Dual battery projecta dc20 confusion

Submitted: Sunday, Nov 16, 2014 at 15:20
ThreadID: 110157 Views:2671 Replies:3 FollowUps:6
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G'day all, I have an old projecta dc20 , dc to dc charger. This is the one without the ignition sensing (new ones have it). Question is, given this unit can accept as low as 9volt input this would no doubt drain my starting battery unless I have an isolator up the line or a vsr?. Now if I do get a vsr or isolator will they work given they won't be able to see the aux battery as the dc to dc unit will be in between. Regards Chris
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Reply By: Allan B (Member, SunCoast) - Sunday, Nov 16, 2014 at 15:48

Sunday, Nov 16, 2014 at 15:48
Hi Chris,

All you need to do is place a relay between the alternator and the Projecta with the coil of the relay controlled by the ignition circuit.

A suitable relay would be a Jaycar SY-4074, 60 Amp at $ 8.95

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Allan

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Reply By: Member - Frank P (NSW) - Sunday, Nov 16, 2014 at 16:34

Sunday, Nov 16, 2014 at 16:34
Chris,

The way I think you may be intending to hook it up, the VSR doesn't look at the aux battery. It looks at the starting battery voltage to decide when to switch on.

Having said that, I agree with Allan above - you only need a "dumb", ignition-operated solenoid.

Cheers


FrankP

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Follow Up By: Chris85 - Sunday, Nov 16, 2014 at 18:40

Sunday, Nov 16, 2014 at 18:40
Thanks guys, appreciate the advice.
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Reply By: ABR - SIDEWINDER - Sunday, Nov 16, 2014 at 21:45

Sunday, Nov 16, 2014 at 21:45
You can use a simple solenoid isolator yes but it will draw 20A as soon as you switch the ignition on.

If you want a automatic system then use a DBi140R and it will only switch on when the engine is running. The DBi senses voltage on the main battery side.

Regards

Derek from ABR

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Follow Up By: Allan B (Member, SunCoast) - Sunday, Nov 16, 2014 at 22:50

Sunday, Nov 16, 2014 at 22:50
If the relay I proposed above is connected to IGN-2 circuit then it does not energise while the engine is cranking, only after the switch is released from the 'Start' position. Hence it will not "draw 20A as soon as you switch the ignition on".

IGN-2 circuit can be accessed from the windscreen wiper motor supply circuit.

Besides, the DBi140R costs $79.95 compared to a relay @ $8.95
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Allan

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Follow Up By: ABR - SIDEWINDER - Monday, Nov 17, 2014 at 00:23

Monday, Nov 17, 2014 at 00:23
Quite right yes if you jump in the car and turn the key in less than a few seconds, I agree.

If the key is switched on at any time and the engine is not running a automatic isolator MAY be the preferred option to some people.

Thanks Allan for your input.
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Follow Up By: Member - Frank P (NSW) - Monday, Nov 17, 2014 at 08:51

Monday, Nov 17, 2014 at 08:51
If the aux battery is down the charger will pull a high current while it is in bulk mode. At low engine speeds the alternator output may not keep up with the demand from the charger so the crank battery voltage creeps down, the isolator drops out, the voltage comes up again and the charger re-starts and the cycle can begin again. Not very efficient.

There is another aspect. The partially charged battery may have a surface charge which the charger sees, and when it re-starts it may not re-commence charging. A good battery may hold its surface charge for some considerable time, especially if it has no load on it or if there is rooftop solar assisting the dc-dc charger, so during that period you miss out on valuable charging while driving.

Same may happen if you take a break from driving before your battery is fully re-charged.

I have found this through experience, particularly with lithium batteries which hold their equivalent of surface charge, even under typical loads such as a fridge.

As a result, I have changed from an automatic isolator to a dumb, ignition-operated one.

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FrankP

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Follow Up By: Allan B (Member, SunCoast) - Monday, Nov 17, 2014 at 08:55

Monday, Nov 17, 2014 at 08:55
I really cannot follow your logic Derek. But thank you also.
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Allan

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Follow Up By: olcoolone - Monday, Nov 17, 2014 at 09:27

Monday, Nov 17, 2014 at 09:27
If you starting battery can withstand an additional 20 amps whilst cranking for a 10 seconds I would suggest a new starter battery.

The 20 additional amps should not kill a battery whilst starting.
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