AC charging for lithium iron phosphate battery

[elewis33]

I'm wanting to replace my SLA battery in my T@B teardrop trailer with lithium iron phosphate. The SLA battery died on me in just over a year, because it's a huge hassle to get to the battery to check water levels and I let it get low and killed it. Luckily you don't need to "water" LiFePo batteries and in mine, the built-in BMS takes care of the concerns over charging when it's below freezing, over charging, over discharging, etc.

I have a WFCO WF8725P all-in-one power center in the T@B and it doesn't have a LiFePo specific charge profile, so I'm looking at options. One option is to replace the entire power center for about $200 with a newer model (WF8725LiS, etc). I looked into replacing the control board alone but WFCO says they don't have an option for that.

I'm also wondering about just adding an AC-DC charger (something like the a Progressive Dynamics PD9145ALV or a Victron IP65) to the current installation. I guess my question is about if you can essentially eliminate the battery charging side of the current converter and substitute a new AC>DC charger in it's place? Or is this more trouble than it's worth?

 

[Mark_K5LXP]

The price difference doesn't appear to be that much. I would see no technical issue with substituting the converter portion of the power center so I guess if it were my project I'd see what the labor difference would be doing a converter-ectomy vs a power center R&R. If there were room I'd consider abandon in place of the existing converter and just splice in the lithium, but if I had to do surgery to the power center then maybe a power center R&R would be the better path.

Mark B.
Albuquerque, NM

 

[elewis33]

I wouldn't need to do any surgery to the existing power center. The new charger would just mount to the floor under the seat, get power from the AC side of the power center and, as you said, just splice the new charger in where the old converter was sending its battery charge voltage.

Of course I'd also remove the fuse for the DC circuit for the battery from the power center so it wasn't trying to send current down the line when connected to shore power.

 

[Isaac-1]

First off a minor detail, if you are checking water in your current battery, you don't have an SLA battery, SLA is Sealed Lead Acid, you probably have a standard deep cycle or dual use lead acid. battery.

Depending on how you use your RV, your existing converter may work just fine. I just skimmed through the installation manual for your WFCO WF8725P, according to the manual it will bulk charge at 14.4VDC for up to 4 hours at 30 amps output, then when either the amp draw drops below 5 amps or the 4 hour timer is up it will switch to Absorption at 13.6VDC for the next 44 hours, then move to float at 13.2VDC. The first 2 stages are fine for charging a LiFePo4 battery, and if you have only about 100-120AH LiFePo4, it should fully charge within that 4 hour in bulk time. The only problem is the float mode, which is at 13.2VDC, which is about 70% state of charge +/-15% on a LiFePo4, which the LiFePo4 actually prefers for long term storage, unlike lead acid they last longer if not fully charged to 100%.

In theory what would happen if you use the current converter, and stay plugged in long term such as at an RV park with full hookups,or in storage with shore power, first the converter would fully or near fully charge a 100AH battery in 4 hours, if that did not do it, then it would get to 98% or higher over the next couple hours in the 13.6VDC absorption stage, then 44 hours later it would drop into float, and slowly discharge down to about 70% state of charge as the battery is drawn down by lights, and other loads.

If you only plug in to charge, and never stay hooked up long enough to float and draw down to 13.2VDC this may work fine for you. The big question if will it go back to bulk 14.4VDC by simply unplugging and replugging the AC power to the converter, or does it need to sense the battery state getting to what it things is low for lead acid, ie below 12.5VDC, which is about 10% state of charge for LiFePo4. If an simple unplug and replug switches back to 14.4VDC bulk, you may be good to go as is. This is easy confirmed with a multimeter once you have the LiFePo4 battery installed.



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[Gary RV_Wizard]

First off a minor detail, if you are checking water in your current battery, you don't have an SLA battery, SLA is Sealed Lead Acid, you probably have a standard deep cycle or dual use lead acid. battery.

Actually, SLA is usually read to mean "Starting, Lights & Accessories", i.e. an engine-starting & running battery. Sealed lead-acid batteries are termed "Valve-Regulated Lead Acid" or VRLA.

The easiest solution to elewiss33 problem is to install an AGM VRLA battery. No further need to deal with adding water or other traditional battery maintenance, and his charging system will work well enough. If he changes to LiFePO4 chemistry, the existing charger won't bring that battery to much more than 80% of rated capacity.

 

[JayArr]

And a VRLA will cost a fraction of the cost of a lithium battery and a new charger.

 

[Isaac-1]

Ah, this is the problem when the same abbreviation is used for different, but related things see What’s An SLA Battery? (Types, Benefits, FAQs)

 

[creativepart]

With a T@B I would also agree that a change from FLA (Flooded Lead Acid) to AGM (Absorbed Glass Mat) would be a super practical change. I'd actually suggest that if you have room it would be wise to install 2-Group 31 12v AGMs and wire them in Parallel to have 200ah of totally maintenance free power (100ah usable to stay above 50% discharge) and can continue to use your existing Converter/Charger.

You can probably make this change for less than $500 saving you probably another $800 over the LiPo4 purchase and the updated Converter purchase.

You'll have more available power, and more money in your pocket for other projects or needs.

 

[Isaac-1]

I personally think we have reached the point where LiFePo4 Lithium batteries may make financial sense if you plan to keep the RV long term. I just paid right at $1,500 delivered for 420 amps worth of drop in LiFePo4 batteries, a smart Bluetooth BMS with 5 year warranty and 10+ year expected service life, if I had bought more generic models without bluetooth, and shorter 1 year warranty, I could have bought a set of 4 100AH batteries for $1,200 delivered.

 

[Rob&Deryl]

I bought 2 Gc2 6v 220ah AGM at Batteries + Bulbs for under $600.

 

[Isaac-1]

So amp hour for amp hour we have reached the point where LiFePo4 is price competitive with AGM, without accounting for LiFePo4 longer service life, more charge cycles, etc.

 

[JayArr]

It's getting closer but it's not quite there yet, The comparison only seems to work if you find the cheapest deal on the internet for the Li batteries and compare it to standard LA from Amazon or bricks and mortar.

If you look at name brand Li batteries the price is still about double.

200 Amps of Battle Born is $1600 - on sale from $1900. The 400Ah kit is $3600! That's a long way from $600 for golf cart lead acids. They would have to last three times as long as lead acid just to match even and that ignores any charger upgrade and the loss from putting all the money up front, instead of 1/3 now, 1/3 in 5 years and 1/3 in ten years.

I'd love a pair of Battle Born 100A batteries to replace my group 24s but at $1600-1900 there just isn't enough upside, I'll go at least one more cycle on LA and check back in 5 years.

There is hardly anything in the world that some man cannot make a little worse and sell a little cheaper, and the people who consider price only are this man's lawful prey.
John Ruskin

 

[John From Detroit]

I bought 2 Gc2 6v 220ah AGM at Batteries + Bulbs for under $600.

Isaac got 420 amp hours of which he can use about 400 for 1500
YOu got 220 amp hours of which you can use only about 110 for 600.

I think Isaac got the better deal over 3.5 times the power for 2.5 the cost.

I would say that depending on how long they last (LiFePo are supposed to last a long time) the line has clearly been crossed and liFePo4 are now the better buy.

 

[Mark_K5LXP]

YOu got 220 amp hours of which you can use only about 110

What happened to the other 110?

Mark B.
Albuquerque, NM

 

[Isaac-1]

JayArr, that price comparison only works if you compare mid-line brand AGM's often with a 3 year warranty vs premium LiFePo4 Like Battle Born with their 10 year warranty, ... I choose to go with a middle of the road, custom spec imported battery with a 5 year warranty, the value of which may be in question as the small company has only been in business for 2 years.

 

[Isaac-1]

...If he changes to LiFePO4 chemistry, the existing charger won't bring that battery to much more than 80% of rated capacity.

Gary, charging at 13.6V with his current charger in the first 2 days of charging, should bring a set of LiFePo4 batteries up to well over 95% charge, probably over 98%. Keep in mind the charge curve of LiFePo4 is very flat around most of the charge voltage profile, and does not spike up over 14VDC until the last 1%, it is only when it drops into float 13.2VDC at 48 hours after starting to charge that the battery would be allowed to drain down to 80% SOC

 

[Gary RV_Wizard]

Gary, charging at 13.6V with his current charger in the first 2 days of charging, should bring a set of LiFePo4 batteries up to well over 95% charge, probably over 98%.

I don't think we know what his specific charger would do, but 48 hours at 13.6v doesn't sound very typical to me. And the sources I read say that an LiFePO4 battery needs a steady 14.2v to reach full charge and that a charger needs to have a lithium-specific profile to do that..

Smart lead-acid chargers may reach 14+ in bulk mode, but they don't maintain that level for lengthy periods. Another factor is that LiFePO4 sustains a higher voltage than lead-acid, quite possibly making a smart charger think the battery is already well-charged and thus going to absorption or float stage almost immediately.

 

[JayArr]

What happened to the other 110?

Agreed! The idea that you can only use half of a LA capacity and that you need to keep it above 50% is FALSE, it's an urban myth.

If I was a conspiratorist I'd think the lithium battery people started that rumor!

 

[Larry N.]

If I was a conspiratorist I'd think the lithium battery people started that rumor!

Of course it started well before there WERE lithium batteries, and has a basis in truth, if you consider how many YEARS it is usable instead of actual hours of use.

 

[Mark_K5LXP]

It does have a basis in truth. The reason I ask is to force people that repeat it to understand what a limited set of conditions it applies to. Virtually none of which exist in the RV house battery world. Another myth that wouldn't die is Nicad battery memory. That was discovered and resolved I think in the 1970's or maybe early 1980's but the admonition for "conditioning" and cycling Nicads to prevent memory effects was believed by many until their ultimate sunset when NiMH came on the scene. So yes, there is an operating strategy for lead acid batteries that involves managing DOD to a particular level (which is likely something other than 50%) but it's not universal, and it's not a marked increase. It's just one factor among many that influence a battery's performance, longevity and cost. But when the topic of LA battery life comes up, it's nearly always the first caution. Don't discharge below 50%, you'll *kill* your battery or the automatic capacity measure of 50% - X amp hours, half of which are "usable". There is nothing about the operation of lead acid batteries that isn't well known and documented, so no mystery about how they'll perform in any application. If you match the battery to the application, the "50% Rule" will have negligible effect. Mismatch the battery and there is nothing that will help.

Mark B.
Albuquerque, NM