Help with sizing batteries.

KTM-Guy

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Aug 24, 2021
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Hello all.

First post here but I have been lurking for a while. And have been watching Will’s videos for a few years now. ( when he lived in a trailer).

I have decided to make the switch to lithium batteries from AGM in my camper and Jeep. Just getting tired of worrying if I’ll run out of power.

I’ll detail what my setup is and then follow up with numbered questions. Hope that will make it easier.

BF1E1C51-3B12-4826-A45E-13619A66BC5C.jpeg

The Jeep has a 125AH AGM that powers a Snomaster fridge/freezer and some device charging, which we will start doing more of. We try to charge when we are driving. We have a Redarc 25 amp DC to DC that is lithium ready. The Snomaster uses from a low of 30 AH a day to the high side of 75 AH a day. It depends on the ambient temperature and what we have the freezer set to. During long drive days I’ll put the temp at 0 then when we stop to 14 degrees. It’s probably a plus and a minus, but the cooling fan on the Snomastercan be used to keep the battery warmer. Last week we had lows in the high 20’s and never seen the battery temp below 43 degree. That may be a disadvantage in the warmer months. I am using a PowerMon to monitor the batteries now, has need working great.

I built the teardrop over the last four years. We currently have about 8,000 miles on it, both on and off road. Probably have taken to places we shouldn’t have. 😂 power use is LED lights, Propex heater, water pump, usb charging, Maxair fan. We use between 10 and 20 AH per day. Winter months we use more because of the shorter days and running the heater. The fan runs all night regardless of the temperature. We have a 55 AH AGM in the tongue box along with a CTEC DC to DC charger that is not lithium capable. We use while driving and as a SCC, same with the Redarc in the Jeep.

In the camper I carry two Renogy 100w portable solar panels. And on the tongue box a 30w panel that probably just makes the lid heavier. As you can see I don’t have the Real estate for more or bigger panels.

I am in the planning stages of what our next build will be. And I would like to be able to use the batteries I am buying now for that. But it will be a bigger off-road trailer or a camper van, of something built on a 3/4 or 1 ton truck. It will use much more power induction cooktop (which is something I would like to move to now), shower, toilet, and do I dare say AC? But the is 3-5 years down the road.

We mostly do weekend trips 3-4 days and 2-3, 7-10 day trips a year. We mostly disperse camp (some people call it Boondocking or dry camping). We rarely stay in one place more than 3 days. We are not opposed to staying in a campground for a few days either.

My plan for the teardrop, a battery big enough that we can go almost the full time without charging at all. Say a 230AH battery. I can put out 1 or 2 panels if we are getting good sun. I would need to add a SCC so probably a Victron 30 AMP.

The Jeep is a bit harder. I would like to use the same idea of not having to charge but am not sure I have the room for the batteries, or the funds to pull it off. So was thinking of a 280AH or a 304/310AH (from Amy, US stock) when they come in. That would give me 3ish days give or take, leaving 40%. We would have to drive after three days or put out solar. Or we could get a campsite for a night with electric.

Am I on the right track with my thinking?

Since most of my charging will be from AC can you recommend a good fool proof charger?

One other thing, I’m in the Phoenix valley, it gets hot here and the tongue box can easily get 140+ degrees in the summer, for storage would I be best to remove the batteries to store? I think this may be why I’m having problems with the AGM batteries.

I’ll come up with some numbered questions in another post, this one is getting long.

Thanks, Todd
 

sunshine_eggo

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For reference sake, take your old battery capacity and multiply it by 5/8. That will tell you the equivalent LFP battery capacity you'll need.

55Ah * .625 =34Ah, so a 230Ah LFP is about 6.7X your existing capacity.

Solar power determines how much power you can use in a day.

Battery capacity determines how long your loads will last without charging.

Consider deployable arrays made with cheap/lightweight PVC frames:


The lower lifespan of the flex panels won't be much of a factor because you're not leaving them in the sun all the time.

LFP is essentially a drop-in replacement for lead acid in many cases. Don't count out your CTEC charger just because it's not Lithium specific. If it charges at or below 14.6V and floats at or below 13.6V, it should work fine.
 

KTM-Guy

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Aug 24, 2021
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For reference sake, take your old battery capacity and multiply it by 5/8. That will tell you the equivalent LFP battery capacity you'll need.

55Ah * .625 =34Ah, so a 230Ah LFP is about 6.7X your existing capacity.

Solar power determines how much power you can use in a day.

Battery capacity determines how long your loads will last without charging.

Consider deployable arrays made with cheap/lightweight PVC frames:


The lower lifespan of the flex panels won't be much of a factor because you're not leaving them in the sun all the time.

LFP is essentially a drop-in replacement for lead acid in many cases. Don't count out your CTEC charger just because it's not Lithium specific. If it charges at or below 14.6V and floats at or below 13.6V, it should work fine.
Thanks for the reply,

I’m not sure what info the multiply by 5/8 would help. I had a 55AH battery that I put in the teardrop. I know it isn’t big enough for my needs.

A8D56D59-509F-4D5F-94DB-570D5029F708.jpeg
CACF1F9C-7A96-4CD9-ADE0-7D82D02338A5.jpeg
This is what I’m using now, I made the stand out of aluminum, they are 100w, lucky to put out 5-5.5 amps. The biggest I can go is about 22X46. I don’t have storage for bigger. Maybe under the mattress I could go bigger.

I didn’t think of that with the CTEK. Below is the charge profile. Will that work with LFP?

C193D7A5-0D4E-46EF-9CF1-BD19B8E9F6FD.jpeg

Thanks
Todd
 

sunshine_eggo

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Thanks for the reply,

I’m not sure what info the multiply by 5/8 would help. I had a 55AH battery that I put in the teardrop. I know it isn’t big enough for my needs.

It's how you compare old to new. The 230Ah capacity battery has 6.7X the usable capacity of your old battery.

This is what I’m using now, I made the stand out of aluminum, they are 100w, lucky to put out 5-5.5 amps. The biggest I can go is about 22X46. I don’t have storage for bigger. Maybe under the mattress I could go bigger.

100W flexible panels are about 22x48.

I didn’t think of that with the CTEK. Below is the charge profile. Will that work with LFP?

View attachment 71765

Thanks
Todd

Hmmm... not a huge fan, but it might work. That "desulfation pulse" stuff is specific to lead-acid batteries; however, it's still charging to healthy voltages (non-AGM profile). I suspect that due to the way that LFP works and responds voltage-wise, the charger can't drive enough current to actually push the battery to the claimed 15.8V. If step 1 can be turned off - it's a winner in non-AGM mode.

Without confirming behavior with measurements, probably best to swap it out.
 

100 Proof

"Please Lord, don't let me do something stupid."
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Feb 14, 2021
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This is what I’m using now, I made the stand out of aluminum, they are 100w, lucky to put out 5-5.5 amps. The biggest I can go is about 22X46. I don’t have storage for bigger. Maybe under the mattress I could go bigger.
Just as an FYI, that little bit of shade on the panels in your photo is possibly enough to COMPLETELY prohibit an useable power from coming from them. They really should not be even partially shaded.
 

KTM-Guy

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Aug 24, 2021
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Just as an FYI, that little bit of shade on the panels in your photo is possibly enough to COMPLETELY prohibit an useable power from coming from them. They really should not be even partially shaded.
That photo was from when I finished the stand in my driveway. On a recent camping trip we spent a lot of time just hanging out in the campground and I spent some time playing with the panel(s). Amazing how even filtered sunlight makes a difference.

Thanks for the reply.
 

KTM-Guy

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Aug 24, 2021
Messages
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It's how you compare old to new. The 230Ah capacity battery has 6.7X the usable capacity of your old battery.



100W flexible panels are about 22x48.



Hmmm... not a huge fan, but it might work. That "desulfation pulse" stuff is specific to lead-acid batteries; however, it's still charging to healthy voltages (non-AGM profile). I suspect that due to the way that LFP works and responds voltage-wise, the charger can't drive enough current to actually push the battery to the claimed 15.8V. If step 1 can be turned off - it's a winner in non-AGM mode.

Without confirming behavior with measurements, probably best to swap it out.
Thanks for the reply,

It’s funny, I spent many hours testing my power usage, adding, subtracting, multiplying watts, AH, etc. to get the same answer you got. I was at 6.3. Lol

I think I will replace the CTEC, not worth the risk. I can probably sell it for $50-80.

Can you recommend an AC charger to charge at home or on the road when we have power?

Thanks.
 

sunshine_eggo

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For DC-DC, Victron Orion line and Renogy has some DC-DC options too.

Victron makes some AC smart chargers that allow you to completely customize the charge voltages.
 
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