Voltage Sensitive Relay

[Liam]

Ariel has a very simple setup with regards to electrics... I have a single battery which is used for both engine starting and domestic power.

I do have a second stand-alone battery which I keep charged with the use of jumps leads in the engine room but this is far from ideal!

I'm thinking along the lines of a simple voltage sensing relay to solve the problem as I would like the existing battery for the sole purpose of engine starting with the stand-alone battery connected up so it gets charged automatically with the engine running.

Something along the lines of a Durite 140A 12v Voltage Sensitive Relay (Midland Chandlers) which would be fairly easy to install neatly.

Is the setup shown below an appropriate diagram to follow? If so, what thickness of cable would I need to use for the + and - connections and because I have no idea about electrics, would the existing battery shut-off switch also work for the new domestic battery?

TIA

[Timleech]

You can isolate two separate batteries/banks with one switch if the Negatives are joined and the switch is in the negative, it used to be common practice and is still allowed but not recommended. Much better to have one switch for each battery or bank of batteries, in the positive leads.

 

Size of cable depends largely on size of alternator and length of run, best to use something bigger than is absolutely needed to carry the max charging current, and the longer the run the bigger it needs to be to get a decent charge rate.

 

Tim

[Liam]

Thanks Tim,

 

The current battery is isolated by the negative if I remember rightly... not clambering around the outside of the boat at the mo!

 

It's a 90a alternator on at the moment, and both batteries would be within a foot of each other so not a great distance apart.

[Scholar Gypsy]

On the question about the isolator, that depends where your current isolator is. If it is on the negative side of the battery then that's fine, you just connect all the negative cables to one side of the isolator, and the negative terminals on both batteries to the other side of the isolator.

 

On the relay, the setup shown is designed for use on a vehicle / camper van I think, where the main (engine) battery takes most of the load and the auxiliary runs a few things. On a boat it's usually the other way around (ie the bulk of the charging current needs to go to the domestic battery) so I think it would be slightly better to connect the alternator direct to the domestic battery, and then connect the engine battery to it using the relay.

Edited January 19, 2014 by Scholar Gypsy

[by'eck]

The diagram looks fine - just make sure the VSR kicks in below the alternator regulated voltage. Most are set to around 13.3 volts though so it shouldn't be an issue.

 

Given just the two batteries (100 Ah each?), a minimum of 35mm² cable from alternator to batteries should be ok assuming they are reasonably close to each other. Make sure the engine negative (return) cable to common battery negatives is at least the same cross-section.

[Liam]

Thanks Scholar Gypsy,

You're spot on, I found the diagram via a Google search and it was on a camper van forum. As far as the usage goes from the 2 batteries, I've only got a bog standard back cabin which has 2 lights and 1 cigarette charging point so it's not a massive load but you raise a valid point about swapping it over.

Richard, batteries are 110Ah (i think!) and the Midland Chandlers VSR I'm looking at cuts in at 13.3v and cuts out at 12.8v.

Google tells me 35mm² is about 10mm cable?

Thanks

[by'eck]

Google tells me 35mm² is about 10mm cable?

 

Thanks

 

Battery cable is sold according to cross-section of copper conductor and is often marked on the insulation. Don't use any other method to determine cable capabilities.

[Liam]

OK cheers Richard.

[Scholar Gypsy]

Thanks Scholar Gypsy,

 

You're spot on, I found the diagram via a Google search and it was on a camper van forum. As far as the usage goes from the 2 batteries, I've only got a bog standard back cabin which has 2 lights and 1 cigarette charging point so it's not a massive load but you raise a valid point about swapping it over.

 

Richard, batteries are 110Ah (i think!) and the Midland Chandlers VSR I'm looking at cuts in at 13.3v and cuts out at 12.8v.

 

Google tells me 35mm² is about 10mm cable?

 

Thanks

 

I should have looked more closely at the picture of your boat. As you are not running a washing machine, inverter, central heating boiler, shower pump, TV etc etc I am sure it will be fine either way.

[smileypete]

Wouldn't hurt to have a cheap voltmeter (plus small value fuse) too, cost as little as a few pounds these days:

 

http://www.buyincoins.com/item/26013.html

 

cheers, Pete.

~smpt~

Edited January 20, 2014 by smileypete