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Old 03-01-2015, 14:29   #16
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Re: Need dedicated starting battery?

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Originally Posted by nimblemotors View Post
No experience here, but getting your engine to start seems like a pretty darn important thing.
Yes, it is. My experience with this is that in the 11yrs we have owned our current boat, we have never run our house batteries so low that they couldn't be used to start the engines, and we have had two starting batteries reach the point that they could not start the engines. In those two cases, we had parallel switches that brought the house batteries into play so they started the engine.

Our experience is starting battery 0, house batteries 2.

My point is to put this in perspective. Starting batteries usually just die unexpectedly because they are not being used and monitored (and they are often low-quality, although this is not the case here). House batteries are typically in constant use and usually monitored, so you know when they are going bad.

It would be highly unlikely that one needed to start their engines quickly and surprisingly found their house bank had completely bit the dust. On the other hand, this is precisely how most people find their starting batteries have gone bad.

Suddenly losing one's house bank is just a low probability occurrence. So low as to be obsessive to worry about it unless one's house bank is very small and in poor shape and under relatively large loads.

The probability of losing an expensive house bank because it is too small and cycled too low and too often is much more of a concern and higher probability than not being able to start the engine because the house bank is dead.

This is game the OP is playing by deciding where to put his third AGM.

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Old 03-01-2015, 21:01   #17
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Re: Need dedicated starting battery?

Thanks everyone for the very helpful comments. Currently my monitor doesn't easily tell me status of starting battery so I agree with comments about lower probability of losing house bank than starting battery. With a 44 hp yanmar 4jhe don't need huge cranking amps but I imagine more than my genset battery which only cranks a 9 hp diesel. I feel like I'd know if my house bank went too low to crank my diesel by other things not working or my low battery alarm. I only have a 40 amp charger but that could be enough to get over the hump for a low house bank?
Given demands of refrig and electronics it seems higher yield to combine the batteries into larger house bank which should make charging more efficient.

What about any effect of voltage drop on house bank when cranking the engine? Can't be more than running AC load off of inverter (like microwave) right? Would this be a problem for electronics with this large of a house bank?

Another option I am hearing hinted at which sounds great is to upsize my genset starting battery and use it for both engines. Anyone else do that?

Cheers!
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Old 03-01-2015, 23:43   #18
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Re: Need dedicated starting battery?

Quote:
Another option I am hearing hinted at which sounds great is to upsize my genset starting battery and use it for both engines. Anyone else do that?
Mike, this does indeed sound like a useful dodge for you. And yes, IME starting from the house battery bank will cause a short term voltage drop (and possibly an inductive spike) that can cause various things to drop out or re-boot, things like GPS receivers which you might want to stay on line!

We have always used a separate start battery with the option of switching to the house bank if required, and this scheme has worked well for many years.

Cheers,

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Old 04-01-2015, 00:07   #19
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Re: Need dedicated starting battery?

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Originally Posted by Jim Cate View Post
Mike, this does indeed sound like a useful dodge for you. And yes, IME starting from the house battery bank will cause a short term voltage drop (and possibly an inductive spike) that can cause various things to drop out or re-boot, things like GPS receivers which you might want to stay on line!

We have always used a separate start battery with the option of switching to the house bank if required, and this scheme has worked well for many years.

Cheers,

Jim
+1 I've had to demonstrate this to a lot of owners of vessels which I; cared for, delivered, & or raced, in order to get them to cough up the coin so that I could put a proper system in place.
Plus, there are some electronics which can be permanently damaged/destroyed by such power spikes, & or drops. That, or you might simply, "only" lose your stored GPS waypoints, including your favorite secret spots.
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Old 04-01-2015, 01:03   #20
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Re: Need dedicated starting battery?

Sure you can, but with the possibility of draining your batteries so far you might not be able to start the engine(s).


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Old 04-01-2015, 09:03   #21
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Re: Need dedicated starting battery?

As it is now, things are okay. Dedicated starting battery with selector switch for starting battery or combined with house bank, or just starting battery.

I just feel an entire 105ah Agm is a waste for that and we'd be better using the battery with a bigger house bank that I could maintain.
Owned and cruised boats for 30 years and never had a house bank die. We haven't cruised for more than a couple of months so I imagine if plans go that way it's a higher risk?


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Old 04-01-2015, 10:14   #22
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Re: Need dedicated starting battery?

I got round the problem on my last boat by installing a Red Flash starter battery. These are dedicated starter batteries and are tiny. The one I used had a CCA (Cold Cranking Amps) of 750amps and was 6.5"x7"x5" and I put it behind the wet locker. They are a Gel battery so no worries about spilling.

A typical small marine diesel draws about 150amps on starting so bags of power to start. It was only 26ah but that is plenty unless you have an engine which is a dog to start. If it starts in 20seconds, which a decent engine should , that amounts to <1ah.
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Old 04-01-2015, 11:28   #23
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Re: Need dedicated starting battery?

IMHO you need two battery systems; one for starting (exclusively) and the other(s) for the house systems. You can never have enough amp hrs. in the house system and this will be your salvation if your starting battery fails. If you don't have "space" - make space. Also, I would install a Victron or Xantrex battery monitoring system to monitor the charging and storage capacity of both starting and house systems.

Why am I so dogmatic on this? Well, years ago I've owned boats that depended on a single system. I got to be an expert on mooring and docking a 48' Marconi schooner and later a Cheoy Lee OS 36 under sail when there was not enough juice in the sole battery to start the diesel after a cruise!
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Old 04-01-2015, 11:56   #24
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Re: Need dedicated starting battery?

No long winded explanation. Have an isolated starting battery.
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Old 04-01-2015, 12:02   #25
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Re: Need dedicated starting battery?

What you need is a way to start the engine when the house batts can't. Like a way to use the genset battery. Or enough of a charger that it'll start off the genset itself (My motorhome will do that) - I like what colemj was saying, seems the most efficient use of the batteries.

Personally, I do NOT have a dedicated starting battery on my boat. Big house bank for everything. But I do have a tiny 'reserve' battery for if the house batts are dead. Gives me max use of the house. (I also have an A4, which I think I could start with a drill in a pinch, it doesn't take much juice. I also have a costal cruiser that if it doesn't start I call the tow.... Needs may be different in the way remote places.)
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Old 04-01-2015, 12:07   #26
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Re: Need dedicated starting battery?

Quote:
Originally Posted by kellyp08 View Post
when there was not enough juice in the sole battery to start the diesel after a cruise!
The OP does not have a sole battery - he will have 315Ahr of AGM in three batteries, if he decides to combine the one used for starting to his house bank.

If a cruise leaves a house bank dead, then the problem is the capacity of the house bank, not the lack of a dedicated starting battery.

For those advocating a small gel or Li battery (not a bad idea), how do you charge these? Particularly Li, which seems like it would be a bad mix.

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Old 04-01-2015, 12:15   #27
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Re: Need dedicated starting battery?

In my opinion it is better to have a separate start battery. But, as already advised above, you already have a separate start battery (for the genset) and it is somewhat redundant to keep 2 start batteries when you are short of amperes in the house bank. However, there is a different consideration that I did not see addressed. Is the current engine start battery exactly identical to the house bank batteries? Not only in amp/h capacity but also in other characteristics? Is it a deep cycle battery? Are they of the same age and condition? If not, it might be a bad idea to connect them to one bank.
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Old 04-01-2015, 12:34   #28
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Re: Need dedicated starting battery?

I would simply switch round so that the genset starts from the house bank and the main engine from a dedicated battery. You never need an 'emergency' start on the genset but may do for main engine!
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Old 04-01-2015, 13:03   #29
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Re: Need dedicated starting battery?

By all means upgrade the house bank. The generator and engine could share a properly sized start battery.
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Old 04-01-2015, 14:48   #30
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Re: Need dedicated starting battery?

Why not consider what 4 wheelers do and use a programmable automatic voltage relay? This gadget can be used to isolate a battery from the house bank when voltage drops to a preset level; The idea being that the start battery contributes to the house but disconnects before draining too low.

On our boat, I've just added a third 125 ah battery. My first idea was to use an AVR as above and use it as a start battery, however my battery supplier suggested I just add it to the existing housebank instead and provided some valid reasons. I now have it connected to a 0-1-2-1+2 switch as battery 2 (with the two existing 120 ah batteries becoming bank 1) and will use the batteries mostly on the switch "1+2" setting. I've installed a battery monitor for battery 1 bank and am so pleased with the monitoring information produced that I intend to install a monitor for battery 2 also. With this setup, I think it will be extremely difficult to run all batteries down to the point where they won't start the engine.

And, being a little paranoid, I also carry an automotive jump starter kit that has sufficient capacity to start the engine whenever we head out into remote areas as a backup.

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