Bill, how long is the cable from the alternator to the second battery?
What is the size of that cable?

I suspect that you are getting excessive voltage drop. If I am correct, a switch will not fix that problem.

Cheers,
Peter

Bill

Alternators are capable of 25 to 30 amps, some much higher, and batteries are not normally charged at that rate for any great length of time, so the power draw of the solenoid does not come in to it IMO. The marine switches are good but not automatic like the solenoid, therefore prone to mistakes (flat battery)

If you have a multi meter, just test the voltage along the line of cable (inc the solenoid) to see if there is a voltage drop. Some cable have bad joints and cause this. Another way is to feel each connection for heat (bad joint) when in use. The solenoid itself could have a high resistance from internal arcing.

You have a situation where you are putting two (dissimilar?) batteries in parallel and that also can be at the root of the problem.

I have used both systems, to good effect, but prefer the solenoid as there were fewer mistakes on my part.

There are twin battery charging units on the market, using electronics to controll the charge, but I have little experience with them, per the KISS principle.

At the risk of repetition "ad nauseum" I never recommend wiring batteries in parallel with out some form of isolation such as marine type changeover switch. More at www.activeboard.com/forum.spark?aBID=51933&p=3&topicID=34676667



-- Edited by jimricho on Friday 26th of March 2010 07:30:55 AM

Peter_n_Margaret wrote:

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He has an isolating solenoid jimricho.

Cheers,
Peter

 

 

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Oops! somehow I misread it as referring to dual (parallel) auxiliary batteries. I'll go and sit in the naughty corner. (thanks Peter for pointing this out)

Back to the drawing board then.  I presume the isolating solonoid is a "smart" relay that does not connect the aux battery to the main until it has reached a pre-determined voltage to ensure that the main gets charging priority, although this is probably not directly related to the problem.  The cable sizes and lengths mentioned should be adequate so there maybe a fault somewhere in the system.

Two things I can think of to check.... 1. Check the battery to see if it is holding a full charge by fully charging it from a mains type multi stage charger and then seeing how long it takes for the fridge to discharge it to 11 volts. This should give you some "feel" for the battery's capacity.  Don't fully flatten the battery as this is not good for its longevity.  Although your battery is relatively new it is unlikely to be in new condition if it has a number of total discharges.

2. What is the alternator output voltage.  This can be measured at the main battery terminals with the engine running, allow time for the main battery to recharge after starting the engine.  If that voltage is under 14 volts it is too low to properly recharge the aux battery (the main won't be fully charged either but as long as there's enough charge to allow reliable restarting it doesn't matter).  Ideally the voltage should be around 14.4 volts.  I believe some vehicles still use the old standard of 13.8 volts. Also measure the voltage at the aux battery terminals to see if there is significant voltage drop between the two.  If the latter you'll need to go on a fault finding exercise.  Check the condition of the terminals including the negative earth return circuit.  You'll need an accurate digital multimeter to do these tests.

Note that the testers that battery suppliers use to test starting batteries are of little use when testing deep cycle batteries.  (I use a 60 watt 12v lamp as a "dummy load" when testing deep cycle batteries, for the techies, this closely resembles the C20 ah rate for a 100 ah battery)

Also note that as you have a solonoid installed, the use of dissimilar main and aux batteries should not be an issue.

Hope this helps,

Jim

PS: regardless of all the above  I have found it to be good practice to regularly recharge auxiliary batteries with a multi stage charger when "ground" or "shore" power is available.