I haven't got my defender yet and I'm so confused already about adding a second battery. I've done a search but no post seem to answer my questions.
Sorry it's so long but I'm just confused.

From what I can tell there are 5 main methods (there might be more that I haven't come across yet) for installing a dual battery system but each have there floors and disadvantages.

1. Run 2 alternators
Advantages, complete seperate circuits no worrys about running the start battery down at all due to relay problems. If start battery fails can manually use jump leads to join them together to get the Landy started. Can run 2 different types of batteries deep cycle, starter, lead acid, agm, etc without any problems. The Ah size of the batteries don't have to be the same.
Disadvantages, you need to install seperate altinator run cables, etc.

2. Ignition fed really, install a rely off the ignition switch that simply joins the two battery together when the ignition is on.
Advantages, simple to fit will isolate both battery's when the ignition is off saving the starter battery.
Disadvantages, not really idea to run different types of batteries or different sizes, soon as the ignition is switched on the weaker battery will start to draw power from the stronger battery, even when the engine isn't running.

3. Run two batteries in parallel.
Advantages, simplest thing to install and fit just join +ive to +ive and -ive to -ive job done, double the amount of power available.
Disadvantages, batteries should ideally be identical size and type, no protection for starting, if one battery develops a broken cell it will drag the other battery down with it.

4. Voltage sensing rely.
Advantages, cheap, easy to fit, isolates start from second battery, can manually use jump leads.
Disadvantages, again batteries should ideally be identical size and type this cause problems when wanting to run things over night due to not being a deep cycle battery. If the secondary battery is extremely low it will receive charge from both the start battery and the alternator until both batteries equalise in voltage then both will charge, this could (theatrically) on short runs drain the start battery.

5. Dc to dc chargers ( battery management system).
Advantages, easy to fit can run different types and sizes of batteries, full isolation of starter battery, can jump lead them together.
Disadvantages, expensive.

The other question is what sort of battery type to use as the secondary battery, starter battery or deep cycle.
Starter battery's should only be taken down to 80% charge.
Deep cycle depending on make can go down to 20% to 50% charge. If going deep cycle, 12v battery or 2 x 6v batteries in series.

Then what sort of battery, lead acid, agm, calcium, all prefer different charging voltages.(I read somewhere)

I know there is people on here that will say you can run different types and sizes with each other and you can but they won't last as long (but does this matter in the long run), I've seen three set up's fail, I can't confirm that it was due to different sizes or types but they haven't failed since we got dc to dc charges installed on two of them and installed two identical batteries on the other. (Boat and two motor homes)

I might be over thinking things (probably am) but the more I look into it the more questions are raised.

If any one could shed light if I'm missing something on one of the methods that might tilt things in that ways favour or I've miss understood something I've read and pieced together.

Wow, what an essay!

Hopefully someone will come along and point out where I am wrong as this isn't something I have actually done, although I have done some research on them.

My thoughts are that by far the best method is to simply use a split charge system. The T-Max ones seem to be popular

T-MAX SPLIT CHARGE RELAY DUAL BATTERY SYSTEM WITH DIGITAL BATTERY STATUS MONITOR | eBay

The best battery to use are the deep cycle ones. The yellow top Optima ones seem to be popular.

Personally I'm going with the dual alternator set up. If a 200/300tdi it's simple.

This will be for a disco but going to remove the air con pump and fit the second alternator there. Run the starter battery from the newly installed alternator , and then charge the secondary deep cycle battery from the original (albeit upgraded) alternator position.

Then I'll run a cable into the passenger footwear to run a secondary fuse box to run any additional equipment fitted.

At least this way you have 2 separate systems. And like you say in option one can easily jump the original system should a problem arrise.

What do you intend using the second battery for?

Starter battery? Good for starting engine/winching not good for running low on charge.

Leisure battery? Good for running your fridge off but not designed for heavy current purposes such as winching or starting the engine.


What about a combined starter/leisure battery such as the Odyssey 1500? Best of both worlds.



The T Max split charge system is a Chinese copy of the original Swiss made IBS system. The T Max does seem to suffer from quality control issues which does not seem to be the case with the IBS.

Twin alternators are really only used for competition vehicles




Brendan

I want to be able to run a fridge and a compressor really. But confidence that the truck will still start. Plus the added confidence that I have a backup battery.
I've had two batteries fail on me in my pervious cars, neither in very good places and a backup battery sounds extremely comforting.

Use a Marine battery selector - so you can choose which battery to run your vehicle from (i.e. if you have a flat battery you just turn a dial 1 click and you'll be running off the second battery)

I'd personally go with either a battery management system or duel alternators. Depending on the engine and use. You mentioned fridges, so, could be 100% draining a battery over night, a standard split charger would put a huge current draw on your main battery if you connect a 100% flat second battery to it just via a split charger relay, the charge needs to be intelligently managed.

no issues joining an unmatched pair through a vsr, people with caravans have been doing it for years

as I dont do offroading in any serious form, I`ve got my disco as an everyday work and pleasure vehicle, but want reliability if I leave it some time parked up.
(I have another car also)
I just went the same type of batteries route - linked as 12 volts (in theory)

got huge amounts of standby power in its doubled capacity.
could fit a battery cut out switch, but would find it a pain in the a$$ switching it..
but, thinking out loud, if I would be using 12 volt power while the engines not running, a switch would then be handy to cut the main "live" battery from the pair....
infact, I`ll put that on my todo list I think..
along with fitting a heavy duty cigar lighter socket thats been in its packet for months now - along with heavy duty plugs for amber beacon and hand held spotlights and such..