Technical Electrical question

[Billycamper]

Hoping there's a sparky or tech who can help with a question I have.
I'm fitting a split charge kit with a voltage sensitive relay that cuts in and out when needed. The relay states it is 140 amp
However my alternator has just been upgraded to 150 amp
The main battery is 110 amp and the aux battery is 120 amp

Am I right in thinking I'll need a uprated relay above 150amp ?

 

[porta]

This is a complex issue, made even more so these days with some alternator outputs being controlled by ecu’s
The output you have stated will be the maximum output of the alternator, the regulator will manage actual output. Maximum output will rarely be achieved and is more likely (on a good battery) and you are running lights, fans, AC etc etc
The next thing to consider is your actual split charger, cheap ones are basically just a relay with secondary regulator or that just reads which battery should get the charge, largely just on voltage. More expensive ones can ‘read’ battery ‘condition and draw’ and allocate charge appropriately and at what amperage, (I had one on mine that also controlled the solar panels, it was about £600 worth but it controlled all the batteries without resorting to switches for cab and cabin like you see on zig panels). A cheap one is the size of a fag packet and weighs about as much, a decent one, like my old one weighed about 3/4 of a brick and was the size of a full brick…oh, and contained at least two internal relays

 

[Billycamper]

Hi thanks for the reply it's a 2003 x244 ducato I don't think the ecu has control over the alternator on this older version.
The split charge relay I've seen is a small one like you mentioned .
I just didn't want to burn it out or worse cause a fire

 

[porta]

Hi thanks for the reply it's a 2003 x244 ducato I don't think the ecu has control over the alternator on this older version.
The split charge relay I've seen is a small one like you mentioned .
I just didn't want to burn it out or worse cause a fire

I think you should be alright on a 2003 model…yes, you have to have something very much more suitable (read expensive) for modern ecu or ‘smart’ alternators
If you want to retain your 140a relay but don’t want a fire, fit fuses to both input and outputs, not very scientific but cheap, but then you do run the risk of, if it blows, you’re not getting any charge… I have had this problem with land rovers, with cheap split chargers to power winches (means you can put them out of waters reach)

 

[porta]

I’ll find the link to a site I’ve used before on basic wiring and explanation

 

[porta]

It seems the world has moved on from my last adventures with wiring in split chargers
For later ecu/smart alternators the options are battery to battery chargers, which you CAN use for earlier types but not cheap.
Link must be on my iPad, which I’m nowhere near, about 15m away to be precise…but have found this company that I’ve bought from for the boat before now and it looks a decent explanation:

Split Charging Guide - caravans, campervans, motorhomes, boats, vehicles

Our guide to split charging for caravans, campervans, motorhomes, boats & commercial vehicles. A look at how it works & the various technologies available.

www.12voltplanet.co.uk

Split Charging Guide - caravans, campervans, motorhomes, boats, vehicles​

Our guide to split charging for caravans, campervans, motorhomes, boats & commercial vehicles. A look at how it works & the various technologies available.
www.12voltplanet.co.uk

 

[Billycamper]

Thanks for all the help. Having a good read of it now thanks again. Will definitely be putting fuses in too.

 

[porta]

No worries

 

[jackwhoo]

Hi,

Don't be concerned about 140amp voltage sensing relay.

As @porta says...
..

Do add a fuse to the wire connecting relay to van battery positive connection (fuse as close as you can to the positive connection)

Do add a fuse to the wire connecting relay to leisure battery positive connection (fuse as close as you can to the positive battery connection)

Make sure that these fuses are rated lower than the maximum current rating as the wire they are fitted to. Fuses are to prevent the wires glowing red hot and starting a fire in case of a dead short.

Connecting two batteries you need two fuses ; because if the live wire connecting two batteries shorts to ground the current has to be stopped from both batteries.

 

[PuntoHowTo]

Don't be concerned about 140amp voltage sensing relay.

Do add a fuse to the wire connecting relay to van battery positive connection (fuse as close as you can to the positive connection)

Do add a fuse to the wire connecting relay to leisure battery positive connection (fuse as close as you can to the positive battery connection)

Make sure that these fuses are rated lower than the maximum current rating as the wire they are fitted to. Fuses are to prevent the wires glowing red hot and starting a fire in case of a dead short.

Connecting two batteries you need two fuses ; because if the live wire connecting two batteries shorts to ground the current has to be stopped from both batteries.

^^^^^^^^^ This is the way !

As stated your 140Amp Split charging relay will be fine.

140Amp is the rated maximum throughput of the relay.
120Amp on the Aux battery is the theoretical maximum constant output of the battery, but it will not charge at 120amps,

Without going into Kirchhoff‘s laws your relay and battery combo will be fine, just make sure you fuse it both ends, one reason being is that if your live cable ever got damaged say in an accident and shorted to ground(ground being the body of the van) you‘ll short the batter and as stated above risk a significant fire.

 

[Communicator]

^^^^^^^^^ This is the way !

As stated your 140Amp Split charging relay will be fine.

140Amp is the rated maximum throughput of the relay.
120Amp on the Aux battery is the theoretical maximum constant output of the battery, but it will not charge at 120amps,

Without going into Kirchhoff‘s laws your relay and battery combo will be fine, just make sure you fuse it both ends, one reason being is that if your live cable ever got damaged say in an accident and shorted to ground(ground being the body of the van) you‘ll short the batter and as stated above risk a significant fire.

The OP, perhaps due to his admitted lack of technical knowledge, incorrectly gave the repective battery ratings as 110A, and 120A. These are battery capacities, and should be stated as 110Ah, and 120Ah. This lax terminology is potentionally dangerous. The starter battery in particular will deliver several hundred Amperes into the starter motor. The short circuit current of this battery could be several thousand Amperes, and which would have the ability to start an electrical fire. The auxilliary battery, if of the leisure type may have have a higher internal resistance, and hence lower short circuit current, but it will still be far above 120A.

120Ah means that in theory you could draw 1.2A from the battery for 100 hours, before it was completely flat. However in with the aim of preserving battery life, it is generally recommended not to exceed 50% depth of discharge (DoD) with lead acid batteries.

As an illustration, the habitation battery of my PVC consists of two 105Ah dual purpose batteries in parallel, which are separately fused. Each battery has a rated CCA (cold cranking Amperes) of 1000A. The possible short circuit current would be much greater that that.

I fully endorse the advice given by @jackwhoo in post #9 above, and saw no need to add to it

 

[PuntoHowTo]

The OP, perhaps due to his admitted lack of technical knowledge, incorrectly gave the repective battery ratings as 110A, and 120A. These are battery capacities, and should be stated as 110Ah, and 120Ah. This lax terminology is potentionally dangerous. The starter battery in particular will deliver several hundred Amperes into the starter motor.

Quite right, 120Ah makes much more sense, and i’d not thought it fully through, the point remains the same.