Direct charge of auxiliary battery from alternator

[Bigdog76]

I have a 2017 triton and was wondering if anyone has a dual battery setup which uses the alternator to direct charge rather than use a dc to dc charger. I have conflicting information as to whether this can be done, and whether the alternator is a smart one which reduces output. Any help would be appreciated, thanks guys

[Maxiy]

From memory, and NFTF can probably confirm this, but if you have a big chunky thing on the negative lead, similar to the fuse block thingo on the positive lead, you have a smart alternator.

If there is just simple cables coming from your negative, no smart alternator.

[NowForThe5th]

Thanks Maxiy.

Vehicles with smart alternators will all have a Hall Effect sensor on the negative cable of the main battery. It will look something like this, noting that this picture is from a Pajero and I've bypassed the sensor with the winch ground cable to minimise voltage drop (plus a couple of other things that draw insignificant amounts of current):



Regardless of whether you have a smart alternator or not, it's not good practice to have a second battery without some means of protection for your main/starting battery. Simple VSR type controllers work well and are not expensive. For example:

https://www.supercheapauto.com.au/p/sca-sca-dual-battery-isolator-kit---12-volt/326830.html?cgid=SCA01040302

[Bigdog76]

Great answer and info! Thanks very much, I will check it out

[Bigdog76]

So is it safe to say that if the sensor on the negative terminal is absent, that I can direct charge, through an isolator, using the alternator instead of a dc to dc charger? I have received other confusing advice which talks about alternator voltage dropping once the car is at a certain speed, does this occur with triton? Thanks

[NowForThe5th]

Yes, you could just connect the second battery to the alternator but what you'd be doing is essentially creating one big battery. They would charge simultaneously but also discharge simultaneously so when the second battery was flat, so would be your main/starting battery. More amp/hours, sure, but one which would, one day, leave you stranded at your camping spot without power to start the engine the next morning.

The alternator output is regulated so not really an issue and having a 'smart' alternator or not makes no real difference to how multiple battery systems should be set up. The most important thing is to protect your starting battery and while you can buy and install expensive DC-DC systems they really don't do the job any better than a simple VSR which cuts the connection when the starting battery falls to a preset level, leaving enough charge to get going again, regardless of how flat the second battery is.

Like I indicated above, setting up a second battery doesn't have to be expensive. I have a 120a/h AGM battery and charging system that cost just over $300 in total and works perfectly. I use a Matson MA98504 which is currently available for about $110 for the whole kit, including wiring. It's essentially a VSR but a bit smarter and has an override so I could start from the second battery in the event that the main battery failed. With adjustable minimum and maximum voltages and a flashy LED display it's fully automatic and sits neat and unobtrusive, except for the colour!

[Bigdog76]

Awesome, thanks so much for info. I assume it’s better to have isolator close to cranking battery? I plan to have my aux battery in the tray with my fridge. I plan to have 2 x 105ah agm in parallel, it’s just for the car fridge really. Interested to know your thoughts. Thanks again

[NowForThe5th]

Theoretically you can put the isolator anywhere - some campers have it in the trailer, for example. The problem with this is it leaves the line live and actual switching points may vary due to voltage drop. Not major problems but better if it's near the starting battery.

[Bigdog76]

Great, thanks. Is it important to run the earth between the starting battery and auxiliary battery? As all the isolator kits only have a short earth lead, presumably to eart on body

[NowForThe5th]

Earthing through the body is the correct way.

[srb]

When installing dual batteries, I usually prefer running a separate negative back to the start battery, especially if the aux battery is mounted in the boot or tub. You can get away with grounding to body if mounted in engine bay as it's close enough to the main battery to get a good connection. Either way, I still prefer to connect straight to the start battery or another good way is to use a bus bar that allows you to free up room on the battery posts.

If you're on a tight budget, I do agree with using a simple VSR for the separation of the two batteries, however a dc to dc charger does a better job of actually charging the aux battery to it's maximum capacity. Especially if the aux battery is mounted in the tub or in a camper trailer or of a gel or Agm style. Dc to dc chargers are not too bad at around $350 ish, and if you consider that your likely to get more life out of that expensive aux battery then I say they're definitely worth it.

Sent from my SM-G970F using Tapatalk

[Bigdog76]

Great, thanks for the advice.