# Digital voltmeter as a battery monitor



## DavidB.UK (Sep 11, 2011)

Hello everybody,

I haven't been on here in months but I am very proud to say that I am living aboard my first little boat!

Nothing flash, but it's home and very cheap to live and that means I'll have a proper Yacht soon enough!

But onto my question, I have two batteries which I need to keep topped up, however the only reason the run the outboard IS to charge the batteries!

My fridge and water heating are all run off a gas bottle, to save money and to protect my batteries from over charging I do not want to run the engine needlessly so I bought a couple of digital voltmeters (1 for each battery).

They are very easy to wire up, but can anybody tell me what levels I should ideally keep my batteries between?


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## Morild (Mar 31, 2013)

I think it depends on the battery type (Wet/AGM/Gel) you have.
My previous wet battery used to be just below 13 volts when fully charged, and then dropping as it was used.
My new (3 rd season now) AGM is rock steady at 12.5 volts from fully charged to almost discharged at around 11.9 volts.


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## Morild (Mar 31, 2013)

Found this chart though

http://www.mmbalmainauto.com.au/PDF/State_of_charge_12_volt_batteries.pdf


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## DavidB.UK (Sep 11, 2011)

That is a huge help, thank you


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## btrayfors (Aug 25, 2006)

David,

What kind of batteries do you have? Are you at anchor or on a mooring, or at dockside?

Charging batteries with any type of machinery -- even an outboard motor -- is very expensive. This is because of the REAL cost of fuel/maintenance/amortization, etc. AND because it takes a very long time to top off any battery, even if you have a great deal of charging capacity aboard.

Also, the alternators fitted to outboards have a very low output, increasing the charging time even more....and the cost!

If you have access to shore power, then a small mains charger is the way to go to keep the batteries charged. If you're at anchor on on a mooring -- or you don't have access to mains power -- then the best way to go would be solar. 

Solar is not all that expensive these days, and with a good controller you wouldn't have to worry about your batteries. It's good even in the high-latitude UK, if you size things properly. And, solar is NO MAINTENANCE and the panels are good for at least 20 years.

Something to think about, anyway. And, if you run the numbers -- as Nigel Calder has done and has lectured/written about -- I think you'll find that maintaining the charge in batteries via a generator or a motor/outboard is far more expensive than you might imagine.

Bill


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## DavidB.UK (Sep 11, 2011)

Hi Bill,

At present I am in a marina with shore power but I am only here for a couple of days more, I booked in here whilst I did some work on the boat.

I plan to return here at the end of the summer (& stay until spring), but until then I will have no shore power.
I have 2 batteries (85ah each)
1 start battery and 1 house battery
1 of them of a sealed for life type and the other is a normal leisure battery, lead acid type.

I would love to go the route of Solar but I only plan on being aboard this boat for 2years max before buying a much bigger/better boat and will then go the solar route.

Having said that, if I already have that in my mind I could rig it in such away that it is easily removable when I come to sell this boat and put the sole set up onto the new one.

The other thing to bear in mind is my power usage is quite low.
A couple of hours of lighting a day
Charge an iPad mini (daily)
Charge a mobile phone (every few days)
Charge an iPod nano (every few days)
Trickle charge to a small 12v Bluetooth speaker

That's it.


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## jameswilson29 (Aug 15, 2009)

Mine stay just under 14 volts when fully charged with my solar panel, according to my digital voltmeters.


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## copacabana (Oct 1, 2007)

David, another option that might work for you is buying a small, inexpensive solar panel that is sized small enough that it can't overcharge your batteries. This way you don't need to invest in a charge controller for the time being. As you live on board you can keep an eye on the charging. Perhaps a cheap 20w panel would be enough to bring your 85amp battery up to a full charge over the day.


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## mdbee (May 2, 2007)

copacabana said:


> David, another option that might work for you is buying a small, inexpensive solar panel that is sized small enough that it can't overcharge your batteries. This way you don't need to invest in a charge controller for the time being. As you live on board you can keep an eye on the charging. Perhaps a cheap 20w panel would be enough to bring your 85amp battery up to a full charge over the day.


Might want to read what even a smaller solar panel can do to a battery:

http://www.sailnet.com/forums/electrical-systems/94547-do-i-need-solar-charge-controller.html


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## aeventyr60 (Jun 29, 2011)

NASA brand electronics sells an economical 2 bank battery monitor. You can always take off the "extras" you add to your current boat and install them on your future boat.


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## Neosec (Jun 25, 2013)

> A couple of hours of lighting a day
> Charge an iPad mini (daily)
> Charge a mobile phone (every few days)
> Charge an iPod nano (every few days)
> Trickle charge to a small 12v Bluetooth speaker


I'm going to guess here that your lighting is the real amp eater. Are your lights AC (using an inverter) or DC (12 volt) and are they incandescent or fluorescent or LED?
I'll assume 12v incandescent. Three lights at 9 watts each = 27 watts, times 3 hours = 81 watts.
iPad mini ~ 20 watts *for 1 hour
Phone ~ 10 watts *
Nano ~ 5 watts *
Speaker ~ 10 watts (Trickle charge makes little difference) *

All this = 126 Watts

Guessing 4 decent hours of sun per day you'll need a 30 watt panel near as I can tell. All guess work.


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