Dual batteries, question for the sparkies

**Mick-Kelly** · 6th June 2004, 11:25 PM

G'day all
Ive just been poking round in te engine bay again and now have some quetions regarding my dual battery system. From what i can tell ive got two regular (cranking type) batteries with all the relevant solenoids etc. the wiring from the second battery runs along the chasis to the rear where it terminates in a fitted plug, presumably for a fridge.
My understanding is that most dual batteyr setups run a standard and a deep cycle and have a monitoring system of some kind. My system was installed by the PO so im not to sure what his goals were. Is there anything there in the way of a monitoring system I just havent foundyet and will it create problems if I run a deep cycle as the second battery.
cheers
Mick

**drivesafe** · 7th June 2004, 04:16 PM

First off, you can use a cranking type battery or a deep cycle type battery as your second ( auxiliary ) battery.
The use of a solenoid, by itself, to connect the auxiliary battery to the charge system is a BIG mistake.
The problem is that solenoids can pick up from a supply voltage as low as 8 volts so if your main battery is on it’s way out or worse has already lost a cell or two, the solenoid will still operate and you actually start the vehicle using the power from the auxiliary battery and you will have no idea that the main battery is in need of replacing.
Not only are you now starting using power from the auxiliary battery but until you are running the motor with some revs, the alternator will not be putting out enough amps to charge the auxiliary battery while trying to charge the main battery so more power is pulled from the auxiliary battery as it also tries to charge the defective main battery.
The end result is that the life span of the auxiliary battery is dramatically shortened and it too spits the dummy. This is usually when you are in the middle of nowhere because you have been driving through rough country at slow speeds and low engine revs.
There's lots more to it but I hope this will be a start and is of some help!
Regards.

**tdi130dualcab** · 7th June 2004, 08:22 PM

I agree with drivesafe

a second battery needs to be properly "isolated" from the main battery. Idealy the "second" battery should be a deep cycle. this is very important to keep the beer cold!.

I make my own isolators for aux batteries but there are a few available on the market that do a good job. 8)

**Mick-Kelly** · 7th June 2004, 09:04 PM

Thanks guys,
i'll try and sortout some pics of what ive got then see if we can figure out how to sort it. It looks like a very proffesional install
cheers
Mick

**tdi130dualcab** · 7th June 2004, 09:13 PM

"professional" does'nt always mean they knew what they were doing.

simonit · 8th June 2004, 07:49 AM

Hey tdi130dualcab,

You say there are some good ones on the market, could you give some examples. Am going down this path at the moment.

Cheers.

**drivesafe** · 8th June 2004, 01:55 PM

Hi simonit, I manufacture auxiliary battery charges and have a web site. I would obviously like to sell you one but that’s not the reason for this message.
If you go to our web site, there is a Q & A section. If you go through that you will have a better idea of what you will need.
If you still have any questions email me or post it here and I will do my best to help you and you don't have to buy any thing.
Regards. 8)
traxide.com.au

simonit · 10th June 2004, 07:57 AM

G'day drivesafe,

Had a look at your website. I noticed on the SC40, that it doesn't isolate the main starter battery until it drops to 12v. Isn't this a bit low? Other ones I've seen do this at about 12.7v.

**Shifter** · 10th June 2004, 08:40 AM

Just a couple of points for clarity: Don't deep cycle batteries require a higher charge rate to keep them fresh? Don't you really need a "smart" charging device to keep the voltage correct when charging a deep cycle and a standard lead acid battery? Don't deep cycle batteries dislike being disharged more that 50% for longenvaty.
I have a deep cycle battery in my motor home and treat it very carefully, with "smart" charging and increased voltage charging, never allowing ti to discharge more than 50% and it has lasted for years. They are an expensive item and require care and attention. From my experience it is normally charging devices, or lack of them, and care and attention that let them down!

Shifter.

**drivesafe** · 10th June 2004, 04:41 PM

First cab off the rank, Sinomit you are comparing our system with a battery isolating type system.
The SC40 and SC80 are designed to do a lot more than just isolate the battery but in brief, they are designed to extend the operating time of your auxiliary battery by using the surplus power available from your main battery but at the same time they will monitor the common voltage and will disconnect the main battery while it still has enough power in it to start your vehicle a few times.
Next, after more than 15 years of designing these devices, working experience has the cut-out voltage at 12 volt as this is more than high enough to start a diesel and a petrol engine can go even lower.
Now Shifter for your query, deep cycle and cranking batteries charged in the same manor. The difference between them is that a cranking battery is made so that it can regularly support a very large discharge for up to 30 seconds at a time and would not be discharged bellow 80% and not be harmed, whereas a deep cycle battery would be damaged and it’s life span shortened if you were to do the same thing to it on a regular basis, but a deep cycle battery can be discharged well below 50% and still come back to full storage capacity after charging while a cranking battery would be damaged if you took it down anywhere near 50%
The trick to good health for both types battery is that you store they fully charged. In other words, no matter what you use, when you are finished, get them charged up again at the earliest possible time.
Now for charging either type battery. Here the main thing to attcheive is to have a high charging VOLTAGE and don’t wary about the charging current. The charging current can be as little as 1 amp , as long as the charging voltage is as close to 14 volts as you can get it.
At 1 amp it will take a long time to charge but by charging at 14 volts, the battery will hold more charge for a longer time.
Here is a simple and relatively cheap way to give you battery the best chance to charge properly. Use at least 8 B & S cable to connect between the main battery and the auxiliary battery and remember that’s for the NEGATIVE as well as the positive wire.
Regards. 8)

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