Second battery setup questions

[mhjackson123]

I am thinking about setting up a second battery for my new FWC Hawk shell. It came with a single Exide FP-AGM24DP (BTW, I see there is also a FP-AGP24DPC but can't find out what the difference is!) and I want to see if the following idea makes sense:

Use a deep cycle trolling motor battery in a battery box (I ideally want my second battery to be an option for some trips, but if the battery can do double duty as a trolling motor, that would be a big bonus). I was thinking of getting a second Exide AGM24DP (or DPC?) or a Canadian Tire AGM Nautilus deep cycle) as the trolling motor battery box battery. I would then use a marine two-battery switch in the battery compartment to allow me to run battery 1, battery 2 or both by running cables to the battery switch from the trolling motor battery.

Does this idea make sense?

The battery compartment of the Hawk is small and I guess I could install a second identical battery in the same box. If I did this should they just be wired in parallel with no switch?

 

[Wandering Sagebrush]

Mike, I will throw my 2¢ worth in. I don't recall how old your camper is, but it would be best to have two identical batteries of roughly the same age. If your Hawk is new (as your post says), then a second Exide AGM24 would probably be just fine. I wouldn't bother with the switch, just wire them in parallel.

 

[Vic Harder]

I'm thinking you would LIKE to be able to remove the 2nd battery easily for use in a fishing boat? If not, then what Sage just said is true.

If yes, then I wonder about the effect of taking battery out of your fishing boat and hooking it up to a fully charged camper battery (which has spent the day charging up via solar). The charge state difference would be high, and they would try to equalize instantly. LOTS of current flow.

 

[mhjackson123]

Thanks Sage and Vic. f I go to the two battery option, then I would go for a second identical battery and since my camper is only 6 months old, they should be pretty similar. If I go for the trolling motor battery option, then I would definitely use the switch. In general I would not expect to connect both batteries together but to switch one over to the other if/when needed.

 

[Vic Harder]

Thanks Sage and Vic. f I go to the two battery option, then I would go for a second identical battery and since my camper is only 6 months old, they should be pretty similar. If I go for the trolling motor battery option, then I would definitely use the switch. In general I would not expect to connect both batteries together but to switch one over to the other if/when needed.

Consider getting the ML-ACR 7622 instead of a simple switch. You can still switch it manually if desired, but the smarts may help you in automatically charging them.

I'd like to hear from a few more others here before being OK with connecting a fully discharged and a fully charged battery together.

Alternately, I guess my system could do that as well.... glad I've got 2 AWG cable between them!

 

[mhjackson123]

Thanks for the ML-ACR 7622 suggestion. That looks like a useful alternative to a switch.

 

[iowahiker]

My two cents, if you are going to treat the batteries differently, i.e. take one out, then have two and always keep them separate:
- Charge one then the other
- Discharge one then the other
If always kept separate, then they do not need to be identical. If you pull one out then they are not identical.

 

[Beach]

As Vic said, your batteries will equalize. If you do go that route( 1house/1troll mo.), you would need an auto or manual method to regulate the charging till the trolling battery charge was equal to the house battery.

 

[baphenatem]

Thanks Sage and Vic. f I go to the two battery option, then I would go for a second identical battery and since my camper is only 6 months old, they should be pretty similar. If I go for the trolling motor battery option, then I would definitely use the switch. In general I would not expect to connect both batteries together but to switch one over to the other if/when needed.

How do plan to charge the batteries? Vehicle, generator, or solar?

 

[mhjackson123]

I was thinking of keeping the main camper battery charged via vehicle and solar, but use the trolling motor battery kind of as a backup - probably charge it via the vehicle or generator if needed.

 

[baphenatem]

If you're not planning on using the second battery on a daily basis it would be easy enough to simply add a 50-100 watt solar module and a charge controller to charge it back up. The problem with relying on the vehicle system is that you have to be running for a long enough period to get the full absorption charge. As for generators...I guess, if you have to.:- {

 

[rando]

Hooking up charged and discharged batteries in parallel is not going to cause a large current to flow between the two. Yes the charged battery will be dragged down a little and the flat one will be brought up a little, but the current flow will be pretty low. The voltage difference between a charged and discharged battery is only ~0.5V, which is not going to drive that much current compared to a battery charger that provides 13.7 - 14.4V. Secondly if the charged battery did provide a lot of current the voltage on the discharged battery would rapidly rise, and the voltage on the charged battery would sag, which would decrease the current.

That being said, I am not sure how practical the system you describe would be. Personally I would just add a second battery the matches the first and fits in the factory battery compartment if you think you will need more capacity. The idea of shifting around a second lead acid battery and finding somewhere to store it doesn't seem that appealing to me.

 

[Bigfoot]

For an off-the-shelf solution that mounts to the truck frame: http://www.torklift.com/index.php/products/auxiliary-battery-storage/hiddenpower.

 

[mhjackson123]

So I ended up deciding to go with a portable group 27 AGM battery (88Ah) that can be moved back and forth between camper and boat (and used as a trolling motor). I have also installed a Victron battery monitor (BMV 700) for the main camper battery which has been very educational. The BMV shows how much I have put into the battery and taken out and gives me a really good idea of how much energy the various systems are using.

What I have found: my ARB 50 qt fridge uses 12-14 Ah of energy a day under light use conditions, which gives me 3 full days before reaching 50% on the battery, assuming no moving and no solar. I have a 40 W portable GoPower panel which, if I move it around 3 or 4 times during a day seems to completely compensate for the fridge. If I lay it flat on the roof and do not move it, it seems to only take care of about half the fridge's draw, but would thus extend me to 5 or 6 days on the one battery. (I am doing this on average April days in Victoria BC (48 degrees N). The 40 W panel is very easy to move around, but is only 40W!
I can see that an 80- 100W roof mounted flat panel would pretty much do the same as the 40W portable panel when moved to face the sun.
I am continuing to experiment and monitor real time power usage and will report back.

 

[DanoT]

I just pulled the trigger on a Coleman portable 40watt solar panel from Canadian Tire, on sale for half price- $135 CDN. (about$100US).

I will mostly will be running a swamp cooler that draws 1.2 amps on medium fan setting and powering it with a Noco Genius Boost HD Jump Starter, lithium-Ion battery.