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Old 06-08-16, 04:51 AM   #13
IamIan
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Quote:
Originally Posted by creeky View Post
BMS musings:

After some initial research: Building my own BMS is probably a bit out of my time/capability ranges. Buying one does get pricey. So right now I'm leaning towards the basic cell monitoring with the option to turn on cell balancing. From what I've read, cell balancers gone bad kill more battery packs then packs left alone.

Some relatively inexpensive made in china offerings. Can anyone check my math/understanding. My brain is a bit fuddled lately. Lot going on besides the new battery fun.

Problem: What amp BMS do I need?
If a cell leveling system fails .. yes it can cause the very problem it is supposed to prevent .. Just a question of Mean Time Before Failure (MTBF) .. OEMs like Tesla, GM , etc... all have a high enough MTBF that their electronics fail very very rarely ... soo they all use cell level balancing & Monitoring.

Basic level would be a Battery Monitoring System that sends a 'top' signal at the top of charge .. to stop from over charging .. a 'bottom' signal on discharge , to prevent over discharge ... show you the current usage volt/amp/watt .. Do cumulative AH/Wh counting ... The Ah part makes a fairly easy fuel gauge as the Ah cycle efficiency is pretty high (around ~99%).

Off the shelf example that looks at whole pack level (not cell level) for all of the above: EV-Display
w w w .electriccarpartscompany.com/EV-Display-Battery-Pack-Monitor-br-Voltmeter-Ammeter-Fuel-Gauge-br-2-inch-round-or-square-br-12V-24V-48V-72V-96V-120V-144V-br-5Ah-10Ah-20Ah-30Ah-40Ah-50Ah-60Ah-br-70Ah-100Ah-180Ah-200Ah-300Ah-br-3-external-outputs-USB-Serial-Port_p_134.html

(remove spaces between the w w w)

Cell level drift / imbalancing should be very slow .. a ~10 mA or so correction rate from a 24/7 device is plenty .. only need higher Amp for the main whole pack battery amps monitoring .. If you have Lithium cells going out of balance faster than that .. you have lithium cells that should be replaced.

One alternative .. that doesn't open the cell level failure risk .. is to only monitor the pack to see if there is an imbalance .. than you can manually fix/service that imbalance yourself ... that can be done very inexpensively and easily by comparing the voltages of the two half packs .. a perfectly balance pack .. the two halves should be the same voltage all the time , even under load .. if there is only a small resistance different between the two halves you can set up your comparor to have a 'fudge' factor .. like 1v or something .. only informing you if the difference between halves is greater than that .. Example of a simple circuit for that attached.
Attached Images
File Type: pdf Lee Hart's Batt-Bridge Battery Balance Alarm.pdf (583.8 KB, 543 views)
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