Simple second battery connection

Submitted: Sunday, Sep 07, 2003 at 22:09
ThreadID: 7092 Views:1591 Replies:8 FollowUps:12
This Thread has been Archived
Recently bought a new TD Prado. I notice it's got space for a second battery (no doubt fitted in some countries). What is the simplest connection method to put another battery there as a back up? Not interested in an expensive, hi tech method. thanks in anticipation
Back Expand Un-Read 0 Moderator

Reply By: diamond (bendigo) - Sunday, Sep 07, 2003 at 22:26

Sunday, Sep 07, 2003 at 22:26
gday bob .
simplest method is a simple solenoid part number st81 from repco $30.
run big wire from your main battery to big post on solonoid and big wire from from other big post on solonoid to your second batery.then run a little wire to the little post on from your ignition on to open solenoid when 4by running and when switched off solonoid closes so you cant run anything from your main battery .cheap/simple and efectivešqUC3<BuIL
AnswerID: 30393

Follow Up By: Slammin - Sunday, Sep 07, 2003 at 22:55

Sunday, Sep 07, 2003 at 22:55
Diamond I take it the above setup means the spare battery is only in use when the ignition is off - and the main is used when driving? When does the charging happen for the spare? or are they both on when ignition is on? Would the above setup mean my radio and clock etc would have to be reset everytime the car starts?
I ask all this as I want something similar to above but had figured an isolator switch was the easiest way to go, just swap back and forth every couple of months or so.
Any help would be great.
FollowupID: 21433

Follow Up By: diamond (bendigo) - Sunday, Sep 07, 2003 at 23:05

Sunday, Sep 07, 2003 at 23:05
gday slammin.
when your ignition is turned of only the second battery is being used so you have all your accesory conected to it spottys/winch/fridge ect.
once your car is running the solinoid opens so both batterys are being normal every day car functions clock ect will not be affected.and problem with isolater switch if something happens when your camping and yopu forget to switch it you run the chance of flatening your main as well.ive had mine 2 yrs no problems and $30+wire isolater switch probly cost nearly thatšqUC3<BuIL
FollowupID: 21435

Reply By: Phil G - Sunday, Sep 07, 2003 at 23:10

Sunday, Sep 07, 2003 at 23:10
If you don't want to reconsider the expensive, high tech options, then you are left with only one choice - wire the batteries in parallel with a 200amp switch in line. This basically doubles the capacity of your starting battery. The switch is rarely used, but can isolate the batteries in an emergency.

You will need a low voltage cutout to prevent your fridge from dumping the batteries below 11.9 volts. At that voltage, with the two starting batteries, you should still be able to start a Prado TD. Main downside of this system is that the batteries should be identical and replaced as a pair. You will find that they'll last a long time.

Apart from that, I have a Rotronics MH10 for my Prado TD - $250 well spent in my opinion, as I never have to think about it. I wouldn't fit a solenoid or other relay system to a Prado TD, as it has an ECU.

AnswerID: 30407

Reply By: Luke - Monday, Sep 08, 2003 at 00:15

Monday, Sep 08, 2003 at 00:15
I'd have a look into what the "high tech options" offer, rather than a solenoid (or less).

There are some around that are not that expensive and offer a very sophisticated battery management system to safeguard you from killing either battery, and to get the best life and usage from both batteries.

Automotive battery management is a science, and whilst a solenoid will certainly do a job, it will not do the best job :-)

Run a search on this topic, and be sure to understand the implications of all methods of battery managament before you go ahead - there is heaps of info out there :-)
AnswerID: 30409

Reply By: Member - Bob - Monday, Sep 08, 2003 at 08:46

Monday, Sep 08, 2003 at 08:46
Thank you to all who responded. Diamond I think the part from Repco seems to be what I'm after. Can't help feeling that some systems make a simple task seem complex. Even the manual marine switch seems a good option. I think some people aren't prepared to remember to do things manually!
Thanks for your help too Luke - can't say I agree about it being a science though as it is in fact a very simple task really. There are a number of electronic devices that are made to sound complicated in order to justify a high price tag!
AnswerID: 30417

Follow Up By: Member - Oskar(Bris) - Monday, Sep 08, 2003 at 16:11

Monday, Sep 08, 2003 at 16:11
Post ID 5991 (July 14th) in the archives covers some of the debate/aspects that you are interested in.
I have used the solenoid setup as I described in there and have been very happy with it and with no problems so far.
The offer a diagram is still there.
The issue of correct charging of batteries for longevity is also worth a search.
Do an archive search though to satisfy yourself.
OskarMrs Oskar on Big Red
FollowupID: 21492

Follow Up By: Luke - Tuesday, Sep 09, 2003 at 00:13

Tuesday, Sep 09, 2003 at 00:13
Hi Bob,

I'll happily agree that some systems are overpriced, but there is more to charging a battery than simply having a switch, or an automated switch using a solenoid.

For example, some battery management features that come to mind that are readily available in many good battery management systems are:

1: Isolate second battery from main battery when vehicle engine is not running so that you don't flatten your main battery when running accessories. (this is about all the solenoid or switch is capable of).

2: Sense the level of auxiliary battery and cut - out if level gets too low to prevent total discharge which can damage a battery.

3: Sense the level of main battery and only allow a charge to be passed to the second battery if the main battery is fully charged, thereby ensuring that your main starting battery is always at it's peak.

4: Sense the voltage level of the second battery, and if the level is low, allow a trickle charge to slowly bring the battery up to a point where a full charge can take place without damaging the battery.

5: Apply a full charge when second battery has a sufficient state of charge to safely accept it.

Anyway, that's why I will be fitting a battery management system to my vehicle instead of just a solenoid. This will give me better battery performance, longer battery life, and always ensure that my main battery in in a good state to charge the vehicle.

There is obviously alot more to charging batteries, and it IS a science, but having said that - like I said in my previous post, a solenoid will do A kob - just not the BEST job :-)

If that's what you want - go for it.



FollowupID: 21559

Follow Up By: Luke - Tuesday, Sep 09, 2003 at 00:17

Tuesday, Sep 09, 2003 at 00:17

>This will give me better battery performance, longer battery >life, and always ensure that my main battery in in a good state >to charge the vehicle.

...that should of course read start the vehicle, not charge the vehicle :-)

...and kob should of course be job....

apologies for any other typos :-D
FollowupID: 21560

Follow Up By: Member - Oskar(Bris) - Tuesday, Sep 09, 2003 at 10:58

Tuesday, Sep 09, 2003 at 10:58
You are correct of course about each of these points and I have no argument with any of these.
My personal preference, however,is "simple is best" and thus I only have a couple of points to trouble shoot if a problem occurs and there are no electronic bits for me to "wonder" about. The cost is not an issue I just can't help myself
Besides I make it my business to know what's happening "under the bonnet" anyway so the simple system suits me fine.
I agree that the store-bought systems are best for most people whether they are technically minded or not.
Don'k worty aboit the typo's we kbow whar you meam.

Mrs Oskar on Big Red
FollowupID: 21589

Follow Up By: Luke - Tuesday, Sep 09, 2003 at 12:23

Tuesday, Sep 09, 2003 at 12:23
G'day there Oskar,

I understand people liking simplicity, and DIYing - I'm an avid DIYer myself, and like to do as much as I can myself on my GQ shorty.

I like to muck around with electronics too, and recently built an electronic ignition for my GQ SWB which is working brilliantly. I've also built a solid state battery management system based on an Electronics Australia (now Silicon Chip) circuit. It will be installed with some additional safeguards to circumvent the well documented potential reverse current problem.

Total cost for my Electronic ignition - around $60 bucks :-) and IF it fails, I can return to standard points while out on the road in about ten minutes :-)

Total cost for my battery management will probably work out to be in the order of $120 +/- fully installed with all cables etc (not including the cost of a battery tray and second battery).

The solid state system will theoretically be more reliable than the mechanical operation of a solenoid, which can be prone to failure after a period of time, as well as having all of the extra functionality and safeguards. The problem is that when it (the solenoid) does fail you will generally not know about it until you need the power and it's not there.

If my battery management fails it will let me know immediately.

Anyway, each to his own - if you like simplicity, that's fine. Like I said in my first response to this thread, I just think it's important to know exactly what you will and will not get with a solenoid Vs a solid state battery management system before you make up your mind.



FollowupID: 21597

Follow Up By: Member - Oskar(Bris) - Tuesday, Sep 09, 2003 at 13:54

Tuesday, Sep 09, 2003 at 13:54
I like your style.
Keep it up.
OskarMrs Oskar on Big Red
FollowupID: 21606

Follow Up By: Member - Oskar(Bris) - Tuesday, Sep 09, 2003 at 14:00

Tuesday, Sep 09, 2003 at 14:00
BTW what system are you considering?
OskarMrs Oskar on Big Red
FollowupID: 21607

Follow Up By: Luke - Tuesday, Sep 09, 2003 at 16:49

Tuesday, Sep 09, 2003 at 16:49
Hi Oskar,

The base kit I have built can be found here:

I'll be adding some modifications and improvements as I go along...but nothing major - it's really quite a good kit.

There is one potential flaw - which is the possibility of a reverse current leakage from second battery back to main battery via the FET protection diodes, but this can only happen under certain conditions which under normal circumstances should not present themselves.

I may take steps to provide absolute isolation to guard against this though - still undecided on that one.

The reverse current 'problem' with this particular management system actually has potential gains also if the condition of the main battery is insufficient to start the vehicle but the second battery is in good condition - but that's another story :-)



FollowupID: 21628

Follow Up By: Member - Oskar(Bris) - Wednesday, Sep 10, 2003 at 15:10

Wednesday, Sep 10, 2003 at 15:10
On a lighter note.........
Some guys I know in the heavy duty electronics business sometimes refer to FET's as Flame Emitting Transistors.
When they do go "poof" they go "BANG"
Have fun
OskarMrs Oskar on Big Red
FollowupID: 21704

Reply By: chopper - Monday, Sep 08, 2003 at 18:21

Monday, Sep 08, 2003 at 18:21

If you run everything off the 2nd battery and it flattens, how do you glow the engine before starting.

I know, you hit the solenoid and youv'e got the starting battery for the glow and the go, however, if you've flattened the 2nd battery, won't the pimary battery attempt to quickly dump its charge to the 2nd battery, possibly leaving you with two batteries with some charge rather than a battery with full charge.

I have a second battery, (with a TJM IBS system) but have a seperate circut coming off it for accesories, the main loom of the car was not affected, lights, glow and starter are all still off the main battery.

This stuff is not as simple as some would like it to be, and what works for one may not work for another.

AnswerID: 30476

Reply By: Ross - Tuesday, Sep 09, 2003 at 14:23

Tuesday, Sep 09, 2003 at 14:23
You've had the money for a new vehicle. Just go out and by one of the sealed battery packs that come with jumper leads and cig lighter or hella type outlets--potable and simple.
AnswerID: 30576

Reply By: Bob Y. - Qld - Tuesday, Sep 09, 2003 at 21:45

Tuesday, Sep 09, 2003 at 21:45

We had a tojo turbo L/C wagon that had a solenoid system on it when we bought it. Had no problems with it. Did read somewhere , that someone had a delay on switching of solenoid. Gave main battery a chance to start engine.

Another alternative would be to get a in-line oil pressure switch, in series with solenoid, that also allows engine to start, before charging secondary battery.

On current vehicle, we have a Piranha Management system, which has been working for 2 years. Dead easy to fit.


AnswerID: 30623

Reply By: Terry - Wednesday, Sep 10, 2003 at 23:34

Wednesday, Sep 10, 2003 at 23:34
I,m not sure if we're understanding how the alternator works. The charge from the alternator is around 14.8v on start up ( Toyota's are around that amount) in a single battery system once the battery is fully charged the alternator drops into float mode, around 13.6 v, which is a trickle charge. In a duel battery system if the batterys are connceted in series ( parallel) the alternator senses the battery as one and doesn't go into float mode until both batteries are fully charged. If a device is placed between the two batteries, the alternator only senses the battery with the greatest charge and tops that up then goes into float mode, the sencond battery then gets a trickle charge. If the current draw on your second battery is above 2amp/hrs it will eventually flatten. Rather than buying an expensive isolator system, it would be more economical to carry a new battery around, far cheaper than the isolation system which can be quite expensive.

Just a thought.
AnswerID: 30738

Follow Up By: Luke - Thursday, Sep 11, 2003 at 10:10

Thursday, Sep 11, 2003 at 10:10
Hi Terry,

I think whether the alternator is going into float mode isn't really important here. What we have is the second battery connected to the primary battery by means of an isolator switch/solenoid, or solid state management system.

When the primary battery is fully charged it is used to supply a charge to the second battery, the alternator will charge the primary battery as appropriate - ie: the level of the primary battery will be kept at an optimum level via the normal operation of the alternator, whilst the secondary battery is kept at an optimum level via the primary battery.

If the alternator went into float mode, and only a trickle charge (less than the current drain) was applied to the second battery - then the battery management systems would be useless (which they certainly are not). Indeed, they work very well.

Anyway, that's my understanding - what are others thoughts?



FollowupID: 21762

Sponsored Links

Popular Products (13)