solenoid dual battery

Kelpi: do a topic search and take a week off work. Plenty of info there, but be warned, it's not exactly a black & white issue. To most of us, anyway. Cheers.

Hi Kelpi, have been using a Rotronics unit for over 15 years now to charge my deep cycle batteries while travelling. The unit has performed trouble free & has fully charged (depending on % of discharge on the deep cycle battery) in a couple of hours travelling after overnight use. The advantage of the unit in a dual battery setup it first checks the charge state of the start battery & will only put a charge to the second battery when the start battery is over 75% fully charged. Cheers

H kelpi, this type of question pops up on fairly regular basis and as all ways, the the end question is not whether a solenoid can or can not charge a vehicle battery properly but how on earth can solenoid have any effect on the voltage flowing from point A to point B other than switching the voltage on and off.

For a system that incorporates a solenoid as it’s main control device and not be able to charge a battery to a full charge then the problem is NOT the solenoid but something related to the over all charging system.

If it is a smart relay or solenoid then it may be the electronics backing it, or it could be the type and / or way the cabling is connected between the two batteries, or it could be that the vehicle’s alternator and / or voltage regulator may not be functioning properly.

The point is a solenoid can not do any more than switch the power on and off, it has no magical way of actually effecting a change in the voltage level as it flows from point A to point B.

A solenoid controlled device is only as good as the power supply provided to it.

Cheers

Unless your using the rotronics that independently charges the main and aux it's going to be no different to any other isolator what ever brand. Just about every isolator on the market has a relay or solenoid (same thing) that joins your 2 batteries together. Even Piranha's so called electronic isolator has a relay inside it to connect the two. The rotonics independent charge isolator may give a better result due to it charging only one battery at a time, but the downside of this is your alternator is no longer connected to your main battery and if your isolator fails you will get no charge to any battery. At least with the other systems worst case is your aux battery won't charge, your main is not affected by a failure.

My Rotronics Battery Isolator died two months ago after working well for two and a half years. I am now running the batteries in parallel and turn the fridge off at night. Will be contacting Rotronics when I get back home from this trip I am on.

Hi folks, if you use a good quality relay based smart charger and the vehicles alternator is up to it, there is no reason why you can’t charge up to three batteries at the same time and get all three up to full charge ( depending on how you use the batteries and then how much time you drive for ) using your standard vehicle alternator and regulator.

You may get some small advantages by putting in some special type of controller that MAY get you an additional 5% out of an auxiliary battery ( and even this small amount is questionable ) whereas for a far lower cost you could simply install another auxiliary battery and give yourself a minimum of 80% additional storage capacity and remember that as you are charging all three batteries at the same time, you are going to have this additional power in a shorter charging time.

Just some food for thought, cheers

I think there may be some confusion here.

Firstly, starting a car engine typically needs less than 3 amp/hours. This energy is replaced by the alternator within 30-60 seconds.

Secondly, a solenoid is simply a switch. It will not affect the voltage that is fed to the auxiliary battery when it closes.

Thirdly a standard alternator/regulator is set at 14.2-14.4 volts. This is insufficient to charge conventional batteries much beyond 70% within reasonable driving time.

There is an argument for the Rotronics in that it separates the auxiliary battery from the starter battery and this has some benefit in that the regulator sees only the latter's voltage and charges accordingly - but the difference is minor.

There are various other solutions.

One is that an AGM battery charges to 100% at as little as 14.1 volts - and quickly. But you do need to use a Redarc or similar a AGMs charges so easily they otherwise initially gobble up the starter batteries share, possibly leaving either or both too low for starting. If doing this it's best to switch out the AGM when it's not needed as they do not need (or indeed like) continuous float charging.

The next one is more complex but works really well. This is to run a small inverter from the vehicle 12-volt system and use that to power a good three-stage battery charger. The charger can also be used where's mains power. This is electrically inefficient but a lot sight less so than trying to push amps with not enough volts.

Yet another way is an interesting new development from NZ that in effect takes the alternator output to well over 15 volts and charges via a varying width pulse, sensing the required voltage by monitoring current flow.

I am about to trial one and will place the results on my own website when I've done so (address is as email except its www.caravan etc...

The TWC three-stage regulator on my OKA works in much the same way except the TWC units pulses are twelve minutes long. It can and does safely charge at up to 100 amps. Oh yes - the Redarc. It's a rugged device, super-easy to fit and works well as long as you accept 70% charging. Or use an AGM (or gel cell).

Finally - how good it is to hear of people using proper-sized cable! 16 sq mm is by no means overkill.
Collyn Rivers

Boy does this subject start some conversation or what.
My background is in electrical and electronics and the statement of keep it simple stupid to me is the best. I have just taken my 4.2P Maverick of 15years up to the top paddock for some well earned rest and the system that I used in it was dual Overlanders with a 100amp Bosh solinoid that used to close once the alternator was charging. This system incorporated 35mm cable between the batteries on both the positive and negative sides as well as 35mm cable to both the body and chasis and starter motor, and to finnish it off I had 10mm cable from the alternator to the primary battery, basically if you keep the cable resistance between the alternator and the batteries as low as possible then the batteries will equalise in charge.
I have placed the same system on my new GUIV Partol with one minor change in that the Bosh solinoid closes as soon as the Ignition is turned on as I could not use the same as before as it upset the indications on the instrument panel (Alternator, ABS, Seat belt etc warning lights) never mind still working on that one.
I have not had any problems on the new one with the Aux battery not fully charging , so as I said befor keep it simple but also big is best on the cable size.

Cheers
Treker

Brett
With constant voltage charging, the charge rate automatically tapers as battery voltage approaches the charge voltage. Given long enough (like a day or two constant running) a starter battery may approach 80% or so, but to exceed that requires around 15 volts. AGMs are a different matter - they charge close to 100% at 14.1 volts or even less.

I'm surprised your three-stage charger goes into float mode at 14.2 volts. Does it have any programmable functions?
Collyn Rivers