Only getting 50% of battery capacity - BMS protecting too early?

[Matty-Lee]

I think my BMS may be cutting out too early.

My 12v system consists of:

BMS (actual product link no longer available, I have a 80A non-smart version)
4 LifePo4 Cells (200 Ah)
EPever Charge controller

I disconnected the solar to see how long it would last and found out that the BMS disconnected the battery with it measuring 12.2v.
I checked the cells individually and they were all nicely balanced.
When I reconnected the solar the charger reported that there was at about 50% charge.

This is disappointing because one of the reasons I went for LifePo4 was that the usable capacity is supposed to be better than lead acid or AGM.

When I look at the spec sheet for the BMS it says the over discharge detect voltage is 2.2, presumably per cell, so the BMS should disconnect at 8.8 volts...is that right?



My charge controller is set up as the chart below except I have changed Boost & Float charging voltages to be 13.6 (to prolong the life of the battery):



When fully charged the battery is reading 13.3v

Can anyone spot if I have made an obvious mistake or what is not looking right?

Thanks,

Matt

 

[BentleyJ]

It sounds like your BMS may need to be replaced or the user settings reset. In any case, the Low Voltage Cut Off 2.2V is TOO low. 2.5V per cell is considered the minimum for LFP chemistry before damage occurs. So if your cells are balanced and they all reach the limit at about the same time 10.0V total should be where the BMS disconnects.

 

[WoodsieLord]

You are not mentioning what your load is. Could it be a low voltage disconnect from the inverter ? Big loads could be pulling battery voltage lower than the inverter's threshold maybe?

 

[MisterSandals]

BMS (actual product link no longer available, I have a 80A non-smart version)
4 LifePo4 Cells (200 Ah)

You have $500+ in a nice set of cells. Saving $50 on a cheap BMS is not the best option.
Do yourself a favor and get an Overkill, JBD or similar with bluetooth. You will save more than that much in alcohol and psychiatry in just a few weeks.

 

[mikefitz]

A battery voltage of 12.2 volts for a lithium battery is not 50%, the Epever SOC is setup for lead acid and makes a poor guess based on voltage.

12.2 volts is around 10% SOC. The voltage will bounce up a bit once any load is removed. For long battery life consider 13 volts or 12.9 volts as the low limit (20% SOC).

Boost 13.8 ( or slightly higher) and float 13.35 would be better. Most BMS don't start balancing until 3.4 volts per cell.

I suggest a replacement BMS, a smart JBD 120A unit, not too expensive and quick delivery with express delivery option,

4S or 3S 12V Li ion or Lifepo4 Battery smart BMS with Bluetooth function UART and RS485 communication with 60A to 120A constant current - LLT POWER ELECTRONIC

1) Application:this smart BMS can work for 3 to 4S Lithium ion or Lifepo4 Battery ,client can choose based on your battery chemistry features 2) Current : constant current of this version BMS can be made with 40A to 120A in accoridng to the different demand 3) Dimension: :Max 138*102*15mm for...

www.lithiumbatterypcb.com

Mike

 

[Browneye]

The standard voltage settings for boost and float on EPEver is pretty good - it measures battery voltage with the charge current on top, and will always show it as higher than the actual battery is.

I have a 230A pack with Overkill JDM bms, and a Tracer Xtra charge controller, in a Winnebago. I have had to adjust charging parameters up a little to compensate for this. But if I leave the boost at 14.5V then I'll usually get one cell that reaches full charge before others - at full knee it shoots up quickly and the bms shuts down charging. If I put it at 14.1 or 14.2V with a 20 minute boost duration then they'll balance and all reach a very high state of charge. I call that 100% charge for my pack, set my meters there. The BMS automatically shows that as 100%, and so does the charge controller.

I don't know about DALY, but JDM default for balancing is over 3.4Vpc, so if you set your charge voltage too low the pack never gets quite charged all the way, and cell balancing never gets a chance to happen.

13.2V float will let your pack settle back to about 50% if there are any loads. 13.4V will keep it about 80-90%, and 13.6V is good if you're supporting loads. This gives you best benefit of your solar charging while you use your battery over an entire 24 hour day.

There's nothing wrong with charging them fully, running them down to 20%, and recharging. It's sitting stored for long periods at 100% in high temperatures that will severely degrade your battery. It's best to charge it all the way up if it's in regular use, you'll get full capacity of your battery that way.

I have two profiles I load in my charge controller - one with lower settings for long term storage - when it's parked unused. This lets the battery drift down into the 70-80% range and maintain, getting a boost charge once a week to top it back up. The other charge profile has higher parameters for daily use when we're actually traveling and using battery power, and want full benefit of solar charging.

BTW, boost duration starts when the battery reaches boost voltage. If you choose a lower boost a longer duration is appropriate, a higher voltage it's fully charged when it gets there making any duration unnecessary. IIRC the minimum boost duration is 10 minutes.

 

[Matty-Lee]

You are not mentioning what your load is. Could it be a low voltage disconnect from the inverter ? Big loads could be pulling battery voltage lower than the inverter's threshold maybe?

Hello, sorry. No big loads at all. Just LED lights, USB sockets and 12v sockets in my van.

 

[Matty-Lee]

Thank you for the replies folks.

The things that I have taken away so far is:

• Consider getting a better BMS. I went for Daly as all the build videos I saw on YouTube seem to use them.
• 12.2 volts is around 10% SOC and not 50% so I'm happier with that.
• My changes to the charging voltages may not be so great so I'll reset them back to default.

Thanks,

Matt

 

[Matty-Lee]

A battery voltage of 12.2 volts for a lithium battery is not 50%, the Epever SOC is setup for lead acid and makes a poor guess based on voltage.

Hi, I definitely set the charger to the Lithium option. I know it's a budget controller, are you saying that it's not a great one for Lithium batteries so won't do a good job?

Thanks,

Matt

 

[Browneye]

Using battery terminal voltage is not a good indicator of state of charge on Lifepo4, due to the very flat charge/discharge voltage curve:

You could be flat in the middle either way and not really know. A smart-shut and meter is recommended to keep track of SOC.