twin battery setup advise-keeping starting battery isolated.

[Mongoose]

Hi i've searched and googled, but can't seem to find out once and for all.
My rib has one battery, doing the electronics and starting. I want to fit 2 batteries and a reliable switching setup that will charge both batteries, but not let me discharge the starting battery when anchored up using electronics\radio etc.
What does everybody recommend??
Thanks

[GuyC]

https://www.force4.co.uk/blue-sea-ad...tery-120a.html

Other chandlers are available

[Mac50]

Hi

You need (something like) this. Each circuit is separate and can be on/off but they can be connected in parallel via the yellow if you manage to run one down.

Incidentally, you can’t quite see a solar panel regulator at the top of the picture. It is connected directly to the house battery so the bilge pump (she’s on a swinging mooring for 6 months) doesn’t run it flat.

Iain

[Pikey Dave]

Originally Posted by GuyC

https://www.force4.co.uk/blue-sea-ad...tery-120a.html

Other chandlers are available

Yup that’s what I have, works perfectly [emoji106] I think mine was more expensive though[emoji6]

[Mongoose]

Thanks.
Just a quick question-im trying to keep things simple!
With the normal manual battery switch- batteries off, battery 1 on or battery 2 on or both on-if you put the switch into 'both on' once engine is started, would that charge both? or is that over simplifying things?

[jwalker]

Originally Posted by Mongoose

What does everybody recommend??
Thanks

Simplicity is the essence IMHO. Two batteries, one main for normal starting and usage and one set aside but always charged along with the main one.

You need a simple switch in a positive cable linking the two batteries positive terminals. That switch will normally be kept open so there is no connection, but you need to wire a diode across the switch poles so that charging current can bypass the switch in one direction to keep that backup battery charged.

A normal diode will have a voltage drop across it of approximately 0.7 volts. Since your charge voltage will be about 14.2 volts that leaves only 13.5 volts for charging which is ok but marginal. Using a Schottky diode, you'll only drop approximately 0.3 volts, (Approximately because diodes, even of the same type, vary.) So you'll have about 13.9 volts for charging which will be fine. Choose a diode with a current rating in excess of you're charging current.

The switch you use can be of your choice but I keep it very simple and use a battery terminal with a screw down contact. Flat battery - tighten the handwheel, start the engine then slacken the handwheel. The little power used will quickly be topped up and all the rest of the charge will go to the flat main battery to recover it.

This topic has been covered a number of times with various solutions, various complexity and various - read crazy - costs.

There a pictures of the above solution somewhere in the archives back around 2004-2005ish, I'll see if I can search them out for you. Still functioning without issue.

Edit: I've given you a slight bumsteer, the circuit I use has the diode in the negative rather that the positive cable. It doesn't matter which cable breaks the battery circuit, it'll work either way but I remember it was easier to source a screw type negative battery terminal so I went with that.

The thread I mentioned doesn't appear to be available any longer.

[Last Tango]

Originally Posted by jwalker

Simplicity is the essence IMHO. Two batteries, one main for normal starting and usage and one set aside but always charged along with the main one.

You need a simple switch in a positive cable linking the two batteries positive terminals. That switch will normally be kept open so there is no connection, but you need to wire a diode across the switch poles so that charging current can bypass the switch in one direction to keep that backup battery charged.

A normal diode will have a voltage drop across it of approximately 0.7 volts. Since your charge voltage will be about 14.2 volts that leaves only 13.5 volts for charging which is ok but marginal. Using a Schottky diode, you'll only drop approximately 0.3 volts, (Approximately because diodes, even of the same type, vary.) So you'll have about 13.9 volts for charging which will be fine. Choose a diode with a current rating in excess of you're charging current.

.

+1.
Exactly the setup I use. When it's sitting in the drive on trickle charge I leave the switch "made" and switch it off when I disconnect the charger to trail.
PS only dropping 0.5V across simple high current rectifier diode.

[Pikey Dave]

Originally Posted by Pikey Dave

Yup that’s what I have, works perfectly [emoji106] I think mine was more expensive though[emoji6]

I’m wrong! I used the “mini” version, so no need to blow the day’s pension

https://www.force4.co.uk/add-a-battery.html