Double battery setup what do I need?

Hi guys,

I'm looking to install an additional battery in my defender to work as a secondary for turning the engine over and winching.

I'm not exactly sure what the best way of doing this would be as I would be hooking them up in parallel positive to positive and negative to negative, leaving no post for positive out and negative out. Any ideas what I will need to make the setup work?

I'm sure you know a winch will make short work of a battery, any reason you can't leave the engine running? This will keep the voltage up and be better for the life of the winch

You'd also be better not connecting in parallel as both batteries can go flat and you'll be left stranded.
That said, you'd want 2 healthy batteries or one will pull down the other over time and you'll end up cooking one

As @kermit_rr indicates, it's complicated.
Just adding a second battery won't increase the vehicles ability in turning the engine over and winching unless the original battery wasn't big enough in the first place.
You need around 200Amps to start the engine, so your battery needs to have a CCA rating of greater than that so that it will start on a cold day.
The C rating of a battery in Amp Hours defines the amount of power (amps) the battery can deliver in X hours. A C20 rating of 100AH means the battery can supply 5 Amps for 20 hours or 1 amp for 100 Hours.

To start the engine then with a C20 100AH battery you have (theoretically) 30 mins of run time. In reality it will be a lot shorter than that as, as the battery loses voltage the currect rises.
Obviously your engine starts within a few seconds (or it should) so you are OK with a lower capacity battery as long as it can provide the required CCA. I believe that LR specify a 70AH 570CCA battery.
Your winch will work fine too as long as the battery provides enough power for as long as you want to run the winch.

If you have 2 identical batteries wired in parrallel (+ to + and - to -) then the CCA and the AH will sum so 2 x 100AH 600CCA batteries will become 1 x 200AH 1200CCA, however matching batteries of that capacity is very difficult and if they are different then things start to go wrong. Connecting them together will be tricky too.
You would be better to fit a 120AH 900CCA battery (or larger if it will fit in your battery box) and uprate your alternator to 100A and run the engine if you need to winch for anymore than 15 mins.

I you do feel the urge to install a 'secondary' or leisure battery (ie one that isn't used for staring the engine etc) then there are loads of systems out there that will allow you to attach long duration, low current applications (like a radio, internal lights, fridge etc) and charge each battery independantly without causing any damage to either. These twin charge systems allow you to fit different types of batteries without any damage or risk of losing power from your primary battery so you can always start the engine and spin up the alternator.
My winch draws around 150Amps when its pulling near capacity, so a similar load to starting the engine, in fact it's greater as the engine starts within seconds and the winch maybe pulling for 10's of minutes.
Once i'm set up and need to pull a load with the winch, I always start the engine.

Just a note on leisure batteries, they are good for sustained low current draw, not the high requirements of a winch

Lej said:

Hi guys,

I'm looking to install an additional battery in my defender to work as a secondary for turning the engine over and winching.

I'm not exactly sure what the best way of doing this would be as I would be hooking them up in parallel positive to positive and negative to negative, leaving no post for positive out and negative out. Any ideas what I will need to make the setup work?

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Have you considered just fitting the largest single battery in the battery tray? You can get quite a good tractor battery in there. Dual batteries always seem to cause issues, at least for all the people I know who have them (many of the reasons cited above).

As 300 bhp above says If you are just using your winch now and then [ not winching comp's ] then one battery with a high CCA of the 663 or 664 type [ just the way around the terminals are] Tractor / small truck battery will do the job. I have one in my Ninety and it fits in the battery box with no mods needed. Sure to be an Agri parts supplier near you that will have them in stock.

tottot said:

As 300 bhp above says If you are just using your winch now and then [ not winching comp's ] then one battery with a high CCA of the 663 or 664 type [ just the way around the terminals are] Tractor / small truck battery will do the job. I have one in my Ninety and it fits in the battery box with no mods needed. Sure to be an Agri parts supplier near you that will have them in stock.

Click to expand...

I currently have a Hankook 12V 1000A, 643 644 in my Landy, it didn't even last a year before I have to charge it every couple of days when it hasn't been running. It's even worse in cold weather. (I've made sure there's nothing draining the battery).

In short I wanted to double up the battery to make sure it will always start, but having done some research and reading the above posts I realise it's not that simple.

Can I ask what make of battery you have? I'll take a punt and give that one a go.

Thank you all for your time, much appreciated.

Battery.
What about using a red top gel battery?
Expensive but reliable

Possibly you've just been draining the battery too much without a full charge. An alternator won't charge a flat battery properly, it needs to go on a proper charger, or it will die

I think most warranties are void if you let the battery go flat