Calder battery sizing & stereo-battery connection

[Ralph Masters]

Radio, on the boat, Rush, NPR, resetting pre-sets.  The admiral tells me all I need to know out on the water.  And I don't have to worry about battery drain, it ain't drained yet..................

Ralph

[mainesail]

With many stereos it is not just station AM/FM pre-sets. EQ settings, aux input settings (on/off, rear/front), balance, fader, face lighting settings etc. etc. can all be affected by not keeping memory power to the unit.

Some brands are worse than others in this regard. With our Kenwood Excelon if we lose battery power we need to go into the full "initial set up menu" and re-do everything. The iPod or TV audio won't work until this step is complete. This set up takes a couple of minutes. We also lose all adjustment settings including any custom EQ settings we've created to dial the sound in just so..

Our unit draws .002A for the memory....

[Indian Falls]

A friend and fellow sailor is installing a dedicated automotive battery with it's own solar charger just for "Boat Entertainment systems"  I thought it was an awesome idea.  No worrying that the stereo and other related accessories (disco ball?) will run your house batts down or your engine start battery down.  The solar keeps this lone system on line and ready all the time.  So with that, there go your "fancy schmancy radio" preset problems!

[Stu Jackson]

From a purely boaty electrical engineering perspective, "dedicated" batteries make little or no sense.  The largest house bank, heck even the smallest one if you only have one house battery, is more than enough to handle a stereo memory.  If he's "afraid" his stereo memory will kill either his house bank or his reserve bank, think about what it'll do to an even smaller dedicated stereo battery.  Sheesh...   Yes, I know, so his dedicated battery goes dead, too, so all he loses his his memory.  His memory?  Sorry, his stereo's memory.   Carrying all the extra lead around, he could use it for his house bank.

[Indian Falls]

Sorry Stu, sounds like you missed  the whole idea.

It's not about the radio memory at all. 

If I'm at the marina with my truck stereo on, it takes the battery so low in 3 hours I can't start my truck. 

If you kill the entertainment battery, you still have an untouched house bank for lights, auto-helm, starting, bilge pumps, whatever.  Secondly, if it takes 5 hours to kill the entertainment battery, it's time for a little piece and quiet anyway!

I like the idea because I don't have a dedicated starting battery or a giant golf cart bank and my 1st mate would love to have a very nice music system on board, the kind that could cause me to worry about how long it'd been on...  sailing is about relaxing and slowing down and I don't want another thing to worry me or at the least bother me to have to keep checking the charge on the house bank, "it's getting low!  lets ruin the mood by starting the diesel for a half hour!"

Many of us out there have a dedicated battery already.  They've just chosen to use it for something else.

[Exodus]

The PO of my boat installed a dedicated battery for the refrigeration.  I may change that when I get around to overhauling my DC system.  So far though it seems to be an okay set up for day sailing and gunkholing on weekends.  Maybe I should run my radio to that battery when I do the overhaul...

[mainesail]

The stereo memory on our boat draws 0.002A or .048Ah per day, or 0.336Ah per week. It takes a full THREE WEEKS to use just 1Ah.

Considering most C34's have a house bank exceeding 200Ah's you'd be using just 0.168% of a 200 Ah bank per week. How many of us go longer than a week between uses or charging??

Even at three weeks, of no use & no charging, though one might question why you own a boat with that much vacancy of use, you use just 0.5% of your Ah capacity of a 200Ah bank for the stereo memory..

I have seen them as high as 0.007A but that is a pretty high one..

[Stu Jackson]

The PO of my boat installed a dedicated battery for the refrigeration.  I may change that when I get around to overhauling my DC system.  So far though it seems to be an okay set up for day sailing and gunkholing on weekends.  Maybe I should run my radio to that battery when I do the overhaul...

Exodus, that actually should be done now.  Why not simply wire your "dedicated" fridge battery with the rest of the house bank, and you'll find your batteries, all of them, will simply last longer.  That's another good reason to avoid "dedicated" batteries, 'cuz for the same amount of lead you're dragging around they'll simply last longer.

Here's why (grouping batteries is better)

IS IT BETTER TO HAVE ONE OR TWO BATTERY BANKS FOR HOUSE USE?
(By Nigel Calder - I DIDN'T write this!!!)

The popular arrangement of having two house banks alternated in use needs scrutiny before I go any further.

LIFE CYCLES: As we have seen, the life expectancy of a battery in cycling service is directly related to the depth to which it is discharged at each cycle - the greater the depth of discharge, the shorter the battery's life.

This relationship between depth of discharge and battery life is NOT linear.  As the depth of discharge increases, a battery's life expectancy is disproportionately shortened.  A given battery may cycle through 10% of its capacity 2,000 times, 50% of its capacity 300 times and 100% of its capacity around 100 times.

Let's say, for arguments sake, that a boat has two 200-ah battery banks, alternated from day to day, with a daily load of 80 Ah. [Note from Stu:  this is a pretty usual daily load.]   Each bank will be discharged by 40% (80 Ah of one of the two 200 Ah banks) of its capacity before being recharged.  The batteries will fail after 380 cycles, which is 760 days (since each is used every other day).  If the two banks had been wired in parallel, to make a single 400 Ah battery bank, this bank would have been discharged by 20% of capacity every day, with a life expectancy of 800 days, a 5% increase in life expectancy using exactly the same batteries!

But now let's double the capacity of the batteries, so that the boat has either two 400 Ah banks, or a single 800 Ah bank, but with the same 80 Ah daily load.  The two separate banks will be cycling through 20% of capacity every other day, resulting in a total life expectancy of 1,600 days.  Doubling the size of the battery banks in relation to the load has produced a 210% increase in life expectancy. The single 800 Ah bank will be cycling through 10% of capacity every day, resulting in a life expectancy of 2,000 days - a 25% increase in life expectancy over the two (400 Ah) banks, and a 250% increase in life expectancy over the single 400 Ah battery bank!

There are two immediate conclusions to be drawn from these figures:

1.  For a given total battery capacity, wiring the (house) batteries into a single high capacity bank, rather than having them divided into two alternating banks, will result in a longer overall life expectancy for the batteries.

2.  All other things being equal, any increase in the overall capacity of a battery bank will produce a disproportionate increase in its life expectancy (through reducing the depth of discharge at each cycle).

FOR BATTERY LONGEVITY, A SINGLE LARGE (HOUSE) BANK, THE LARGER THE BETTER, IS PREFERABLE TO DIVIDED (HOUSE) BANKS.

[Stu Jackson]

The stereo memory on our boat draws 0.002A or .048Ah per day, or 0.336Ah per week. It takes a full THREE WEEKS to use just 1Ah.

Considering most C34's have a house bank exceeding 200Ah's you'd be using just 0.168% of a 200 Ah bank per week. How many of us go longer than a week between uses or charging??

Even at three weeks, of no use & no charging, though one might question why you own a boat with that much vacancy of use, you use just 0.5% of your Ah capacity of a 200Ah bank for the stereo memory..

I have seen them as high as 0.007A but that is a pretty high one..

Thanks, Maine Sail.Some folks may benefit from the "math."  Some will choose to ignore it. Darn.

[Indian Falls]

Memory drain on any battery is like draining lake Ontario with a straw. 

Why has no one done the "math" on a 100w watt stereo, cd/player in operation?

[Stu Jackson]

Dan, if you have, we'd be interested.

Our Sony maybe 50W system with four speakers and apowered subwoofer pulls out all of 2 amps when running.

This includes the powered subwoofer!!!

The bigger the load, all the more reason to have a larger, non-dedicated house bank to power it.

Your boat, your truck, your choice.

[scotty]

So, if I understand it correctly, bigger bank, less discharge, longer life.  Right?  Then, if I keep my battery switch on "both", I'm running off both banks, which should get me that advantage?  Is that correct?  Thanks.

[Stu Jackson]

So, if I understand it correctly, bigger bank, less discharge, longer life.  Right?  Then, if I keep my battery switch on "both", I'm running off both banks, which should get me that advantage?  Is that correct?  Thanks.

Nope.  The Calder discussion was originally to provide the explanation of why it was better to have a larger house bank and a small reserve bank than it was to have two separate, but equal, house banks.  It was written in the early 90s, when two separate equal house banks were still the norm in many boats.  It was copied and posted 'cuz the concept applies to "dedicated" batteries as well.

What seems to work best is a large-as-you-can-make-it house bank with a small reserve bank.

Using B on the switch should essentially never happen, because you want to hold the reserve bank in, uhm, well, reserve.  Only reason we see for using B is for when your echo charger breaks and you want to charge your reserve bank, and should ONLY be used when charging.  All the rest of the time, use the house bank.

[scotty]

Thanks for the answer, Stu.  Luckily there is a wiring diagram for my boat made when some re-wiring work was done.  I'll get it out (I'll make a copy and keep it at home) and look at what I've got - and check it with the actual wires.  I've spent a bit more time on the electricity 101 wiki, but I'll have to think a while before I can figure out how the Calder discussion applies to me.  When I have a sense of what's going on (and believe me, I'll need some help!) I'll start a new thread so that I don't hijack this one.  I know that I have two banks of house batteries (each of which has two 6 volt batteries in series), and one (12 volt) dedicated starter battery.  Any suggestions where to start looking?  Thanks,  Scotty

[Stu Jackson]

Thanks for the answer, Stu.  ... I know that I have two banks of house batteries (each of which has two 6 volt batteries in series), and one (12 volt) dedicated starter battery.  Any suggestions where to start looking? 

Scotty, you're welcome.  Where to start would be to post your wiring diagram and also, for starters, tell us how many switches you have.  From what you've said, you could either have two equal house banks and a separate reserve bank OR one large house bank and a separate reserve bank.  It could well be that the PO had two separate house banks of (2) 6V banks, and then added the separate reserve.  It will be fun helping you to figure out what you have.  Let's give it a go.