Dual Battery Components?

hey all,

i'll be putting a dual battery system in and so far i haven't really a clue...

all i know is that i'll need to buy a battery tray and a 2nd battery (been recommended a HYBRID battery which can be used for cranking, but which is also a high capacity battery).

also i'll need some sort of cut off system for the 2nd battery since the engel doesn't have an auto cutoff in it.

so what isolator systems are recommended and exactly what function do they have?

i've been told i need a special relay/system that makes it so once the 2nd battery has been used up to say 11v cut off it doesnt have the main battery equalise with the 2nd battery meaning the main battery has a low voltage and thus a risk of not being able to start.

all info is appreciated and i'm working on a bit of budget (arent we all?)

cheers

my setup TJM tray (after comparing i think looks more robust then piranha), supercharge all-rounder (i agree you should go the hybrid type), Projecta 150amp electronic dual battery isolator kit (good value and full featured). thats all you need not a hard install.

I have got a VSR relay in mine, basically the same as a redarc (do a search on redarc, lots of people here use them) If you mount the battery and relay, you can get an auto sparky or similar to make up cables for you to run inbetween. It is really really simple, if I can do it anyone can!

I have got a 120ah AGM sealed battery and am really happy with it.

maddogrc said:

so what isolator systems are recommended and exactly what function do they have?

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Mine connects both batteries until the voltage drops to 12.8 volts. It then disconnects them so that the front battery will have cranking power left. The fridge will then only run off the rear battery. When I start the car, the isolator won't reconnect and charge the aux until the front battery reaches 13.2 volts. Its a nice and simple system that works just fine.

when i went to the 4wd shop for quotes they had a crap load of dual battery gear.

they wrote down on a lil business card the prices of items, one of those being the REDARC for $177...

i've looked around at REDARC and i don't know exactly what model it is, but i cant see any for $170 bucks, only the REDARC smart start for like $90 on ebay, lol

hope that isnt nearly a 100% mark up ugh

I think that'll be the same one mate, they are heaps cheaper on ebay.

Don't think the shop is trying to rip you off though, there would be a hell of a lot more overheads in their shop than the bloke on ebay probably selling them out of his shed.

Just remember one golden rule about the second battery - it will never, ever reach full charge.

Neither will your car's main battery, but that's essential.

Here's why. Car batteries will start fizzing their electrolyte away at somewhere around 70-75% of their capacity. This results in the necessity for more frequent maintenance - and to avoid such high maintenance, your car's regulator will restrict the charging at around 14.4V (batteries need around 15V to fully charge).

Now, since you shouldn't discharge a battery to below 50% of its capacity, your 100Ah battery - charged to 75Ah - can safely deliver to you just 25Ah.

The way around this is to not use a battery isolator at all, and "endure" the higher maintenance. Run power cables directly from the battery through a thermal fuse and into a relay, which only turns "on" when the vehicle ignition is on. The relay passes this power via 6mm automotive cabling (minimum) through to the tub and to an inverter. The inverter powers a 3-stage battery charger - which will deliver 100% charge, giving you 50Ah out of your battery instead of just 25.

That's what I have in my caravan, and what I'll be doing in my tub as I get the time.

Oh, an added advantage of this: the batteries in the tub can never drain the starter battery. You should never rely on the starter battery for power anyway: it's designed to deliver a short burst of high energy and then be recharged. Discharging the starter too much and too often will drastically shorten its lifespan.

Which is the best battery to get? AGM is great. Gel is also very good. Marine batteries are up there, the normal "deep cycle" are just sturdier versions of the starter battery and will handle cycling a bit better but not really enough to justify the cost, compared to gel or AGM.

normal deep cycle batteries take too long to charge up though? no? i believe thats why most people suggest the dual purpose ones.

i just dont see how you cant 'not' run an isolator tony, if you got both batteries (starter and dual purpose) hooked together and run a fridge the whole night what if the voltage becomes too low and you cant start the car at all with either battery since both are low?

surely you'd need to isolate the starter battery so it stays on full charge to be able to start the car up?

That's what the relay is for: it only passes power down from the main battery to the tub when the ignition is on.

Deep cycle batteries can charge up at decentish rates, but you actually get better charge rates from a good 3-stage charger than you do from a cheap "smart" charger.

maddogrc said:

i just dont see how you cant 'not' run an isolator tony, if you got both batteries (starter and dual purpose) hooked together and run a fridge the whole night what if the voltage becomes too low and you cant start the car at all with either battery since both are low?

surely you'd need to isolate the starter battery so it stays on full charge to be able to start the car up?

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Lol, thats when you need jumper leads.

Old.Tony said:

That's what the relay is for: it only passes power down from the main battery to the tub when the ignition is on.

Deep cycle batteries can charge up at decentish rates, but you actually get better charge rates from a good 3-stage charger than you do from a cheap "smart" charger.

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I very much agree with your method Tony

WIR35 said:

I very much agree with your method Tony

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so you need another special charger system also???

i thought the aux battery would charge once the car is running as it'd reconnect to the other battery in parallel?

maddogrc said:

so you need another special charger system also???

i thought the aux battery would charge once the car is running as it'd reconnect to the other battery in parallel?

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I believe the relay (the coil of which is powered by ignition of vehicle) powers the 12VDC -> 240VAC inverter, which in turn powers the 240VAC-12VDC battery charger, which charges the auxillary battery. You could just have a 2 pole relay (or contactor) which directly powers the auxillary battery when latched (as you suggested), but as Tony said the deep cycle batteries charge better with a charger rather than from your vehicle's alternator. Plus if the relay latches in when you hit the ignition in your vehicle, thus parallelling your two batteries, if your auxillary battery is empty then half the charge from your main battery will instantly go into the empty auxillary battery (rather like if you had a full rainwater tank and an empty rainwater tank with a pipe between them at the bottom, then opened the valve - half the water would go into the empty tank to make them both half full). This will reduce the starting capacity of your main battery to effectively start your vehicle's engine, especially if there is some cabling distance between your main battery and your auxillary battery (voltage drop due to large load current for engine start up)

Yep, the last thing you want is for the aux battery to be driving the starter motor. Most deep cycle or AGM/Gel batteries can't dump that much power, and more importantly, trying to pump a couple of hundred amps down 6mm cable over 5 metres long is going to cause a fire.

It's not a risk, it's a guarantee. Whatever you do, don't try to rely on your aux batteries to start the vehicle.

The reason for charging batteries with a 240VAC charger is so that the battery actually gets 100% charge. Your alternator/regulator can't do that - it's designed not to!

well the 2nd battery is going under the bonnet and i'm leaning more towards the hybrid batteries since i'm on a bit of a budget...

you talk about the start battery dumping half its capacity to the aux battery when the aux battery is low leading to the inability to start the car...however isn't the redarc isolator unit designed so that when you start the car it disconnects the aux battery leaving the starter battery only to start the car and thus not allowing the equalisation of the start and aux battery?

That's how the redarc works, yes, but WIR was referring to the simple relay, which while a lot cheaper, wouldn't provide that sort of protection.

Just bear in mind the reduced capacity you'll have because you're charging from your alternator. If you can live within that amp budget, go for it.

if its not too much hassle can you link me to a product that does the whole 240v charging setup?

i'd appreciate it

you could simply connect the contacts of a delay-on timer in series with the active wire for the auxillary battery relay, which will delay the latching of the auxillary battery relay until the engine is running. for example you could get a 30 second timer which would give your engine a bit of time to warm up a bit, and time for your main battery to get the charge back that it lost starting your engine. It would be a compromise at best, but should work quite well.

woznme said:

Projecta 150amp electronic dual battery isolator kit (good value and full featured). thats all you need not a hard install.

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I agree with these Projectas however the one thing I will say is the 100 is more than enough. I understand that many people will disagree but if your running a standard alternator which is only about 90 Amps you can save yourself quite a bit of cash. The 100 is still capable of 125 Amp short term and and providing the battery is right will still be able to be used to crack the car over if the main fails.

When you consider that even at a full 90 Amps your vehicle is taking the majority of the Alternators output there is no need to go to the expense of a 150 and like cable. I'm running my system off a 100 and 10mm2 cable with a self resetting fuse and I've not had a problem since last Monday running the fridge in +35 heat on a cycle of +3 - -3 degrees and keeping the battery voltage above 12.5 even over night with the fridge running.

Considering I brought my Projecta100 for $54 it's a hell of a saving over the 150 which I'll never fully utilise. That's just my two bob's worth anyway, some may disagree but it's worked for me in the Nav and other utes before the Nav so I'm sticking with it.