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Old 06-28-2019, 10:13 AM   #1
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Managing batteries and switching/charging same

I recently took delivery of a new trailer which included a new group 24 battery rated at a paltry 75 amps. Additionally I own a older (2 y/o) but still working very well, group 27 battery that when new was rated at 105 amps.


I have enough room to mount on the trailer tongue beside the existing the g24, the older g27. I am of the belief that it is not a wise idea to parallel these two batteries due to the difference in size and age.


I plan to install a multi bank battery switch and thus isolate one battery from the other. My yet to be mounted, but in hand, battery monitor can monitor each bank (of one battery each) and alert me when to switch from one bank to the other.


My primary question is: when one bank (of one battery) becomes low and I need to charge it, how can I charge it while at the same time using the other full bank? I have available for charging purposes, (1) the trailer's onboard charger on either shore power or a generator. (2) a portable battery charger.


I posed the question above because if the battery switch is isolating the batteries, isn't it also isolating a bank from receiving a charge? Should I just plan on using the portable charger on the low and now, isolated battery?



TIA
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Old 06-30-2019, 09:05 AM   #2
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If your reason for installing the switch is because of the difference in the batteries whey not just get two batteries of the same type and rating and operate your rig as designed?
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Old 06-30-2019, 09:38 AM   #3
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your isolater does isolate the batteries... charge from a separate charger not your built-in unit and hook to whatever needs to be charged... 8 amp electronic chargers are reasonable price

or buy 2 new group 27 Interstate battery at Costco for about $80 each and save your isolater and charger purchase money...
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Old 06-30-2019, 09:48 AM   #4
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Mismatched pair of batteries will never provide what you want. You're correct they need to be separated from each other and the system otherwise the weaker battery will continually siphon power from the stronger. If run separately you're likely to over-discharge one battery before switching to the other. Note the dozen's of threads regarding discharging batteries too low and damaging them.

But you already own these batteries so why not just use them? The switching scheme, a simple Off-1-Both-2 switch (common on small boats) will work for the near future afterwhich you can get a pair of new matched batteries and run them combined as one big battery.

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Old 06-30-2019, 09:59 AM   #5
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If your reason for installing the switch is because of the difference in the batteries whey not just get two batteries of the same type and rating and operate your rig as designed?

To be clear;
1) I own the batteries and chose to use them. Some would consider me a cheap S*B
2) If were to get another battery, I would buy only one. It would match the newer G24 I have.
3) I have always installed a battery disconnect switch, irregardless of the style, type or age of the batteries. Goggle will yield many reasons its wise to do so. YMMV.


Charging in my chosen configuration was the question. Can you help with that?


Thanks
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Old 06-30-2019, 10:05 AM   #6
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your isolater does isolate the batteries... charge from a separate charger not your built-in unit and hook to whatever needs to be charged... 8 amp electronic chargers are reasonable price

I may wind up doing just that. Thanks


Quote:
or buy 2 new group 27 Interstate battery at Costco for about $80 each and save your isolater and charger purchase money...

I can't fit two 27's with out some surgery I don't want to do............Thanks
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Old 06-30-2019, 10:18 AM   #7
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Mismatched pair of batteries will never provide what you want. You're correct they need to be separated from each other and the system otherwise the weaker battery will continually siphon power from the stronger. If run separately you're likely to over-discharge one battery before switching to the other. Note the dozen's of threads regarding discharging batteries too low and damaging them.

But you already own these batteries so why not just use them? The switching scheme, a simple Off-1-Both-2 switch (common on small boats) will work for the near future afterwhich you can get a pair of new matched batteries and run them combined as one big battery.

-- Chuck

Thanks. My Victron can be set to alert of a over or lower discharge than a [%] event. Tested twice with success yesterday. Once with the onboard charger and once with a separate (very old) non automatic (no float) charger.

Yep. Using them is the plan and your near future remarks are well noted, that also fits into the bigger picture plan. Thanks again.
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Old 06-30-2019, 10:18 AM   #8
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A 6v GC2 (?) battery has the same footprint as a Group 24 battery and much more storage. Just a little taller. I've run a pair of these as one big battery for 15 seasons.

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Old 06-30-2019, 10:31 AM   #9
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A 6v GC2 (?) battery has the same footprint as a Group 24 battery and much more storage. Just a little taller. I've run a pair of these as one big battery for 15 seasons.

-- Chuck

I have considered going that route but it will result in an appox 180 lb weight gain. Maybe down the road and after I lose some weight.
15 seasons? On the same set?
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Old 06-30-2019, 10:43 AM   #10
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Choosing to use what you have, the best solution is to isolate the two batteries and use a separate automotive battery charger to charge the one not in use.
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Old 06-30-2019, 11:24 AM   #11
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I've run a pair of these as one big battery for 15 seasons.

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15 seasons? That's longer than some marriages last
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Old 06-30-2019, 12:10 PM   #12
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My second set of 6v batteries. I doubt they weigh 180 pounds total and that wouldn't be an additional 180 pounds, just the difference between the weight of the smaller batteries.

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Old 06-30-2019, 12:54 PM   #13
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My bad and I'll blame it on my HS math.
The 6v T105's? I was looking at, according the sales guy were 90 each. X2 = 180. I forgot to minus the one 45 lb group 24 currently onboard. The corrected difference would be 135.
I still need a diet plan.
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Old 06-30-2019, 01:07 PM   #14
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your isolater does isolate the batteries... charge from a separate charger not your built-in unit and hook to whatever needs to be charged... 8 amp electronic chargers are reasonable price

or buy 2 new group 27 Interstate battery at Costco for about $80 each and save your isolater and charger purchase money...
BTW, can you point me to one of those reasonable 8 amp chargers? Two battery output would be a plus!
Thanks.
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Old 06-30-2019, 01:18 PM   #15
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Yep, an additional 135-ish pounds on the tongue wasn't a problem for me.

Battery capacity in a lead-acid battery is directly (but not perfectly) related to the amount of lead in the battery. In this case the 45 pound battery has X-amp hours and the 180 pound battery will have 4X-amp hours in a perfect world.

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Old 06-30-2019, 01:18 PM   #16
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My bad and I'll blame it on my HS math.
The 6v T105's? I was looking at, according the sales guy were 90 each. X2 = 180. I forgot to minus the one 45 lb group 24 currently onboard. The corrected difference would be 135.
I still need a diet plan.
A quick way to get rid of LOTS of weight would be to replace the battery with a single 100 amp hour Battleborn LiFePo battery. Just over 30# and you can use almost all of the 100 amp hour's stored. Even if you go to T-105's you should only use 50% of the rated capacity for best battery life or around 110-115 amp hours. Even better, the LiFePo's charge much faster so if you have to run a generator you don't have to run it all day to obtain a TRUE 100% charge.

I just got back from a trip Friday where I ran for 5 days on two 100 amp hour Battleborns without charging from either Generator or Solar. Batteries were down to 34% and when I plugged in at home, two and a half hours later the batteries were up to 100%. (measured with Victron Batt. Monitor).

If still camping I would only have to run the generator an hour or so, every other day (or so) to keep me in power for an extended trip. Sure beats the up to 10 hours necessary to achieve a true 100% charge with lead acid batteries.

Back to weight, the two Battleborn batteries only weigh the same as ONE 6 v GC-2 Lead Acid battery and I have them mounted inside where they can stay nice and warm during freezing weather. Lead acid batteries can loose up to half their capacity when subjected to cold weather which is exactly the opposite of what you want for your furnace which will be running much more.

Yes, they do cost more but I figure I'm all done buying batteries unless I live another 20 years or so which is when I figure I'll use up the number of "cycles" possible with Lithium batteries. I guess it's not WHAT you pay, it's how OFTEN you have to pay it.
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Old 06-30-2019, 01:27 PM   #17
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My pair of Trojan 6v batteries (I think they're T125s) routinely last us 5-day extended holiday weekends at state parks where neither power nor water is available.

The T125 batteries "only" weigh 66 pounds (not 90) which helps explain my lack of weight issues.

We tend to find full service sites more frequently these days but all the amp hours are still very handy to have.

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Old 06-30-2019, 02:33 PM   #18
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I just looked them up.

T105 - 62 lbs
T125 - 66 lbs

So, in my scenario the 105's at 62 pounds twice would be 124. Minus 45 would be 79 lbs add.
The 6v route is looking better and better and the diet may be out.
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Old 07-01-2019, 05:27 PM   #19
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Choosing to use what you have, the best solution is to isolate the two batteries and use a separate automotive battery charger to charge the one not in use.

about the only choice you have
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Old 07-01-2019, 05:49 PM   #20
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I have considered going that route but it will result in an appox 180 lb weight gain. Maybe down the road and after I lose some weight.
15 seasons? On the same set?
I have eight Trojan T-105 6v, 225 amp gc batteries. They are 62 lbs each.
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