Materials needed for DIY LiFEPO4 100Ah battery build

[mrcotton73]

Hello

I am new to this and trying to put together a 90Ah/100Ah LiFePO4 battery with BMS. This will replace two SLAs in my travel trailer - and I will be charging them with 2 100W solar panels mounted on the roof. I have been researching Will's website, battery builds / videos etc. but I am not 100% clear on all parts needed to test the battery - so I thought I would put out a question to this forum. I want to work out the costs to compare if its better to build v/s buy one

Things I need :

Battery cells -- purchase from AliExpress (seeing deals for 4 cells around $170.00 -- can't confirm if these are grade A or not)..
BMS system with bluetooth and low temp sensor
Bus Bars, Wires, hardware etc..

For charging / testing / balancing:
Variable high-current power-supply? (I have a NOCO genius charger - will that work)
Battery capacity tester
Cell Balancer?
Anything else?

Thanks again for the help in advance!

 

[snoobler]

Top Balancing LiFePo4 Cells using a low cost benchtop power supply.

To get the paper, click on the orange at the top of this page. This is a tutorial with detailed steps on how I top-balance my LiFePO4 Cells using a low-cost benchtop power supply. To get the tutorial, click on the orange "Download" button...

diysolarforum.com

 

[mrcotton73]

Thanks snoobler for the document and the links - very helpful.. Will be reviewing these for my build.

 

[Dzl]

In terms of what is the minimum necessary to build, balance, capacity test (off the top of my head):

Battery:

Cells

BMS low temp sensor (recommended), Bluetooth (totally optional)

Bus Bars, and Hardware (included with the cells normally)

Way to Secure/Protect the cells

For standard balance method:

Multimeter and/or Clamp Meter

CC/CV Power Supply
*There are other methods (i.e. just buy and use an active balancer, hobby balancer)

Capacity Test:

Way to charge pack (or individual cell)

Way to discharge pack (or individual cell)

Way to measure energy in/out (like a battery monitor, shunt, hall sensor)
*An actual capacity test as a pre-commissioning step is a good idea, but not necessary, many people skip it. If you have a battery monitor in your system your first full cycles can double as a practical capacity test.

 

[mrcotton73]

Update on this -- I purchased a 4 pack of 120AH Grade-A new cells from ShenZen Basen for $160+shipping (total $233.50). I have the cells - they look new and bar-codes are present. I am about to do a top-balance with a benchtop 10A power-supply -- but I have a question on pre-charging.

Individual Cell voltages are 3.282 / 3.283 / 3.282 / 3.282 V ; Should I pre-charge these before starting the top-balance procedure? For pre-charging - can I wire them in series and use my NOCO genius charger -- without using a BMS (will the charge cutoff charging once charging is complete)? Or should I just wire in parallel and start top-balancing? Once top-balanced - I want to do a capacity test on the cells using the variable load circuit that I purchased off Aliexpress.

Thanks!

 

[Dzl]

Individual Cell voltages are 3.282 / 3.283 / 3.282 / 3.282 V ; Should I pre-charge these before starting the top-balance procedure?

By 'pre-charge' do you mean wire in series and charge partially or until first cell hits HVD?
If so, the only reason I am aware of that people do this is impatience / speeding up the process. It doesn't do anything useful beyond speeding up the process. But with high capacity cells and low amp chargers many folks find it more than worthwhile to shave some time off the total.

How many Amps is your NOCO charger?

For pre-charging - can I wire them in series and use my NOCO genius charger -- without using a BMS (will the charge cutoff charging once charging is complete)?

No, always use a BMS for this. You don't know how out of balance your cells are and things happen very quickly towards the top or bottom of the voltage curve. This is one of the most common causes of new cell damage that we see here (not using a BMS when cells are wired in series).

Or should I just wire in parallel and start top-balancing?

Comes down to time/patience.
If you aren't in a rush, nothing wrong with just setting your power supply to 3.4 or 3.5, hooking it up to the parallel connected cells and letting it do its thing until current drops to zero, then step up to 3.6 or 3.65 for the final shorter step (where you should be present/attentive).

And if you are in a rush, nothing wrong with hooking up your BMS to your series connected pack, and charging in series.


The TL;DR is neither is wrong so long as you use a BMS if you 'pre-charge' in series, comes down to priorities.

 

[mrcotton73]

DZL - thanks. Yes - for pre-charging I meant connecting the cells up in series and doing a pre-charge to speed up top-balancing. I have a NOCO Genius 3500 - which is rated for 3.5A. I was looking up its documentation - and it does say need to use with a BMS.

I have the overkill type BMS - but haven't hooked it up -- and I'll need to start with crimping lugs to the BMS wires and understanding the settings -- so for now - I'll just connect the cells in parallel and start top balancing at 3.5V as you recommend!

 

[Dzl]

DZL - thanks. Yes - for pre-charging I meant connecting the cells up in series and doing a pre-charge to speed up top-balancing. I have a NOCO Genius 3500 - which is rated for 3.5A. I was looking up its documentation - and it does say need to use with a BMS.

I have the overkill type BMS - but haven't hooked it up -- and I'll need to start with crimping lugs to the BMS wires and understanding the settings -- so for now - I'll just connect the cells in parallel and start top balancing at 3.5V as you recommend!

Yeah just connect in series parallel, your NOCO charger would not really help speed things up much anyways (unless I'm mistaken)
If I understand this correctly, In series you are charging a 100Ah bank at a maximum of 3.5A (NOCO), in parallel you are charging a 400Ah bank at a maximum of 10A (Benchtop). Not worth the hassle of setting up the BMS and configuring and reconfiguring the pack.