Battery Voltage - when to charge?

Twister_Ken

Twister_Ken

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My Twister had no way of monitoring battery voltage, so I never worried about it. The trip up and down the river after a day's sailing always seemed to put sufficient sparks back. New boat, though, has a meter showing battery volts. Again, I haven't worried about it, doing mainly day sailing. However a longer offshore trip is planned next year, and the new boat drinks more sparks than the old one. So, at what voltage level should I consider doing a bit of motoring (or turning stuff off) to keep the charge up to sniff?
 
add solar?

I'd say at about 12.4V you need to be charging. Alternatively stick 100W of solar on; I can sail with zero net power consumption (with nav kit, autopilot, VHF and fridge running (30% cycle)) on an average broken cloud sunny day. If constantly on a shady tack (mainsail putting panels in shade) then that is not possible. Portable panel then gets deployed in to dinghy on the davits to maintain supply!

ps - meant to say that it is difficult to get a sensible V reading when underway because if there's a load on the battery the V reading will be lower than when in a no-load state. My 12.4v suggestion is for a no-load state.
 
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Battery Charging.

No doubt you will get quite a few replies from experts in Batteries. However with all the trouble I've had over the years with them I'll offer my bit. Letting batteries go below 50% state of charge shortens their life and letting them go to 25% seriously does so. I would therefore suggest you charge when the no load voltage goes below 12.2 volts when the battery will be at about 50% state of charge.
 
You can't readily measure no-load voltage whilst under way. Your best solution is to fit a battery monitor which can measure actual Ah in and out of the batteries, then recharge when capacity drops to about 50-60%. When initially setting a battery monitor, be aware that (depending on their age) your batteries may no longer have the capacity they started off with.
 
Twister_Ken said:
My Twister had no way of monitoring battery voltage, so I never worried about it. The trip up and down the river after a day's sailing always seemed to put sufficient sparks back. New boat, though, has a meter showing battery volts. Again, I haven't worried about it, doing mainly day sailing. However a longer offshore trip is planned next year, and the new boat drinks more sparks than the old one. So, at what voltage level should I consider doing a bit of motoring (or turning stuff off) to keep the charge up to sniff?
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Is it just a basic voltmeter? I've got a battery monitor that was original fit nearly twenty years ago and find it very handy. It sums up the current in & out. Also very handy for working out what takes the most current. So it would not only help you make the decision when to charge but it also helps you manage the current draw.
 
Twister_Ken said:
I can switch off all heavy consumers before checking voltages, but would prefer to leave basic nav instruments on, which I suspect consume very little. Would this help get a meaningful reading?
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Theoretically, no. A no-load voltage reading means just that - no load at all. Even worse, you're supposed to leave the battery without load for a while (an hour or so) before measuring the voltage.
 
pvb said:
Theoretically, no. A no-load voltage reading means just that - no load at all. Even worse, you're supposed to leave the battery without load for a while (an hour or so) before measuring the voltage.
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And, bit of fred drift I suppose, all the angst and posts and arguments are caused by that simple bit of information not being taken heed of! NO_LOAD VOLTAGE. Peeps look at their volt meter whilst on board and see 12v, 12.4v etc and go AAARGHHH my batteries are flat! BUT what they havent taken in to account is the fact that the fridge is on, or the kids have left all the lights on. As you quite rightly say, the voltage should be checked with no load and with all loads switched off for a while before checking.
Stu
 
If it's an analogue voltmeter (moving coil) then it may not be accurate enough. Digital voltmeters are usually better but still need calibration. With the one you have, it would be worth noting the voltage when you are sure the batteries are fully charged. Then a one volt drop from there would represent about discharged. It's fuzzy, because a fully charged battery may show 13.2 - 13.6 volts unloaded, but after a few minutes loading, only rises to 12.6 - 12.8 volts, zero load. My one volt suggestion applies to the latter figures. Really anything below 12 volts, no load, is not good news.

If the battery has accessible wet cells, try a hydrometer works well to measure charge state.
 
Just a thought,depending on what ampere/hours you already have available,might be worth adding another battery to reduce the angst. I live aboard ,so not really relavant to you situation,but I have 6X 110 ah :o Boat floats low in the water!
 
Ludd said:
Just a thought,depending on what ampere/hours you already have available,might be worth adding another battery to reduce the angst. I live aboard ,so not really relavant to you situation,but I have 6X 110 ah :o Boat floats low in the water!
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Adding battery capacity will not necessarily help if you have no means of charging them. If you have a solar panel, you could add a cheapo regulator. The one I bought has two LEDs amber means 'charging' green means 'fully charged' none means 'its dark'. If you see the green on during the day you know the batteries are charged.
 
The mnemonic is, as far as I remember, "12.7-you're in heaven, 12.5- still alive, 12.2- lots to do". That's for a battery at rest i.e. no charge or discharge current.
 
Although I have a multimeter I normally rely on the volts reading on my Raytheon instruments which are switched on much of the time anyway. The two Raytheon meters give different answers and both are higher than the true voltage, so I just subtract a little from each when I need to. With a 36w solar panel and only 3x65hr batteries for service I can still run a fridge for nearly a week on a mooring before the voltage goes below 12.4 after a night's rest.
 
skipper_stu said:
And, bit of fred drift I suppose, all the angst and posts and arguments are caused by that simple bit of information not being taken heed of! NO_LOAD VOLTAGE. Peeps look at their volt meter whilst on board and see 12v, 12.4v etc and go AAARGHHH my batteries are flat! BUT what they havent taken in to account is the fact that the fridge is on, or the kids have left all the lights on. As you quite rightly say, the voltage should be checked with no load and with all loads switched off for a while before checking.
Stu
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And rest for a long time, though to be fair this is approaching from the other direction, charging, recovery from discharge may well be different.
http://www.pbase.com/mainecruising/battery_state_of_charge
 
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