Split charge relay

[owas]

I'm a tad confused,
I had one of these 30 amp voltage sensing relay things in my 'might come in handy' box so I've thrown another battery in to run my accessories.

https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.co.uk/ulk/itm/370837856330

My alternator is 60 amps iirc, so the relay kicks in when the engine is started from the increase in voltage, if say the second battery was flat, what's stopping it trying to put 60 amps through the relay?
It's fused both sides and hasn't blown a 20 amp fuse yet.
If it's not enough, I will get a large relay and just use this voltage sensing one as a trigger.

 

[rtbcomp]

... what's stopping it trying to put 60 amps through the relay?
...

The flat battery may not need 60 amps and if the alternator looks like it's going to be overloaded it will drop its output voltage to keep the current to a safe level.

 

[owas]

The flat battery may not need 60 amps and if the alternator looks like it's going to be overloaded it will drop its output voltage to keep the current to a safe level.

I should be ok to leave the 30amp relay then?
It seems to work fine in normal use.

 

[rtbcomp]

I'd leave it, the 20 A fuses will protect the relay.

 

[123hotchef]

I used one of those smart sensing things to trigger a 200 amp relay for my split charge system, I also added a in cab switch so I can jump start myself too

 

[Matrixx]

It depends what charge current your battery requires, this will depend on state of charge and internal reistance.

if you had a short on the second battery circuit you could have hundreds of amps flowing, as the primary battery would aslo feed the fault.
idealy you would want 30A fuses on both sides of the device. And that the cable is rated at more than 30A .ie. 4mm sqd cable

 

[ajlelectronics]

Remember that conventional alternators already have a "load sensor" built in! Just wire your relay across the charging light WL terminal to ground. It will only switch on with the engine running. I do that for providing caravan power feeds to the 'fridge etc.

 

[rtbcomp]

^^ I've done that on older cars with complete success, but I'm led to believe that modern cars with the alternator controlled from an ECU don't work with this setup.

I'd mention that the position of the fuses is important too, they should be connected as close as possible to each battery, not close to the relay.

 

[owas]

Currently it is,

Primary battery - 20amp fuse - 30 amp wire- 30amp switch (to isolate the circuit) - Voltage sensing relay - 20amp fuse - auxiliary battery

Then I have 25mm2 cable between each battery with a kill switch and 200amp fuses

The fuses are right next to each battery

So if I want to bridge the 2 batteries to start it, I have to isolate the relay and turn on the kill switch.

Hope I've got this right

 

[rtbcomp]

^^ Sounds about right - you have to make sure the starting current doesn't go through the relay.

On one vehicle I had (Rangey with a Perkins 4236) I connected a starter solenoid between the two batteries so I could jump start without lifting the bonnet. The switching was arranged so they were only connected when the starter was pushed and the split charge isolated.

 

[123hotchef]

^^ Sounds about right - you have to make sure the starting current doesn't go through the relay.

On one vehicle I had (Rangey with a Perkins 4236) I connected a starter solenoid between the two batteries so I could jump start without lifting the bonnet. The switching was arranged so they were only connected when the starter was pushed and the split charge isolated.

on mine the current does go through the relay but it is a 200 amp relay so hopefully it is all good