I am building a 4S LiFePo4 200Ah for my VW California. I have 4 100 watt portable solar panels and a 2000 watts 12 volt inverter. To my understanding, my solar charged controller will stop charging as soon as it reaches the maximum charging voltage. My inverter will also shut down when the voltage drops to a certain level. With this constellation, I think that my batteries will be safe. I will need only a LiFePo4 charger to balance the cells as soon as the cells are not in a balance state. Do you think that II still need a BMS?
DIY LiFePo4 without BMS unit
- Thread starter mrswr350
- Start date
Yes.
You still need to monitor individual cell voltages and have a method to disconnect load / charge source if any of the cells are under / over voltage. In series, the cells will have different voltages.
This can be done simply without using a traditional BMS, a cell voltage monitor in conjunction with one or two contactors is sufficient. You will need to manually keep cells in balance if there is significant difference in self discharge.
This can be done simply without using a traditional BMS, a cell voltage monitor in conjunction with one or two contactors is sufficient. You will need to manually keep cells in balance if there is significant difference in self discharge.
acdoctor
Solar Enthusiast
The 4 individual cells have the potential to charge to output of the charge source 14.4 volts. As well as over discharge the entire pack or just one cell if they get out of balance. BMS is a wonderful thing and not expensive so why not?
WindWizard
Solar Enthusiast
Yes Yes Yes.
I like the overkill bms due to the fact that it has easy connection with bluetooth on the phone. It allows me to see what is happening while in the house. The range is really very good. When the cells start to go over 3.4 volts then they can quickly go into over voltage. A BMS will prevent that from happening. If a Cell gets overcharged then it can bloat and be permanently damaged.
joanna_lannie
Maybe sleepwalking now
- Joined
- Sep 14, 2021
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- 188
Is still needed. BMS is the most important accessory to ensure battery safety, and it has a good effect on preventing overcharge, overdischarge, and overcurrent. Besides, LiFePO4 batteries are not very cheap items, so be prepared.
Ampster
Renewable Energy Hobbyist
Yes, this is known as, "You are the BMS." I would rather spend my time doing something else than doing what electronics can do for me, but we all have choices about how we want to spend our time.
Me too, but it is a viable option for some.
DerpsyDoodler
Solar Addict
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- Jan 10, 2021
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It's not a viable option. Neither practical nor realistic. Tending (babysitting) batteries, every time they process through an x amount of hours charge or discharge cycle will not happen.
Thanks for all the inputs. I am really liking this forum as you will get a lot of feedbacks. From what I have read in these feedbacks, the most critical one is the overcharging of the cells. The electronics of my solar charge controller will stop charging when it reaches the maximum charging voltage but it is not enough because it is not monitoring each cell. My solution to this is to connect an active balancer and a voltmeter that monitors each cell. Regarding the over discharging the cells, I think this will not happen as my inverter will shut down when the voltage drops to 11 volts. Your thoughts on this idea?
registertolonlyonepicture
New Member
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- Mar 19, 2022
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This is not enough, new balanced cells without BMS may be can be used for months/years with luck without any error. But they should not be used in such way.
LiFepo4 cells are very "rugged?" but they can differ in SOC depending from many variables. Beginning with temperatur differences from the one side of the Pack to the other, than the differences in aging or SOC and capacitiy differences at the beginning of usage of the accupack.
The fact that the voltage alone is not very meaningful to determine the "real state of charge" SOC makes it even harder to balance this cells. But there are hard limits which should not heappen to the cells. Undercharge and Overcharge. If you have 15 full charged cells and the 16th one has only 2,5V and you discharge the block, the Inverter thinks "all is ok, the Pack voltage is high enough" but in reality this 2,5V cell will be discharged under 2,5V and can be also inverted in voltage. It can get an negativ voltage and instantly die. For this case it will be good to have the BMS to switch off before something like this will happen. Also the BMS can warn you if the temperatur for charging this cells is to low or to high. Or it warns you if one cell loose charge because of an internal Cell error. All this resons are enough for me to use an BMS.
LiFepo4 cells are very "rugged?" but they can differ in SOC depending from many variables. Beginning with temperatur differences from the one side of the Pack to the other, than the differences in aging or SOC and capacitiy differences at the beginning of usage of the accupack.
The fact that the voltage alone is not very meaningful to determine the "real state of charge" SOC makes it even harder to balance this cells. But there are hard limits which should not heappen to the cells. Undercharge and Overcharge. If you have 15 full charged cells and the 16th one has only 2,5V and you discharge the block, the Inverter thinks "all is ok, the Pack voltage is high enough" but in reality this 2,5V cell will be discharged under 2,5V and can be also inverted in voltage. It can get an negativ voltage and instantly die. For this case it will be good to have the BMS to switch off before something like this will happen. Also the BMS can warn you if the temperatur for charging this cells is to low or to high. Or it warns you if one cell loose charge because of an internal Cell error. All this resons are enough for me to use an BMS.
You need to ensure your voltmeter that is monitoring each cell has the ability to disconnect a cell as soon as it goes out of voltage range.
Monitoring pack voltage (eg using inverter LVD) isn’t sufficient as one cell in a series pack can be below critical voltage while the pack voltage is still within range.
Using an active balancer in conjunction with cell level monitoring capable of cell disconnect if required is equivalent to having a BMS.
acdoctor
Solar Enthusiast
I have trouble understanding some newbees to the form. “ first time DIY LFP that want to run without a BMS”. They ask the options of people that have experience and are given sound reasons why it’s necessary. Challenge every point, and at the end still think running without a BMS will work for them. Perhaps their’s cells come with some Devine blessing that others do not.
RV10flyer
Solar Wizard
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- Mar 8, 2021
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Many of us have ruined LFE cells going over 4.2v/cell. It happens fast above 3.5v. ?? For the price of one diy 280ah cell, you can buy a BMS and know exactly how your battery is performing, whether you have loose or corroded connections and protect cells in case of an out-of-limit condition. I have a 132kWh off-grid bank with protection on all 160 cells, plus battery monitors on each battery. It's worth it.
Why do you think running a cell level voltage monitor with the ability to disconnect upon cell level high or low voltage, in conjunction with an active balancer is worse than a traditional BMS?
I use quality cells that often go many years without balancing. If i was using cheap cells i would use an active balancer, and individual cell voltage monitors that can trigger disconnects.
I’ve seen too many batteries destroyed by cheap BMS units to go down that path. I think it’s reasonable to ask if there is a better way.
Thanks for all the wise advices. I am coming from an AGM power source and it has served me since 5 years of use without any issues. No monitoring using Bluetooth or WiFi. I just leave it and it works. As my power requirements is increasing, I will need 4-95Ah AGM batteries and this requires more space and more weight, which is not the right way to go. This is the reason why I opt to go for a 200Ah LFP. Now, you all have convinced me to get a BMS. Many thanks for those who patiently replied to my post.
MisterSandals
Participation Medalist
If you don't want to deal with a BMS you can buy LiFePO4 batteries with an internal BMS. The are sold as drop-in replacements.
Some are very very good (BattleBorn) and some are more "affordable". DIY batteries from cells are not for everybody or every situation.
jsmetalcraft
New Member
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- Jul 7, 2021
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- 185
I am sure i will get a lot of sluff but here it goes. 3 years ago I built a camper. Added 4 CALB cells. Every thing was 12vdc. 600w of solar and Victron controller. Top balanced, set the charge 90% discharge 20% and let it go. I never hit the 20%SOC. But I would every now and then check the voltage. The pack remained balanced. I don't think I seen more then 12 amp discharge and don't remember what max charge was. Controller was 50 amp. Guess was 20-25 amps. NO BMS. Coldest night was 14f. But i had the stove vent right next to batteries with an external temp gauge. Morning coffee would heat compartment up.
I now have added a BMS because I know more and can, but with low draw and diligent monitoring it can be done. Like someone posted, there are now better things to do.
I now have added a BMS because I know more and can, but with low draw and diligent monitoring it can be done. Like someone posted, there are now better things to do.
Thanks for the advice. The 8 - 200Ah cell that I have ordered is already on his way. I will build 2 - 12 volts system as I will still use my 12 volt inverter. For the moment my old setup in my camper is still ok as I can boil water for my coffee and also for my home office (actually a camper working remotely in front of the lake). Which BMS brand would you recommend that can handle at least 200 amperes for 4 cell configuration?
MisterSandals
Participation Medalist
I think a lot of people are happy with Overkill, JBD and JK.
