Dual Battery Installations

Posted on 09 Dec 2014 by cliveweatherhead@batterybusiness.com.au

Why a dual battery set-up?

The usual reason to install a dual battery set-up in a vehicle is to provide power to accessories, eg a fridge, without draining the starting (cranking) battery. There are other reasons - a back-up starting battery for example - but usually the 2nd battery in a dual battery set-up is to run fridges, lights, camping equipment etc.

How does a dual battery system work?

In the simplest form, power is taken from the alternator (where it comes into the cranking battery) and is fed through an "isolator" (see below) so that when the engine is running (and the alternator is charging) the 2nd battery is sharing the charging from the alternator, but when the engine is stopped, and the cranking battery voltage drops to (say) 12.8 volts, the isolator opens the circuit between the two batteries and disconnects the cranking battery from whatever accessories are running on the dual battery system, so that they cannot drain the cranking battery below the voltage that is needed to start the vehicle.

What are the isolator choices?

Generally the isolator is either a voltage sensing relay (VSR), or a DC to DC charger (ie takes in DC current at around 12-14.5volts and outputs a charging current to the 2nd battery). This is where it gets a bit complicated though, so we'll break that down below.

What are the types of a VSR?

The VSR comes in a few versions. One will just sense the cranking battery, so it closes (completes a circuit) at above (say) 13.3 volts (alternator running), and opens at below 12.8 volts. The reason for the gap is to stop the relay opening and closing frequently and wearing out quickly. Version 2, though, is a DUAL SENSING VSR, which sense the voltage on both sides of the relay so that if either battery is above (say) 13.4 volts then the relay closes. This would be useful in a boat, for example, where there are two batteries separated by a 1/2/both switch, and either battery can be used to start the motor. With a dual sensing VSR, whichever battery is used to start, the motor will charge both batteries when running. The next version of a VSR is like those above, but programmable, so you can set the voltages that it opens and closes, and the delay times between sensing and acting on what it finds. Programmable VSRs have many uses, especially when different battery types are being used (such as a lead acid battery on one side and a lithium battery on the other). Finally, some VSR's have an override facility - either a built in switch or a remote switch - that allows you to join the two batteries in an emergency...a very nice feature if your cranking battery dies in the middle of the bush, and much easier than using a jump lead to join the batteries.

What are the types of DC to DC charger?

There are many brands of DC to DC charger. Some have the facility to take a solar panel input as well as alternator input, some are suited to under bonnet installation (typically called "potted" units, others need to be somewhere cool and dry such as under the dashboard. Different models have different amp charging rates, typically somewhere between 20 and 40 amps, although there are ways to add extra bits to boost the throughput to 100amps or more (which is seldom needed). In general you'll pay about 3-4 times more for a DC to DC charger than a VSR, but in the overall cost of a dual battery installation, the difference might be $300, give or take.

How do I choose between a VSR and a DC to DC charger?

It's horses for courses. A VSR typically allows 100-160amps of flow through from an alternator (more than most alternators put out), whereas a DC to DC charger allows, typically, 20-40amps so, theoretically, a VSR allows quicker charging. BUT, and it is a big BUT, most 2nd batteries are going to be around 100 to 120amp capacity, and will typically only accept a charge rate of about 20% of their capacity for a while, if heavily drained, quickly falling to around 10% of their capacity, so in most cases it doesn't matter whether you use a VSR or a DC to DC charger in terms of charging speed. In both cases, if your battery is heavily drained, it'll take a few hours of engine running to get it well charged again (not the case with lithium batteries by the way). On the other hand, a VSR only allows the alternator to do its job, and that is basically bunging as much as it can in a steady voltage (usually) at the battery. This is fine to get a battery to about 80% or even 85% full, but to get the last of the charge into a battery it needs to be pulse charged, which only happens with a DC to DC charger. So, in summary, a DC to DC charger costs more, and will give a better charge overall, but a VSR is simpler and can be useful if there is a lot of battery capacity to charge (eg when you are charging a couple of batteries in a trailer via the VSR).

Battery choices

Lead acid batteries come in a couple of different styles, and are built for a few different purposes (we'll come to lithium batteries later - they are a bit more exotic than good old lead acid). Firstly, for the purposes of under-bonnet use, the options are wet accessible (batteries that you can top up), wet maintenance free (no access to top up) and AGM (dry, absorbed glass mat). Accessible batteries have largely had their day, but they still might suit where conditions are very hot for the secondary battery (Nissan Patrol Turbo Diesel owners please note!). Maintenance free batteries are, by far, the most common starting battery for cars, and very often also used for the deep cycle battery, and AGM batteries tend to cost a bit more, punch a bit harder and last a bit longer, all other things being equal, but AGM batteries, in particular, don't generally like a lot of heat.

Cranking batteries and deep cycle batteries should be different internally. If someone tells you otherwise well, bless them and let them enjoy their delusion. Cranking batteries are designed (plates, electrolyte, internal contacts) to deliver huge chunks of power quickly, and to get recharged before they have given up more than a few percent of their total charge. Deep cycle batteries are designed to deliver small amounts of power, consistently, for much longer, and to accept a much deeper discharge without damage (there are varying types of deep cycle design, too, but that's too much information for this article). Some manufacturers create a hybrid between cranking and deep cycle, and all lead acid batteries can perform both functions to some degree, but generally should stick to their knitting.

In general, the positive post of the battery should be towards the highest part of the bonnet - so generally towards the rear and Centre of the engine bay, with negative nearest the metal. Battery manufacturers accommodate this by making most (not all) deep cycle batteries with positive on the left (when the terminals are nearest you) which works in most cases. This is a common trap, so worth a second look if you are doing a DIY installation.

In general, you'll be trying to fit as large a second battery as possible, and most under bonnet conditions allow for something of between 100ah and 120ah capacity (the 100ah is generally called an N70 or size 27, whereas the 120 will be a size 29 or 31 in battery parlance). In general, also, it is worth matching the batteries as closely as you can, in terms of size, style of battery etc, so maintenance free for both is typical. You can use AGM for starting (there are several excellent brands such as Optima, Odyssey, Duracell, Fullriver HC, Braille etc) and you can use an AGM under bonnet for deep cycle if it is positioned away from direct heat and, here, the brand choice is critical, so we like Duracell, Fullriver and Optima for the deep cycle application (but note that Optima batteries are a compromise between deep cycle and cranking so you won't get as much capacity in the Optima, but you will get colossal starting power).

When it comes to battery quality, you get what you pay for. Take advice from someone that fits a lot of batteries, because they'll know the brands that fail prematurely. Buy a brand because it is known to be good, not just because you have heard of it, and make sure you aren't buying old stock. We see examples, all the time, of people coming in with an internet bargain battery that doesn't seem to do what the advert promised and, if it seems too good to be true, it probably is. Make sure there is at least one retail store that you can contact with real people to talk to who do this stuff for a living. 

Topics for future writing include:

Battery location



Cables and cable running

Battery installation and location

Lithium batteries

Tricks of the trade

Accessory sockets

Wiring for an on-board dual battery AND a trailer or caravan