AGM battery?

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   If a cell in one of two parallel batteries shorts you will get circulating currents as the good battery dumps it's charge into the battery with the bad cell trying unsuccessfully to charge it. The current can get really high. Ask me how I know. 

You also have twice as many cells  to fail with the parallel 12v configuration.

Large trucks use parallel batteries because they need high starting currents. It's a SLI (starting, lights, ignition) not a deep cycle apucation like the rpod is. Ok. Skip the "I" part for diesels...

Off grid solar is a deep cycle application, and standard practice is to avoid paralleling batteries when possible. Of course, it's not always possible in large battery systems, so battery paralleling is done when needed. 

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offgrid Members Profile Send Private Message Find Members Posts Add to Buddy List Senior Member Joined: 23 Jul 2018 Online Status: Offline Posts: 5290	Topic: AGM battery? Posted: 22 May 2021 at 7:23pm
	If a cell in one of two parallel batteries shorts you will get circulating currents as the good battery dumps it's charge into the battery with the bad cell trying unsuccessfully to charge it. The current can get really high. Ask me how I know. You also have twice as many cells to fail with the parallel 12v configuration. Large trucks use parallel batteries because they need high starting currents. It's a SLI (starting, lights, ignition) not a deep cycle apucation like the rpod is. Ok. Skip the "I" part for diesels... Off grid solar is a deep cycle application, and standard practice is to avoid paralleling batteries when possible. Of course, it's not always possible in large battery systems, so battery paralleling is done when needed.
	1994 Chinook Concourse 1995 RV6A Experimental Aircraft 2015 Rpod 179 - sold

offgrid Members Profile Send Private Message Find Members Posts Add to Buddy List Senior Member Joined: 23 Jul 2018 Online Status: Offline Posts: 5290	Posted: 22 May 2021 at 6:52pm
	I'll give you 3 answers to the first question. 1) Batteries aren't identical, and as they age they tend to vary more. Specifically, each one will have a different internal series resistance. If they are in series that won't matter but if they're in parallel the one with lower series resistance will take (and provide) more current and so will get more cycling over time. 2) Two 6v batteries have a total of 6 cells. Two 12v batteries have 12 cells. So for the same amphour capacity the 6v configuration will have half the cells with twice as much lead and acid in each. The battery plates will be thicker and more robust. 3) This one is specific to our use case. 6v golf cart batteries are very common in this size so available from many manufacturers at good prices. Also, two GC2 size golf cart batteries are the largest that will fit in the rpod rack so give you the most capacity. Re how to be sure that you don't overcharge AGM batteries? Use a good quality charger with a setting for AGM's, which will be slightly lower in voltage for each charging stage than for a flooded battery. Will the WFCO charger in your rpod do the job? Debatable. All this bring said, if you don't feel you will remember to check the water level in flooded batteries they by all means get AGM's. If you do get 12v AGM's then take the positive output lead from one and the negative output from the other, that will keep the wire resistance the same for both.
	1994 Chinook Concourse 1995 RV6A Experimental Aircraft 2015 Rpod 179 - sold

podwerkz Members Profile Send Private Message Find Members Posts Add to Buddy List Senior Member Joined: 11 Mar 2019 Location: Texas Online Status: Offline Posts: 966	Posted: 22 May 2021 at 6:11pm
	The only way to always avoid overcharging AGMs is to chronically undercharge them. Nature of the beast, but if the level of constant under-charge is in the 80 to 95% range, then you are good to go. But, in this application (tongue mounted battery on a travel trailer) where the batteries are outside (no interior venting concerns) exposed to temperature extremes, but wont be turned sideways in any normal use (AGM are generally spillproof) and the water refill ports are reasonably accessible, flooded batteries really are the most bang for the buck, very robust, easily replaceable, and almost 100% reliable, usually made in the good ol USA. I take a different fork in the road when it comes to 6v batteries than offgrid...I recommend and use 2 12v batteries in parallel and it works fine. If one cell in one battery shorts or opens, you can still use the remaining battery for some time if needed... And if your tow vehicle battery dies in the middle of nowhere, you can drop in your RV deep cycle (assuming it has starter posts) and continue on your merry way. Virtually all class 8 OTR semi truck tractors use 4, 12v batteries in parallel. It's proven, it works, and the replacements are reasonably priced and available just about everywhere you are likely to be. But with a pair of 6v GC batteries, if a cell shorts or opens in one battery, you are 'dead in the water'...so to speak. I know, I tried it with an old motorhome I had. Not gonna do that again. At any rate, your choice...pros and cons for each solution.
	r･pod 171 gone but not forgotten!

OurPodJourneys Members Profile Send Private Message Find Members Posts Add to Buddy List Newbie Joined: 01 May 2016 Online Status: Offline Posts: 8	Posted: 22 May 2021 at 2:40pm
	Thanks. That's helpful. Two questions: why use 6V instead of 12V? And ... I keep reading cautions about not overcharging AGM. But how does one avoid overcharging?

offgrid Members Profile Send Private Message Find Members Posts Add to Buddy List Senior Member Joined: 23 Jul 2018 Online Status: Offline Posts: 5290	Posted: 22 May 2021 at 2:31pm
	Pros: no maintenance Cons: no way to add water so they are sensitive to overcharging Expensive Hard to get in 6v configuration, so the batteries have to be connected in parallel, which is not ideal because they don't get equal charging and discharging when in parallel. I'd recommend 2 6v GC2 size golf cart batteries in series. But you have to be willing to check and add distilled water at regular intervals. If not, go for your AGM's.
	1994 Chinook Concourse 1995 RV6A Experimental Aircraft 2015 Rpod 179 - sold

OurPodJourneys Members Profile Send Private Message Find Members Posts Add to Buddy List Newbie Joined: 01 May 2016 Online Status: Offline Posts: 8	Posted: 22 May 2021 at 12:16pm
	Sorry, that 1st post was mine, sent without a message. My question: I use two 12V lead acid batteries for 2016 179. They need to be replaced. We do a fair amount of boondocking. Am thinking of switching to AGM. Are any modifications needed, or do I simply switch the batteries? Are there important pros and cons to AGM I should consider before committing to AGM?

OurPodJourneys Members Profile Send Private Message Find Members Posts Add to Buddy List Newbie Joined: 01 May 2016 Online Status: Offline Posts: 8	Posted: 22 May 2021 at 12:11pm