How to charge a second battery

Where are you placing the second battery?. . .you would have to be wary about leads vibrating off under the bonnet, or gasses from the charging battery if you intend placing it inside your car, or are you talking about just to start the car in the event of the main battery running flat?. If you connected it to the second battery via smaller cables you would need some sort of relay that would isolate it during starting or your cables would overheat.

In the past i have used a solar trickle charger placed on the dash to feed the second battery while you drive along. About $35 from jaycar, dick smiths or the like

How far are you discharging it? That will be the big thing we need to know. If you have discharged the battery 50% and you are charging your battery at a camp site if could take 5 hours or more to recharge it to about 70% of its maximum capacity.

PeterD

Hmm 6 or 8 mm cables are not going to allow much current flow if you actually need to jump start the car. I've had a set of HD Matson anti-zap cables for years . .by memory they are about $140 from Repco. You really need to take a car for a 15 minute drive to recharge a flat battery and get the alternator charging it at various RPM's.

Yes Jethro, you could charge your second battery that way but you would need to be very careful with the jumper leads to avoid bad connections or short circuits.
Particularly if children are present.

The alternator produces only a very low charge rate at engine idle speed so, and depending on the state of charge of your cranking battery, it could take several hours to get even a small amount of charge into your secondary battery. If you were to attempt to charge using jumper leads while travelling it would be very hazardous due to short circuit possibility.

All-in-all, not a good way to go. But I do not have any suggestions for you other than installing a proper dual battery system. Sorry.

Hi Jethro

I have a second battery for my waeco which, while at the campsite, I have a switch that isolates it from the main battery/alternator. I turn it off while the vehicle is stationary, and turn it back on again while driving to charge up the second battery. One time when camping the fridge cut out (you can set the cut out voltage on the fridge) as the vehicle hadn't been driven for a while. I turned the switch on and ran my car for 20mins or so and the battery kept the fridge running until the next morning when we packed up and left. I think you could do what you are suggesting, but I used 25mm2 cable.

Adam

Just a thought but when it goes flat why not swap it into the car give it a

jump and go for a drive. Use the car one for the fridge and keep alternating

(no pun intended) them.

The answer is yes.

Start the motor and connnect the spare batt. The alternator is capable orf 80 or so amps and the engine at idle will handle that that no prob. If not the motor would start to stall as the alternator put the load on. The alternator will only supply 13.8 volts at any r.p.m. which limits the amount of amps you can push into the spare batt

In practice, the spare batt would not ask 80 amps, maybe 15 or 20 in my experience with a 13.8 vehicle supply. One hour of idle will get that 15/20 amp/hours . Ask yourself how many amp hours will you drain off the spare.

Damned expensive way to charge a battery. The energy it takes to idle a petrol cruiser is way and above the energy you will put in the battery.

But it's a horse for a course and any port in a storm.

And remember that draining a battery too much will destroy it a lot quicker than normal use will.

Owen

Owen,
The amount of charge that would go into an empty battery at idle may not be much in your 80series.

Easiest way to maximise it is to hook up a multimeter to the flat battery and increase the engine RPM until the voltage peaks.

We measure the voltage on my daughter's 80series and the voltage drops off easily at idle. Often only about 13.5V after the fridge has been running overnight, but give it 1500-2000rpm and its up to 14.5V.