Partially discharged batteries

[ontherocks83]

So I know everyone can agree that leaving a battery in a partially discharged state is bad. My question is how long is a partially discharged state?

Let's say you have a 870ah bank and on a passage you use 240ah per day (10amps per hour). You have solar panels that put 60ah back into the bank each day. So by my math that means your net usage is 180ah per day which would give you 2.4 days of battery power before needing to be recharged. So that leaves your batteries in a partially discharged state for 57 hours. Now my assumption is once every now and then is ok but if you are on a 30 day passage that means you are constantly leaving your batteries in a partially discharged state for most of the passage. At what point does this become detrimental?

To me it would seem ok during the passage but on the flip side if I were to partially discharge my bank and then leave the boat for 2 days without any charging and then come back I would feel like I was damaging the bank.

So how much is too long to leave a battery in a continuous discharged state in this fashion?

[St. Elsewhere]

Others may respond with more quantitative information, but battery life is inversely proportional to the amount of discharge, and length of time at a discharge. I have run lead acid batteries for 13 years that were rarely discharged more than 25%, and brought back to full charge within 12 hours. When I replaced them they were still fully functional, but I did not want to risk their end of life occurring when they were greatly needed.

They were connected to a three stage charger that held them at float voltage most of the time, with periodic bulk charging. This is not practical for boats on anchor.

[btrayfors]

ontherocks83,

A 30-day passage in a Catalina 30??? Where ya going :-) And 870AH house battery bank on that boat? I think we must be talking about another boat or just "figurin" :-)

Seriously, an average 10 amp load on an 870AH bank is less than 1/4 the discharge rate for which the batteries were rated (870AH / 20 hour rate = 43.5amps load). That means the real capacity is much more than 870AH.

And, with a battery bank like that if you have solar panels with only a 60AH per day charging capacity, then you are WAY under-paneled. How are you going to make up the rest of the charging need? Engine? Generator? Wind generator?

Will a 30-day partially charged FLA bank suffer? Probably, but you'll not be able to measure the effect in that short a period without doing a 20-hour discharge test or using a sophisticated battery capacity testing instrument.

Only thing to do is to keep the batteries as fully charged as you can reasonably do, and don't lose any sleep worrying about 30-day passages.

Bill

[ontherocks83]

Originally Posted by btrayfors

ontherocks83,

A 30-day passage in a Catalina 30??? Where ya going :-) And 870AH house battery bank on that boat? I think we must be talking about another boat or just "figurin" :-)

Seriously, an average 10 amp load on an 870AH bank is less than 1/4 the discharge rate for which the batteries were rated (870AH / 20 hour rate = 43.5amps load). That means the real capacity is much more than 870AH.

And, with a battery bank like that if you have solar panels with only a 60AH per day charging capacity, then you are WAY under-paneled. How are you going to make up the rest of the charging need? Engine? Generator? Wind generator?

Will a 30-day partially charged FLA bank suffer? Probably, but you'll not be able to measure the effect in that short a period without doing a 20-hour discharge test or using a sophisticated battery capacity testing instrument.

Only thing to do is to keep the batteries as fully charged as you can reasonably do, and don't lose any sleep worrying about 30-day passages.

Bill

Its just figuring and trying to learn more about the characteristics and properties of batteries. If you also look you will see I'm on a lake so obviously this is theoretical. Im really just curious about the effect on batteries in that type of situation for future reference in case my wife and I do have the opportunity in life to upgrade and move to the ocean on something bigger. I really just picked rough numbers to get an idea of what happened so I'm sorry if the reality is a bit off.

[ontherocks83]

Alright so let me generalize my question since the "reality" of my original post is in question.

Lets say you have A boat that has a battery bank of X hours and your draw on that bank and solar input allows for 4 days before reaching 50% SOC at which point you would have to start a generator or engine to charge. Does that hurt the bank being in a partially discharged state for that long and to what degree?

My assumption is that your solar or wind should be able to bring it all or almost all the way back during the day but if this is not feasible due to available real estate and you don't want to run the engine every day are you hurting the batteries?