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Dual Batt isolator test

GQ Beast

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I really question the use of battery isolators - especially if the second battery is used for small loads like fridges, lights etc.

If you have a second battery thats say at 50% charge and you start the engine, the isolator will 'join' or 'short' both batteries together in parallel causing high current to flow from starter/alternator to the second battery. The two batteries will eventually equalise in charge if left in this state but I cannot see this as being anywhere near ideal conditions for the second battery.

IMHO a DC/DC charger - like the redarc 1220 or 1240 or the ctek Dual 250S is a much much better way to charge the second battery. Not only does it completly eliminate the massive current that would be involved but it also boosts the voltage to ideal charging levels. In the case of the ctek it also manages battery temperature with respect to charge current.


Edit: If the main purpose of your second battery is to winch where you join both batteries via a relay then use of an isolator is completly valid.

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Yeh agree, to a point , the redarc closes liink at under 12.7 v then opens , i use 2B&S cable in my setup and also have a heavier cable from Alt to start batt. I get 14.45 V when running to start batt. and 14,43 V at Aux. when link opened. A drop of .02V so in my case the Aux. is getting its recommended charge voltage. The dc-dc would be better, but iv,e allways had a fully charged Aux. after a normal 2 hour drive to fishing / camping spot. Run my Evakool 60 litre on freeze if catching fish and fridge// freeze if not. lol The light that shows batts are still linked stays on for near two days EG: batts havnt dropped to 12.7V to isolate start batt. After day 5 my aux. is at about 12.4 V. and ime ready to go home I also have solar when i stay longer.Also a good rule of thumb is to charge your batts. on a bench charger at least every 2 months. I have a Crown (USA) 120 Ah aux. and its specs on voltage charge is to be atleast 14 V.

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