Battery Isolator amp rating

[towpathrider]

I've looked through the previous topics on isolators but couldn't find a specific answer regarding the rated amps of battery isolators.

 

Should this be relative to the size of the battery bank or to the maximum possible load drawn from the batteries. - or both. (obviously the two are related) i.e. a 500a isolator for a 500ah battery bank?

 

I may completely have the wrong end of the stick here!

 

 

[alan_fincher]

Should this be relative to the size of the battery bank or to the maximum possible load drawn from the batteries.

 

 

 

The latter - it is about the maximum current flowing at any point in time, and far less about how many hours your battery bank might be able to deliver it for.

 

The only caveat, I suppose, is that if you momentarily draw somewhat more current through an isolator than it is designed for, it will probably survive it - try and do it for minutes or hours at a time, and it is more likely to be damaged. But if you go for a switch rated continuously above the highest current you ever expect to draw, then how long you draw that current shouldn't matter, (other than you may trash your batteries!....)

[nicknorman]

Yes it relates to the maximum current flowing, not the battery bank size. There are 3 considerations:

 

1/ How the switch tolerates the contacts opening with a high current flowing, since this is likely to cause arcing and burning of the contacts. But you are unlikely to open the switch with a high load on, except in an emergency.

 

2/ How hot the connections and switch get with a high current flowing - if things get too hot, something is going to start melting.

 

3/ How much of your precious 12v is lost through voltage drop in the switch at high currents.

 

Really it is 3/ that is the most important consideration. An over-rated switch will lose less voltage which will help deliver the maximum energy to the circuits under high current flow.

 

Blue Sea Systems switches seem to have a very good reputation.

Edited October 7, 2014 by nicknorman

[by'eck]

4/ How long the high current is flowing through the contacts

 

BEP isolators have as many as three ratings - continuous, intermittent and cranking (limited to 15 seconds and over four times the continuous rating)

 

My bowthruster one worked apparently without issue for two years until it melted part of the casing recently - it was rated at 1250 amps cranking but only 275 amp continuous. Now replaced with the next size up.

[alan_fincher]

My bowthruster one worked apparently without issue for two years until it melted part of the casing recently - it was rated at 1250 amps cranking but only 275 amp continuous. Now replaced with the next size up.

 

You are doing too much bow-thrusting!

[by'eck]

 

You are doing too much bow-thrusting!

 

Well it happened after just 10 seconds of use and only after two years in service. Nevertheless you are probably right

 

Once it happened and I'd checked the spec - 275 amps continuous - BT fused at manufacturers recommended 600 amps I wasn't too surprised.

[smileypete]

Just avoid the cheapo 'red key types' whatever the claimed rating is. As said Blue Seas, BEP, Durite should be good enough, if on a budget the 'TIR' types with a metal handle should do.

 

Something decent around 300A continuous should handle a typical narrowboat engine and inverter up to 2000W just fine. Seems like the intermittent ratings should be taken with a pinch of salt.

 

cheers, Pete.

~smpt~

Edited October 7, 2014 by smileypete

[alan_fincher]

Once it happened and I'd checked the spec - 275 amps continuous - BT fused at manufacturers recommended 600 amps I wasn't too surprised.

I thought Mr Hudson was supposed to never cut corners!

 

(Sorry, couldn't resist1).

[nicknorman]

I thought Mr Hudson was supposed to never cut corners!(Sorry, couldn't resist1).

Richard has a pretty "non-standard" electrical installation of his own design, though I don't know who specced the isolator.

[by'eck]

I thought Mr Hudson was supposed to never cut corners!

 

(Sorry, couldn't resist1).

 

Nothing to do with Mr Hudson. I supplied the hardware and it was wired to my spec. - I thought something that could stand 1250 intermittent cranking amps would be more than adequate. New one capable of twice that.

[towpathrider]

Thanks all for the responses. Looks like a 300amp should do the job then as we will only be using a 1200w inverter.

 

Really it is 3/ that is the most important consideration. An over-rated switch will lose less voltage which will help deliver the maximum energy to the circuits under high current flow.

 

Out of interest what specifically is it about higher rated switches that causes them to loose less voltage? different design of switch or just bigger contacts?

[by'eck]

Thanks all for the responses. Looks like a 300amp should do the job then as we will only be using a 1200w inverter.

 

 

Out of interest what specifically is it about higher rated switches that causes them to loose less voltage? different design of switch or just bigger contacts?

 

Generally bigger contacts and terminals capable of taking appropriate sized cable terminations, although there are mechanical and self cleaning actions to consider. The one I have just fitted uses 12mm stud terminals.

 

Blue Sea and BEP isolators for example use a twisting action as the contacts come together when making the switch as well as being ignition protected. This in preference to cheaper types that just push contacts together with a cam.

 

Another issue is breaking the arc when an isolator or contactor is switched off under heavy or fault load. Spring assistance for the breaking of contacts helps here plus distance between which requires the arc to be stretched to around 1mm per volt to extinguish.

 

If an isolator drops voltage with high current flow this is power loss so it will get hot and eventually melt or even catch fire.

Edited October 8, 2014 by by'eck

[nicknorman]

Thanks all for the responses. Looks like a 300amp should do the job then as we will only be using a 1200w inverter.

 

 

 

Out of interest what specifically is it about higher rated switches that causes them to loose less voltage? different design of switch or just bigger contacts?

Bigger connectors and switch contacts, which create a lower resistance and hence less voltage drop, plus possibly better (self-cleaning/wiping) contacts with surface material less prone to resistance-increasing corrosion.

[NorthwichTrader]

Not actually knowing the eventual size or spec of our electric set-up, yet, combi size, etc.? Is there an isolator switch which would be a coverall regarding amps? It would be better to over-rate at this stage, than go back and under issues later.

 

Can anyone suggest a coverall switch, or is it not as simple as that?

[nicknorman]

Not actually knowing the eventual size or spec of our electric set-up, yet, combi size, etc.? Is there an isolator switch which would be a coverall regarding amps? It would be better to over-rate at this stage, than go back and under issues later.

Can anyone suggest a coverall switch, or is it not as simple as that?

It is fairly simple. You just want a quality product that has a high current rating and is explosion proof. Something like this, but there are other possibilities

 

http://www.aquafax.co.uk/html/product_specification.asp?ID=10285

 

That one is 350A continuous / 600A intermittent which should be plenty. Hopefully you are putting the isolator in the +ve feed in which case you will need 2 (one for the engine battery) or maybe 3 if you have that devil's creation a bowthruster.

 

Ed: that diagram makes it look like a multiway switch but in fact it looks like this

 

http://www.aquafax.co.uk/html/product_details.asp?ID=10285

Edited October 13, 2014 by nicknorman

[by'eck]

Or this heavy duty one available from E C Smith agents (most good chandelries) and 600 amp continuous rating - ignition protected as is the equivalent Blue Sea one and also in motorised version for remote control - use mine for bowthruster batteries remotely controlled via cunning relay system from panel isolator.

 

Edited October 13, 2014 by by'eck

[NorthwichTrader]

Usual thanks for all your help and advice! Really kind, brilliant!