Optimal Size of Starting Battery

[Lars_L]

That is a good question if the charging capacity is enough. I have shifted up the alternator on the engine, so count that it could deliver 30 A @ cursing speed. I have two 60 W solar panels and I have a Super Wind wind generator.

I try to hold down my daily needs and thereby get a low duty cycle on the batteries (and a long life). The biggest user is the freezer, which I have not tested in hot climate yet. The manufacturer claims that it shall use less than 2 A in average when its hot. On second place comes the laptop with an average of 1.5 A. The wind vane do the steering so I do not have any on the third place.

[daddle]

Originally Posted by Lars_L

Have you EVER met a sailor with to large batteries?

Yes. My foolish Previous Owner installed more battery capacity that could ever be reasonably charged. So it never was. So I tossed more than half of it out. Now it's all happy.

Batteries do not create energy. They store and waste energy.

[Rick]

You can use a battery combiner to automatically disconnect the start battery when voltage exceeds 14 Volts and automatically connect when above a chosen value such as float voltage.

One of the best combiners is made by Magnum, they call it the SBC, smart battery combiner. Can set what you want.

Operating a start battery AGM on float voltage for flooded-cells is not optimal yet will work if you allow a time of charge periodically above 13.3. They "like" 13.7V to 14.0 as long as they are kept charged. When full the AGM will not charge accept any significant current even above 14 V. Consider that.

[Cheechako]

Whatever small starting battery is on sale this week, or that will fit in the space you have for it!

[Lars_L]

A battery combiner don’t sound good. If I understand it right, the start battery only sometimes get full charged and mostly of the time the battery is only partly charged. Since a start of the engine only need ¼ Ah I don’t think there will be any problem to start but what due the battery say? How long will it survive?

I was thinking to have a charger of DC-DC converter type. This takes its energy from the house battery as long there is any. IOW as long the house battery is over 10.5 V. This way the start battery is always fully charged and will live forever. This charger is very small since it only need to have a max charging capacity of 1 A. And it will have done his job in 15 minutes after the engine have started.

[daddle]

I more of a hands-on kind of sailor. I have a small dedicated start battery. If I think it needs charging I just flip the battery switch to both for a while. Which I rarely do since a start takes so little and I do it so rarely.

Saves all the expensive, heavy, gimmick gear...and it's eventual maintenance.

If you can't handle that simple task....um...should you be out there?

[Lars_L]

That’s one of the big risks with the simple system with a 1 – 2 – 0 – Both switch. If the house battery is, maybe not empty, but low, then when you puts the switch in the position Both, you will discharge the start battery to the level of the house battery. If you then fully charge the whole package of battery everything is ok. But if the house battery is big, witch it usually is, it will tale a long time to get a full charge. Since the start battery is much bigger then it’s needed to be, it will manage to start the engine. I want to go down in size of the start battery (I have not room for a big battery) and then I will have problem with this simple system. And further it doesn’t work if I have different type of batteries.

What I advocate is an “advanced” charger that charge the start battery. It’s simple to connect with its tree thin cables (they only have to manage 1 A). The start battery is always fully charged, independent of type. And there is not any switch that’s easily forgotten. My problem is, do such charger exists? Or shall I extend my list with tings to do and build it myself.

Imagine a battery that has a live like this. Every time the engine is started, the battery will be discharged by ¼ Ah. Twenty minutes after start, the battery will be fully charged. The engine is during high season started two times a day. Of season it will always be fully charged. I would not be surprised if the battery would be ten years old before it withdraws.

[jkleins]

Originally Posted by Lars_L

What I advocate is an “advanced” charger that charge the start battery. It’s simple to connect with its tree thin cables (they only have to manage 1 A). The start battery is always fully charged, independent of type. And there is not any switch that’s easily forgotten. My problem is, do such charger exists? Or shall I extend my list with tings to do and build it myself.

Imagine a battery that has a live like this. Every time the engine is started, the battery will be discharged by ¼ Ah. Twenty minutes after start, the battery will be fully charged. The engine is during high season started two times a day. Of season it will always be fully charged. I would not be surprised if the battery would be ten years old before it withdraws.

What you are describing sounds like an Echo-Charge. It only kicks in when the voltage on the house is high enough to indicate charging and works separately from the regulator or house charger. Mine works as advertised.

Jim

[Lars_L]

I have read on Xantrex home page about Echo-Charge. But as you say it start charging the start battery when it feels that the house battery get charge. I want the start battery to be charged independent if there is any charge to the house bank or not. This way the start battery will be fully charged even over the winter. Another drawback is that it’s need to be the same type of battery in booth banks.

[hellosailor]

Lars, i think that by researching only the numbers, you may be suffering from tunnel vision. There are more considerations than you are looking at, such as what repeated use will do to battery longevity.

Yes, I have used a 17AH AGM battery to start a Volvo MD7A, which is similar to your engine size. It fired right up and turned right over, but I would never use that battery for that purpose regularly. The current being pulled by the starter would eventually damage the plates in the battery. As an emergency start or to get the last pound out of a racer, sure. But for a 1kW satarter motor, in the US we would probably specifcy a "Group24" SLI battery for that engine. Same battery that is used for compact cars, which typically have a 1kW or 1.5kW starter motor.

The larger battery takes less damage from each starter use, and provides a more reasonable lifetime.

You CAN use the house deep cycle batteries for starting as well, once their combined capacity sifficiently exceeds the draw from the starter, it makes no difference that they are deep cycle. But all in all, if you want a dedicated starter battery (what we call an SLI Starter-Lights-Igntion battery) your best bet is just to go to an auto supply store and buy whatever they sell for a sub-sompact or compact car. The load is about the same, the non-marine price will be significantly lower.

[Lars_L]

Yes I know I’m quite fixed to the numbers. That is little of the reason I started this thread, to see what the theoretical limit is for a small start battery. The risk with a test on the net is negligible. In real life it probably ends up with that I buy a standard battery in the nearest auto supply store. (And build my own charger for it.)