Replacing Trailer batteries

[Trooper#1]

Hi everyone,
Its the time to replace my RV trailer batteries. I have been very lucky, as I've had 2x Trojan deep cycle batteries on board for 10+years with no issues!!
Now one is bulging slightly, so before the season really gets going I'd like to replace them. Rather than going with the same Trojan's (even tho they have served me well) I'm wondering the pro's and cons of going with a single 12v Lithium? Will I need to make changes to my on-board charging system if I do this? Can I use just one 12V?
Any help with these questions would be greatly appreciated.
Thanks,
Simon

 

[Lou Schneider]

Welcome to The RV Forum, Trooper!

I'd go with one or two 100 a/h lithiums. Prices are extremely cheap right now, with 12 volt 100 a/h batteries available on Amazon for less than $300. Be sure to get one with cold temperature charging protection (most include this now, even the inexpensive ones) as they will be damaged if you try to charge them when their internal temperature is below freezing.

Since lithium voltage doesn't sag as they discharge and you can use all of the rated capacity without hurting the batteries, a 100 amp-hour lithium will give close to or better performance than a pair of 200 a/h golf cart batteries that you can only discharge 50% before they start to brown out. At current prices you can buy two 12 volt 100 a/h lithiums for less than a pair of Trojan T=105s and get much more usable capacity.

LiFeP04 batteries take 14.6 volts to reach full charge. If you have a multistage converter it will do this during the Bulk charging phase. If your converter is a straight 13.5 volt single stage charger it will take a while to fill the batteries to about 85%. This doesn't hurt the batteries, you just won't get the final 15% capacity unless you charge them to 14.6 volts. If you want you can replace your converter with one designed for lithium batteries - all this means is it puts out 14/6 volts instead of 13.5 volts. Or add some solar to fill them that last 15%.

If you don't have a shunt based battery monitor you'll have to get one to keep track of the battery's state of charge. Lithium's voltage changes so little between 90% and 10% charge that it's next to impossible to keep track of the state of charge using a voltmeter. Battery monitors are available anywhere from $50 to $300 on Amazon - I like the LITime at $65 or the Renogy at $90.

 

[Trooper#1]

Thanks very much Lou....just a few more questions if I May,
Do I NEED to get 2 12V Lithium? Just wondering what the advantage is?
Is "deep cycle" only relevant with lead/acid, or do they make a deep cycle lithium in 6V?
Thanks again,
Simon

 

[Trooper#1]

I also notice that lithium inverters come in different amperage's....how do I determine how many amps I need?
Thx
Simon

 

[xrated]

Just as a follow up here....LiFePO batteries do not NEED 14.6V to fully charge. Anything over 13.8V will charge them up fully. That 13.8V is 3.45 volts per cell which is generally considered to be 100% SOC. The difference is that it will take a considerably longer amount of time using 13.8V than it will using 14.6V. The trade off of time vs stress on the cells though is something that many of us that use LFP batteries take into consideration. Personally, in order to not stress the cells by hitting them with their highest rated voltage, which is 3.65 per cell (14.6 volts for the battery), I use 14.2V and keep the cells away from their upper limit voltage-wise. So for me personally, I sacrifice a bit of time during the charge cycle, in order to have the cells last for a longer period of time

 

[Gary RV_Wizard]

I also notice that lithium inverters come in different amperage's....how do I determine how many amps I need?
Thx
Simon

The charger amperage only affects the charging time, so "need" depends on how much of a hurry you are in.

 

[Lou Schneider]

Thanks very much Lou....just a few more questions if I May,
Do I NEED to get 2 12V Lithium? Just wondering what the advantage is?
Is "deep cycle" only relevant with lead/acid, or do they make a deep cycle lithium in 6V?
Thanks again,
Simon

All lithium batteries are "deep cycle" - you can repeatedly drain them to the last 10% or less of their rated capacity without harming them. You'll pay lots more for 6 volt lithiums if you can find them - 12 volts is such a basic building block that economies of mass production really come into play. The connections don't corrode and lithium batteries have enough consistency from one battery to the next that there's really no advantage to 6 volt series connections. Multiple batteries in parallel work fine as long as you pay some basic attention to the wiring

As far as the converter's amperage, that just depends on how fast you want to pump electrons into the batteries. Lithium batteries don't taper off much as they gain a charge, they'll take whatever the converter can put out. And you can charge them at higher rates than lead acid if you want.

 

[xrated]

I also notice that lithium inverters come in different amperage's....how do I determine how many amps I need?
Thx
Simon

Here is a real world example, based on my battery (a DIY 302AH built from four 302AH cells)

So if the battery is fully charged and is at 100% state of charge (SOC) and I run it down to 10% SOC, I have used right at 272AH out of the battery. I have a Meanwell Converter/Charger that is rated at 750W and will produce 43.xx amps while charging in the bulk mode. So the math is fairly simple.....I need to refill those 272AH that I've used so divide that number by 43 and you can figure about 6 hours and 20 minutes till the battery is back to 100% SOC. Hope that helps

 

[John From Detroit]

I also notice that lithium inverters come in different amperage's....how do I determine how many amps I need?
Thx
Simon

You ask the battery supplier for recommendations Both fastest and slowest. You do not want to charge Li batteries too fast.. (in fact the Battery's internal electronics should prevent that but .... why take the chance) nor do you want to do it too slow. The Battery maker should be able to give you a range.

 

[Mark_K5LXP]

All lithium batteries are "deep cycle" - you can repeatedly drain them to the last 10% or less of their rated capacity without harming them.

Lithiums have a similar cycle life vs DoD curve as lead acid does, just that the numbers are greater. So, if lithium doesn't suffer cycle life 'harm' down 90% DoD, PbA doesn't either...

No question lithium delivers more Ah at higher terminal voltages. 2x 100A lithium @ $600 vs 210Ah FLA @ $230. OP got 10 years calendar life which implies very tepid use. Barring some specific use like high power inverter operation or faster charging, lithium would cost more but not offer much more.

Mark B
Albuquerque, NM

 

[xrated]

Lithiums have a similar cycle life vs DoD curve as lead acid does, just that the numbers are greater. So, if lithium doesn't suffer cycle life 'harm' down 90% DoD, PbA doesn't either...

No question lithium delivers more Ah at higher terminal voltages. 2x 100A lithium @ $600 vs 210Ah FLA @ $230. OP got 10 years calendar life which implies very tepid use. Barring some specific use like high power inverter operation or faster charging, lithium would cost more but not offer much more.

Mark B
Albuquerque, NM

The spec sheet for the EVE cells that I bought to build my battery list the number of cycles at greater than or equal to 3500 cycles at 25C. At that rate, and me not being a full timer, the battery could possibly outlive me!

 

[thefid]

This is my setup... If you go Lithium, change out your converter for much better results. Lithium lasts longer than lead acid and requires less maintenance. They can go months without discharging to zero and even if you did forget to place your rig into storage mode and had a power drain, they can handle it better.