Leisure Battery and Electrics
A leisure battery does what it says on the tin. It's dedicated to non vehicle tasks such as lighting, power ports, water pumps etc. This lets you perform these tasks without worrying about the charge of the battery that will start the car when it's time to move on. This was an addition that transformed the feasibility of extended trips. This page covers the installation of the battery, the very simple system that charges it, and how to switch couple of simple pre-installed circuits to run off the leisure battery, instead of the starter battery.
DISCLAIMER: I am not an auto electrician. Below is what I have picked up from various people and my own learning. I accept no responsibility for the success or failure of anything you do related to what I have written here.
What's the difference between a starter and leisure battery?
I know it's not a very sexy topic but it is useful to understand some basics, in order to ensure your system works well. Starter batteries are designed to deliver a high current output for a short amount of time (turning a starter motor) before being immediately re-charged. Conversely, leisure batteries are designed to serve low current functions for an extended period and endure repeated discharge to lower levels. The inside of the batteries differ to serve these two different functions. A starter battery will be damaged by repeated discharge to low levels without prompt recharge, while a leisure battery will be damaged by repeated high current demands. So start your vehicle with a starter battery and use a leisure battery to power your other stuff.
Choosing a battery
Which battery you go for depends on what you want it to do. It also depends on the size of the space you can fit it in. Battery capacity is measured in amp hours (Ah). This is a slightly strange unit to get your head around but a battery with 1Ah is able to produce a current of 1A for 1 hour, or 0.5A for 2 hours etc. Basically the more Ah, the more you can do with it before a recharge. Essentially you should get the battery with the highest Ah capacity that will fit in the space you want it to go in. Ours was somewhat of an opportunistic acquisition, it's a 75Ah hybrid battery and has run our systems well so far. Things get complicated if you would like a fridge, as they draw a lot of current. We don't have one and I'm not well informed on the subject so it's best to go elsewhere for more on that topic. If you're buying a battery second hand, spend a tenner on a multimeter first so you can check it's charge. It's also an incredibly useful tool to have later on in your electrical adventures. A voltage of 12.6V or above indicates a fully charged, healthy battery. More on battery charge levels later.
Where should it go?
In an ideal world, our leisure battery would have gone under the bonnet. There are two potential positions for this in a discovery 2; the first is to modify/replace the original battery box to accommodate both the starter and leisure batteries. I believe this requires the ECU to be re-sited, something I am not currently bold enough to attempt. (certain electrical components are a little emotionally unstable in these cars and therefore if they aren't broken, I don't touch them...) The second option is to put the second battery in a special box near the bulkhead on the passenger side of the vehicle, next to the turbo. Both options require either the purchase of a specially designed battery box or the ability to make one yourself. Neither of these was realistic for us. Therefore ours sits in the foot well behind the drivers seat (above). There is the convenient option of earthing the battery using the bolts which secure the drivers seat (below).
How do we charge it?
The simplest way to charge a leisure battery is off the alternator, just like the starter battery. Batteries have a maximum voltage of about 12.6V. The alternator typically produces about 14V. Thus current flows from the alternator to the battery and charges it when the engine is running. There are various ways to link a leisure battery to the alternator, we opted for the simplest option. A manual kill switch links the two batteries together when the engine is running, and the alternator charges them both. When the engine is turned off, we turn the kill switch off, separating the two batteries. This lets us run circuits from the leisure battery without draining the starter battery. The image below is from this link, which has an excellent guide to the various different charging options: https://www.12voltplanet.co.uk/split-charging.html . Link your batteries with large bore cable to ensure efficient charging, we cannibalised a set of jump leads for this purpose. Make sure your switch is rated for a high current. Our kill switch is mounted on the side of the bed, behind the drivers seat. Here it's within easy reach from the driving position.
Below is the cable connected to the starter, with the crimp terminal on the end, maxi blade fuse holder and 30A fuse. Fuses are vital. Do not forget to include them. Locate them as close to the battery as possible, so that the vast majority of the wire is protected by a fuse.
This system works well for us, you just need to remember to connect and disconnect the battery at the appropriate times. Simple and cheap.
Our power cable runs along inside the engine bay and enters the cabin through a convenient large rubber grommet (below) in the bulkhead on the drivers side of the vehicle. This brings it out behind the cabin fusebox, beneath the steering wheel, and can then run back through the center console to our leisure battery.
Using the leisure battery to power existing circuits in the car.
The first things we wanted to power with our battery were the interior lights and rear 12v power socket. First you must identify which fuse in the original car fusebox (located underneath the steering wheel in a disco 2) is for the circuit you want to power.
In this case; Fuse 32 (25A) powers the rear accessory socket, Fuse 20 (15A) powers the internal lights and radio memory. Find out your radio code before removing fuse 20 as you'll need to re enter it afterwards. Check the component works before removing the fuse, remove the fuse, check it now doesn't work. If indeed it doesn't, then you probably have the right fuse.
The easiest way to power them from your new battery is to have a wire running from the leisure battery (via an appropriate fuse, the same rating as the one you've just removed) with a male spade terminal on the end. Plug this into the downstream terminal on the car fuse box. The circuit should now be drawing power from the leisure battery, not the starter. This is shown in the very simplified diagram above; 1 is the original circuit running off the starter, 2 is the socket running from the leisure battery. So you can now charge your phone without affecting the starter. Our fusebox is shown below. In hindsight, a design with a single input cable would have been more practical. We used one such box elsewhere and it was much easier to use.
NB: When figuring out which fuse you need to hijack, get hold of the wiring diagrams (example below) from a Haynes manual. They are complictaed at first glance, but start at the component and work backwards until you find the fuse. Many circuits are activated by relays which are run off a different fuse. If you power the relay, not the component, with your aux battery, the circuit will now be complete without the ignition being on, but the power actually operating the component will be from the starter, and you could drain it without realising. A good way to know for sure, is to disconnect the starter battery. If the component still works, then you've got it right, with power coming from the leisure battery. The radio in a discovery 2 has several fuses (3?) which control it, and is a good example of a situation where it is easy to get it wrong. If you are trying to run that off the second battery then do your homework first!
