Optimal Size of Starting Battery

[Lars_L]

I want to have a battery dedicated for start of the engine. My motor is a Yanmar YSB12, a one cylinder diesel engine on 12 hp. Today I have a slow blow fuse of 125 A to the start motor and it holds. But the starter can take 250 A, that I don’t now. I have heard from persons how have tested, that on 1 Ah I can do many try to start the engine. I cont that a start takes 0.5 Ah.

Of this I come to the conclusion that the battery should have:- A capacity of 2 Ah. Then I have some margin.
- Bee able to deliver 300 A MCA.
I believe that it must be an AGM battery.

To this battery I have a charger that takes its energy from the big house battery (10.5 to 15 V). It shall be able to deliver 1 A, so the charging would take less than 0.5 hour. Even if the house battery should be flat, I think it will be able to deliver 1 Ah.

Is there anyone who knows if there exists anything that is near my desires?

[GordMay]

Greetings and welcome aboard the CF, Lars.

The Yanmar YSB Operation Manual (pg 14) requires a minimum 70 AH Battery.
➥ http://sailboatpower.com/files/yanma...ion.manual.pdf

More info’ ➥ Blank
http : //sailboatpower.com/EngineManuals.html

[MarkSF]

Clearly your smaller engine doesn't need many AH to start it. However you're going to find it easiest just to get a common starting battery size, which will have the lowest cost, and a large reserve of capacity, which is not a bad thing. As the manual says 72AH minimum, suggest you get one around that size.

[djmarchand]

Lars:

A Group 24 starting battery, the smallest of the marine engine starting batteries, will start your engine fine. Marine cranking amps, not amphours is the important specification. A Group 24 typically has a spec of 660 MCAs and an amphour rating of 79AHs

It doesn't have to be an AGM. It can be a flooded cell, AGM or gel cell battery.

A typical start uses a negligible amphour amount and will be quickly replaced by any charging source- the engine alternator or a shore power based charger.

I would not put a fuse in between the starting battery and the starter. It is an unnecessary complication, might blow at an inopportune time and is not required by the American ABYC standards. If you must use a fuse, use a big one- 300 amps or so.

David

[saben]

I had 4 group 27 wet cells, one was the dedicated start battery. I realized I could increase the house bank by 25% if I could shoe horn in a small AGM somewhere for the start battery. I'm not much a believer in AGM house banks, but for start batteries they have all the qualifiers. High discharge rate, low maintenance and most importantly they live in a fully charged state 99.9% of the time which will insure they enjoy a long life. The engine, 4 cylinder, 38 hp Kubota, I installed a Lifeline AGM 10"x5"x7" to satisfy the CCA requirements of the starter. I felt it unnecessary to oversize at all, because if it fails just turn the switch to house bank. I used the digital duo charge to allow mixing of different bank types although I believe the charge parameters for wet cells and AGM's are so similar one could safely use the standard isolating relay instead. As for fusing the start battery? Why not. I used the Blue Seas thermal breaker. Ya never now when the starter may decide to short out or the resident mechanic on board is a bit clumsy with that big conducting wrench.

[Lars_L]

I have also read in the manual that the engine shall have a 70 Ah battery. But the engine (and manual) was made 1978 and at that time there didn’t exist any AGM battery. I believe that the manufacturer say 70 Ah because it is much easier to say that than invent the term MCA and say it should have 200 A (my guess). As djmarchand say, I believe that MCA is the important specification.

I am in the same situation as saben say, I have a limited amount of space. I have a small 30 feet boat. To day I have four Trojan T-105 and whit that house battery capacity of 450 Ah, so the engine see a battery that is big enough. There is space to put T-145 and then get 520 Ah. I have no separate start battery today, and even if it would be cheapest to put a Group 24 starting battery, I do not have the space for it.

Even if not required, the alternatives to a fuse when I have bad luck, is a fire in the cabels, and I prefer a burnd fuse.

[barnakiel]

Exactly:

Is there a way to tell how much CCA we need for a particular starter motor? Given we know the power of the starter motor?

barnie

[Lars_L]

As I said before, my engine gives 12 hp on one cylinder. That is quite hard for the start motor on a diesel engine. I have enclosed a performance diagram for my start motor. In the diagram you can see that the max output power is 820 W and the motor use a current of 190 A. But the battery that have been used gives only 8.5 V. That will be 1600 W taken from the battery. This motor is rated for 1.0 kW. Logically if I use a battery with better performance, either bigger or an AGM, I will get a higher voltage and thereby higher current and power.

Normally is a MCA value 20 – 25% higher than a CCA value, this because of that the battery is more efficient at a higher temperature.

An electrical motor takes as much current that it needs to get the engine to rotate. The torque is direct related to the current. If the motor is designed wrong, if will burn. In the performance diagram you can see that the maximum torque is 0.93 @ 370 A. But at that torque do the motor not rotate and thereby the output power 0 W. The input power is still 2.0 kW, so pretty soon it will be hot.

If you have a bigger engine, they do seldom take out more than 12 hp per cylinder, and that is what dimensions the start motor. In practical use have my 125 A slow blow fuse last for my 1.0 kW motor.

[Rick]

The short answer is to choose a battery having a CCA rating equal to or greater than the stall current rating of the starter motor.

Amp-hour rating is only indirectly relevant: The two ratings of interest when choosing a start battery is CCA and the internal resistance (often not found on the data sheet). In general, a good start battery will have approximately 10 milli-Ohms internal resistance per 100 Amp-hour rating. A 200 Amp-hour battery will have a resistance close to 5 milli-Ohms, etc.

The stall current is close to twice the peak power current rating of the motor. Without complete information on a particular start motor assume that the peak power is at, or slightly above 8Volts. One needs to know the peak power rating.

Consider using AGM or gel-cell batteries rated to operate at the alternator regulator voltage setting (around 14V for internally regulated engine alternators). This is because these batteries have a higher equivalent specific gravity than do flooded-cell constructed start batteries and, therefore, have higher operating voltages for all currents. Higher voltage (even 100mV) translate to lower start motor current and faster rotation (what is needed to start dieseling at cold temperatures). This starts the engine faster with less stress on the start motor.
Regards,
Rick

[Lars_L]

I found a battery Shuriken SK-BT20 at Sonic Electronix. They claim the battery to have a cranking amps of 850. The capacity is 20 Ah. It’s a little big but near my desires. I couldn’t find Shuriken homepage. Have anyone tried this battery as a start battery for a diesel engine? At a price $59.99 it could be worth to test, it’s only a pity with the high shipping cost to Sweden ($80) (plus our VAT 25%). If feels wrong to buy lead battery on the other side of the Atlantic.

Many years ago I heard about NiCa battery with sintered cells and that they should have extreme performance. Something that a battery of 2 Ah hade blown a 70 A fuse. If I have a charger just for the start battery, the battery could be of any type. Its good if it gives a voltage around 12 V but even 14 – 15 V would work.

[daddle]

An interesting datapoint: My Kubota 3 cylinder 20hp diesel tractor has a tiny 400CCA starting battery. More like a motorcycle battery than a small auto battery. Smaller than a six-pack. Starts every time. Even after months of sitting. I think boaters maybe over do it.

[Rick]

Lars, check out Optima and Fullriver AGM batteries.

[Pete7]

Originally Posted by Lars_L

I have also read in the manual that the engine shall have a 70 Ah battery. But the engine (and manual) was made 1978 and at that time there didn’t exist any AGM battery.

Lars, I think you are reading too deep into this. It's a single cylinder 12 hp diesel, anything will do as a starter battery. The manual gives a figure of 70AH, well batteries are still sold with these labels:

Halfords

http://www.barden-ukshop.com/powerma...ange-105-c.asp

Originally Posted by Lars_L

I am in the same situation as saben say, I have a limited amount of space. I have a small 30 feet boat. To day I have four Trojan T-105 and whit that house battery capacity of 450 Ah, so the engine see a battery that is big enough. There is space to put T-145 and then get 520 Ah.

That's a huge house bank for a 30 foot yacht, why? and are you really thinking of increasing the house bank size?

Perhaps just stick a normal car engine starting battery in for the engine and leave the house alone. Nor would I suggest an AGM if your house bank are ordinary flooded lead acid batteries (FLA).

I have an AGM engine battery but its a pain because I want to charge our house bank quickly at a high voltage and amperage but can't because the AGM starter battery. When it dies it will be replaced with a quality small FLA.

Pete

[Lars_L]

Have you EVER met a sailor with to large batteries?

I understand the problem with different type of battery’s and to get the house bank quickly charged. That’s way I want all charging capacity to go to the house bank. Then I have an little 1 A charger that takes its energy from the house bank and charge the motor battery. If the motor battery should be of another type doesn’t mater, it’s just a question about to get a charger with the right characteristic.

[Pete7]

Lars, how are you going to charge this increased house bank, not with the alternator on the little diesel surely?

There is another thread on here within the last week discussing cooking an alternator trying to charge a large bank, might be worth reading.

Still not sure why you need 450 AH on a 30 foot yacht, let alone 520 AH. Do you have a really good charging system to meet your daily needs? say solar/wind/genny etc. I don't think you could run a large alternator off the little diesel.

Pete