Battery Upgrade

by Greg Zimmerman

It was time to upgrade the battery and I wanted more capacity without jumping to a group 31 which requires modifying the hold down (they’re really big and heavy). Trojan batteries are made in the USA. They normally supply much larger batteries for golf carts, mega yachts, and big trucks. Trojan batteries need to be ordered from a dealer- find your nearest by clicking here.

UPDATE: I now recommend the Trojan 27-AGM battery as it’s sealed and maintenance free. It cost more, but it’s worth it. If you’re up for a challenge and need the capacity, you can use the bigger AGM-31, but you’ll need to modify the hold down bracket for clearance. No need to cut the terminals on the AGM either.

Another option is the much cheaper TMX-27 wet cell. It has more capacity, but needs distilled water added every 6 months. To fit it, we need to cut off the threaded post on the terminals because they stick up too high to fit in the battery compartment. A pair of off-set terminals are used to give the required clearance.

TIP: Don’t forget that the white and green wires are negative and the black is positive!

The TMX-27’s capacity at a 20 hour discharge rate is 105 amp/hours which is quite a bit more than the 87 amp/hour rating of the original Interstate battery. The AGM’s capacity is 88, pretty much the same as the original. I also recommend the Little House Custom Battery Disconnect, click here for the article on that.

Just fits!


Gary Wells January 28, 2019 - 12:01 pm

Great job on the battery change out. What size wire from the battery to the distribution posts. And the batter disconnect (RED) block – what size? Or were these items already on the Casita?


Greg Zimmerman January 28, 2019 - 1:09 pm

It’s 8 gauge wire. It was included in the Little House Customs battery disconnect kit along with the terminal block and connector.

Gary Wells January 31, 2019 - 12:39 pm

Thanks Greg.

Kirby Gchachu June 2, 2020 - 8:24 am

Thank you for this information. Your article gives me expected roadblocks to deal with & I appreciate that.

Steve Hilliard June 27, 2020 - 8:25 pm

Hey Greg, I just noticed something in the bottom of the last photo–is that a remote thermometer?

Greg Zimmerman June 28, 2020 - 9:17 am

Good eye! Yes it’s one of 3 accurite thermometers I had. One in the fridge, one in the freezer, and the battery cave was the best place I found to get outside temp.

Donna Parker September 9, 2020 - 8:46 pm

Hi Greg. I am thinking of buying the Trojan 27 AGM. I have a 2009 Casita with the original converter. Before I pull the trigger, I want to confirm a couple of things that I think you’ve partially answered for me in various posts. I need your succinct answers though, since I value your expertise.

1. It WILL fit in my battery case without being altered, right?
2. Can I continue to use the original converter without damaging the battery, or should I just go ahead and upgrade converters?
3. Will the LHC battery cutoff switch fit?
4. I usually like to run a dehumidifier during the winter. Which means being plugged in almost all the time. Will that hurt the Trojan? Dumb question alert!! Can I flip the breaker and turn off the converter and just run the dehumidifier since I’m on shore power?
5. If I don’t run the dehumidifier, can I just plug in once ever two weeks or so to charge the Trojan? I’m not very good at taking a batter out and using a smart or trickle charger. I’d like to just keep the battery charged enough so it won’t freeze.

I hope you’re enjoying Italy! What an adventure!!

Thanks so much.

Greg Zimmerman September 10, 2020 - 1:31 am

Yes to all except #4- leaving it plugged in will not hurt the battery. You can turn off the converter, but no need to. The converter in a 2009 is smart enough to not overcharge.

Alan Day March 16, 2022 - 10:58 am

Greg, your answer to Donna needs to be more concise…
Just kidding!!!😀

Karen Dennehy October 6, 2020 - 2:58 pm

Hi Greg! Once again, what a great site! Thank you for your time and effort into providing such valuable information.

I am trying my very best to understand the world of batteries and RV power. I feel like I’m trying to drink from a fire hose as a 1st time travel trailer owner with a newly-purchased 2014 Casita SD 17’.

Unfortunately I have found my “new“ Casita has a bad battery that needs to be replaced. It has had a deep cycle battery in it up to now. (Carquest Deep Cycle 27)

In addition to your excellent advice here, I realize the groups/forums talk a lot about batteries, battery maintenance, dual batteries, trickle charging, plugging-in at home, etc. Therefore, I feel bad asking you when there are these resources. However, my head is about to explode because I only comprehend bits and pieces at this point and I don’t want to make an expensive mistake! After combing through your site, I feel confident you are an absolute EXPERT and sound source of advice.

My end-goal is to equip the Casita for plenty of dry camping both in Texas (hot) and Colorado (cold)…. and anywhere else! The former owner mainly used shore power and had a generator as backup. As it stands, I do not have a generator or solar. So, once I have a battery solution, that’s the next thing to figure out. Based on all this, would your top recommendation still be 1 Trojan 27-AGM with a Little House Custom Battery Disconnect?

Although I am a bit in shock over the expense involved with equipping a trailer, I’m committed to investing in the best choices for longterm benefits. The task is figuring out exactly what those decisions should be while trying to understand how it all works. If you have time, I would hugely appreciate your help in confirming the best approach!

Confused but Learning Casita Owner,
Karen (but not a “Karen”🤓)

Greg Zimmerman October 7, 2020 - 2:20 am

A proper answer would require a new article! If you plan to boondock a bunch, I would up your battery to the group 31 size. You’ll have to modify the metal bolt in bracket by drilling a new hole. It’s a tight fit but has more capacity. AGM (Absorbed Glass Mat) actually has less capacity that a comparable wet cell, but they’re maintenance free (and more expensive). So if you’re on a budget- you could get a deep cycle 31 but you have to check the water levels every 6 months and did I mention they’re heavy?

If you plan to boondock more than 2 days, solar is a great addition. Get a Renogy 100 watt panel and either install a solar port (I have an article on it), or you can open the battery door and clip it on, or get an adapter and plug it into the trailer vehicle connector- anyway to get the panel the connect to battery. Solar isn’t the end-all solution, but 100 watts will top up the battery with a full day of sun.

Karen Dennehy October 7, 2020 - 1:15 pm

Greg, thank you so much for your reply! This gives me a great start. I am going to do exactly what you recommended. I have read your article about the solar port and the solar panel you mounted on top of your Casita. You are very skilled. I am prone to getting a little “analysis paralysis” but seeing your detailed projects is inspiring me to go for it – just do it! 😎

I did notice in your article about adding your rooftop solar panel that you used a different brand. Any reason you recommend Renogy now vs what you used? Also, I see that some people have added panels to their Casitas’ roof and swear that it get a charge on overcast days and partial shade. If I haven’t worn out my welcome, I’d love to know your thoughts on that.

Thank you again for taking time out to reply to me. I’m off to get a deep cycle 31.

Grateful to you,

Greg Zimmerman October 8, 2020 - 2:33 am

I used the roof panel that I picked because it was narrower than the Renogy so it didn’t stick out so far on the sides when mounted on the roof. Roof panels need their own solar controller and wiring. A “suitcase” panel, like the Renogy 100 watt folding one comes with a controller built in, so you just hook it up and it works.

As for charging on an overcast day, sure it does, but at a much reduced level. That’s why I liked the Bogart battery monitor, it shows exactly how much the panel is putting out or how much the battery is being drained. It’s expensive though ($400) just for the controller and monitor. A portable panel is nice because you locate it where the sun is. The roof panel is fixed and if you’re under trees, won’t work. I liked that it was always on though- I could leave it in a parking lot and it would be charging.

Karen Dennehy October 9, 2020 - 3:19 pm

Thanks again, Greg! Discovered your new reply when coming back to re-reference your advice to me, as the local Trojan distributor was trying to persuade me into a 30 size flooded battery because they don’t carry the 31 you recommended. (I was thinking “no” due to size constraints of the box which is why I came back to look again at your comments about having to modify the bolt in bracket.)

Anyway, about the solar panels, good points all the way around. I am really loving learning about all this and am slowly starting to see a bigger and bigger picture. Sure appreciate you and your giving spirit in providing such excellent advice!! Solar is next!

Back to the battery, I may have to order the 31 (as you advise in your article) vs picking one up as I had hoped. I’m about 100lbs…. this is going to be a test for sure. If I were getting the AGM, wouldn’t get a sliding drawer but I might have to with this beast since it’s supposed to be topped with water every 6 months!

Not for me now necessarily but maybe later… what do you think about people putting 2 golf cart Trojans in their Casitas?

Intrigued by your answers,

Greg Zimmerman October 10, 2020 - 6:54 am

For you I would do a AGM 31 and no slide- it’ll stay in there for 5+ years with no maintenance. As for dual golf cart batteries- it’s a lot of work- they need to be a compartment sealed off from the inside of the Casita, yet vented to the outside. Some folks use Lithium, but they’re $1000 batteries and require a new converter. Casitas are not the ultimate trailer for maximum off-grid battery capacity, but the Trojan AGM31 will get you 102amp/hrs which is 20% more than the original group 27.

Scott Loomis April 4, 2021 - 1:16 pm

You touched on Lithium, can you out line what that would take in regards to new converter, any suggestions?
I’m taking the plunge and getting a Lion 105Ah LiFePO4 that fits nicelyv (smaller actually) and only weighs 24lbs, it has it’s own BMS build in too. Do I need to fully swap out the Castita Parallax 7100 converter charger or just a Li 14.4 standalone charger?

Greg Zimmerman April 4, 2021 - 1:30 pm

That’s a good question. Let me do a bit of research and get back to you. I know the latest converters can do lipo

Greg Zimmerman April 5, 2021 - 2:23 am

It looks like the Parallex cannot handle LiFePO4, or any lithium battery. Starting in mid year 2019+ Casita has changed to the Progressive Dynamics PD4060KV, which does have a switch to handle LiPo batteries. So in your case, a converter upgrade is in your future to run that battery. Another concern with Lipo, is that they won’t accept a charge when below freezing.

Dan August 31, 2021 - 6:04 pm

Hi there, I just installed the Trojan 31AGM. All seems good… but I check my voltage meter and it says around 13.5? Doesn’t that seem high? Is that because the Casita is plugged in and charging? I was thinking 12.6 was going to be the max I was looking for? (Referring to little house customs battery depletion chart). But now I’m not sure how I’ll know when it is 100% charged? Thanks for any advice!

Greg Zimmerman August 31, 2021 - 6:20 pm

That’s perfect. You should see mid 13’s while plugged in and charging and mid 12’s and running only on battery.

MichaelM May 2, 2022 - 9:32 pm

Hi Greg,
I’m looking at your picture of the battery cave. And am assuming this is a Casita, yes? The pic shows the white and green wires (neutral/negative) as well as the black (positive on a Casita) all wired to the same post on the terminal block. Am I not seeing this correctly? Thanks.

Greg Zimmerman May 2, 2022 - 10:57 pm

My photo is a little confusing because I’m running a battery monitor and shunt. The thinner of the 2 black wires in the photos is the Casita black going to the far terminal on the block. The big fat black you see is going from the terminal block to the shunt, and it’s a negative wire which is why the white and green are on there too.

Jenna August 9, 2022 - 12:50 pm

Thank you for all your tutorials! Your site has helped me so much. Do you have Venmo? I’d like to buy you a cup of coffee 🙂

My question for you is…I’m about to buy the Trojan 31 AGM for my 2008 Liberty Deluxe. I know you mentioned the converter in the 2009 would work fine, does the same go for 2008? I plan on buying the solar port and already have the battery cut off from LHC, as well as a Renege 100W suitcase. Am I missing anything? A specific trickle charger for the winter months? Thanks in advance!

Greg Zimmerman August 10, 2022 - 12:59 am

That all sounds like a great plan. If you want a trickle charger, I like the “battery tender” brand. A small weatherproof 2amp will be fine. And the plug will fit the solar port you are adding.

Jenna August 9, 2022 - 3:16 pm

Forgot to ask one more question…does the TV charge an AGM battery like it does a deep cycle?

Greg Zimmerman August 10, 2022 - 12:59 am

Yes it will. AGM is still a lead acid battery, just a better one. The lithiums are the ones that need higher voltage to charge.

Greg Gendall March 13, 2023 - 4:22 pm

To confirm, the converter/charger from 2008 on is adequate to provide tiered charging for a new Trojan Motive Overdrive AGM 12 V battery? It appears that 2008 seems to be a pivotal year with several different changes….some of which need to be improved upon?


Greg Zimmerman March 21, 2023 - 8:59 pm

I looked at the factory changes list and the converter didn’t update until 2014. I had the same converter in my 2009 and it charged my Trojan battery with no issues. You should be fine with your AGM. When/if you go Lithium, then you need a newer converter.

Bob Wade June 13, 2023 - 10:05 pm

Hi Greg – been on your site for quite a while and thank you for sharing your experience. Just did our maiden voyage with 2023 SD. Put out the new Renogy 100 watt suitcase in the morning sun and the supplied battery monitor showed well over 14 volts and the indicator went from green to yellow. The panel has a 20 amp controller, so I don’t understand why it would go that high. I unplugged it for a while and back in later but we were still over 13. We used lights sparingly and pump a bit, but in the middle of the my wife turned the pump on to flush, and she said it went yellow (on the low end this time). I know these are seemingly separate issues, but what do you make of this? Thanks!

Greg Zimmerman June 14, 2023 - 6:16 am

I looked through the manual at it seems normal to me. The yellow bulk charge and green is charge over 12.5v. It should be charging around 14 volts in order to fill the battery. The battery should show 12 volts after it’s left alone, not charging for about 30 minutes (as a test).


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