dual battery,do it yourself

I've built two of those kits. The mosfet protection diodes allow current flow from the aux to the main battery so isolation is not complete. If the main battery is in bad condition it can drain the aux battery.Hooroo

And if you try to crank the motor in this condition, the start current will be too much for the protection diodes and destroy the FETs.

If you use a deep cycle battery, this should not be an issue as the unit can handle 100 amps and a deep cycle would struggle to produce that much. It is also only for a short period of time - whilst the starter is engaged. (This is if you have the sense line runing of the alternator). I think they are great value at $39, just need to be aware of this small design issue.

Whoops, what I said above is not exactly true, if the voltage of the aux bat is higher than the main, current can follow back into the main bat. Just need to make sure you have a good main bat before heading off.

I've built a Jaycar 12V battery manager kit - haven't installed it yet. The reverse current problem via the protection diodes is well documented in the kit instructions.

Quote:

".....the MOSFET internal protection diodes will allow a 'reverse' current to flow if the auxilliary battery's terminal voltage is significantly higher than that of the main battery. While this is obviously not the case during normal charging operations, there can be circumstances where the auxilliary battery is fully charged yet the main battery's voltage has fallen due to a high current load - such as the vehicle's starter motor.

If the auxilliary battery can deliver a high current (most deep cycle batteries can' by the way), then the MOSFET diode current is only really limited by the conecting lead resistance Rl, and may rise to a high level - say 60amps. This in turn means that the MOSFETs will dissipate around 30watts each, thanks to the 1V drop accross the diodes. If this curent were present for an extended period, the MOSFETs would become rather hot and bothered due to the modest heatsinking capability of the diecast box.

Note thatthe MOSFETs dissipate less than 10W during normal charging operations.

While we were initially a little concerned about the above scenario, and considered a number of circuit configurations that would provide full two-way isolation between the batteries (by the use of a high current relay), we found that the additional expense and circuit complexity of the alternatives was difficult to justify.

And as the condition would only occur if

a) the auxilliary battery was of the high current type.
b) the load was applied for an extended period (unlikely with a vehicle's starter motor).
...and c) the main battery voltage dropped substantially under load (indicating a less than healthy battery), we reasoned that there was not too much cause for concern.

Of course, we'd have to addd that it's extreme folly to set off in your 4WD, camper or car/caravan combination with an unhealthy main battery - or for that matter, a vehicle that's difficult to start and needs to be cranked for an extended period.

Under normal circumstances, the potential for reverse current flow could be considered as somewhat of an advantage in a dual battery system, since any excess charge in the auxilliary battery will be automatically passed to the main battery via the MOSFET diodes. This in turn ensures that the main battery is always in the best possible state of charge for the conditions at the time, and can therefore crank the engine into life......."

End Quote.

Anyway, sorry for the long-winded post, but I thought the reverse current 'problem' should be documented correctly here.

I'm sure there are better (and more expensive) alternatives to the DIY kits. But I'm also sure that the DIY kits offer a good cheap and reliable alternative.

Cheers,

Luke.

One just has to be aware that if the main battery carks it for whatever reason it could possibly flatten the Aux. If this happens then you have no one to blame but yourself for installing this setup.Hooroo

You're absolutely right Ray.

You've built two of those kits - were they for others or yourself? I'm interested to hear, aside from the reverse current problem, how you found the overall performance of the EA Battery manager kit?

Perhaps a slight modification of the kit to include a high current relay for absolute isolation would make the kit a worthwhile proposition, considering it's sensing capabilities and ability to apply low current charging as well as normal charge rates depending on the condition of both main and aux batteries?

The addition of said relay should be reasonably simple...

Cheers,

Luke.

Just to expand on this subject, buying a cheap setup with questionable reliability would not make you sleep to well out under the star's miles from anywhere. Does anyone have info on which Battery management system's to definately (stay away from) or recommend (the one to buy) all prices aside ?
Rod.

Rod
I have Rotronics RDC12M and can thoroughly recommend it. Top dollar though but I need reliability and to date have lasted 2 years without either battery replacement whereas before both were going under every 12 months. I have friends who have had same batteries for 4 - 5 years with Rotronics and another who is into 8th year and only now looking like a replacement is necessary. Therefore I can see it paying for itself easily and the extra peace of mind made it all worth it.so many places, so little time !!!

We've had marine switch systems on two old 4WD's over the years and yes they are simple and reliable BUT the onus is on the operator to manually swith between batteries as appropriate. We had occasions when we'd forget and it was a pain! For the sake of ease and convenience we would not go back to a manual system.

:o) MelissaPetrol 4.5L GU Patrol &
Camprite TL8 offroad camper

Hi Oskar,

I'd be very intersted to read your circuit diagram please.

swbmav@yahoo.com.au

Thanks very much,

Luke.

Hey Oskar,
Me Too Please
waynepd@yahoo.com.auCheers
waynepd
%
%:-)
%

I'm pretty interested in this set up as well Oskar, could you send me the circuit please?? I am thinking of battery management as we are a couple of weeks away from buying our fridge.... I was going to follow Australian 4X4 monthly's Roothie and install a 3 way marine switch.. apparently cost under $40...your suggestion may yet find its way under my bonnet...
Cheers
Brian
chapples@primus.com.au

yeah i better have the cicuit diagram as well thanks .

cheers

decuisine1@iprimus.com.au

1999 NISSAN NAVARA DUALCAB
DIESEL 3.2 & SPRINGY CARLTON TOY

Hi Oskar,

I'd be very intersted to read your circuit diagram please.

cdchook@hotmail.com

Thanks very much,

Colin.

No worries it's on its way
Please remember, however, that the circuit as it stands is presented as an idea that may or may not work. I cannot take any responsibity for what anyone else may do with it in its current form or in any modified form. If you choose to use it it is your own responsibility.
All I can say is that I have installed it in its current form in my own vehicle and I am very happy with it. I take responsibiltiy for my own mistakes only.
cheers

Howdy

As mentioned above with the Dick Smith kit, you are getting a bag
of electronic components, bare circuit board, a bunch of wires,
a die cast box that you have to drill and fit components into. You have
to solder the components into the circuit board, once assembled it
has to be tested and if it dosn't work then you have to do some
fault finding, Worse case you fry one of the components and or
damage the circuit board. You will need some basic knowledge
in assembling electronic circuits and be able to identify and read components, also you will need to know how to use a multimeter etc.
This sounds like a cheap way to go at $40, but if you have any doubts about your circuit building capabilities then steer clear unless you know
someone that can help you. If you are keen to learn go for it.

Phil

Legerlos
Did you get my E-mail?
I lost the original one and I had to scrounge it back from one of the others. I'm not sure I used the attachment system/thingo correctly so if you didn't get it - sorry.
I can send it again if you want.

Cheers

Hi Legerlos,

I'd be very intersted to get a copy of Oskar's circuit diagram please.

cdchook@hotmail.com

Thanks very much,

Colin.