Thread: Cheap second battery setup

Hi Rick

You need to be careful with low cost dual battery systems, especially home made ones. The connection between the +'ve terminals on the main and secondary batteries should only be made when a charging level voltage is detected on the main battery (typically above 13.5v). When the main battery is under load and its voltage drops the link to the secondary battery has to be disconnected.

If you have a manual switch or ignition controlled relay then you could end up with batteries connected together during starting (or maybe winching) and end up sending very high current from the secondary to the primary batteries, if the wire between them is too thin it will get hot and could even start a fire.

I'd recommend a proper dual battery split charge kit that will disconnect the link between primary and secondary when the secondary is not being charged, places like Supercheap (Edit- Just noticed your location, try Halfords) do some reasonable ones, so do a few others.

If you do go for your own system then be very careful with wire gauges and maximum current ratings on all components. the simplest dual battery system is two batteries directly connected with heavy gauge wire but obviously you'd have no protection for starting if the load is left on for too long.

Yeah, though about it and picked up a Narva 140amp VSR today ($36 surprised were that cheap!) and will probably go with 6 or 4 B&S cable. I can get a lone of a battery cable crimper tool. Can't see this vehicle ever getting a winch. Fridge only....and maybe a USB charger.

I can see what you mean with disconnecting batteries when cranking. On my Suzuki with simple solenoid, it activates off a power to the wipers, which drops out while cranking so only cranks off main battery. It does have big 00 B&S cables tho, so maybe not so much of an issue if it did.




On a different note - my Rangie still has a simple 200amp solenoid system which currently has the trigger disconnected as it now has separate alternator for each of the two batteries. It will get a winch soon, so was thinking maybe have a switch to arm winch which also activates solenoid to link both batteries. Winch connected to aux (cranking, not deep cycle) battery. OK idea or not?

Winching should always be done with the engine running so you'd need to be careful with two batteries connected and each having its own alternator. One alternator would likely stop charging due to seeing the charging voltage from the other but you could get other electrical problems too. The ability to link the batteries would be useful if one alternator fails but otherwise I'd suggest never using it.


Winch current can be 400A+ so I'd connect it to the closest battery with the shortest cables possible, fortunately the duration of winching tends to be short enough to not discharge the battery.

You could use a solar charge system. This is one of hundreds on Fleabay

30A 12V 24V Solar Controller Regulator Charge Battery Safe Protection CE CERTIFY | eBay

Down side is that the charge current is limited, but this means that you only need a certain size of wire. 10amps to 1mm2 of copper.

These units will cut off the 2nd battery from the load at usually 11 ish volts (some are adjustable), so if you put a couple of diodes (series) in the line from the main battery (connected to the solar cell terminals) you should be able to keep the main battery at about 1.2volts per diode (check the diode specs) above the 2nd battery. I'd bench test all this first though.

Quite a few years ago (7ish) I bought a 80amp VSR kit from Jaycar cost about $30, works a treat, but might not be available anymore.

as in use the solar controller to control the charge to the second battery in the vehicle, not with a solar panel?

I sorta pondered on if that would work for a while.....since they can be had so cheap these days....Do they limit the currant?

Hi Dorian and the lowest you can safely take a deep cycle battery is 11.58v, not just 11 ish.

The use of such thin wire will mean you will never replace more than a very small proportion of the used auxiliary battery capacity while driving and you have absolutely no chance of fully charging any battery.

Next, if you fit diodes in the line, even if the cable was much thicker than the 1mm2, you would create a 1.2 to 1.4v drop between the cranking battery and the auxiliary battery but the problem is that once you start the motor, you still have the 1.2v drop so even if the alternator was running at 14.2v, you would only have 13.0v at the auxiliary battery.

13.0v will charge a battery, but will take at least 30 hours of constant charging to fully charge a low battery and this again, is with thick cabling.

Rick if you are trying to save your friend some money, fit an “idiot” switch.

This is simply a solenoid controlled by a switch.

The solenoid connects the batteries when the switch is set to on.

The reason it’s called an idiot switch is if you forget to turn the switch off, you can then drain both batteries and if you do, you will feel like an idiot.

You can wire the solenoid to the ignition circuit but this has a number of major potential drawbacks which can still lead to two flat batteries.

Hi Rick
Yes that is what I meant, it's a cheapish solution. They limit the current draw, this has the down side that the battery doesn't charge as fast, the upside is that there are plenty of batteries that have a charge limit anyway. As to wire size i meant 1mm2 per 10 amps so if you have 20 amps you need 2mm2. Although I have looked up the specs for some of wiring we use and 4mm2 will only carry 36 amps, 6mm2 os 52 etc.
As another post said, putting diodes in gives you a charging voltage drop, no argument there, but if its just week end type stuff and your friend uses a battery charger to top off the battery every so often there should be no damage to the battery long term and any driving they do while away will put some charge in the battery.
As to the lowest voltage you can discharge a lead acid battery I was always told that 10.5 was the lowest but at that voltage the battery had a reduced life span, hence 11ish because there is not a lot of power between 11v and 10.5v. ( I am assuming that 1.54 was a typo.)

Having said all of that you can get a Navara VSR on Flea bay for about $50 delivered, which should give you a bullet proof solution. Give us a few days and I recon I can get a circuit diagram together using one of those solar charges and a relay to to the VSR thing, and get around the diode voltage drop.

Cheers Glen