Adding 2nd starter battery

[Tucson38]

I'd like to add a secondary battery with and isolator the function of which would be to start the truck if the primary battery should die. Lots of information available on using a 2ndry battery for accessories but I'm only interested in using it as a back-up starter battery with some very occasional winch use.

Has anyone had any experience with this? It's probably pretty simple but I'm not really up on how it would work.

[wilkin250r]

I knew a guy (don't we all?) that tried a real fancy setup. He bought a really cool battery switch with 4 positions on it. So he could switch from battery 1, to battery 2, to both batteries in parallel, or a complete shut-off. It sounds great, right?

Problem was, his "battery switch" was indeed a battery switch, but it was designed for trolling motors. Those trolling motors typically pull anywhere from 30-50 amps.

He didn't fry his switch on the first start (amazingly), but it didn't last long. Typical amp draw for a starter motor is around 150 amps on a nice warm day, and can exceed 600 amps in cold/adverse conditions. If you're getting a switch, be sure to look at the ratings carefully.

[Tucson38]

I found an isolator (Battery Doctor 100 amp) that has a manual override that allows the current to flow from the 2ndry to the main starting battery. I think that will do the trick. Hopefully the 100 amp limit will accommodate the current.

[Nessmuk]

A starter on a diesel pulls a lot over 100 amps.
You could charge a battery with an isolator and only connect it when needed. Military battery clamps are good for this.

[Tucson38]

My truck is gas operated so it should be drawing less than a diesel.

[Nessmuk]

I was once told an isolator charges to a level less than the primary battery. If you went to a dual alt system with an isolated ground you would get full voltage, but I just did this on mine, a lot of work and money.

[Nessmuk]

A quick search here shows 200-350 amps. Just a starter solenoid going bad can pull over fifty.
If you can find a set of the jumper cables that plug together that would work. I wonder if JC Whitney still sells the old school switches that have the handle and swing over to make the contact?

https://www.amazon.com/3B-Scientific.../dp/B001F3BID4

[Tucson38]

Originally Posted by Nessmuk

A quick search here shows 200-350 amps. Just a starter solenoid going bad can pull over fifty.
If you can find a set of the jumper cables that plug together that would work. I wonder if JC Whitney still sells the old school switches that have the handle and swing over to make the contact?

https://www.amazon.com/3B-Scientific.../dp/B001F3BID4

Yeah, I thought that would be the easiest solution but thought I'd try something slightly more high tech. BTW, I found a unit from the same company that handles 150 amps so I'll go that route.

[wilkin250r]

Originally Posted by Tucson38

Yeah, I thought that would be the easiest solution but thought I'd try something slightly more high tech. BTW, I found a unit from the same company that handles 150 amps so I'll go that route.

Gimme a link to that isolator, I'm curious about it.

The 150 amp rating might apply only to charging currents, but might be able to handle significantly more when both batteries are connected in parallel.

[wilkin250r]

Okay, I've been doing a little bit of research on the topic.

It seems isolators come in two varieties. First, there is the "old school" isolators that utilize diodes, and exactly as Nessmuk referred to a few posts ago, they don't charge to full voltage because there is a voltage drop across the diodes. These types of isolators usually have large heat sink fins.

The next style of isolators is a VSR, or Voltage Sensitive Relay. Basically, when your electrical system gets up over 13.5V or so (indicating that your engine is running and your alternator is spinning), the VSR connects both batteries in parallel so your charging system will charge both, to full voltage. When your engine stops and your alternator stops spinning, the VSR disconnects, isolating your batteries.

The VSR has two advantages: 1) full battery voltage. 2) You can manually override the relay to connect both batteries together even below 13.4V (engine not running), essentially jump-starting your dead battery with your secondary battery.

[Tucson38]

Here ya' go:

It is a VSR, not a diode style

http://www.batterymart.com/p-acc-200...-isolator.html

[wilkin250r]

A VSR can often handle MUCH more than it's rated amperage. The rating is usually the amount of current it can reliably interrupt. If you exceed that rating, you generally don't run a risk of fire, or melting, or explosion and death. You just weld the contacts together, and now the batteries aren't "isolated" anymore, now they're connected in parallel. That's not really dangerous.

And since the problem is HEAT, and heat builds (it's not instantaneous), you can actually handle spikes of huge current without problem. For example, your starter motor might have an inrush current of 650 amps, but drops to 150A within a tenth of a second as it gets spinning. So even though you're exceeding the "rating" of your VSR, that 0.1 seconds isn't enough time, and enough heat, to weld those contacts together.

[Tucson38]

Thanks for the info Wilkin.....You are obviously light-years ahead of me on this topic. Really appreciate your input.

This is a great website.

[Nessmuk]

Good to know!
I'm an old truck kinda guy. I still listen to disco on my AM transistor radio with the extension antennae.

[wilkin250r]

I just know electricity in general.

Don't misunderstand me, I'm not saying "Everything will be just fine and always work properly". There is the small possibility that you will ruin your isolator the first time you use it to jump-start your vehicle. But it doesn't pose a danger, it's not like it's going catch fire, it's not going to ruin your battery, it's not going to kill your entire electrical system and leave you stranded. It just won't "isolate" anymore, your batteries would then be connected together 100% of the time.

But I'm seeing a lot of common specs like "140A Continuous, 400A peak". or similar numbers. Although I don't see a spec like that on the isolator you posted, I would imagine it's similar, I think you can handle a momentary peak MUCH higher than 140A.

[Nessmuk]

I enjoy wiring, I have a nice relay switching controll panel for every heavy load in the truck, and anything I want off / on with the key.
I set up my dual tank switching switch with a triple pole, double throw to have tank indicator LED lights. I used double poles similarly for simpler circuits
When it comes to resisters, diodes, etc, I look things up.