Do I need to use a BMS if I bottom balance first?

[TommyG-UK]

I'm building a 200W solar shed at the bottom of my garden, I'm about to install 4x new 100ah 3.2v LiFePO4 cells and after watching some of Will's videos it got me thinking.

Do I need to use a BMS if I bottom balance first?

I won't be pulling any huge loads in the shed, typically about 70-100 watts for a few hours a day to power things like shed/garden lights at night and phone chargers.
I'm also about to purchase a Victron MPPT SCC (which I intend to program to only enable charging between 20-80% to hopefully elongate the life span of the cells), along with a low temperature disconnect sensor.

So, because of my system’s relatively small size and power draw, will I need to use a BMS after I have bottom balanced the cells? or could I get away with only using an active balancer permanently connected to the cells?

 

[MagicToolbox]

I'm still learning, so take my input with teaspoon of salt.

The BMS does other things besides monitoring cell balance. A good BMS also monitors temperatures to prevent charging at low temps, prevents overcharging, prevents over-discharge, and can shut off the battery if there is an over-current draw.

Your system design is for small loads, but it may grow, and the BMS can help prevent expensive damage if another part of the system does not work as planned. The Daly BMS that Will is banging on about (in a good way) is around fifty bucks, new ones may or may not have low temp protection, but your SCC has this covered. It seems to me like cheap insurance to include the BMS.

 

[Supervstech]

without one, it should work fine, but it might not.

and, without one, you have no way of knowing if they are balanced or not.

at a minimum, you should have a monitoring setup to track the cell status.

 

[TommyG-UK]

Thanks for the responses guys, I think I'll wait to see Will's next update video on whether the new Daly BMS has low temperature protection or not, because it would be great to have that feature included instead of having to buy a separate unit.

 

[Maast]

Since it sounds like you'll have more panel than battery I'd recommend you top balance instead, without a balancing bms your biggest danger is going to be driving an individual cell over 3.65v/cell even if the overall pack voltage is in the safe zone.

A $15 Tenergy 5in1 cell monitor/balancer can balance (slowly) but more importantly it'll show you your individual cell voltages. You can connect it with a standard 4s balance lead. It has no protection circuitry though. If you're okay spending a few bucks more the $33 ISDT balancer has a much better display - it what I use when I'm looking a packs.