Second battery bank system question....

[Burb One]

Hi everyone!

Been a lurker here for quite awhile... Have a 2004 Suburban with a pretty fair amount of "features" for overlanding/ we call it “luxury camping” (140w worth of solar, winch, 14bolt rear end with e-locker, 33's leveled, onboard air, homebuilt hot water and cold water filter system, radios, outside lighting etc. etc.) I am going to write a build thread soon as most of the ideas are all taken from this forum, and either made it my own, or just stole them I hope I can inspire someone as a lot of you all have done to me.

Anyway…

My next purchase is going to be an ARB style 12v fridge, and I have been looking into boosting my reserve capacity for my power storage.

I already have the dual battery/ dual alternator setup under the hood with a large deep cycle battery for the house battery. In normal operating they are completely separate wring systems, with a “jump” switch/ solenoid connecting them together in an emergency use or in winch use.

The solar I have, should be enough to run my current power needs with the fridge as I do have 120w solar going to top off the house battery under the hood (20w goes to the starter battery), but as with everything, I believe just enough is not enough, especially when I go camping in winter and sometimes where I am with tree coverage or weather, getting all the sun is not possible. I also do not want to have to worry about my power usage and running other power hungry devices, such as my water filters and pump, speakers, etc. The solar I have is about the max I can get away with the space on my roof, and I am not interested in removable panels to setup in camp. I figured, more power storage, can get me at least 2 or 3 days of no car starting on the solar and storage alone. I calculate for worst case scenario and to do 3 days I need 100ah more. (realistically this will probably be good for at least a week of limited solar, but in “worst case” it will be good for 3 days with my maximum power usage.)

As such, I have been looking to boost the capacity of deep cycle house battery I currently have

I walked into RadioShack the other day, and they have a serious sale on their 15ah 12v sealed deep discharge batteries for UPC's (first one is 25% off, second is 50% and third is 75%.) Turns out to be like 8 bucks for each battery. More so, I haven't been able to find a good place to add a matching deepcycle house battery like the one under the hood as there no more room under the hood, and a huge battery in the interior space isn't going to work. These 15 ah, fit perfectly under the middle row seat, and I could fit 8+ of them easily and neatly,

Sorry for the long intro…my question is, I know connecting unlike brand/size/age batteries is a pretty big nono, as they will self-discharge on each other. Is there some type of inline device, so I can connect this new radioshack house battery bank to the existing house battery circuit but eliminates the batteries discharging on themselves (IE the radioshack batteries discharging the large deep cycle battery and viceversa?) I would like to keep the house system as one system so the solar keeps them all charged, and the devices connected to the system have all the available storage at any given time, without human intervention of having to switch the banks etc.


Thanks!
Harrison

 

[Haf-E]

Hummm.... Not to excited about the ideal of putting eight 15 ah batteries together and then paralleling them with a bigger battery way up front. I think you will have serious issues with them not charging and discharging equally, resulting in disappointing performance and poor life.

If you do want to do it - I would consider having the smaller AGMs set up as a separate, third, battery bank which is connected to the house battery/alternator via a contactor (big relay) when the engine is on. Keep them separately wired to outlets as a "back-up" battery back perhaps in case the main one gets too low. There are lots of inexpensive LED displays you can setup to let you watch all three batteries at the same time.

Also - wiring the smaller 15 ah batteries together gets tricky as the terminals are very low current rated. I would bring equal length leads (#10 AWG) off of each terminal to a centrally mounted positive and negative terminal bus bar where the larger wiring to the loads would be connected. Might also consider putting in a 30 amp or so fuse for each battery as well in the positives to keep problems from becoming fires - could use a small fuse panel to do the combining of the batteries by wiring it "backwards" i.e each battery connected to a load terminal and then all combined together by the fuse block.

 

[Burb One]

Haf-E thank you for the thoughts...

I never thought of the current at the terminals being an issue, as with parallel, theoretically the current seen should be limited at each of those contact points to the current produced by the battery, I guess in the case of a cell failure, the current could become too high if the other cells attempt to take up the load or the cell itself creates a short circuit. I like the idea of adding fuses to each terminal before the distribution point. I was planning on having them go to a large distribution point next to the batteries, and then being attached to my main distribution point under the hood through a 2 gauge wire, with a fuse of each end , near the distribution points. I did plan on equal length wiring between the batteries and distribution points, minimizing discharge between them.

One of my thoughts was just that, when the main battery gets below a certain voltage it trips a solenoid connecting the third bank with the house battery. However, I like the solar being able to charge and top all of them off. I guess, it makes sense to do this, as if the main voltage goes below a threshold, the third bank will start charging into the circuit, and will stay connected until the voltage goes back up (due to solar, which means that both the house battery and third bank are both charged to that point since at this point it is sensing the complete system voltage) I could add a second relay off the alternator output so that the alternator can charge the third bank when the engine is running.

I was just thinking there was some type of regulator I had never heard of that reduced discharge between batteries (almost like a diode based isolator for alternators and different battery banks, but maybe the above is the best solution.

 

[Haf-E]

Connecting the two battery banks together (the eight 15 AH in the back and the auxiliary battery in the front) all the time would work OK - you just wont get 100% of the combined capacity. I don't think there would be a lot of issue with the charging - other than it being difficult to do the correct charging profile for the AGM batteries (especially when the engine is running and the batteries are fully recharged as few alternators switch down to a "float" voltage level). That would be the only real way I can think of that would allow charging of all the batteries from the solar array when operating loads (i.e. camping).

 

[Burb One]

Not too worried about the longevity of the AGM, they are so cheap, and will be replaced at my ~3 year service intervals.

Just as a follow up…
Ended up making the aux bank, with Haf-e's suggested fuses on the individual cells. I diverted one of the 20 watt solar panels to charge this bank, and also diverted my interior LED lighting, “sleeping area” fans, and water and filter pumps to run off this aux bank, on their own circuit. I then added a 4 gauge wire with 30amp resettable breaker connecting this Aux circuit to the house circuit with a relay switched to the ignition. This way, most of the daily use items that could possibly take away from the house capacity (and subsequently the fridge) will be disconnected from the house circuit (Other than my exterior LED lighting, but I don't use those much without the engine running). With the engine running, the relay will trip, and the house alternator will also charge the aux battery bank.

Did a mock up, and it seems to be working well.

Only issue I forsee, would be having a large load on the house side, with the house battery removed or discharged and also the house alternator broken or not pushing out the required amps in combination. In this situation the breaker will flip. The more likely case will be that, I have flipped the relay for the house and starting circuit to be connected and so the starting alternator and battery will take up the slack and the demand on the aux circuit will be negligible. In the case, my house battery, house alternator, starting battery, and starting alternator are all gone, I will be walking home so the flipped breaker won't even matter.