Myths and facts on “Dual battery” setups.

Myths and facts on “Dual battery” setups.

Hi all,

I’m in the process of fitting my center console with a Dual battery system and have had a look at some of the questions and comments placed onto this web site. Just to give my comments some credibility, I have been an Electronics tech for the best of 22 years and have spent the last 12 years in IT so I hope that I can remember my Ohm’s law. Firstly the myths about how much current your electronics is drawing. If you are unable to measure the current drawn by a device or don’t have the manufacturers specs, then using the rating of a fuse is a poor indication of how much current the device draws. There is NO hard rule or formula on manufacturers as to how they rate fuses. If a piece of equipment has a 5 amp fuse (depends on type of fuse) then you can only assume that the current consumption of that device once it has settled down would be 40 – 50% of that i.e. 2 amp. So when I see people place comments on the web about a sounder using 5 amps I think that is a bad guess. I have had a look at the manuals of some of the popular Sounders, GPS’s and Radio’s and this is what I have found.

 battery switch bank c/w VSR

Furuno 600L color sounder                      1.3A with full backlighting on

Humminbird Paramount wide sounder      0.6A (No specifications figures available)

Lowrance LCX-15MT GPS/Sounder          2 to 3 A (No specifications figures available)

Navman 4500 sounder                            0.8 A with full backlighting on.

Garmin 182 GPS                                   0.4 A

Navman 5100 GPS                                 0.6 A with full backlighting on

Icom M45 VHF radio                              0.8 A in standby. 6 A in high power Tx.

GME GX294 27 Mhz                              0.35 A in standby, 1.6A in Tx

OK you can see that the real power users if only for a short period of time are the transmitters in radios, GPS’s have no power circuits in them and as such use bugger all power. Sounders, well some use more than others.

 Now for the outboard myth. For those who have a 4 stroke outboard, you may think that it can put out some colossal amperage and charge your battery quickly, WRONG. The modern alternator charging system is designed to be kind to batteries. The system relies on the condition of the battery itself to determine the current flow, only the voltage is regulated. A discharged or partly discharged battery will consume a relatively high current for a short time only until it reaches a state commonly referred to as “surface charged”. The current it demands then drops to a lower value, typically around 5 amps. This equates to a 100 amp/hour battery that is half flat, taking 8-10 hours of engine running time to fully charge. In a situation when power consumption may average 5 amps constant, the total capacity of an auxiliary battery may be used in less than one day. Running the engine for a short time (even up to two hours) WILL NOT come near recharging that battery.

 As for 2 stroke outboards, most like my Yamaha 70 have a rectifier and are only capable of delivering only about 5 amps. Maybe the 4 stroke has the edge over the 2 stroke as it is able to bring the battery to the “Surface charge” quicker, it’s chalk and cheese.

After doing all this web browsing I have come to the conclusion that I will be leaving my starting battery which has a CCA (cold cranking current) of 700A as is, and adding a lower current deep cycle battery as the auxiliary. I’ll be employing a ‘BEP battery management cluster’ with a voltage sensitive relay. In my case the wiring changeover should be relatively simple as I wired a separate cable form the main isolation switch to run all the electronics and lights. This cable will now come from the auxiliary battery. As can be seen from the figures that I  have provided, current drawn from an auxiliary battery for a typical setup would be around the 3 to 5 amp mark and maybe double that if out at night with navigation and cabin lights on. I hope this has cleared up some of the guesswork that seems to be rampant.

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