Two batteries worth it?

[KeithB]

Last year I purchased a new battery for my 2108 DS trailer. Nothing fancy, 100 amp hr , deep cycle AMG battery. I replace the marine battery that came with the trailer when purchased. Now we are thinking of doing more off grid camping. My AMG is good for about 24 hrs, minimal lights and running a small fan at night. If I go tandem with the original marine battery I am thinking I might get two nights out of the system even though the original marine version is not as “good”. Two questions; is it worth giving the tandem approach a try? And is it even advisable to join two batteries of different type?

[creativepart]

Two “new” batteries is great. Adding a worn out used battery to another one year old used battery can just produce two compromised batteries. The weakest battery will bring all other batteries down to its level.

If you want to add another battery make it a newer one, of the same type, not an older one. But really two new ones is the way to go.

And I’m sure you mean AGM. AMG makes some terrific automobiles but haven’t branched out into batteries. I’m sure they will soon though.

PS. 100 amp hour deep cycle AGM is a pretty special battery.

[No1Hunter]

PS. 100 amp hour deep cycle AGM is a pretty special battery.

My Lifeline GPL-31T AGM Battery I used for my trolling motor back when I had a boat has an AH rating of 105. I had hoped to use on my TT, but being close to 10 years old, I am not sure if it would be a good idea. I kept it charged by charging it every few months.

Any idea how to test it? I know how to test the voltage, but wonder is the AH capacity has degraded over time.

[creativepart]

You can take it to an auto shop and have it load tested. But they are more concerned with starting abilities. It’s a 10-year old battery. You know that it’s at the end of it’s lifecycle.

[BobC]

The easy answer is Yes, it's worth it. Creativepart is right, adding the old marine battery back in isn't a good idea. On the other hand, your AGM battery is only a year old so I don't thing adding a new one of the same type, size, etc. shouldn't be a problem. It's best to stick as close to identical as possible.

Also, based on my reading, the old rule of thumb of not going below 50% state of charge (SOC) isn't necessarily valid. It is for the ultimate maximum in battery life in terms of discharge cycles but most battery manufacturers state that a 20% SOC (80% discharge) is the threshold beyond which battery damage will occur. Discharging lower than 50% SOC won't have an appreciable effect on battery life unless you do it consistently, pretty much every day. Check out some of the posts on pages 2 and 3 of this thread for more info:

https://www.irv2.com/forums/f56/firs...-535245-3.html

By adding a second battery, you'll be adding 60 lbs or so to your tongue weight so, before you do so, make sure this won't be an issue.

You might also want to research adding solar to the mix, either rooftop, portable or both. And make sure your tow vehicle is supplying a charging current to your 7-pin connector. Some do, some don't.

https://www.northtexastrailers.com/f...RV-Diagram.jpg

If you don't measure a charging voltage between pin 4 and ground, you'll need to deal with your tow vehicle. All vehicles are different but here's a Youtube video about a Ford F-150 that was pre-wired but not "activated":

https://www.youtube.com/watch?v=PVHiWWzc_RA

Adding a charging circuit from scratch is a bit more complicated but doable. Here's a link to an etrailer article/video on the subject:

https://www.etrailer.com/question-64598.html

[No1Hunter]

Originally Posted by creativepart

You can take it to an auto shop and have it load tested. But they are more concerned with starting abilities. It’s a 10-year old battery. You know that it’s at the end of it’s lifecycle.

Yeah, the 10-year age it what concerns me.

I tested the voltage and it at 12.38 volts (approx 50 to 75%, depending on who you believe). My AGM Charger says it is fully charged. I'm going to leave the charger on it for a while, let it sit, and test it again, but being that old, with that reading, I'm thinking it is toast.

[Marine359]

Originally Posted by KeithB

Last year I purchased a new battery for my 2108 DS trailer. Nothing fancy, 100 amp hr , deep cycle AMG battery. I replace the marine battery that came with the trailer when purchased. Now we are thinking of doing more off grid camping. My AMG is good for about 24 hrs, minimal lights and running a small fan at night. If I go tandem with the original marine battery I am thinking I might get two nights out of the system even though the original marine version is not as “good”. Two questions; is it worth giving the tandem approach a try? And is it even advisable to join two batteries of different type?

Curious about a couple of things:
What DOD % do you take your AGM down to? You should be able to take it to 80% DOD without significant degradation of cycles.

Using as much power as you do in one day would signal to me that you have a 12v compressor fridge, which uses 50ah/day nominally. Check that you have it set for performance level 1. I’ve tried all other settings, and this one tends to use the least power, but keeps things very cold.

If that’s the case, adding a second AGM discharged to 80% DOD would still only give you two days boondocking without generator.
Have you considered adding some solar? A single 120w portable panel with an mppt controller can cost less than an AGM battery. And it will collect 60 amps or more on a good day.

Additionally, I have found that I have extended my battery SOC significantly by offloading a lot of my 12v load onto a cheap power station. For $200 you can get a LiFePo4 300wh powerstation that will run a cpap and charge all devices for several days. You can throw this thing into your tv and charge while driving, or charge with portable solar, or charge it up when you have some SP. It’s probably the best gadget/hack I’ve bought for my micro Minnie.

[BobC]

Originally Posted by Marine359

Additionally, I have found that I have extended my battery SOC significantly by offloading a lot of my 12v load onto a cheap power station. For $200 you can get a LiFePo4 300wh powerstation that will run a cpap and charge all devices for several days. You can throw this thing into your tv and charge while driving, or charge with portable solar, or charge it up when you have some SP. It’s probably the best gadget/hack I’ve bought for my micro Minnie.

Will Prowse, a generally well-respected Youtuber on the topic of solar, batteries, etc. has reviewed many of these power stations aka solar generators. Here are a couple of searches of his channel. He's even devised DIY versions:

1. "Portable Power Stations" - https://www.youtube.com/c/WillProwse...ower%20station

2. "Solar Generator" - https://www.youtube.com/c/WillProwse...ar%20generator

There are a ton of other reviewers on Youtube but I don't know anything about them. If you find something interesting, I'd see if Will has reviewed it. In contrast to many reviewers, he will often dig in with a tear down.

Keep in mind, that, despite the term "solar", there's nothing solar about a "solar generator" other than they can be charged from a solar panel (not included) in addition to a charger, etc. That's why I prefer the term "portable power station" but it doesn't have the same cache.

I was skeptical when these first came out but can see their usefulness as described by Marine359.

One difficulty I see in these in comparing these units is they often only quote their maximum output wattage, 500W, 1500W, etc. Watts just tells you how big a load it can run but not for how long. To adequately compare units you need to know their watt hours, kilowatt hours or amp hours capacity. Amp hours divided by the output voltage will give you watt hours. Both of these depends on the capacity of the internal battery. I almost forgot, the best ones have LiFePO4 (Lithium Iron Phosphate) batteries. Sometimes, cheap ones have conventional lead acid batteries. Weight is a dead give away on this issue.

Here's an article from Popular Science that gives a down to earth overview of them along with some reviews:

https://www.popsci.com/reviews/best-solar-generators/

[Marine359]

Bob,
Good points. I would add to that some important things if you’re considering buying a portable power station.

Most being offered are lithium ion, but newer offerings have LiFePo4 battery. Buy only LiFePo4, as it charges much faster, and doesn’t suffer Peuket as voltage approaches 100% DOD. And the LiFePo4 will give you several times the cycles as Li.
Make sure the all outlets including 12v are tested psw. Many models are not.
Ensure the charger is mppt and can accept an acceptable amount of charge current.
Make sure it has pass-thru allowing you to discharge while charging.

[BobC]

Costco and Sam's Club have some Massimo units for sale online. I know absolutely nothing about them but both Costco and Sam's Club have a no-questions-asked 90 day return policy on electronics.

[creativepart]

Originally Posted by Marine359

Curious about a couple of things:
What DOD % do you take your AGM down to? You should be able to take it to 80% DOD without significant degradation of cycles.

Jim, I know that the 50% rule is not a hard and fast one, but I think you may be overstating things here.

Sure take your batteries to 70% every great once a while. But normalizing a 80% DoD and claiming “no degradation” goes against the information supplied by virtually all battery manufacturers and industry experts.

Lots of folks have no idea that severe discharging can cut a battery’s life short in their RV. And this is why so many RVers find their batteries lasting 2-years or less.

Can you backup that 80% DoD claim from more than one or two commenters? ‘Cause I think we could provide about 3-dozen that have a different take on this topic.

A simple search will show hundreds of responses such as this:

Battery life is directly related to how deep the battery is cycled each time. If a battery is discharged to 50% every day, it will last about twice as long as if it is cycled to 80% DOD. If cycled only 10% DOD, it will last about 5 times as long as one cycled to 50%. Obviously, there are some practical limitations on this - you don't usually want to have a 5 ton pile of batteries sitting there just to reduce the DOD. The most practical number to use is 50% DOD on a regular basis. This does NOT mean you cannot go to 80% once in a while. It's just that when designing a system when you have some idea of the loads, you should figure on an average DOD of around 50% for the best storage vs cost factor. Also, there is an upper limit - a battery that is continually cycled 5% or less will usually not last as long as one cycled down 10%. This happens because at very shallow cycles, the Lead Dioxide tends to build up in clumps on the the positive plates rather in an even film. The graph above shows how lifespan is affected by depth of discharge. The chart is for a Concorde Lifeline battery, but all lead-acid batteries will be similar in the shape of the curve, although the number of cycles will vary.

[Marine359]

Good thing to point out Creativepart, and I understand the concern.

It’s not conducive to long FLA long life to take them to 80% DOD regularly.
However for the amount of time that OP boondocks, he will not see severe degradation for the few times he does a deep discharge. I’ve never owned AGM, but have taken down FLA wet cell to 80% DOD and they bounced back to a point. But they were so cheap, that replacing them was easier and cheaper than buying a bigger battery bank.

Maybe I’m stretching it a bit, but OP has to weigh effects of DOD vrs buying more a bigger bucket. From his OP, he may be better served by sticking with one AGM, and occasionally discharging to 80% DOD. It likely won’t really significantly diminish his cycles for his type of use. And of course, depending on the quality of the AGM, higher quality means lower Peukert as he approaches 80% DOD. This site may not be conclusive, but it’s a start:

https://www.power-sonic.com/blog/the...oads%20quickly.

IMHO, Although AGM are a good fit for many, they are expensive compared to gc2. For my money, I’ll always choose LiFEPo4.

Thanks for clarifying for members.

[creativepart]

Originally Posted by Marine359

Good thing to point out Creativepart, and I understand the concern...

Balance is the goal in all these things. Otherwise people start saying, “I read on the RV forum that I can discharge my batteries all the way every day without any problems, so that’s what I plan to do.”

[BobC]

Does this help with documenting depth of discharge and cycle life?

This is from the FullRiver battery company and refers to their 6V/200Ah AGM battery. I've seen similar numbers from Trojan and other manufacturers but this came up first in my Google search:

http://resources.fullriverbattery.co...ts/DC250-6.pdf

See page two of the document for the graph entitled "Cycle Life vs. Depth of Discharge". This graph portrays what happens to cycle life based on constantly discharging to the specified Depth of Discharge levels.

There's nothing magic about the 50% level, it's not a "cliff". In fact, the decrease in lifetime cycles between 30% and 50% is greater than between 50% and 80%, so why not stick to 30%? On the other hand the 80% depth of discharge (20% state of charge) is a cliff, beyond which, most manufactures cite as the point beyond which true damage can occur.

Since it's a graph, I can't be totally accurate but it appears that if you always discharged to 50% depth of discharge (50% state of charge), your battery will last about 1300 cycles. Always discharging to, but never exceeding, 80% depth of discharge (20% state of charge) will reduce your battery life to about 800 cycles or 61.5% of the 50% battery's cycle lifetime.

Looking at this another way, based on the above, a 100Ah 12V battery (with the same life cycle graph as above) routinely discharged to 50% depth of discharge would deliver a total of 1,300 x 50Ah = 65,000Ah over its lifetime. The same 100Ah battery routinely discharged to 80% depth of discharge will deliver 800 x 80Ah = 64,000Ah over its lifetime or about 98% of the the total lifetime Ah at 50% SOC. Even if my graph reading is way off, the ratio is still going to be in a ninety-some percent range.

The other factor to consider is, as depth of discharge increases, voltage levels decrease, which may be an issue for some voltage sensitive devices. This possibly may be where the 50% rule of thumb came from. There's a graph on the same page that shows the 50% state of charge "open circuit voltage" to average a little below 12.25V.

My take on this is, be careful and don't ever discharge below 80% depth of discharge but don't obsess over staying at, or above the 50% depth of charge.

[creativepart]

Bob, not all batteries have the same DoD life cycle. And, not all marketing materials are completely trustworthy.

The 50% rule of thumb came from a lot of sources. Is there anything else but Fullriver’s statements?

I don’t think the 50% DoD rule is iron clad. But I certainly don’t think an 80% DoD is just fine for lead acid batteries over their lifecycle.

That’s all I’m saying.

[Marine359]

OP is asking for how to get more boondocking time out of his existing battery setup. Obviously the answer to paralleling his new AGM with his OEM marine deep cycle is “no”. So, short of buying another AGM, is there another solution. That’s hard to say because OP has not told us what kind of fridge he has, what DOD he goes to, or what other loads he puts on the battery so as to blow through his available amphrs in one day. I think if he clarifies these things for us, the answer may become obvious.

[BobC]

Originally Posted by creativepart

Bob, not all batteries have the same DoD life cycle. And, not all marketing materials are completely trustworthy.

The 50% rule of thumb came from a lot of sources. Is there anything else but Fullriver’s statements?

I don’t think the 50% DoD rule is iron clad. But I certainly don’t think an 80% DoD is just fine for lead acid batteries over their lifecycle.

That’s all I’m saying.

I don't think we really disagree, I was trying to present some empirical data on the effect of discharging further than 50%. Here's a link to some graphs from Trojan showing similar cycle life curves:

https://www.trojanbattery.com/pdf/Si...tLineSheet.pdf

And another from Trojan that puts things into perspective (see "Discharging"):

https://www.trojanbattery.com/tech-s...y-maintenance/

I do note that this Trojan document cites 80% as where damage will occur, not the point "beyond which" damage will occur, as I was previously assuming. Therefore, I agree that 80% isn't "just fine" and, personally I wouldn't want to go anywhere close to it.

We've really departed from the OP's question so I say we put this to bed and let everyone decide on their own comfort level with some understanding of the data and realizing they're not going to ruin their batteries by discharging to 60% or 70% depth of charge to get through a few nights of boondocking.

[creativepart]

I agree Bob. Just trying to balance the “80% is fine” statements.

That Trojan document really answers this question:

Shallow discharges will result in a longer battery life.
50% (or less) discharges are recommended.
80% discharge is the maximum safe discharge.
Do not fully discharge flooded batteries (80% or more). This will damage (or kill) the battery.
Many experts recommend operating batteries only between the 50% to 85% of full charge range. A periodic equalization charge is a must when using this practice.

[MadMaxWinny]

Battery Age: No fast and sure rule. Years don't count. Everything hinges on if the battery (regardless of chemistry) was charged "perfectly," the number of discharges, and the AVERAGE depth of discharge. Of the pallet load of AGMs I got from Lifeline for testing in 1995, about half are still in service and are working in every current application I have.

The 50% rule: Still pretty much an industry standard, but Gels and AGMs were found to be a bit more forgiving. Ultimately, the shallower the depth of discharge, the more discharges you get. This is one of the reasons flooded golf cart batteries got so popular. 1000 average discharges vs 200-300 on conventional construction.

[No1Hunter]

Of the pallet load of AGMs I got from Lifeline for testing in 1995, about half are still in service and are working in every current application I have.

What application are you using them in? Are they in an actual battery powered application or in a UPS type system where they are discharged very seldom. Granted, my trolling motor application is one hard use of the battery with significant discharge at times, though it was charged promptly upon returning home or the dock. On a side note, I have an APC 1500 Smart-UPS with AGM batteries on my home network that is seldom used and it seems it's batteries need replacing around every 5 years even though they are monitored and charged as needed.