12v Lithium to replace 12v LA-Deep in RV with constant feed from 48->12 Converter

[RvNaut]

The current set up is a 48v Lithium battery feeding a 48v -> 12v converter that is connected 24/7/365 to a 12v deep cycle battery that feeds the entire 12v system(s) in my RV. The 12v battery is inline to act as a load buffer for when I use the high draw 12v systems IE the Hyd motor for the leveling system, the slide motors. The big draw is from the Hyd at 45+ amps. The best converter I could afford for the build was the 30amp Victron. Thus the need for that buffer battery to suck up the load.... and FYI, it is very short time usage. The 48/12 converter is currently feeding 13.2 volts to the LA battery.

To the query. I propose to to replace the LA batter with a 280ah/100amp 12v Lithium battery that I am building. Once I have the Lithium battery built, balanced and fully charges, what voltage would be best to set the 48/12 converter to? There is a BMS in the battery, but once in service I propose to just feed it the voltage I set the converter to.

Thoughts?


(Why? Becasue the old LA battery is not in great shape as it was abused in a former life. The 280ah replacement will allow me to disco the 48v system from the 12 for an extended time to facilitate other work. )

 

[RvNaut]

(edit - the load on the 12v systems is very light 98% of the time just running lights, fans and the controllers for fridge/furnace/H2O heater )

 

[Vigo]

So im guessing the 48>12v step down converter you have is adjustable? Do you have a link or model number (just my curiosity)?

I have heard that the 'safe to leave hooked up forever' voltage for lifepo4 is 3.37v/cell. That works out to ~13.48v. The question is, can your BMS effectively balance the pack if the voltage never rises above that point?

Another pretty cheap option is to set the voltage to something like 14.5 or 14.6 and then put a cheap PWM SCC between the converter and the battery to implement the 'multistage charging' and set float voltage to something like 13.4. I probably wouldn't bother doing this UNLESS you need to raise pack voltage above 13.48 occasionally for your BMS to be able to balance. The higher voltage may allow slightly faster charging but it seems like charge rate is not an issue in your situation even with the current degraded amount of capacity you have. With more capacity it will matter even less.

 

[RvNaut]

So im guessing the 48>12v step down converter you have is adjustable? Do you have a link or model number (just my curiosity)?

It is a Victron Orion-Tr IP43 48/12-Volt 30 amp

Orion-Tr DC-DC Converters Isolated - Victron Energy

Provide a stable DC output voltage using the, easy to install, Orion DC-DC Inverter from Victron Energy. Find a dealer near you.

www.victronenergy.com

Yes it is adjustable, so the question remains, what is the ideal float level in this situation.

I will fully charge the new 12v lion and then balance the cells, and reassemble, prior to setting it all up in place.

Yes, charge rate is a non-issue. As an exmp. I have been in one spot now for 5 weeks and haven't used any of the high draw items in that time.

I am surprised at the lack of responses... where are the people to tell me this is a bad idea or that I am doing it wrong, surely this is a bad idea in some way shape or form???

( cost, yes.. cost is a bad aspect... I could but a lot of 12v deep cycle no-maint batts for this money)

 

[Vigo]

If you dont need 100% of the available capacity and dont care about the charge rate then 13.4 is fine. The only caveat is that you may want to occassionally bring it to a higher voltage so the bms will balance. I read that trying to balance at only 13.4 is either ineffective or counterproductive. Since you wont be cycling much it should take a long time for the cells to drift apart in the first place.

 

[0truck0]

I can't be of any help.

I am curious though.
Can you explain again why you have a 48 volt lion feeding (through a converter) a 12v lead acid battery? I get the 45 amp part, so I guess your converter cannot handle 45 amps from the lion....
Sounds like you can build a battery. Can you take one apart? Why not take the 48 one apart and make a 12 volt lion? I'm just not understanding the desire for the middle battery.

 

[timm]

Wouldn't it be easier to add a second Orion converter in parallel?

 

[justinm001]

I'd set it at whatever the float voltage is. It'll keep the lfp charged up 100% and only use it when there's more than 30a. If lower it won't fully charge the battery and it'll be like 80% the whole time and if lfp doesn't fully charge it doesn't equalize the cells.

I have mine at like 13.7

 

[Sfletcher]

Hello, Don't want to hijack, but you all have some great information. So, perhaps you could lend a hand. I am going to convert my 6V deep cycle lead batteries over to Lithium. My current set up in the coach is four(4) Trojan 125/240AH that run it. So, in doing this do I still need to carry the same 240AH per battery for a total of 960, when converting over to lithium. I will as be adding a four panel solar set to the roof of the coach if that helps in factoring. We do quite a lot of boon docking, and the most we operate in the coach is a coffee pot, TV, phone, laptop, the most common items. Any advice on battery selection would be much appreciated. Thank you, Shawn.

 

[justinm001]

Hello, Don't want to hijack, but you all have some great information. So, perhaps you could lend a hand. I am going to convert my 6V deep cycle lead batteries over to Lithium. My current set up in the coach is four(4) Trojan 125/240AH that run it. So, in doing this do I still need to carry the same 240AH per battery for a total of 960, when converting over to lithium. I will as be adding a four panel solar set to the roof of the coach if that helps in factoring. We do quite a lot of boon docking, and the most we operate in the coach is a coffee pot, TV, phone, laptop, the most common items. Any advice on battery selection would be much appreciated. Thank you, Shawn.

960ah at 6v is only 480ah at 12v. I'd go with a couple SOK batteries

 

[Sfletcher]

960ah at 6v is only 480ah at 12v. I'd go with a couple SOK batteries

Thank you sir! If a guy had room for 2 of the 8D's would that be better than 4 of the smaller 12V 100AH ones?

 

[Sfletcher]

Thank you sir! If a guy had room for 2 of the 8D's would that be better than 4 of the smaller 12V 100AH ones?

Also I see that Renogy is having a sale on some of there batteries, would they be comparable to SOK?

 

[justinm001]

Also I see that Renogy is having a sale on some of there batteries, would they be comparable to SOK?

Renogy is junk. SOK is good for the money. It's best to have at least 2 batteries for redundancy and not more than 4 because of wiring complications. Bigger is better, I prefer rack batteries

 

[Sfletcher]

Thank you! I'll steer clear of them. I just watched some of the SOX videos. Of course I will need battery heaters(in WY) will there be any issues with hooking two of them parallel and making sure they stay equalized? I really like the 12V280Ah. Two of those should do the trick I would think.

 

[justinm001]

Thank you! I'll steer clear of them. I just watched some of the SOX videos. Of course I will need battery heaters(in WY) will there be any issues with hooking two of them parallel and making sure they stay equalized? I really like the 12V280Ah. Two of those should do the trick I would think.

Nope it's very typical to have 2x in parallel. You just wire the positive of one then the negative of other to the connection so power flows equally through all 2.

 

[Vigo]

If a guy had room for 2 of the 8D's would that be better than 4 of the smaller 12V 100AH ones?

The lifepo4 batteries that are 8D size are 280ah, BUT they are now making 300ah "mini" that are even narrower than the 8D 280ah. So if you buy 2 8D-sized prebuilt batteries that's 560ah and should fit your battery trays exactly (if you already have an area setup for 8Ds). If you buy two 300ah "mini" they would fit in the 8D battery area and leave a couple inches empty on the sides, and that's 600ah. Either way that's a lot more than the 400ah you were thinking.