RV Lithium Conversion

[magnet]

Hi guys! New to the forum but very familiar with Will's videos. Anyway, I live in Colorado in my RV full time, after dealing with this shitty lead acid house batteries that wont run my furnaces through the night, I decided lithium is the way to go. I scored 16x LYP260AHA cells for a great deal. Currently my RV has 140amp alternator, 2x 12v deep cycles, a starter battery, a 55 amp progressive dynamics converter, and a 6kw genny. My plan currently is to put the 16 cells in a 4s4p configuration under the bed in the back. It will have a daly BMS on it. I also ordered a 70amp non-isolated DC-DC charger. I would like to be able to charge the Lifepo4's from the alternator as well as the generator. Here are my issues. The RV is 33' long, In order to tap into the existing 12v system, i need to run wire from the existing house battery bay to the bed, Since the largest loads will be under 30 amps total, I dont think sending power will be an issue. Here is where things get confusing for me. I want to add the DC DC charger to the bay where the house batterys were, since the output is 70 amps, I will run appropriately sized wire back to the bank. My original thought was to leave the lead acid batteries in the bay and hook the DC-DC charger to them, but now I think I will eliminate them. My first question is, will the 55amp power converter run the 70amp DC-DC charger ok or will it cause issues? Now, if the power converter needs to be larger, it seems to me that just getting 100amp power supply and feeding it to the DC-DC charger would be more cost effective than getting a larger power converter (that has charge stages etc..) as the DC-DC charger will control the charge curve. Am I overlooking anything?

Thanks.

 

[Partimewages]

You should probably draw this out and post. It will be easier and quicker to answer your questions.

 

[Partimewages]

One thing I'm not understanding is how your lifepo4 bank is going to interact with your lead acid bank.

 

[Rocketman]

Here are some thoughts- I assume you are in a Class C RV?

Something’s to make sure you have figured out.

Batteries: have one bms for each 12v battery (so four bms’s). Daly for a while did not have a good reputation on this site - I don’t know if it has changed. I used Overkill bms on a smaller battery and liked it. Just make sure you can see the cell voltages some easy way - Bluetooth usually.

I am a little confused on why you are putting the Dc-Dc charger between the lithium’s and the loads. It should go between the alternator/starting battery and the Lithium battery. Then break the connection between the alternator and the old house battery. And your lithium’s will completely power the 12v house. Use a larger cable there to compensate for the distance.

Get a good shunt based battery monitor- like a Victron Smartshunt or BMV712. That way you can know if your battery is mostly full or mostly empty. The ones on bms’s can have issues with low currents - which is what you will have.

draw out your plan - include wire size, fuses, etc. it is much easier to catch mistakes and errors than in paragraph form.

 

[magnet]

Thanks guys. I must have been vague, the DC -DC charger will be after the alternator. Will the 55amp feeding a 70 amp dc-dc charger cause issues? I am working on drawing it out and will post when completed.

 

[Partimewages]

The PD 55 is separate from the alternator.
Alternator charges chassis batteries.
PD 55 charges house batteries.
Some motor homes have a relay that allows the alternator to charge house batteries while running.
The PD 55 charges when connected to shore power.

 

[Rocketman]

Yes it will cause issues. (And probably bad ones).

It will cause your starter battery to drain into the house battery via 15amps per hour. This will also cause the engine, lights, and everything engine side to be running on approx 12.x volts instead of the 14.3-ish volts they are use to, until the house batteries go into absorbing mode, then the alternator can catch-up on the starting battery.

The other issue is that whole time the alternator will be running flat out!! If/when it gets too hot you will burn up the alternator.

Do you really only have a 55a alternator in your RV? I have heard you only want to use half for charging the house and leave 50% for running the engine- that would indicate a 30a max Dc-Dc charger…

Edit… @Partimewages was typing at the same time I was. I don’t know how that piece of equipment will work.

 

[magnet]

Yes it will cause issues. (And probably bad ones).

It will cause your starter battery to drain into the house battery via 15amps per hour. This will also cause the engine, lights, and everything engine side to be running on approx 12.x volts instead of the 14.3-ish volts they are use to, until the house batteries go into absorbing mode, then the alternator can catch-up on the starting battery.

The other issue is that whole time the alternator will be running flat out!! If/when it gets too hot you will burn up the alternator.

Do you really only have a 55a alternator in your RV? I have heard you only want to use half for charging the house and leave 50% for running the engine- that would indicate a 30a max Dc-Dc charger…

The RV has a relay that connects and disconnects the starter battery/alternator from the house battery when the key is in the off position. I have a 145 amp alternator and a 55 amp power converter.

 

[Partimewages]

How often do you drive and for how long?
Generally unless you are traveling everyday it's not terribly efficient to use the alternator as the source. $ per watt gained.

 

[magnet]

The PD 55 is separate from the alternator.
Alternator charges chassis batteries.
PD 55 charges house batteries.
Some motor homes have a relay that allows the alternator to charge house batteries while running.
The PD 55 charges when connected to shore power.

Mine does have the relay. The motorhome has a 145 amp alternator. So when the motor is running, it would send current to the DC-DC charger from the alternator. When the Genny/shore power is supplying, the 55 amp power converter will supply power to the DC-DC charger. Will supplying the 70 amp DC-DC charger only 55 amps cause issues?

 

[magnet]

How often do you drive and for how long?
Generally unless you are traveling everyday it's not terribly efficient to use the alternator as the source. $ per watt gained.

I have been driving like 300 miles a week lately.

 

[time2roll]

Thanks guys. I must have been vague, the DC -DC charger will be after the alternator. Will the 55amp feeding a 70 amp dc-dc charger cause issues? I am working on drawing it out and will post when completed.

Yes the converter may overload.

Do you really need/want 70 amp DC-DC charger? I recommend 40 amps max and not run the alternator so hard. Adding 50% load to the alternator could push it over the edge.

 

[time2roll]

If there is much voltage sag getting to the DC-DC charger it will draw more amps to upconvert the power to 70 amps at the proper 14.x volts and could require 105 amp input.

Even a 40 amp could be pulling in 60 amps. Consider even smaller.

May need more generator time and increase the roof solar. Both of which can operate as you drive.