RV Lithium setup on the cheap - charger and BMS?

[Xoliul]

Hi,
I just registered here as it feels like the right place to ask, there seems to be a real DIY attitude here and plenty of people wh know their stuff.

My situation:
I have a self-built RV (van) with a (mismatched) lead acid battery bank (200ah), a Victron BlueSolar MPPT 75/15 and a decent 120w solar panel, and a Cyrix Ct smart relay separating the alternator. The lead-acid batteries are now dead due to my own mismanagement/treatment (old, way too strong charger used often).
I want to switch to Lithium and do it right, but on the cheap. I could replace the batteries for about 250-300 euro, but have come across someone selling Winston 100Ah LFP cells from an EV experiment, at 360 euro for 4 cells, saving me nearly 200 from new. That seems worth the risk for second-hand cells, as they were not used very long and were hooked up to an expensive BMS (Emus). I've put down a small deposit since he ahs a lot of demand for them and it seems like a good opportunity.

I understand I will need a BMS and charge solution. I'm pretty sure the Victron MPPT supports charging LiFePo4, so no change but a reconfigure is needed there. As my van would draw 40A or so max (direct to a 500w inverter), and the MPPT Load limits the rest of the house to 10A, I'm under the impression I don't need big expensive solutions for a BMS.

This is where I need advice:
I'm considering a cheap AliExpress BMS around 50 euro, something like this or this. I'd prefer a system with display/logging possibilities, as I think proper monitoring is key. I also think it would be best to go for separate charging and load rails?
Is this stupid, are these things a scam and is a decent BMS at least 200 euro? Or is this stuff good enough for my application?

I really want to keep alternator charging (not enough sun here often), but I understand I need to at least limit alternator current to the battery, and be able to switch it off.
I'm considering a decent DC-DC Lithum charger from the RC world to put between Alternator and BMS charge rail, they can be configurable to LFP often, and I believe this would limit current and switch off when the battery is charged. I'm less certain about this and can find very little info about people attempting this. Would this work together with my BMS choice? Some chargers support charging without balancing (the BMS could handle this), or is that a stupid thing to do? I'd have an ignition-switched relay feed the charger's DC-in, as to not drain the vehicle batteries.

Both of these options would put me under 500 euro for the whole system including battery cells. That seem very good and almost too good to be true, so I'm expecting to be told I'm very wrong...

I hope I can be enlightened a bit here, I've spent many nights googling but have not become much wiser, other than the fact that spending 5000 euro on a full Victron system is the safer and easier choice. I might have to sell the van to afford that option however

TL;DR

Are cheap 50-euro smart BMS's from Aliexpress worth it for a simple 4S LFP setup in an RV?
Can an RC-hobby DC-DC LFP charger be used between vehicle Alternator and BMS charge input, reliably?

 

[jonyjoe303]

yes you can do it cheaply. I'm using a 6 dollar 30 amp bms on my 220 ah lifepo4 house battery. I been using it non stop almost 2 years. But I also have voltage meters everywhere and always monitor the voltage. I only charge with solar otherwise I would get a bms that can handle a higher charge current.

With lifepo4 the voltage is within the range of the alternator (usually 14.4 volts), lifepo4 fully charges at 14.6 volts, you shouldn't need a rc charger unless you plan on trickle charging it. If your alternator puts out 80 amps, the bms needs to be able to handle 80 amp charging. Myself I would use an overvoltage protection relay (6 dollars) and connect that to a solenoid(30 dollars) that can disconnect the battery when it reaches a certain voltage, they are fully programmable so it disconnect the charging at the voltage you want. This overvoltage relay acts like a deadman switch and will protect you if the BMS were to fail.

One thing I notice with my lifepo4, it drifts out of balance when charge with high amps. I had to get a 4s active balancers (cost 100 dollars). The built in balancers on the BMS, most of them balance at 60 ma, they won't keep up if your charging at high amps. That is something to consider if your charging with your alternator. Active balancers are the only ones that can keep up with high amp charging, they balance at up to 6 amps per cell. The bms on your link, balances at 35ma , its almost like having no balance function at all.

As far as monitoring your lifepo4 get a tk15 (25 dollar for 50 amp model, they have u to a 300 amp model), it counts all amps in/out of battery, they are very accurate. Lifepo4 will always read about 13.1 volts between 20 and 80 percent, so you need a coulomb meter to track the amps. To monitor individual cell voltage get a ISDT BC-8S Lipo Cell Meter (20 dollars), it shows cell voltage and also has an alarm if the voltage goes to high or low.

No need to spend alot for your lithium battery system. Get a BMS that can handle your alternator output and you will be good to go. Thats the biggest consideration. If you only plan to use solar and an rc charger, you can get away with a 6 dollar bms like I use.

tk15
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overvoltage relay protection