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Battery charging and BMS questions.


New Member
Apr 12, 2024
Hello, I am definitely new to designing power packs and I have a couple of questions.

I was planning on getting a bms for a 4s LiFePo4 battery pack. Each battery is a EVE 304Ah. I was thinking about getting a 100 balance 40A bms. I have multiple sources of power this will be in a camper and I will have power from either the alternator, a 12v power supply (from hookup power), and 12v from solar power. I believe that the battery has a voltage of 14v (batteries in series). Will I need a battery charger for this or will the bms take an input of 12v and output 14v (3.5v per cell)? If I do need a battery charger is there any that have an input of 12v and an output of 14v? Or can I disregard a battery charger and use voltage step-up boards (20A) for each source to boost voltage? Or can the BMS do all of this. I want to be safe and make sure I am doing this properly. If I do need a battery charger please let me know if you have any suggestions for this setup. Thank you guys in advance.
Your setup is similar to what I setup and I also don't have a great charging source. For my build I used this guys video (HERE:
) and followed a lot of what he did. I went with a bigger 2000w pure sine inverter. I also use a 20amp MPPT solar charge controller instead of the all in one unit he uses for charging. For the bigger inverter I got a DALY "Like" 200amp BMS. Basically the same thing but no name on it from China. Otherwise my system is exactly like his. I can tell you what has worked for me and what hasn't charging my LifePO4 S4 batteries. I have 4 sources but only 1 works as I hoped. 2 and 3 work but not as well as I hoped and 4 is a failure I don't have a solution.

1. Connected 100w 18v 5.56amp solar panel to solar charge controller. This works as I hoped but only at the camper when it's sunny. For your setup you'll need a charge controller otherwise the BMS will shut off your solar panel most likely with too much voltage.
2. Connected car battery charger directly to LifePO4 batteries. This also works as I hoped but only pushes 60 watts on the 10amp setting so at most half what I expected.
3. I used a laptop computer wall charger. It is 20v and 6amps for 120 watts. I use through the charge controller but it has issues. It will start charging and I see the amps increase and in about 20 seconds it will go from around 3 amps to 5 amps to 8 amps to 10 or 11 amps and then go to zero. It will stay at zero for around 30 seconds and then repeat. What I think is happening is the computer charger itself is shutting off because 10 amps at 13 plus volts is over the watts (120) it is supposed to produce so it shuts down. This is just my guess.
4. I made a car cigarette charger plug. This has been the most disappointing because my hope was to charge the batteries while driving to go camping around an hours drive. I'm not sure what the issue is here. The car charger plug is providing 13.8 volts while the car is running which is great. I initially tried using it through the charge controller but it didn't work at all. I then used it directly to the batteries and it will only charge the batteries to about 13.3 volts then there is no charging at all. Which is not enough. Not sure why this isn't working. I can't imagine why the BMS would shut off a source of 13.8 volts at 13.3 volts .... but it does or something does. Anyone have ideas why?
This was somewhat helpful. I am wondering more about what voltage do you need to charge a battery at and if you need to charge a battery to 14v can you have an input of 12v on the bms that will boost the voltage to the necessary charge voltage. Do I need a battery charger if I have a bms?
The BMS can only turn the power on and off, it can't change it. Using the AGM profile on any basic car charger will get you around 95% full so it's a good 2nd best option.

As for the BMS, a 40a on a 300ah battery is really, really small. If it's not too late, get at least a 100a. That rating is not only the continuous DISCHARGE rate, but usually is also the max CHARGE rate. Recharging a 300ah battery at 40a max is a 10 hour process no matter how much charging capability you have available.

As for the alternator charging, the alternator usually puts out 13.8-14.5 volts, but the battery is going to want to draw ALL the amps it can. When you see the DC-DC converters out there for alternator charging, those do 2 things. First they convert the 13.8v from the alternator up to the 14.6v that batteries like and second it throttles the amperage draw so you don't burn up the alternator.

Your car plug would have worked better until it blew the fuse trying to draw ALL the amps through the port if you didn't have the charge controller. Most charge controllers need about 5v over charging to turn on their bits and start working. Unless your lighter port is putting out 20v, the controller isn't going to do its job. The voltage drop you're seeing to 13.3 is either the cells hitting the flat of the curve and they'll sit at 13.3 for a lllloooonnngggg time drawing amps but not changing volts, and/or the load on the wires is dropping that half volt through resistance. Much like when you turn on the vacuum the room lights dim a bit. You can let it sit at the 13.3 and see if the amps are still going in via the BMS app or amp clamp (charging curve) or if it's getting to 13.3 and no amps are flowimg (voltage drop). Not uncommon.

Same with the laptop charger, you're getting that 20v to start but then the brick is topping out while the controller is asking for more.

Ok, that's a lot of phone typing, hope it helps.
Thanks RNT. I did think the car port charger wires were too thin so your probably right the resistance eats up some voltage. Not sure if its worth trying to salvage using the car cig charger.