries you have in series to effectively make a single 12V 120Ah bank?

PS As SailAboutVic says, a 50 - 100W solar panel (or thereabouts) and a cheap controller will make a big difference without breaking the bank.

 

[Dutch01527]

If you do not have a engine start battery isolator I would turn off all non essential electrical drain immediately. Start the engine and run it in neutral at medium / high revs for a couple of hours ( the alternator will work off the engine, not the gearbox so no reason to be in gear).

There is a real danger that you could quickly drain the batteries to an extent that you can not start the engine especially if you are using cool box and radar. Been there, done that.

For the future I would suggest fitting a start battery isolator, swapping to led bulbs and only using the radar (and maybe the cooler) when the engine is running except in short bursts if absolutely required. I also carry a mini power pack which will enable me to start the battery in an emergency.

 

[steve yates]

It's two 12v 120ahr batteries in series, either can be used to start the engine, depending on the selector switch setting.
Yes a solar panel is on the cards, but I'm sailing now and it's not an option immediately. Just wondering ta what point I should shut my batteries off while stuck on the harbour wall with no shore power to ensure the engine starts when I want to get out. The portpatrick entrance is 35m wide with shallows, rocks and ruined breakwaters all around I don't intend sailing out!

 

[PaulRainbow]

Start the engine and run it in neutral at medium / high revs for a couple of hours ( the alternator will work off the engine, not the gearbox so no reason to be in gear).

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.

 

[TQA]

If they were in series you would have 24 volts. I suspect you mean parallel.

 

[PaulRainbow]

It's two 12v 120ahr batteries in series, either can be used to start the engine, depending on the selector switch setting.

You will find the batteries are not wired in series, if they were they would give you 24v.

If you have an old 1-2-both switch, everything (engine and electrics) will run from whichever battery is selected. If you select both then the batteries will be connected in parallel and everything will run from both batteries. This would be a bad setting to use in your circumstances.

Set it to one battery, make sure that will start the engine. Then change to the other battery, safe in the knowledge that the other battery will start the engine when you want to leave.

Fit solar panels and get rid of the 1-2-both switch. Fit a new battery for the engine alone, with it's own isolator and the other batteries connected in parallel with their own isolator. Add a dual sensing VSR and you have a vastly better system than now.

 

[steve yates]

You will find the batteries are not wired in series, if they were they would give you 24v.

If you have an old 1-2-both switch, everything (engine and electrics) will run from whichever battery is selected. If you select both then the batteries will be connected in parallel and everything will run from both batteries. This would be a bad setting to use in your circumstances.

Set it to one battery, make sure that will start the engine. Then change to the other battery, safe in the knowledge that the other battery will start the engine when you want to leave.

Fit solar panels and get rid of the 1-2-both switch. Fit a new battery for the engine alone, with it's own isolator and the other batteries connected in parallel with their own isolator. Add a dual sensing VSR and you have a vastly better system than now.

Thx Paul, yes someone reccomended that in a previous thread ( may have been you, or vic or prv) and it's on the winter list.
So what sort of voltage is required to start the engine? And if the engine is charging the battery as it runs, say you started it on selector 1, once that is charged, will it start charging battery 2? Or does that only happen if the engine is started from battery 2 ?
Fwiw, the voltmeter tells me the voltage of whichever battery the selector switch is set to, I guess it would show 24 or so if set to both? And does that shed light on whether they are in series or parallel? I was told they are in series.

 

[johnalison]

The 1-2-both switch was fine when needs were simple, as on our last boat with no fridge or radar, but really doesn't work for more complex systems. It seems inconceivable that the batteries are in series, otherwise they would always both be in circuit. I doubt if anyone can give an exact voltage for engine starting because othe factors come into play, but I would be unhappy to go to sea without at least one battery being near to full charge. I'm assuming that you are not close to shore power, which would be the obvious way to fully charge up.

 

[VicS]

Thx Paul, yes someone reccomended that in a previous thread ( may have been you, or vic or prv) and it's on the winter list.
So what sort of voltage is required to start the engine? And if the engine is charging the battery as it runs, say you started it on selector 1, once that is charged, will it start charging battery 2? Or does that only happen if the engine is started from battery 2 ?
Fwiw, the voltmeter tells me the voltage of whichever battery the selector switch is set to, I guess it would show 24 or so if set to both? And does that shed light on whether they are in series or parallel? I was told they are in series.

My preferred procedure with a system that has two batteries and a 1,2,both, off selector switch is to:

Designate one battery as the engine start battery and the other as the domestic battery ... (It makes some sense for the domestic battery to be larger than the engine start battery......... it also make some sense to have two the same ....... take your pick!)

Select the engine start battery and start the engine. Allow that battery to recharge ... a volt meter on the system is a useful guide to when that is so.

Switch, via the "both" position to the domestic battery ...... Keep this selected to supply all your domestic equipment, until you next want to start the engine or shut down and go home.

If you switch batteries while the engine is not running it does not matter whether you switch via "both" or "off" ... but NEVER, NEVER, EVER, NEVER switch to "Off" while the engine is running!


Others will suggest different preferred procedures ... but choose whichever you feel happiest with and stick to it


Unless you have some odd ball engine like the Watermotor Seapanther then you batteries should connect together in parallel when "both" is selected.

( Some Sea Panthers had a 12 volt system but used two batteries in series to provide 24 volts for engine starting. )

 

[Chris_Robb]

Steve, its rather a waste to have 2 batteries of 120 ah, one of which needs to be dedicated to the engine. It would be better if you connected the 2 as "house "batteries and installed a small 75ah starter battery which you only use for the engine.

The advantages of this are:
1. Less likely to run below 50% full (which ruins a battery)
2. Because there is greater capacity you will get more amps going in from your alternator than if just charging one battery.

You should check when you are charging what the voltage gets to. It should get somewhere near 14.4V. Below 14V is not good and you will be charging for ages with a much smaller ampage going in. So check this out and come back here to ask what you then should do to improve things.

 

[steve yates]

Steve, its rather a waste to have 2 batteries of 120 ah, one of which needs to be dedicated to the engine. It would be better if you connected the 2 as "house "batteries and installed a small 75ah starter battery which you only use for the engine.

The advantages of this are:
1. Less likely to run below 50% full (which ruins a battery)
2. Because there is greater capacity you will get more amps going in from your alternator than if just charging one battery.

You should check when you are charging what the voltage gets to. It should get somewhere near 14.4V. Below 14V is not good and you will be charging for ages with a much smaller ampage going in. So check this out and come back here to ask what you then should do to improve things.

Hi Chris, yes that's the intention, after advice on here, but it's a winter job really. The electrics are in a bad way, so going to do it all in one hit over the winter. The voltmeter reads 14.4 v when the engine is running.

 

[Chris_Robb]

Hi Chris, yes that's the intention, after advice on here, but it's a winter job really. The electrics are in a bad way, so going to do it all in one hit over the winter. The voltmeter reads 14.4 v when the engine is running.

That's one good sign then!

 

[sailaboutvic]

Steve no one really have answer your question , which was how far can you let the battery run down before it won't start your engine , so I give it a go .
Just for this occasion just to get you out of trouble, I would have it switch to battery one once the volts get to around 12.2v I would try the engine if it start let it run for a why in gear , if it don't , then use both battery to start , then turn it off and turn the switch to battery 2 , that way you should have enough power to start the engine when the time comes to leave , these are new battery's you have so don't mess them up by running then under 50% that's about 12.1 v
.
People correct me if I am given bad advise .

 

[steve yates]

Thx vic

 

[lpdsn]

Just keep one of the batteries isolated for engine use.

You really don't need even a single 120Ah battery to start the engine. It's a waste of an expensive battery. When you get around to the refit fit an additional small battery with high CCA. I just use a 55Ah one for a 4-cyl diesel. You could get away with smaller battery. And once you've fitted that only ever use it for starting the engine.

In the meantime, when the volts of the battery you're using as the domestic start to drop from 12.5 or 12.4 charge it.

 

[[2574]]

Be aware that if you have a load on the circuit your voltmeter reading is not representative of the state of charge (SoC) of the batteries. Turn all loads off, let the circuit rest for thirty minutes and then check the voltage, this will give a much truer voltage and give better indication of state of charge. There are plenty of voltage to state of charge tables on the interweb to give you an idea of the SoC from the reported V.

Here is a table link: http://www.energymatters.com.au/components/battery-voltage-discharge/

Don't go below 50% SoC.

 

[PaulRainbow]

Thx Paul, yes someone reccomended that in a previous thread ( may have been you, or vic or prv) and it's on the winter list.

Good

So what sort of voltage is required to start the engine?

Hard to give a reliable answer. I'd try starting it now from the battery with the lowest voltage. If it starts easily, charge it for a little while then switch to the other battery, keeping the first one for engine starting when you want to leave. Given present circumstances, don't use the "both" setting, other than for an emergency start.

And if the engine is charging the battery as it runs, say you started it on selector 1, once that is charged, will it start charging battery 2? Or does that only happen if the engine is started from battery 2 ?

The alternator will only charge the battery that is selected. Vics system is as good as any, i'd go with that. Some people will say switch to "both" with the engine running, to charge both at once. IMO this is a bad policy. If you forget to change the switch you can end up with no way of starting the engine. If a battery fails you can also lose both.

Fwiw, the voltmeter tells me the voltage of whichever battery the selector switch is set to, I guess it would show 24 or so if set to both? And does that shed light on whether they are in series or parallel? I was told they are in series.

No, if you set it to both it will parallel the batteries, definitely not series. If you parallel your two batteries you get 12v and 240ah. If they were in series you would get 24v and 120ah. Turn the switch to both (briefly) and you'll see for yourself.

 

[sailaboutvic]

In the meantime, when the volts of the battery you're using as the domestic start to drop from 12.5 or 12.4 charge it.

12.5v is about 80% charged so he still got plenty of power in that battery ,
Off cause all this advise he getting is just to get him out of trouble this time , I think Steve know him self he needs to get it all sorted .