Wiring a Lithium and a Lead Acid battery

[MikeDeAngelo]

I'm buying a 2002 Coachmen Leprechaun 31 foot motorhome. I'd like to upgrade the existing house battery using a Lithium Iron Phosphate, but I'd also like to keep the existing lead acid as a backup and for starting the generator.

I think I have a handle on what I want to do, but I want to see if anyone else has ideas...

The Lithium battery will likely be inside the coach and the Lead Acid will continue to be in the existing external compartment.

First I'm going to replace the existing converter/charger with a Lithium specific one such as this: https://www.etrailer.com/RV-Converte.../PD9130LV.html

Second, referring to the following wiring diagram I'm going to move the connections to the converter/distribution panel and probably to the power step and radio memory over to the Lithium battery. The connections to the auto battery, the generator, and the emergency start switch will continue to be at the Lead Acid battery.


Finally, I'll use something like this two bank charger/maintainer connected to both the old house lead acid battery and also the vehicle battery. That way whenever I am running the generator or connected to shore power I'll top off both batteries. Over the winter when I'm not using the camper as much the "maintain" mode will keep both the lead acid batteries fresh. https://smile.amazon.com/dp/B079SZFFL2/

[MikeDeAngelo]

I wanted to follow up on this.

When I actually picked up the coach I realized that the house battery was actually under the steps into the coach. My previous experience had always been with battery compartments on the outside. When I opened it up I discovered that there was plenty of space for two batteries. There was an Interstate 24DC Marine/RV battery in there that has approximately a 58Ah capacity. Since it is a hybrid, it probably shouldn't be discharged more than 50%. So I was really looking at about 30Ah of power. This is a battery that is available for $80 or so at Costco.

I purchased a pair of Renogy 12 AGM 100Ah deep cycle batteries that just barely fit into the space. Flooded batteries should only be discharged to 50%, but AGM can be discharged to 80%. So the pair gives me roughly 160Ah of usable capacity. They were $180 each on Amazon, so I got the pair for $360.

At the same time, I knew that the charger/converter in the coach was not going to be great for these batteries. Old chargers generally have only one or two charge modes. Batteries are charged relatively slowly and then can be overcharged once they are full. AGM batteries should have a different charge profile than flooded lead acid batteries, and so using the old charger/converter was going to reduce the life of my batteries. Also I wanted to avoid sulfanation if I could.

I replaced the 2002 vintage Magnetek 55A charger/converter with the Progressive Dynamics PD9260C 60A charger/converter. The PD9200 series has an advanced charger 4 mode charger. When the battery is low it uses "boost mode" that should get the battery up to 90% charge in just a few hours. AGM batteries charge more quickly than flooded batteries anyway, and with the right charge profile it will be even quicker. If I'm boondocking I can get most of my charge with the generator in just a few hours. When the battery is close to fully charges it goes into a lower charge rate to finish off the battery. When the battery has not been used in several days it goes into "storage mode" that is even a smaller trickle to avoid "boiling". While in storage it occasionally does a 15 minute higher voltage charge to avoid sulfanation and stratification.



The PD9260C was about $250 but I had a coupon. All together it cost me about $600 to upgrade from 30Ah to 160Ah of power. Since the PD9260C uses an appropriate charge profile for AGM and Gel batteries, the batteries should last much longer before needing replacement. I'll keep the Interstate 24DC Marine/RV battery on a trickle charger in my garage and use it as an emergency replacement for a car battery.

My wife has a health condition that is exacerbated by hot weather, so she needs access to air conditioning. Consequently in the summer boondocking is not going to be common. In the cooler months 160Ah will be more than enough to allow us to run the heat overnight without needing the generator. My understanding is that the circulator fans for a typical RV furnace can draw 7A or so. 160Ah would then be enough to run for 12 hours and barely hit 50% of my usable capacity. I'm sure lights and other appliances will use a bit too, but I'm not anywhere close to the max.

[twinboat]

Originally Posted by MikeDeAngelo

I wanted to follow up on this.

. My understanding is that the circulator fans for a typical RV furnace can draw 7A or so. 160Ah would then be enough to run for 12 hours and barely hit 50% of my usable capacity. I'm sure lights and other appliances will use a bit too, but I'm not anywhere close to the max.

Unless your in sub freezing areas, your furnace isn't going to run full time.

Running 50% of the time will cut the AHs consumed in half.


When I got my MH, I pulled 2 brand new GP24 batteries and installed 2 GP31s. I used Maintance free, flooded acid, deep cycle batteries.

Just about doubled my AH capacity.

Flooded and AGM batteries can both be drawn down to 0% state of charge, only going down to 10% is safer.
The cycle life is about the same for both chemistries, half the cycles as only using 50% capacity.

AGMs will accept a higher bulk charge but only if deeply discharged. If they are at 70% state of charge or higher, the higher internal resistance slows the charge rate on both types.

[Bobby F]

"All together it cost me about $600 to upgrade from 30Ah to 160Ah of power."


Money well spent. Nice plan.

[Ed_G]

Amazing amount of misinformation in these posts regarding depth of discharge on wet cells and AGM batteries! Both types are the same Lead acid chemistry and are spec'd the same........manufacturer recommended practice discharge to 50%. Discharging a lead acid battery below 50% shortens the life.....the more often you do it, and the deeper the discharge , the shorter the life.

[SteveJ.]

There is a thread over on the class B forum where somebody set up a hybrid lithium/lead acid system. He set it up so the lithium battery would do the absorption charging phase of the LA battery. This way one could let the LA fully charge without the genny running for hours, just a final short run at the end of the LA full charge to top off the lithium battery, if desired. Solar also integrates well as well as an under hood genny.

A bit complicated but all of the info is there, including in the field results.

Good kit for extended boondocking.

No dog in this hunt.

[tankcj]

Originally Posted by Ed_G

Amazing amount of misinformation in these posts regarding depth of discharge on wet cells and AGM batteries! Both types are the same Lead acid chemistry and are spec'd the same........manufacturer recommended practice discharge to 50%. Discharging a lead acid battery below 50% shortens the life.....the more often you do it, and the deeper the discharge , the shorter the life.

I have Crown AGMs and the mfr states to "optimize battery life by limiting duty cycle depth-of-discharge to 75% or less of available discharge".

To OP, $600 for over 5 times the Ah is amazing!

[Persistent]

I would go with the battery manufactures recommendations over the conflicting advice of others. If the battery manufacture does not make test information available, use a different manufacturer.

[Ed_G]

Originally Posted by tankcj

I have Crown AGMs and the mfr states to "optimize battery life by limiting duty cycle depth-of-discharge to 75% or less of available discharge".

Yes, Crown has spec'd a discharge of 80% -- they also spec an expected life of three years. If Depth of discharge is commonly kept to 50% one will most likely double that life expectancy.
.

Cycles vs Lifespan
A battery "cycle" is one complete discharge and recharge cycle. It is usually considered to be discharging from 100% to 20%, and then back to 100%. However, there are often ratings for other depth of discharge cycles, the most common ones are 10%, 20%, and 50%. You have to be careful when looking at ratings that list how many cycles a battery is rated for unless it also states how far down it is being discharged. For example, one of the widely advertised telephone type (float service) batteries have been advertised as having a 20-year life. If you look at the fine print, it has that rating only at 5% DOD - it is much less when used in an application where they are cycled deeper on a regular basis. Those same batteries are rated at less than 5 years if cycled to 50%. For example, most golf cart batteries are rated for about 550 cycles to 50% discharge - which equates to about 2 years.

How depth of discharge affects cycle life on batteriesBattery 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.

[MikeDeAngelo]

According to this site - https://batteryuniversity.com/learn/..._glass_mat_agm - an AGM battery can be discharged to 80% and have the number of cycles as a flooded cell discharged to 50%. So in an apples-to-apples comparison the AGM will provide more useful power over its life. Likewise is we hold the DOD constant at 50% an AGM will last more cycles than a flooded.

Do you have other references I should look at?

[twinboat]

Originally Posted by MikeDeAngelo

According to this site - https://batteryuniversity.com/learn/..._glass_mat_agm - an AGM battery can be discharged to 80% and have the number of cycles as a flooded cell discharged to 50%. So in an apples-to-apples comparison the AGM will provide more useful power over its life. Likewise is we hold the DOD constant at 50% an AGM will last more cycles than a flooded.

Do you have other references I should look at?

Google Trojan battery and search for DOD charts. They have a few that show each type of battery cycle graphs.

[Bobby F]

I'm not a battery guru by any means, so if what I'm about to say is somehow laughable, well . . . be gentle.

It almost seems to me that the cycling/life numbers indicate that a battery is good for delivering a set amount of amp-hours over its life time, regardless of how many cyles you use to deliver those aH's.

Thus, you take a statement such as " . . . If cycled only 10% DOD, it will last about 5 times as long as one cycled to 50%..." This seems to say that it doesn't matter how many times you charge it up - it's going to deliver X number of aH's before it dies.

Meaning, if you have a 100aH battery, and you take 10aH's from it 5 times, (charging it up in between), you have delivered 50aH's from it, and you have decreased its life exactly as much as if you had taken 50aH's from it all at once - because we're defining "lifetime" by "number of cycles."

I realize the math isn't working one-to-one when speaking of the comparison between 50% and 80% DOD, but it's close.

What's wrong with my logic here? Seems too simple.

[twinboat]

Yes your right.

A deep cycle battery is like a bucket of energy.

Every time you use 1/2 the energy, you throw a hand full of sand in the bucket as you refill it.

If you use all of the energy, throw in 2 hand fulls of sand.

If you use 1/4 a bucket of energy, you throw in a half a hand full of sand.

Over time, as you drain and fill the bucket with energy, you lose capacity, until the time when you don't have enough room for enough energy to do what you want to do with it.

Time to get a new bucket.

[Smitty77]

So basically. A battery will 'Kick the bucket!'...

I love all this techie talk!

Twinboat - That was so simple even a caveman could have read it in the evening and understood it. Of course, after 'Man make fire!'... days.

Best to all,
Smitty