Dedicated Starting Battery

[Stephen Butler]

We have had our C34 for 7 years and have often thought of adding a dedicated starting battery to our circuit.  However, we have 4 x 6v batteries, which are monitored closely, as well as a Honda 1000 genset for emergencies, and have never come close to being unable to start the engine from the house bank.  So, knowing that I am asking a near heretical question, why do we need a dedicated starting battery?

[mainesail]

We have had our C34 for 7 years and have often thought of adding a dedicated starting battery to our circuit.  However, we have 4 x 6v batteries, which are monitored closely, as well as a Honda 1000 genset for emergencies, and have never come close to being unable to start the engine from the house bank.  So, knowing that I am asking a near heretical question, why do we need a dedicated starting battery?

Dedicated as in direct hard wired or a reserve battery that can be "selected" if you were to kill the house bank?

Never a bad idea to have a reserve battery especially if you don't have a well calibrated battery monitor. You don't "need" to do anything but what would happen if you did kill your bank can you sail back and into your slip? Do you have SeaTow?

[Kyle Ewing]

A dedicated starting battery adds additional redundancy.  Are your 4 6v batteries in one bank or two?  If one, it's possible that a failure of one of the four could affect the entire bank.  If you have two banks and a battery goes bad, then you can switch to the second bank.

Depending on how you use your boat is a consideration.  If you go long periods without running your engine (anchoring for days or sailing long distances) the risk of completely discharging the batteries becomes higher, especially as the batteries age.

Your propensity for risk (worry factor) is also a factor.  Would you feel better knowing you have a dedicated start battery without risk of additional draw?  A stuck bilge pump switch or leak in the water system could cause the pump to run continuously and unexpectedly run down your batteries, especially if older.  The battery monitor can mitigate this, but it won't necessary catch a failure.

Worst case:  You're entering a harbor under sail or in another situation that you need to start the engine to avoid a serious problem.  If the primary battery is too weak to start the engine, what are your alternatives?

[Roc]

Here's what I did...I also have (4) 6V golf cart batteries.  I used to think I needed a starting battery and wire it up in such a way to start the engine with it and call it THE STARTING BATTERY and I would have the house bank separate. Also, I would need a way to charge both using the alternator, echo charger (or similar device) and battery charger, etc.  When a friend (C350) had his boat wired up like this since new, it made sense to me in its simplicity.  Also, knowing that when he cruises, he NEVER goes to a marina and anchors out for a week or more at a time.  This is the set up.... I combined the golf carts into one house bank and they are connected to Batt 1 on the selector switch.  I installed a "reserve" battery and tied that to Batt 2 on the selector switch.  I highlight the word "reserve" because in effect, that's what it is. It's just there in an emergency, to either start the engine or if needed run the 12v house power.  The alternator wire stays the way it came from the factory.  I keep the selector switch always on Batt 1 to start the engine and for all the 12v power.  To charge the 'Reserve' battery, I bought a Xantrex True Charge 10 amp charger (a little over $100 from Defender) and hooked it up soley to the Reseve battery.  Now, at first, I thought the Reserve battery needed to be always topped off when motoring.  But on a cruise, after a few days if needed, you might go into a marina and plug in for the night.  However, you might end up going a week or two, never using shore power and I feel that Reserve battery should still have enough power to start the engine.  I realized this because f I left my car in the driveway for a week or two, I still would be able to start the car.  So basically, my boat is wired just like it came from the factory, except I now have a reserve battery tied into selector #2.  It is charged when plugged into shore power with the Xantrex 10 amp.  It's not charged while motoring, based on knowing it was charged via shore power before heading out.  By the way, I do have a 100amp alternator. 

[Ken Juul]

The PO had a wierd set up, 3 -12v on Bat 1 and 2-12 on Bat2.  Normally I would even/odd the banks to try to get the most out of them.  As they got old, even when I selected Both, when starting the engine the electronics/AP would drop off line.  That is the main reason I chose to go with seperate house and starting batteries.  Wiring diagram was recently posted in the other thread will be changed based on advice recieved.

[waterdog]

I have four T105s and an Optima battery as a reserve.  The alternator feeds the house bank directlty.  A paralleling relay designed to detect when the batteries are charging and connect in the reserve battery remains in the electrical bag never installed.   I discovered i dont really need it.  A good battery monitor lets me keep an eye on the voltage.  The shore charger keeps it topped at the dock.  The occasional switch "both" keeps the reserve topped up on extended cruise.   Pretty simple.  Reasonably bullet proof.  I would spend money on a battery monitor before relays or echo chargers.

[mainesail]

I have four T105s and an Optima battery as a reserve.  The alternator feeds the house bank directlty.  A paralleling relay designed to detect when the batteries are charging and connect in the reserve battery remains in the electrical bag never installed.   I discovered i dont really need it.  A good battery monitor lets me keep an eye on the voltage.  The shore charger keeps it topped at the dock.  The occasional switch "both" keeps the reserve topped up on extended cruise.   Pretty simple.  Reasonably bullet proof.  I would spend money on a battery monitor before relays or echo chargers.

Steve,
It's not the best practice to mix battery types with a voltage sensitive relay. You can do that with an Echo but it's not generally advised to combine AGM/Wet. If what you have is working I would just stick with it.

Yandina has an interesting work around: http://yandina.com/mixedbattery.htm

[waterdog]

Mainsail.   I understood the reason you shouldn't mix AGM and wet cell batteries was that the AGMs have lower charging voltage and you either end up under-charging the wet cell if you set up for AGM or overcharging the AGM if you set up for the wet cell.   Is that correct?   

In any case, when I designed the system I selected the Optima because it was tolerant of the higher charging voltage.  Should I be suspicious of their specification and treat it more gently?    Is there any other reason not to mix types when charging?   

Yandina's work around might be OK for wet house / AGM reserve as you describe.     I wouldn't touch it in the application they describe!   If I had spent all the money on AGM house batteries, I would want all the benefits and would spend the money on the smart charging regulator and high output alternator and optimize the system around charging the house bank.     

[waterdog]

Back to Steve's original question.    There may not be a compelling reason to add a reserve.   If you do spend three days on the hook drinking margaritas without checking your battery monitor, you can always fire up the Honda.   Do be sure that it can handle the load of the charger when it's running full out into a discharged bank.   (I had a generator that couldn't handle that load once.)   You are not protected against something that goes wrong on the house system - and things do happen - accidental short, you burn up your last fuse, whatever.   It's an unlikely event, but it could happen.   It might be a good idea to have one of those jump start battery packs tucked away in a locker, that way you have a way to get the engine going with the house bank down.   A dedicated reserve gives your alternator a place to output to if you do have to isolate the house bank, but realistically you are probably deep into trouble shooting, rejumpering, finding out what is going to burn your boat down at that stage anyway...

[sailaway]

Steve I have a dedecated starting battery mounted in the copit locker with a red battery switch mounted under the sink in the head. I can start off the house bank or the starting batt. When the engine runs I am charging the starting or house bank. I have 3 deep cycle batt in the house bank and 1 cranking batt for starting the engine. with the short distance from the batt to the engine lots of cranking power. With the  universal engine you need lots of batt for glow plugs. Charlie

[WTunnessen]

Steve

See Jim Moe's article in the Tech Wiki about adding a dedicated starting/reserve battery.

Note - I have updated Jim's original wiring diagram to reflect recommended changes and other best practices.

See:

http://www.c34.org/wiki/index.php?title=Catalina_34_Electrical_System_Upgrade