Installing an Isolator Relay to an EXISTING dual battery system. Where do I start?

[Kasu]

Disclaimer: I searched the forum and online for the answer to these questions, yet unfortunately the only ones that keep coming up are how to install dual battery systems, not how to modify an EXISTING dual battery system.

I have a GMT800 2006 SSV (police) Tahoe that came with a stock dual battery system, RPO Code 8Y9. The battery system is parallel, connected at the alternator (red bracket in picture). This vehicle is my DD, camping rig, and already on a great start for an Expedition build (stock included HD everything, skid plates, 4.10 gearing, rubber floors, etc).

I need an isolator relay solenoid to keep the starting battery safe while the house battery deals with accessories. This would give me time and less frustration to isolate drainage issues. I was planning the addition anyway to prepare for the addition of future add-ons such as winch, fridge, lights, etc.

The problem I am running into is that there is a parasitic drain coming from somewhere that is killing both batteries overnight. As you know in a parallel system both batteries will feed of each other if one has a lower charge than the other, essentially killing each other (and halving their lives). I haven't been able to find the issue thanks to a lack of a garage and heavy spring thunderstorms, so I am moving forward with this step.

My questions:
1. I have a 160 amp alternator. What size isolator do I need? And what size fuses?
2. Which battery should be the starting battery? Which is the usual stock location? Does it matter? Mine came with the dual setup. I have the driver's side disconnected for now.
2. How should I rewire the stock wiring? See the picture for the stock bracket setup at the alternator. The POS wire from the passenger battery goes under the engine to this bracket. Should I put the isolator here, or by the passenger battery at the old siren screw holes?
3. Should I relocate the grounds? The passenger battery ground goes to the engine. The driver side battery ground goes somewhere below. Or do I ground the batteries to each other?

Thank you!

 

[DiploStrat]

Some Contrarian Thoughts

... I need an isolator relay solenoid to keep the starting battery safe while the house battery deals with accessories.

No, you don't - but this is a common misconception.

-- You don't need to keep your starting battery "safe" unless you plan on having a camper battery that is too small to carry the load. You need a battery this is large enough to cover your load; an isolated system won't give you more power and if your battery is too small, it is too small.
-- The absolute easiest solution to your "problem" is to replace the two existing batteries with a matched pair of deep cycle batteries that are, between the two, large enough to carry your load. Done.
-- An isolated battery system IS useful if the starter and camper batteries are of different formulations, that is one a starter battery and one a deep cycle battery.

The problem I am running into is that there is a parasitic drain coming from somewhere that is killing both batteries overnight. As you know in a parallel system both batteries will feed of each other if one has a lower charge than the other, essentially killing each other (and halving their lives).

Your problem is that you have a parasitic load and you will continue to kill batteries until you locate and correct this problem. Dual batteries don't make this better or worse. Remember, you actually have twelve 2v batteries, not two 12v batteries. A load connected to one is connected to both.

My questions:
1. I have a 160 amp alternator. What size isolator do I need? And what size fuses?
2. Which battery should be the starting battery? Which is the usual stock location? Does it matter? Mine came with the dual setup. I have the driver's side disconnected for now.
2. How should I rewire the stock wiring? See the picture for the stock bracket setup at the alternator. The POS wire from the passenger battery goes under the engine to this bracket. Should I put the isolator here, or by the passenger battery at the old siren screw holes?
3. Should I relocate the grounds? The passenger battery ground goes to the engine. The driver side battery ground goes somewhere below. Or do I ground the batteries to each other?

ANSWERS:

1. You want a relay (NOT a solid state diode isolator) rated at at least 160A.
2. Makes no difference. I would use the one closest to the alternator, assuming that you haven't damaged it.
3. The isolator goes between the starter battery/alternator and the battery that you choose to use as a camper battery. Physically it can be anywhere that it is convenient. Depending on whether you use a key controlled relay or an intelligent relay, perhaps with manual override, give some thought to the need to run wires to the dashboard area.
4. No. As long as you don't damage them, the grounds should be fine.

NOTES:

-- Assuming that at least one of the existing batteries is reusable as a starter battery, then you will need to buy a deep cycle battery that is large enough to cover your anticipated load with a reserve.
-- For longest battery life with a lead acid battery, the rule of thumb is that the camper battery should not be drawn down more than 50%. There is a contrary argument that it is better to sacrifice battery life rather than carry the weight of extra battery. In your case, this may be moot as you probably can't fit that large a battery into the factory carrier.

Hope this is clear and understandable Don't do anything until you find and correct the parasitic load, remembering that the problem could be as simple as dead batteries after years of police use.

 

[Kasu]

Thank you for clearing that up Diplostrat! And for answering my questions.