Prompted by a question on the Mud Club website today, I wrote a quick summary of the options for installing a dual battery setup. This is a slightly longer version of that.
Like all things, there’s no one right answer to this. The right dual battery configuration depends on what you want to do with it, and what vehicle you have.
What’s it for?
Typically, a second battery is used for one of a few applications:
- Lighting, radios etc when parked up.
- Winches
- Fridges or other in-car appliances which may be left running when the vehicle is unattended.
Winches are the biggest challenge, with high current draw and often needing to be recharged quickly.
Second Battery Location
The biggest constraint is where you place the second battery. On some Discos, you can turn the main battery around and fit a second one. Some Classic Rangies have space under the air intake. Under the spare wheel works on a P38A. In my Dakar, there are twin battery boxes in the back of the cabin. Find a spot, and secure the biggest battery you can find.
Type of Battery
The other important decision is what type of battery. For most purposes, a deep discharge type is better than a starting battery. Starting batteries produce higher current at peak load, but can be damaged by deep discharging (e.g., running a fridge all night). If you’re not getting into huge demands on the winch, a starting type battery may still be OK – and cheaper – than an expensive Yellow Top Optima. Which leads me on to battery types: Optima are held up as the gold standard for both starting (Red) and deep discharge (Yellow) types. But they’re expensive – over £125 each. You can throw away a £40 cheapy every year for the duration of the Optima warranty for that. So think before spending the money.
Dual Charge Control
The final decision to make is what kind of control circuitry you want. There are a few options again.
- The simplest option is just to wire the batteries in parallel. +ve to +ve, -ve to -ve. Job done. Each battery will drain and charge more-or-less equally. But there will be slight battery drainage when left unused as imabalances develop and drain the batteries. This isn’t likely to be noticeable in normal use, but it’s worth disconnecting either or both if you’re leaving the vehicle for any length of time.
- The mid range solution is to use a high current relay, powered by the alternator, to switch the second battery on when the engine is running. When the engine is off, the main battery and the vehicle is dead, and a separate isolator controls power to the accessory circuits. This can be augmented by a voltage controller for things like a fridge which will kill the power once the battery discharges to a certain level to avoid completely draining the battery.
- The high end solution is a dual battery controller and monitor. These are best kept for expendition vehicles, since most people can find better ways to spend £300 or more.
Alternator
Ok, so there’s one more thing to think about, especially on older vehicles. With two emtpy batteries there will be a huge load on the vehicle’s charging system. You may need to think about either uprating the existing alternator, or fitting a second one.
So what did I do?
My requirements were pretty straightforward. I wanted to power a winch, and provide secondary power for a radio, external work lights and a fridge without discharging the starting battery. I added the largest standard car battery that would fit, with the X-Charge relay. The work lights, dash power to the CB and +12v sockets, and +12v sockets in the back are all connected to the second battery isolator. I’m going to add a bypass alternator at some point, and possibly a set of X-Jump leads for easy jump starting of other vehicles.