Pros and Con's of Realy dual batt setup????

[lowndsie]

A friend was just quoted 100 bucks to install a relay for a dual batt setup in his 80 series. I'm currently using a Jaycar kit which i dont seem to have any problems with. My question is, what would be the advantages of his setup over mine or vice versa?????

[chimpboy]

Is this a voltage sensitive relay?

[lowndsie]

he didnt say that it was. is a normal relay prone to voltage spikes?

[chimpboy]

Dual battery set-ups are prone to voltage spikes, but the danger is a bit overrated.

There are two typical relay setups for dual batteries, one is a normal relay which is probably what your mate is getting for $100 I would think. It just connects the two batteries when the ignition is on.

The other relay option is a voltage sensitive relay (VSR). What this does is measure the voltage on your main battery and only connect the second battery when the main battery is fully charged.

What this means is that if for some reason your main battery is a bit drained, your alternator will fully charge it before it even attempts to charge your secondary battery.

This is a safer bet because the most important thing is always to have a fully charged starting battery - so you can start the car!

A VSR should only cost about $130 or even less, plus some cable and fitting - but it's really easy to fit for yourself. My dad used to be a distributor for these but I don't know if he can still get them.

If you have the Jaycar dual battery kit, that's a different animal altogether. It is not quite as good as a VSR if you have two normal batteries, because it limits charging of the secondary battery to a relatively slow rate. However if your secondary battery is a "deep cycle battery", the Jaycar setup is better because deep cycle batteries need to be charged slowly, otherwise their lifespan is reduced drastically.

Normally your secondary battery would be a deep cycle if you are running things like fridges, DVD players, etc. Your secondary battery would be a normal cranking type battery if you were running things like winches, which draw lots of current.

The other complicating factor here is that I am told (I haven't sussed this out fully) that most of the batteries sold as deep-cycle batteries these days are NOT true deep-cycle batteries; they are essentially just normal cranking batteries.

Having said all that, in anything except extreme conditions, your Jaycar kit is going to work just as well as a VSR, and better than a normal relay, even if your secondary battery is a normal cranking type.

If I were setting up dual batteries in my car today, I would use the jaycar style electronics or a VSR, but wouldn't use a normal relay or solenoid.

Jason

[lowndsie]

thanx for that. just the info i was looking for.

[murcod]

If you have the Jaycar dual battery kit, that's a different animal altogether. It is not quite as good as a VSR if you have two normal batteries, because it limits charging of the secondary battery to a relatively slow rate. However if your secondary battery is a "deep cycle battery", the Jaycar setup is better because deep cycle batteries need to be charged slowly, otherwise their lifespan is reduced drastically.

The Jaycar kit can supply up to around 25-30 Amps- it only current limits when the aux voltage is below a certain figure (10.0V IIRC?) Realistically speaking the charge current to the aux in any system is more likely to be limited by the alternator output current, the resistance of the wiring and the internal resistance of the battery.

Relays are mechanical devices and as such can wear. Contacts can also pit from the arcing caused by switching high current loads resulting in resistance or failure. It will cause voltage spikes through your electrical system (from the high currents being switched)- whether or not that will cause any damage.... who knows??

[chimpboy]

If you have the Jaycar dual battery kit, that's a different animal altogether. It is not quite as good as a VSR if you have two normal batteries, because it limits charging of the secondary battery to a relatively slow rate. However if your secondary battery is a "deep cycle battery", the Jaycar setup is better because deep cycle batteries need to be charged slowly, otherwise their lifespan is reduced drastically.

The Jaycar kit can supply up to around 25-30 Amps- it only current limits when the aux voltage is below a certain figure (10.0V IIRC?) Realistically speaking the charge current to the aux in any system is more likely to be limited by the alternator output current, the resistance of the wiring and the internal resistance of the battery.

Relays are mechanical devices and as such can wear. Contacts can also pit from the arcing caused by switching high current loads resulting in resistance or failure. It will cause voltage spikes through your electrical system (from the high currents being switched)- whether or not that will cause any damage.... who knows??

These are fair points, I agree.

[Cossie]

If a solenoid is rated to, say, 100 amps - how is it going to handle a winch pulling over 400 amps?

I know the load will be drawn from both batteries, thus reducing the load going through the soenoid buts still gonna be over 100A?

[murcod]

It will most likely not handle it for too long.....

[chimpboy]

The solenoid/relay is not on the heavily cabled connection between the winch and the battery, it's between the main battery/alternator and the secondary battery, which are not so much running the winch as trying to charge up the secondary battery.

I am pretty sure that the cable acts as an effective current limiter here protecting the relay; B1 can't pump more than 100A into B2 down the cable, just like you can't start a car using sh1tty jumper leads.

But I have wondered about this many times and would like to hear how it goes in practice with the solenoid (a) connected and (b) not connected during winching.

Jason

[murcod]

Personally I wouldn't like to rely on the resistance in the wiring / solenoid to limit current from the second battery when winching. With the current levels a winch draws (and the time period it could be drawn over) something overheating / catching on fire is a very real possibility.

[Cossie]

The solenoid/relay is not on the heavily cabled connection between the winch and the battery, it's between the main battery/alternator and the secondary battery, which are not so much running the winch as trying to charge up the secondary battery.

I am pretty sure that the cable acts as an effective current limiter here protecting the relay; B1 can't pump more than 100A into B2 down the cable, just like you can't start a car using sh1tty jumper leads.

But I have wondered about this many times and would like to hear how it goes in practice with the solenoid (a) connected and (b) not connected during winching.

Jason

But if you have the same size cable linking the batteries (through the solenoid) as you have running from B1 to the winch then the cable cant be limiting the current.
Do we know for certain if the winch draws equal current from both batteries or if it uses the main battery and the 2nd battery just keeps battery 1 topped up?

[murcod]

If all the cabling is the same type and length (resistance same), both batteries exactly the same, and the solenoid has no internal resistance (ie. in an ideal situation) then the current would be shared equally.

Given that the cabling to the second is generally longer then there would be more resistance and the main would supply more current. Differing battery types (construction and capacity, as well as age) would also affect the current sharing.