Starting Battery

[Pete O Static]

This question may have been asked already but I can't find anything on the search. If there is already a thread, I would appreciate any link or advice.

Having read Nigel Calder's book and the posts written by Rod Collins, I have decided to update my batteries and charging system. The new house bank will consist of three Oasis Firefly AGM's and the charging system will consist of an externally regulated, temp compensated, 160 amp alternator with a serpentine pulley kit.

All sources tell me that in the absence a dedicated alternator for the start battery, I should have the same battery type for my starter as my house. This way they share the same bulk, absorption and float regime.

My issue is that I feel having an expensive, AGM, deep cycle battery dedicated to start a small 3 cyl diesel is way overkill and a wasted resource.

So my question is, what is wrong with having all charge sources directed to the expensive house bank and having a DC-DC charger top up a smaller much less expensive start battery? Even if the chemistry is different and the float voltage slightly off, it would take a while to kill it and I can justify replacing a smaller cheaper start battery on occasion.

It just seems crazy to me to have a $500 Carbon Battery doing such a menial task.

[Lepke]

That would work for me. One option would be a small solar panel, just for the starting battery. I have a power boat, but carry a small i/o boat that often sits for many months. I have a small solar panel (about $25) on the engine box that keeps the battery up. Even when the bilge pump is draining rain water in the winter.

[john61ct]

Originally Posted by Pete O Static

So my question is, what is wrong with having all charge sources directed to the expensive house bank and having a DC-DC charger top up a smaller much less expensive start battery?

What gave you the idea there's anything wrong with that? It's a perfect configuration. The dc-dc charger can be relatively inexpensive as long as the voltage output matches the needs of the start battery.

If you want to spend a lot of money and get maximum flexibility then the Sterling battery to battery chargers are fully adjustable through the custom profile, so can be used to charge any chemistry you like.

[belizesailor]

You dont necessarily need exactly the same type/brand of battery...just a start batt that will not be damaged by the same charge profile. For example, there is no reason for a start batt to be deep cycle. There are AGM start batts in the automotive industry that would probably work just fine. Just research the charge voltage profiles.

[john61ct]

Whole point of a real DCDC charger is that unlike voltage-following tech like VSRs, it delivers a completely **independent** charge profile,

One bank could be LiFePO4 the other lead, whatever.

Step-up boost or step-down buck conversion is part of what defines the category.

House could be on hi-amp Bulk stage at 14.8V, while Starter is being Floated on 13.4V, or just as easily vice-versa.

[smac999]

Nothing, get a xantrax echo charger. This is exactly what it's for.

[Wotname]

Originally Posted by Pete O Static

This question may have been asked already but I can't find anything on the search. If there is already a thread, I would appreciate any link or advice.

Having read Nigel Calder's book and the posts written by Rod Collins, I have decided to update my batteries and charging system. The new house bank will consist of three Oasis Firefly AGM's and the charging system will consist of an externally regulated, temp compensated, 160 amp alternator with a serpentine pulley kit.

All sources tell me that in the absence a dedicated alternator for the start battery, I should have the same battery type for my starter as my house. This way they share the same bulk, absorption and float regime.

My issue is that I feel having an expensive, AGM, deep cycle battery dedicated to start a small 3 cyl diesel is way overkill and a wasted resource.

So my question is, what is wrong with having all charge sources directed to the expensive house bank and having a DC-DC charger top up a smaller much less expensive start battery? Even if the chemistry is different and the float voltage slightly off, it would take a while to kill it and I can justify replacing a smaller cheaper start battery on occasion.

It just seems crazy to me to have a $500 Carbon Battery doing such a menial task.

Why even have a dedicated start battery, your house battery will easily start your engine even when at 50% SOC. Just a have smallish dedicated "standby" battery that can be switched in to start your engine if your house battery ever gets unintentionally discharged. Keep the "standby" battery topped up with either small dc-dc charger or small solar panel.

JMO.

Start batteries have a place when you have big diesels with heavy current starter motors or small house batteries.

[daletournier]

Originally Posted by Wotname

Why even have a dedicated start battery, your house battery will easily start your engine even when at 50% SOC. Just a have smallish dedicated "standby" battery that can be switched in to start your engine if your house battery ever gets unintentionally discharged. Keep the "standby" battery topped up with either small dc-dc charger or small solar panel.

JMO.

Start batteries have a place when you have big diesels with heavy current starter motors or small house batteries.

This has been discussed a couple of times recently, not conventional wisdom but i believe it has some merit. Cruising has changed due to technology. My own boat, 500w solar, honda generator and lithium jump start battery, yet i still seperate my house and start due to my ingrained conventional thinking, the net effect results into limiting my house bank by a third.
A64 first got me thinking about this.

[john61ct]

Yes, designing a second normally isolated bank as Reserve for House loads is wise, and justifies investing in true deep cycle and higher capacity.

Starter service is so minimal it really doesn't matter which bank feeds it, in fact with a solid high-amp bank switch like BEP or Blue Sea's, cycling between them is a good way to periodically check that all is well.

But otherwise no loads at all in normal usage on Reserve.

--------
Or another design option, make both banks matching keep as one bank at higher SoC to reduce average discharge level.

Fit a load LVD to isolate the Reserve/starter bank when SoC drops, long before there will be any difficulty cranking, only the "House half" continues to carry non-essential loads.

With another lower cutoff to prevent that bank from going below 50% in normal usage, say 30-minute override for when needed for something critical.

These devices are often fitted to emergency vehicles.

One of those little jumping power packs kept charged is a useful belt & suspenders measure.


Traditionalists, especially those spending time far from shore, will continue to argue for a dedicated Starter-only bank.

[Wotname]

Just to be clear, my concept of a "standby battery" is only to start your engine rather than run any other "essential" loads. The premise is that once the engine is started, it can power any other required loads.

The standby battery should be adequate for the engine in question and as it is unused (apart from periodic testing), it should be at 100% SOC and floated by an external source.

The house battery can be sized as big as you need for house (& engine) loads.

Note, this is not suitable for engines that have large demands on starting current. In these cases, a suitable starting battery is required - again IMO

[dhays]

Originally Posted by Wotname

Why even have a dedicated start battery, your house battery will easily start your engine even when at 50% SOC. Just a have smallish dedicated "standby" battery that can be switched in to start your engine if your house battery ever gets unintentionally discharged. Keep the "standby" battery topped up with either small dc-dc charger or small solar panel.

Other than not every actually using it, what you have described seems to be exactly what a designated start battery is. I'm not sure I see the distinction.

[Wotname]

Originally Posted by dhays

Other than not every actually using it, what you have described seems to be exactly what a designated start battery is. I'm not sure I see the distinction.

Well there isn't much difference and what differences that do exist are subtle.
Conceptually a start battery is there to start the engine every time and is installed accordingly where as a standby battery is essentially never used and is not connected to any electrical load though out it's life. Of course it still has to be maintained to ensure it remains at 100% SOC but this is not difficult.

In practice it will be a little smaller than a start battery and it's life should be considerably longer.

I accept there are many situations where it isn't the best solution.

[john61ct]

I use the term Reserve for the standby idea, and have no problem paralleling it as part of House as long as SoC is high.

The key is ensuring Reserve is never depleted so much it would ever have difficulty cranking. And having it in use to support House's lifespan by reducing daily DoD to me helps justify the extra cost and carrying the extra weight

Cranking is such a light load it's barely relevant to longevity, and IMO should be easily switchable to any bank on board.

I guess Standby is good as a term for a battery that is never used at all. But to me that is just as wasteful as a large expensive heavy dedicated Starter battery.

[ErikFinn]

Hi Pete,
I asked something related a few years back and got some good answers, here is the link if you are interested:

http://www.cruisersforum.com/forums/...-a-131466.html

Best regards,
Erik

[Dockhead]

You've gotten good answers and a couple of viable choices!

My own personal opinion is that a diesel engine needs a dedicated start battery which is used for nothing else. A start-type battery with thin plates and high cold cranking amps. You CAN start your engine with your house bank, but to me this is just wrong. In my opinion, ability to start your diesel engine should not depend on the state of your domestic systems. Opinions vary on this, and you'll have to make up your own mind, but that's mine.

There is nothing at all wrong with your idea of having such a thing and charging it with a DC powered battery charger. Sterling make a good one. As someone else said, such a device provides an independent charging profile and eliminates the need to make the start battery the same type and size as the house batteries.

A separate alternator is still much better than that, but can be difficult and/or expensive to arrange. But in your place I would investigate this, at least. The house bank will be much better charged with a large-frame, externally regulated school bus alternator. Adding one of those and leaving the original car-type alternator to deal with the start battery, would dramatically improve your system in a couple of different respects.