Battery volts during anchorage

C

cmedsailor

Well-Known Member
Joined
10 Sep 2007
Messages
1,830
Location
East Med...
Visit site
During anchorage and while running the fridge and occasionally the water pump etc I have the habbit to look at the voltmeter from time to time. If it shows above 12V then I feel OK, otherwise it's time to run the engine for a while.
Having no any other sophisticated equipment to monitor the batteries, is that adequate or should I be concerned with remaining ampers rathen than voltage (therefore need to install an amperometer)?
Thanks
 
Battery voltage should be checked at no load. ie switch everything off, wait 5 mins, then check voltage.

At college, many moons ago, we had to learn a little ditty for flooded cell batteries, gel batteries are slightly different but near enough IMO.

Battery Voltages as your battery tells it:

12.7 i'm in heaven.
12.5 still alive.
12.1 some work needs to be done.
12.0 i'm not a hero.
 
Measure the voltage under known load if possible. This graph will give you an idea but there are lots of variables such as type of battery and temperature. In the graph 'C' means the battery capacity in Amp hours. You will need an accurate voltmeter good to nearest 0.1V
cell.jpg


(Reason for measuring under load is that, as you can see, the light load line is quite flat. The no-load line is even flatter)
 
Last edited:
The manual for the AGM batteries I have states they should be measured after resting for 4 hours.

Also, the voltage measured is dependant on the temperature of the batteries. Most of the charts I've seen, maybe also the one above, are for batteries at, I think, 25C (or was it 20C?). At 15C, which is more likely in the UK at this time of year, the voltage would be lower. If you Google you should be able to find charts for Lifeline Batteries at different temperatures.
 
Oh boy now I am getting confused because my battery knowledge is really limited. I only want to avoid problems (OK for this I always have the engine battery switched off).
The batteries for everything else are two deep cycle Delkor DC31m 100amp connected in parallel. Same age, same size, exactly same batteries. Temperature around here where I live (med) is definately above 25C.
Voltmeters are quite accurate.
 
Last edited:
I agree with the "leave it 4 hours" mentioned above (to allow for diffusion) since last major charge/discharge.

Call the capacity 200 Ah. and aim to use the C/100 line (2A). Wire a 24W bulb across the batteries (24/12 = 2) and measure the voltage. Look the voltage up on the C/100 line on the discharge set of curves to give % full.

I welcome other forumites to review the method above, particularly the math!
 
cmedsailor said:
Oh boy now I am getting confused because my battery knowledge is really limited. I only want to avoid problems (OK for this I always have the engine battery switched off).
The batteries for everything else are two deep cycle Delkor DC31m 100amp connected in parallel. Same age, same size, exactly same batteries. Temperature around here where I live (med) is definately above 25C.
Voltmeters are quite accurate.
Click to expand...

Your 12 volt is OK.

In normal use, with average loads 5 - 10 % of battery capacity, 11.85 volt is around 50% battery capacity. It is not precise, but it is a good rule of thumb, and cheap, and will normally keep you out of trouble.

Brian
 
battery voltages

tinkicker0 said:
Battery voltage should be checked at no load. ie switch everything off, wait 5 mins, then check voltage.

At college, many moons ago, we had to learn a little ditty for flooded cell batteries, gel batteries are slightly different but near enough IMO.

Battery Voltages as your battery tells it:

12.7 i'm in heaven.
12.5 still alive.
12.1 some work needs to be done.
12.0 i'm not a hero.
Click to expand...

Add on 12.2 ... lots to do. Not quite consistent with the 12.1 caption though.
 
Thanks. I think I'll stick with my method. Above 12volts OK and when approaching 12V start worrying.
Plus my isolated engine battery (I turned it off even if it's isolated to feel more safe; don't ask me why)
 
Battery volts

I think your checking the voltmeter under load is good. (Assuming the fridge is running each time you check ie not in its rest phase)
You may be being a little cautious and perhaps slightly lower voltage would be OK.
If you let the voltage fall more you will get more charge from the engine (for a given engine run time) but letting it fall too much will abuse the batteries.

I would suggest you try to measure or estimate your current drain for a period typical of the time between engine runs for battery charging. Watch that on off ratio for the fridge. If you can estimate the AH used and relate that to the voltage drop then you will have a better idea if your 12v limit is a good one.

eg if your average current drain over 10 hours is 5 amps (50AH) and you have 200 Amp hour batteries then after 10 hours you have 25% discharged the batteries. (safe) what is the volt meter reading? If you go for 20 hours then that volt meter reading will tell you more. But beware older batteries lose capacity so 25% might be a safer level to discharge.
good luck olewill
 
You must log in or register to reply here.

Share this page

Top