Dual Batteries

Submitted: Monday, Feb 11, 2002 at 01:00
ThreadID: 747 Views:1902 Replies:3 FollowUps:8
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I know this subject has been covered quite a few times already, but I still have a question. A few years ago I purchased a Piranha 150Amp electronic Isolator for my Kombi for around the $150 mark. For the few years that I used it it never let me down, so I was happy with it's perfromance. Now I want to set up dual batteries in my 4wd. I went to purchase the same piranha isloator and was quoted $260. I bought my previous isolator from out of town and I can't remember where I bought it. Does this price sound right? Anyway, I then went to a local auto electrician to get a quote. These guys have a contract with a motor home manufacturer to install the 12V electrics, and don't fit electronic isolaters as in their opinion they are unreliable. The guy told me that the best way is to keep it simple and use a (as they do to all new motorhomes) 160amp heavy duty relay. This works the same as a solenoid set up. The guy I spoke to said that there is too much that can go wrong with the electronic isolators, hence the decision to use the simple relays. The cost of the relay is only $44, so I don't believe this guy was telling me a story just to get my money (especially as it is supply only, I am fitting it myself). I know I will get lots of replies praising electronic isolators (I read the archives), but can anyone tell me if there are any real problems with this relay setup?
Cheers.
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Reply By: rich - Tuesday, Feb 12, 2002 at 01:00

Tuesday, Feb 12, 2002 at 01:00
mark,
Im in the same boat (or car) as yourself. Im not too familiar or an expert with any of this, these thoughts may help, 1. I've heard more 'bad ' stories about isolators than heavenly raves. 2. The more simple the setup, the less can go wrong. 3. In regard to price, Piranha have a national advertising campaign to support, does your local auto-electrician? cheers, rich
AnswerID: 2059

Follow Up By: Mark - Wednesday, Feb 13, 2002 at 01:00

Wednesday, Feb 13, 2002 at 01:00
Thanks, you have some valid points and are extremely honest!
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Reply By: Mal Try - Tuesday, Feb 12, 2002 at 01:00

Tuesday, Feb 12, 2002 at 01:00
Mark,I have had both an electronic Hella system and a relay system let me down. My auto electrician uses the KISS principal and recomended using veeeeery heavy duty wires and a battery isolation switch (the type with large red key) mounted somewhere obvious and handy on or under the dash board. For every day running around town (not running a fridge etc) the switch is on and the whole electrical system is running off, and charging both batteries (three in my case). When running a fridge on a trip the switch is on while the engine is running and when you stop you turn the switch off. This leaves the start battery for starting and the other runs your accesories. I have a volt meter in the accesories circut so I can monitor the state of the battery. When you start to drive again you have to make a decision. If your accesories battery was very low and you turn the switch on the start battery is going to supply charge to the acc.bat. as is the alternator. So if you are only going 10 minutes up the road and will need to start again, leave the switch off. If you have re-started your trip and will be driving for more than an hour,turn the switch on and the whole system will be charged. Yes you do have to remember to turn the switch on or off but that is what the person in the passenger seat is for. It should cost you less than $100.00. Mal.
AnswerID: 2061

Follow Up By: Grant Taylor - Tuesday, Feb 12, 2002 at 01:00

Tuesday, Feb 12, 2002 at 01:00
Spot on Mal. My trusted local auto-electrician recomended this same sort of setup and has installed heaps of this kind in the last decade. Never had a complaint.This is the system I will put in. And soooooo much cheaper and reliable. I guess these companies that sell the expensive setups rely on the laziness and/or gee whiz factor in some humans.
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Follow Up By: Mark - Wednesday, Feb 13, 2002 at 01:00

Wednesday, Feb 13, 2002 at 01:00
Thanks Guys. A mate of mine has this set up too and says it works great. Thing is when I set up camp I've already started emptying the fridge, so I don't trust myself to isloate the battery manually!
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Follow Up By: Mick - Wednesday, Feb 20, 2002 at 01:00

Wednesday, Feb 20, 2002 at 01:00
Mal,

Do you know of any problems with this setup using a deep cycle battery for the second battery? Also, can the isolator switch be turned on and off while the car is running? (I assume this would ease the load on the deep cycle battery as you could crank purely using the car battery and then charge the deep cycle)

Thanks
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Reply By: Mark - Tuesday, Feb 12, 2002 at 01:00

Tuesday, Feb 12, 2002 at 01:00
Thanks for the info guys. I have done a little more research and have found; supposedly the relay idea is not good because when using two batteries of a different size, they will 'equalise' and the larger battery (in my case the auxillary) will not get a full charge. I rang another auto electrician to get an opinion/quote. He disputes this claim and says they will both charge fully and the amount of charge and charge time will simply rely on how low each battery has been run down ie. if one battery is at a lower state of charge, it will receive more charge than the other until unlimately they are both fully charged. According to him it is common knowledge among auto electricians that electronic isolators have a 0.6V drop across them, so neither battery will ever get fully charged anyway. He too told me that a simple relay is the answer.



Does any body know what a smart relay is? I found some info on these but cannot work out what exactly what they do better than a simple relay (cost is $140-$170). According to the sales brochure,"Smarter than a continous duty solenoid".

Cheers.
AnswerID: 2067

Follow Up By: Mark - Tuesday, Feb 12, 2002 at 01:00

Tuesday, Feb 12, 2002 at 01:00
Mark,
I fitted an electronic isolator about fours years ago and have had no drama's with it. Cost around $100 and I fitted it myself along with an 2nd 80 amp battery, I believe this unit uses a relay rather than diode (diode 0.6v drop) and I've never had to worry about it as it disconnects from the main battery when ignition off and switches the 2nd battery into charging circuit only when the main is fully charged. Batteries still in good shape and getting 3 days out of 2nd batt with my Evakool 60L. Made in Sydney by GSL Electronics http://www.gsl.com.au/
Cheers
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Follow Up By: Mark - Wednesday, Feb 13, 2002 at 01:00

Wednesday, Feb 13, 2002 at 01:00
Thanks Mark. Had a look at the GSL site, I hadn't heard of them, but will get in contact with them. Thanks!
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Follow Up By: Nigel - Sunday, Feb 24, 2002 at 01:00

Sunday, Feb 24, 2002 at 01:00
Isn't it amazing how the "experts" will hear something and then repeat it without understanding the theory behind the arguements. If ever you are using batteries in parallel (connected together) then should not only be of the same type and capacity, but also of the same age. Breaking this rule will not cause any immediately obvious problems, but you will never get the full capacity or life from your batteries. Electronic isolators do not have a voltage drop. Some electronic isolators use a diode (one way value for DC electricity) and a diode has a 0.6 volt drop that will have a worse effect on a deep cycle than a starter battery, but will cause either type to not charge properly.
There are a couple of electronic isolators that have no diodes and totally isolate the aux from the main battery at all times, but you are looking at $400+. The way I see it you have 3 choices. Option 1 - identical batteries with a manual switch, relay, solenoid or cheap electronic isolator (make sure it doesn't use a diode) so that you can isolate one battery and then parallel charge when the car is running. This is still not ideal as parallel charging when one battery is flatter than the other will have an effect on battery life, but this is an cheap and easy option. Option 2 - all out independant charge system. You can use the most appropriate battery for each purpose and expect a long life from each, but you are looking at $400+. Option 3 - put two battery isolator switches in (one for each battery) and connect the alternator and all standard vehicle electrics between the two switches. This way you can manually connect one battery at a time to the alternator. You should leave your starter motor hard wired to the starter battery (not switched). Cheers
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Follow Up By: Mark - Monday, Feb 25, 2002 at 01:00

Monday, Feb 25, 2002 at 01:00
Hey Nigel, not sure if you'll get to read this but I ended up calling Rotronics (www.rotronics.com.au)in Brisbane. The guy I spoke to (Rod Street I think) knows all there is to know and I ended up buying one of their parallel charging units. I would have loved the independant charge system but it was just too much. By the way GSL Electronics in Sydney actually maufacture the Piranha unit. Rotronics manufacture, sell and service their own units & have a 3 year warranty. Enough for me. Thanks for the reply.
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