I need HELP!!!: Dual Batteries - must type be matching ?

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Dual Batteries - must type be matching ?

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Forum Name: I need HELP!!!
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URL: http://www.rpod-owners.com/forum_posts.asp?TID=15252
Printed Date: 08 Jun 2025 at 8:00am
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Topic: Dual Batteries - must type be matching ?

Posted By: 77 Sunset Strip
Subject: Dual Batteries - must type be matching ?
Date Posted: 02 May 2022 at 12:06pm

If adding another battery - and the original is lead acid, can you add an AGM as the second battery or must the battery type be matching ?

Thanks for you help - great forum - looking forward to sharing info.

******* And yes- would want to hook them together ********

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2019 Rpod 189
2022 Nissan Frontier

Replies:

Posted By: jato
Date Posted: 02 May 2022 at 2:51pm

I don't see a problem as long as you don't try to wire them together or charge them in parallel. That being said, I have had a pair of 12v lead acid batteries that were used separately from one another. Even though they were only 6 months apart age wise I still did not wire them up together. Oldest one died last August, the other is still going strong at 11 years. Replaced the dead one with a Trojan T-1275 golf cart battery. (lead acid)

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God's pod
'11 model 177
'17 Ford F-150 4WD 3.5 Ecoboost
Jim and Diane by beautiful Torch Lake
"...and you will know the Truth and the Truth will set you free."

Posted By: offgrid
Date Posted: 02 May 2022 at 4:33pm

It's bad practice to mix battery types, sizes, or even batteries of significantly different ages. They won't charge the same so will be imbalanced and the one which charges fastest can be damaged. Buy like for like or start over.

If you do start over consider getting two 6V golf cart batteries in series. Unless you can't bring yourself to check the water level in your batteries AGMs really don't do much for you, they don't cycle as well as the golf cart batteries, have lower capacity, and cost lots more.

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1994 Chinook Concourse
1995 RV6A Experimental Aircraft
2015 Rpod 179 - sold

Posted By: jato
Date Posted: 02 May 2022 at 8:42pm

Originally posted by 77 Sunset Strip

******* And yes- would want to hook them together ********

Hmmm, that wasn't in your original post. Regardless +1 and +2 to offgrid on this. Do not mix batteries unless you have a fat wallet that needs to be on a weight loss program.

That was a reason why I never tied mine together. Even though they were exactly the same battery they were purchased 6 months apart and the original had been used quite a bit by the time we purchased the second one. And the results speak for themself: The original lasted 10.5 years and the other one turned 11 this month and still going strong. In fact it received a 36 hour 2 amp charge Oct.31-Nov.1 last year, since then it has sat directly on a concrete floor in an unheated garage in northern Michigan. Just checked the charge today and it reads 12.39v, not bad for sitting around for 6 months.

The replacement battery purchased last August is a Trojan T-1275, same scenerio as above, and it read 12.45v. Maintenance is key. Keep outside and posts clean with baking soda. Keep water levels up with distilled water and don't draw down below 12.2v before recharging. Simple

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God's pod
'11 model 177
'17 Ford F-150 4WD 3.5 Ecoboost
Jim and Diane by beautiful Torch Lake
"...and you will know the Truth and the Truth will set you free."

Posted By: Colt
Date Posted: 03 May 2022 at 2:47pm

You can add an AGM battery in parallel to a flooded cell battery, but since the AGM operates at a few tenths of a volt higher then most flooded cells, the 2 will not discharge evenly. That means you won't get all the capacity you paid for and most likely they will both have a shorter life. Best to match batteries in both type, chemistry and age.

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John
'16 R-Pod 180

Posted By: offgrid
Date Posted: 04 May 2022 at 4:21am

Under discharge conditions it's true that AGMs will be a little higher in voltage at the same depth of discharge, meaning that if they are in parallel with a flooded battery the AGMs will be at a lower state of charge during discharge. No good but not the end of the world.

What's worse is that
it's the other way around during charging. You can see that by looking at your solar charge
controller voltage settpoints.

AGMs need bulk and float charge voltage settings that are lower than flooded batteries. And there is no equalization charge setting with AGMs.

That means that if you set your solar charge controller for flooded batteries you will be overcharging the AGMs. That's bad because there is no way to add water to them to make up for electrolyte losses from overcharing.

Worst case AGMs can overheat from excessive overcharging, while flooded batteries don't. Reason is that overcharging converts water to hydrogen and oxygen in the battery. That shows up as the bubbles you see in flooded batteries while being charged. So in flooded batteries the two gasses just disappear into the air, and you have to add water periodically to compensate.

But in AGMs and other "sealed" batteries they are recombined into water within the battery. That is good up to a point as you don't need to replace the hydrogen and oxygen with water. But it's very bad if excessive charging occurs because converting hydrogen and oxygen to water is a combustion reaction, creating lots of heat.

The heat has nowhere to go so AGMs can overheat much more easily that flooded batteries. They can get hot enough to blow battery cases apart. Ask me how I know...

If OTOH you set your solar charge controller for AGMs to avoid all those bad things then you will be undercharging the flooded batteries. That's a safer choice than overcharging the AGMs but you're basically throwing away money on the flooded battery, it won't last long not getting charged properly.

Bottom line is you can't win by mixing the two types, just don't do it. Even the smaller variations from different battery models of the same type or different ages of the same model can cause significant imbalance so avoid that too.

Either just start over or swap your two different batteries into the rpod circuit one at a time like Jato does. If you do the latter be sure to reset your charge controller for the battery type youre running every time you make the swap.

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1994 Chinook Concourse
1995 RV6A Experimental Aircraft
2015 Rpod 179 - sold