Battery banks - weird arrangement or not?

[richardm47]

I'm planning to re-do the charging and batteries on my boat this winter, mainly because batteries don't last me very long. First job was to document what I've got. That's when I started finding a few things that look a bit odd to me.

I've got a starter battery, and a domestic battery, by the chart table. So far that's normal. There's also a third 12v battery under one of the forward bunks, which is for the bow-thruster and the windlass, also normal. The slightly weird thing is that this bow battery is paralleled with the domestic battery.

It seems to me that the bow battery needs to be a good cranking battery so as to supply short-term high current. Just like a starter battery. So it would be more logical to parallel it with the starter bat of the same type. Not with the deep-cycle domestic, which would tend to be damaged by the heavy current taken by thruster or windlass.

What do you think? Is paralleling the bow and the domestic weird or normal? Perhaps there's a good reason for doing it that way. Or would you prefer to parallel the bow and the starter together? Or do you think the bow battery would be better as a third bank on its own?

[Side note: the paralleling is done with really massive cables. And I would always have the engine running when using the thruster or the windlass.]

 

[prv]

My understanding is that a deep cycle battery will not be damaged by trying to draw a large current from it, it may simply have a lower limit to what it will supply. With less surface area to the plates (fewer thick ones instead of lots of thin ones) the reaction can't happen as quick.

The whole point of an engine-start battery is that it doesn't get run down and so is always available to start the engine. So paralleling it with anything defeats the object and you might as well just have one big bank (which a handful of people do).

Basically, your existing setup sounds reasonable to me.

My personal preference is to keep things simple and just have large enough cables rather than a bow battery, but with large thrusters I admit this starts to become impractical.

You may not always have the engine running when using the windlass. At the start of this season I had to anchor, and then raise it and re-anchor, with the engine unavailable due to an electrical fault we couldn't diagnose at the time. Without a motor, sails and anchors become your manoeuvring tools (I had the kedge out that night as well) so it's as well to keep them independent.

Pete

 

[Alpha22]

Pete's logic sounds good to me.
I would be tempted to increase your domestic battery capacity by adding another one and then splitting off the thruster/anchor battery with a VSR to isolate that so you are not left unable to raise the anchor.

Most leisure deep cycle batteries are not true deep cycle batteries and will not be damaged by starter type loads. True deep cycle batteries are very specialist and expensive bits of kit. They also need a lot of care and attention to prevent damage, hence the 'myth' that starting your engine from the domestics will damage the battery.