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Old 05-18-2017, 02:56 PM   #1
Flagman
 
Join Date: Nov 2015
Location: East Aurora,n.y.
Posts: 11
Flagstaff shamrock hybrid

I recently purchased a 23ss with 2 - 24 batteries. Would I get any favorable additional battery life if I wired my "old" 27 battery from previous Starcraft hybrid in parallel to the 24's ? Thank you for any " light" on this subject.
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Old 05-19-2017, 11:18 AM   #2
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Non-identical batteries in parallel cause problems - some trivial, some not.

The voltages of the parallel batteries will try to equalize, with the higher voltage battery trying to "fill" the lower voltage battery. In cases of extreme inequality (one cell bad in one battery), the current between the batteries can be enough to melt the insulation off the battery cables and start a fire from the off-gassing (happened to me).

When charging, the battery with the lowest internal resistance gets the most charging current - which may or may not be a good thing.

When discharging, the battery with the lowest resistance path to the load will carry more of the load. This is why the length of battery cables should be the same to the two identical batteries.

Bottom line: some batteries are more equal than others - and they get to do most of the work. So the output of parallel batteries is really not quite equal to the sum of the individual batteries. The more identical the parallel batteries, the better they work.

Use of 2 series 6V golf cart batteries (GC-2 are very close to Group 24 in physical size but have more AH) gets rid of a lot of these issues. And my Interstate GC-2s from Costco were cheaper than any 12V Group 24 deep cycle batteries.

just my experiences
Fred W
2014 Rockwood A122 A-frame
2008 Hyundai Entourage minivan
camping Colorado and adjacent states one weekend at a time
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Old 05-19-2017, 01:21 PM   #3
Flagman
 
Join Date: Nov 2015
Location: East Aurora,n.y.
Posts: 11
reply to battery question

Thanks for excellent info. Would there be any significant increase in amp-hrs. as long as all 3 batteries are in good condition and all cables are the same length ?
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Old 05-19-2017, 02:01 PM   #4
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A reasonable guess is 10% less than the individual amp-hrs added together.

That said, what are you really trying to accomplish? You are attempting to put 3 used batteries in parallel to what end?

On the demand side, how many days without recharging? About how many amp-hrs per day to you anticipate using? What is your method of recharging?

As an example, in my little A-frame, the planning was like this: max of 4 nights with worst case 30 deg nights, 50 deg high during day. Max of 5 hrs run time on heater per night, none during the day, heater fan draws 4 amps. That's 20 AH per day for heater alone. In warmer weather, Fantastic Fan draws 0.8 amps on low for 19AH per day instead of heater.

Propane/CO alarm is 0.25 amps, 6 AH per day. Fridge cooling fan is 0.07 amps, or less than 2AH per day. We use mainly flashlights, minimal use of LED lights and water pump. No stereo (cutoff installed), no CPAP, no computer or phone charging while dry camping.

Worst case is 28AH per day, times 4 days/nights is 112AH. My GC-2 6V golf cart batteries are 232AH each, or 116 AH usable (stay at 50% or above). So I have enough battery to get by for the 4 nights. We don't carry a generator or solar because we are usually out doing things during the day - the camper is a place to sleep at night. If we are dry camping, battery, food, and water are all running low after the 4th day, and it's time to move on - which is less than an hour with the A-frame. In the real world, we have never dry camped for more than 3 nights in the same campsite.

The larger campers generally use more than 28AH per day, so you need to adjust your numbers.

Something to consider - if your batteries are not matched, switch them in and out individually. As long as your loads are small (5-7 amps or less), you don't lose much of the non-linear benefit of a large parallel battery bank. Use a switch like boats do to run 12V batteries separately (see photo). In my case, with 6V batteries, I just use the switch as a battery cut-off. In your case, I would use the same switch to go between your matched batteries (kept as a parallel pair) and your extra larger battery. When recharging, don't put the switch to "both" unless the batteries are reasonably equal in charge.

just my thoughts
Fred W
2014 Rockwood A122 A-frame
2008 Hyundai Entourage minivan
camping Colorado and adjacent states one weekend at a time
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Old 05-21-2017, 03:09 PM   #5
Flagman
 
Join Date: Nov 2015
Location: East Aurora,n.y.
Posts: 11
battery useage

We will be boondocking for 2 weeks each in two different spots and 1 week with shore power. I was hoping to use my Honda 2000i as little as possible and using electricity "very" little. My thought was by using the 27 in parallel with the 2 24's in parallel, I would gain some amp- hrs. on a daily basis.My estimate for each 24 is approx. 60 amp-hr. Thanks for all the good info. you have taken to send me. ART
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