RV lifePo4 house battery conversion + charging

[Hillhater]

The perenial question, !…with no definitive answer to date
…converting an RV house battery system from AGM to Lifepo4, and charging options..
most current Lifepo4 12 v batteries (100/200+ Ah) claim to be able to charge from existing RV alternator system (14.4 v),..and there are lots of RVs that have used that effectively.
but.. many installers suggest an expensive ($3-400) dc/dc charger is required to full charge (14.6 v)
if a solar charger is already installed also, ( PWM with a 14.6 boost charge function) ,would that not effectively “top off” the charge if the alternator voltage is not quite enough ?
Is the DC/DC charger really bringing anything to the party….other than expense ?
EDIT. Has anyone tried using aa solar chage controller as a dc/dc charge controller ?
IE,..simply inputting alternator output dc in to the solar panel input terminals ?

 

[Hillhater]

IE,..simply inputting alternator output dc in to the solar panel input terminals ?

FYI.. answering may own question here after a little testing…
using a cheap ($20) , 20a , solar controller, and feeding it the 14.4 volts of alternator output into the solar input terminals, it charges my LifePo4 ,12v battery at 15 amps.
and yes, these cheap solar controllers do have a LifePo4 charge profile setting .
Now i wait to see if it can fully charge the battery ?

Edit/update…. Charger cut off at 13.6v at the pack,..which the capacity meter tells me is 98% !
All good, but that was running the 14.4v input from a power supply.
Next test will be the real thing from the RV alternator

 

[neptronix]

...this is why i think about something more along the lines of a 110v battery pack ( or higher if you've got 220v coming out all your sockets )

..you could just clip the top and low end of the voltage curve, accept some voltage drop near the end, and just run DC to AC since no voltage conversion is necessary..



..your only challenge then is to find a very high voltage solar inverter and some way to convert a much smaller current from the 14.4v alternator and convert it up to 110v.. 220v or whatever.

Why would you do this? you'd get maximum efficiency on discharge of the battery, since you've taken one conversion step out of the equation.

 

[amberwolf]

and convert it up to 110v.. 220v or whatever.

Why would you do this? you'd get maximum efficiency on discharge of the battery, since you've taken one conversion step out of the equation.

Keep in mind there are many RV items powered directly from 12v systems; even air conditioners, refrigeration, heating, etc etc. So depending on the RV or mobile home it may not even need an AC inverter.

I looked into getting one of those small 12v air conditioners to use on the enclosed dog trailer I was going to build to be able to carry all four of the dogs when i still had Yogi, Kirin, PeanutButter, and JellyBean. But they were way out of my budget.

Dunno what Hillhater's got...

 

[neptronix]

Ah that's true. My bad, i wasn't thinking about that.
If that's the case then it may be better to just run off 12v direct..

I was thinking we'd expect wall outlet power somewhere.

..okay, back to baseline, to answer the question, i do think solar could help top things off as you're thinking. If you have kind of oversized your battery, this could work really well

 

[amberwolf]

There may be--one of the RVs I helped someone with a while back had 12v "everything" but also had a small few-hundred-watt inverter with four outlets on it to run small stuff; it was built into one of the walls (it's heatsink was "outside" as part of the outer wall; don't know if it had any active cooling). IIRC they used it to charge their laptop and stuff that didn't have 12v-powered wall-bricks, but ddint' need it for any of the usual appliances. Even a little clothes washer (that might've done two shirts or one pair of pants :lol ran off the DC "mains".

I don't know what the DC source was (batteries, gen/alternator, solar, etc; wasn't part of what I was dealing with), but knowing what I do nowadays I'd guess a bank of lead-acids charged by the engine's alternator, with a separate AC-powered charger they'd plugin at whatever opportunities they had).

 

[Hillhater]

...this is why i think about something more along the lines of a 110v battery pack

I get what you are suggesting, and why, but those high voltage pack systems are in another league of safety and component availability.
24v is a common upgrade for RVs, and i could see 48v being possible but even then solar chargers , AC inverters, etc are getting limited sources and high prices.
12/24v dc stuff is so cheap, proven , and readily available.

 

[BiggKidd]

I would NOT try and push the batteries that far myself and I have been living off grid with solar and batteries for 17 years. If you really want to get the most life out of them. You only want to fully charge them once every few months. Do a little research most people go with a basic parameter of roughly 25-75% or 20-85% state of charge no lower or higher other than the occasional balance charge. That's for maximum battery life to the best of my knowledge with LifePO4 cells and I've got a lot of them! I actually tend to run mine a little different @ 30-85% state of charge.

 

[Hillhater]

I actually tend to run mine a little different @ 30-85% state of charge.

How Do you guage those 30-85% limits, ?… by voltage, or capacity (wh) monitoring ?
The battery bms has low voltage cutoff and overcharge protection (13.6v), so some form of external charge voltage limiter would be needed to keep down to that 85% max level (13.35v ?)

 

[BiggKidd]

By voltage. My systems are all 16s or 48 / 51.2 volts I charge up to 53.5 battery volts and go down as low as 51.5 volts. Nice and healthy.

 

[Papa]

Page 9 from my Li-Time manual 12V 100Ah LiFePO4.

 

[Hillhater]

There are obviously differences between brands.
My DC Mont 135 ah battery lists 3.6v resting voltage at 100% capacity, and there is a “%” readout on the pack that corresponds .
but i have never confirmed that 100% is actually 135 ah ?
Also 30% capacity is stated to be 12.9 v,…but again i have not verified that figure.
It is a little worrying in that 0.1v can be more than 10% difference in capacity.
A good enough reason to keep well away from the voltage extremes. !

 

[ZeroEm]

Ah that's true. My bad, i wasn't thinking about that.
If that's the case then it may be better to just run off 12v direct..

I was thinking we'd expect wall outlet power somewhere.

..okay, back to baseline, to answer the question, i do think solar could help top things off as you're thinking. If you have kind of oversized your battery, this could work really well

Just to remind everyone, alternators are AC then convert that to DC. People used to have AC plugins to run AC tools off their Alternators.