Safe voltage for restarting a 25 hp Diesal

JohnShannon · Feb 6, 2016

http://www.lifelinebatteries.com/manual.pdf page 30 link to the lifeline technical manual. 11.9 V open circuit is 25% charge so that is not good. Is that 11.9V under some load? If so then you are somewhere over 25% of capacity. You want to avoid discharging under 50% as the cycle life of the batteries gets bad quickly.

But what are your options here? If you don't start your motor does a freighter run you over? In that case "risking it" might be OK. Fancy boats will have a dual battery system with a start and a house battery. The alternator charges the house battery and there are various schemes to recharge the start battery (find a MS post and follow it) with as I recall the best being an ACR (automatic charge relay) in this case start battery only ever has to start the motor which is a trivial amount of Ahr and should really never find itself at 11.9V.

Stu Jackson · Feb 6, 2016

John's right, but it ain't only "fancy" boats. Most of us have done this over the years if we're not marina hoppers. If you want to sail long distances (i.e., for more than daysails) and also like to anchor out, it's good to have a robust house bank large enough to support your electrical loads, and have a separate reserve bank just in case. Many C30 skippers have done just this.

Stu Jackson · Feb 6, 2016

Yes, but...

Not quite sure what's driving your question. If 11.9V actually starts your engine, then...

If you've been sailing for a few hours sometimes/most times, it's been my experience that you don't need to use the glow plugs (if you have a Universal engine, not a Yanmar), so there's less juice required, too.

What are you using to measure the voltage? The cockpit voltmeter or instruments down below?

Are your two batteries on a 1-2-B switch and do you use just one at a time? How do you operate your system?

This is to better help us to help you.

Stu Jackson · Feb 6, 2016

mytime said:
based on reading my digital voltmeter when should I question weather I have enough volts to start motor without going down to switch to battery 2?

Thanks, now we understand more. What you describe is a "two house bank" arrangement, alternating by days or coming / going.

Using a battery as low as 11.9, which is very low, may start your engine, but is not good for the battery. That's really all there is to need to know. ITWMB, I'd simply use the "fuller" battery if that is what is happening.

Now, to go somewhat further, and only if you're interested in going along, what are you running on your daysails that gets either of your battery banks down that far?

If it's a fridge, your individual battery banks working alone are sadly undersized.

With no fridge, and just the usual sailing instruments including an autopilot, shouldn't drop a battery bank that size down that much.

For sake of clarity, a battery bank is either one or more batteries operating together. In your case, the way they're switched gives you two banks of one battery each.

JohnShannon · Feb 6, 2016

Refer to page 32 of your lifeline techincal manual http://www.lifelinebatteries.com/manual.pdf
show voltage vs depth of discharge at various discharge rates (as opposed to an unloaded condition)

What it says is that the capacity that 11.9V represents varies depending on your discharge rate.

11.9V when you are cranking no problem. Run a tiny LED for a week and the battery reads 11.9V = almost nothing left. What you need is a battery monitor i think you can get them in the store.

Failing that it sounds like you always have a full battery in reserve to start.

Stu Jackson · Feb 6, 2016

12.1 It's half and what you should use as a guideline. Just switch to the other. I'd try, at least once, to see what happens when it's 11.9. Just so you know.

mytime said:
considering adding third battery dedicated to the head

Don't do this. Please. "Dedicated" batteries are not a good idea. Your third battery should be added to one of the existing two, although depending on age, you should get two new ones, wire them together and have one of the old ones as your reserve, or Batt #2.

There's tons to learn about electrical, and without retyping it all here, unless you have already, Charlie Wing's electrical for boats is a very good read.

You could try some of the stuff here, too:

Electrical Systems 101 http://c34.org/bbs/index.php/topic,5977.0.html

Davidasailor26 · Feb 6, 2016

Also keep in mind that the 20A number for the head is likely the maximum instantaneous current draw, but it's only going to draw that for a couple of seconds per use. Even if we said 3 seconds per use and 1 use per hour, that's still only about 0.25 AH in a 16 hour sail - not a significant percentage of your battery capacity. Having said that the "2 house + 1 reserved" arrangement that Stu describes is certainly more robust than what you currently have and should translate into better long term battery durability.

Benny17441 · Feb 6, 2016

I don't know if I understand the question but when I push the start button the way the starter motor reacts lets me know if I have enough power to start the engine or not. If the starter is turning the engine more slowly than usual it is my cue to get up and engage the second battery. I'm not going to ponder the accuracy of a voltmeter reading depending on the location of the gauge or whether the battery was at rest or not. I think there are too many variables to categorically define a minimum battery voltage to start every engine under any weather condition. We all know the difficulty of turning over a cold engine when the oil in the crankcase is at its thickest; it definitely requires more battery power than in the middle of the summer while the engine is still warm from a prior run.

Maine Sail · Feb 7, 2016

mytime said:
Is there a recommended battery voltage to stay above to ensure restart based on 95 degree air temp? I have seen charts that show as low as 11.9 volts acceptable? We have 2 batteries and probably will add a third. But curious what people think is the "safe" zone. Thanks...

With a healthy battery, sufficiently sized to start and run house loads, it should easily start your motor at 50% SOC.

The real question should be;

What should my lowest voltage, under typical house loads, never dip below?

The answer to that is usually 12.1V BARE MINIMUM and 12.2V as the normal point to begin recharging... This should put you at right around 50% SOC under typical house loads if the bank was sized well...

Maine Sail · Feb 7, 2016

mytime said:
I think I am looking at my voltmeter as a "Fuel Gauge"

A volt meter won't be an accurate fuel gauge unless YOU have done the home work to determine what the SOC/Voltage should be at each load & air temp. My guide above is a "safe" value for a recharge point with a healthy & well sized house bank. The typical cruising boat is discharging the battery at approx 25% to 35%, or less, of the 20 hour discharge rate.

For a 100Ah battery the 20 hour discharge rate is 5A so the typical boater ideally should not be discharging a 100Ah battery, at at rate that averages more than about -1.25A to -1.75A. For a 400Ah bank this becomes 5A to 7A for the "average" discharge rate. At these discharge rates 12.2V is a typical safe bottom voltage for a healthy bank.

mytime said:
Maine sail I think this is what I was looking for Thanks!

Here's a test I ran on a Lifeline Group 31 Battery that helps put voltage vs. SOC into perspective but remember this discharge was at the 20 hour rate.....

Loaded Battery Voltage vs. SOC

Stu Jackson · Feb 7, 2016

mytime, in addition to my link, read more on Mine Sail's website (included there, too) and his link to it. Many of the topics in my ES 101 link refer to material written by Maine Sail, on both his website and this forum.

Brian D · Feb 7, 2016

mytime, do you by chance have solar panel(s) on your boat? I run a 50w solar panel on my house bank. During a normal days outing the solar keep the batteries topped off. I basically run the same thing as you, nav, radio, autop. Just a small panel will help to keep the battery you want to start the motor topped off.

Brian D · Feb 7, 2016

mytime, it depends on your budget and use. However, if you are just using the basic tools on the boat while out and about, then 50 watts should be enough.