Keeping a start battery charged

[valhalla360]

Originally Posted by Happ

Simple answer to why you have a dedicated start battery is the battery type should be different. Your house batteries are typically Deep Cycle. They are for slow drain. A start battery has a high initial output to start the engine but drains quickly. For instance when your car doesn’t start you can see the power in the battery weaken very quickly.

Charging the batteries can be done through Controllers and through shore power battery chargers. Alternators are often enough to keep your start battery charged. Just like in a car.

It's not that they "should be different". It's design demands are different for different use cases. If the deep cycle battery bank has sufficient CCA and cabling to carry it, there is no harm in using deep cycle batteries to start an engine.

In a truck:

A starting battery makes a lot of sense. You only need a lot of amps for a a few seconds and then the alternator is running providing for the long duration relatively low amp vehicle electrical draws.

To use deep cycle batteries in place of a truck starting battery, you would likely need multiple deep cycle batteries to get a bank to get enough CCA to turn over the engine. This would be more expensive to purchase, use up payload and take up space.

In a cruising sailboat boat:

You already have a large bank of multiple deep cycle batteries to accommodate house loads and the engine tends to be smaller with lower CCA needs, so the deep cycle batteries can happily supply the required amps without any negative effects.

To justify a separate starting battery, you need to presume some sort of failure in the house bank.

[a64pilot]

I’ve seen golf carts used to tow aircraft, once it’s out of the hanger, they plug in and start the Garret G10 engine, that draws 1000 amps at 28V (I’ve measured it).

Garrett’s are so difficult to start because they are a single spool turbine, so to start one you have to spin everything including the propellor up to a high enough speed so that you get enough air flow through the engine to crank it without burning it up, so it takes enormous power to crank one.

Deep cycle batteries can easily provide huge amps, but it does take more weight in batteries to do so, but if you have 300 lbs of deep cycle batteries, cranking a baby Diesel is no problem.

[Midas H28]

In my H28 I have a new AGM start battery and a older FLA house battery and manage them in exactly the way described here by Mike. Maintaining battery charge is my responsibility, that's the way I like it and it works just fine.

[Fineline]

Regarding the manual battery switch.
If the manual switch is set to off (possibly accidentally), does that disconnect the alternator from all loads?
If so, it's a good way to fry the alternator diodes.
Learned that the hard way as the saying goes!

[wingssail]

Originally Posted by Fineline

Regarding the manual battery switch.
If the manual switch is set to off (possibly accidentally), does that disconnect the alternator from all loads?
If so, it's a good way to fry the alternator diodes.
Learned that the hard way as the saying goes!

What is different is that you must go all the way with that manual battery switch.

The switch must cut off all connections to the batteries INCLUDING power to the regulator which in turn drives the field of the alternator. When the regulator cuts power to the alternator field there is no current flowing to fry the diodes.

I know it sounds impossible that the whole thing can shut down in an instant, but it does.

[malbert73]

So I am in the camp of single large battery bank and start off of a starter battery. I just had all of this rewired professionally and the electrician recommended the Duocharge from Balmar which I believe achieves the original goal- bleed off a tiny amperage charge into starter battery. Unlike a less sophisticated ACR which combines the batteries.

[Frankly]

Have you considered a small nuclear reactor/ steam system. You could remove the water heater and fit it in that space. Hot water and electricity, what not to like.


Frankly